Graphic summary
  • Show / hide key
  • Information


Scientific and technological production
  •  

1 to 50 of 912 results
  • Symmetry-preserving discretization of Navier-Stokes equations on collocated unstructured meshes

     Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Perez Segarra, Carlos David; Verstappen, R.W.C.P.
    Journal of computational physics
    Date of publication: 2014-02-01
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    A fully-conservative discretization is presented in this paper. The same principles followed by Verstappen and Veldman (2003) are generalized for unstructured meshes. Here, a collocated-mesh scheme is preferred over a staggered one due to its simpler form for such meshes. The basic idea behind this approach remains the same: mimicking the crucial symmetry properties of the underlying differential operators, i.e., the convective operator is approximated by a skew-symmetric matrix and the diffusive operator by a symmetric, positive-definite matrix. A novel approach to eliminate the checkerboard spurious modes without introducing any non-physical dissipation is proposed. To do so, a fully-conservative regularization of the convective term is used. The supraconvergence of the method is numerically showed and the treatment of boundary conditions is discussed. Finally, the new discretization method is successfully tested for a buoyancy-driven turbulent flow in a differentially heated cavity.

  • Numerical and experimental study of a flat plate collector with honeycomb transparent insulation and overheating protection system  Open access

     Kessentini, Hamdi
    Defense's date: 2014-02-14
    Universitat Politècnica de Catalunya
    Theses

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    En esta tesis se presenta un captador solar plano (FPC) con aislamiento transparente plástico (TIM ) y un sistema de protección al sobrecalentamiento de bajo coste. Este captador está destinado a suministrar calor en el rango de temperatura de 80 hasta 120°C. Un canal de ventilación es introducido por debajo del absorbedor con una compuerta térmicamente accionada mediante un sistema de resorte de tipo aleación con memoria. Este canal permite proteger el colector en condiciones de estancamiento, preservando al mismo tiempo un buen rendimiento durante el funcionamiento normal. Para este objetivo, un prototipo ha sido construido y probado experimentalmente. En paralelo, diferentes modelos numéricos se han implementado con el objetivo de predecir el comportamiento térmico de este captador. La presente tesis consta de seis capítulos detallados a continuación. En el primer capítulo, se ha realizado una revisión de la literatura con el fin de presentar el estado del arte más actual en el campo de calor solar a temperaturas medias. Esta revisión ha permitido apreciar los últimos hallazgos y desafíos clave relacionados con el tema estudiado y presentar la contribución de este trabajo al conjunto de conocimientos existentes. El segundo capítulo está dedicado a la descripción del dispositivo experimental: descripción técnica del captador, de los diferentes sensores utilizados y los procedimientos de prueba adoptadas. En el tercer capítulo, se ha implementado un modelo numérico de cálculo rápido. Este modelo está basado en la resolución de los diferentes componentes del captador por medio de una plataforma modular orientada a objetos. Pruebas experimentales de interior y exterior se han llevado a cabo y han demostrado la eficacia del sistema de sobrecalentamiento en condiciones de estancamiento. La comparación de los resultados numéricos con experimentos ha demostrado que el código puede reproducir con precisión el funcionamiento térmico del captador. Varias simulaciones paramétricas se han realizado con el fin de optimizar el diseño del captador: se han evalúado 3125 diferentes configuraciones por medio de prototipos virtuales. Los resultados han permitido proponer el diseño más prometedor del FPC con TIM de plástico capaz de trabajar a temperatura de funcionamiento de 100 °C con eficiencia prometedora. En el cuarto capítulo, los elementos más críticos del colector (canal de ventilación y cámara de aire&TIM) han sido sustituidos por objetos CFD en el código modular implementado. En las simulaciones CFD, se ha utilizado Large Eddy Simulation (LES) modelo. Las soluciones numéricas se han validado primero con casos de referencia y a continuación se han verificado los resultados del modelo general del captador por comparación con las pruebas experimentales que han mostrado buena concordancia. Este análisis preliminar ha permitido entender la transferencia de calor y dinámica de fluido a diferentes temperaturas de funcionamiento del colector estudiado.En el quinto capítulo, se ha llevado a cabo un análisis de la transferencia de calor por radiación y conducción en el aislamiento transparente de estructura alveolar. Este analisis ha sido llevada a cobo mediante de la resolución de la ecuación de energía en su forma tridimensional de forma acoplado a la ecuación de transferencia por radiación (RTE). Se ha utilizado el método de volúmenes finitos para la resolución de la RTE. Los resultados numéricos han sido comparados con resultados experimentales de varios TIM dados por diferentes autores en la literatura mostrando acuerdos aceptables. Un estudio paramétrico se ha llevado a cabo para investigar el efecto de la variación de los parámetros ópticos y dimensionales más relevantes del TIM en la transferencia de calor. Finalmente, el último capítulo resume la contribución de esta tesis y presenta las posibles direcciones de investigación futura.

    In this thesis a flat plate collector (FPC) with plastic transparent insulation materials (TIM) and a low-cost overheating protection system destined for heat supply from 80 to 120°C is presented. A ventilation channel with a thermally actuated door is inserted below the absorber allowing to protect the collector from stagnation conditions, while preserving good performance during normal operation. For this objective, a prototype has been constructed and experimentally tested and in parallel, numerical and CFD models have been implemented with the aim of predicting the thermal behavior of this collector. The present thesis consists of six chapters and a brief summary of each one is given below: In the first chapter, a literature survey is carried out in order to present the most updated R&D status in the field of solar heat at medium temperatures. This literature research has allowed to appreciate the latest findings and key challenges related to the studied topic and to present the contribution of this work to the pool of existing knowledge. The second chapter is devoted to the description of the experimental set up. The problem of overheating for FPC with TIM is first pointed out and the technical description of the studied FPC is then presented. The different sensors used and the test procedures adopted during the experiments are presented. In the third chapter, a fast calculation numerical model is implemented. This model is based on the resolution of the different components of the collector by means of a modular object-oriented platform. Indoor and outdoor tests are performed and have shown the effectiveness of the overheating system being able to maintain low enough temperatures at the collector preventing thus the plastic TIM from stagnation conditions. The comparison of the numerical results with experiments has demonstrated that the code can accurately reproduce the performance of the collector. Several parametric simulations are then performed in order to optimize the collector design: 3125 different configurations are evaluated by means of virtual prototyping and the results have allowed to propose the most promising design of a stagnation proof FPC with plastic TIM able to work at operating temperature 100°C with promising efficiency. In the forth chapter, the most critical elements of the collector (ventilation channel and air gap&TIM) have been substituted by high-level CFD objects in the implemented modular object-oriented code. For the detailed numerical simulations, Large Eddy Simulations (LES) modeling is used. In order to speed-up the simulations, parallelisation techniques are used. The numerical solutions are firstly validated with benchmark cases. Then, the general model of the collector is validated by comparison of the numerical results with the indoor experimental tests showing a reasonable agreement. The preliminary CFD simulation results have allowed to understand the heat transfer and fluid flow at different operating temperatures of the studied collector. In the fifth chapter, a heat transfer analysis of the honeycomb TIM is carried out. The combined radiation and conduction heat transfer across the isolated cell is treated by means of the solution of the energy equation in its three dimensional form which is coupled to the Radiative Transfer Equation (RTE). The Finite Volume Method is used for the resolution of the RTE. The numerical results are compared to experimental measurements of the heat transfer coefficient on various honeycomb TIM given by different authors in the literature showing acceptable agreements. Finally, a parametric study is conducted in order to investigate the effect of the variation of the most relevant optical and dimensional parameters of the TIM on the heat transfer. Finally, the last chapter summarizes the contribution of this thesis and discuss the possible directions of future research.

  • Three-dimensional numerical simulation of fluid flow and heat transfer in fin-and-tube heat exchangers at different flow regimes

     Paniagua Sánchez, Leslye
    Defense's date: 2014-07-30
    Department of Heat Engines, Universitat Politècnica de Catalunya
    Theses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    Esta tesis tiene como objetivo unificar dos ramas de trabajo dentro del Centro Tecnológico de Transferencia de Calor (CTTC). Por un lado, se ha realizado un amplio trabajo experimental durante los últimos años. Este trabajo experimental se ha complementado con modelos numéricos para el estudio de intercambiadores de calor de tipo aleta-tubo. Tales modelos numéricos pueden considerarse una herramienta numérica de bajo coste empleada con propósitos de diseño principalmente. Por otro lado, los científicos que trabajan en el centro han desarrollado con éxito un código de Dinámica de Fluidos Computacionales (TermoFluids). Este código de alto rendimiento ha sido ampliamente utilizado principalmente para predecir flujos complejos de gran interés académico. La idea de unir a estas dos ramas, proviene de la necesidad de una plataforma numérica fiable con datos locales propios del flujo y de la transferencia de calor en diversas aplicaciones de intercambiadores de calor. Ser capaz de generar coeficientes locales de transferencia de calor para abastecer con datos propios los modelos existentes de bajo coste, permitirá la correcta predicción del rendimiento de dichos dispositivos.Para lograr estos objetivos, se han hecho varias contribuciones al código TermoFluids que está en continua evolución. Algunas de las mayores cuestiones que se plantean implican la generación de mallas adecuadas y asequibles, la implementación y validación de la condición de contorno periódica tridimensional y el acoplamiento de los diferentes dominios para el estudio de casos con diferentes comportamientos físicos, como desarrollo transitorio e inercia térmica. La turbulencia está presente en la mayoría de los flujos de ingeniería, y los intercambiadores de calor de evaporadores para refrigeración no son una excepción. La presencia de muchos tubos (que actúan como obstáculos para el fluido) colocados en diferentes configuraciones y el hecho de que el flujo también está confinado por aletas, crean características de flujo tridimensionales complejas que tienen generalmente régimen turbulento o en transición. Por lo tanto, se analiza la convección forzada turbulenta en una matriz de pines delimitados por paredes. simulando las grandes escalas de turbulencia y modelando las pequeñas (LES) con el fin de evaluar el desempeño de los tres modelos seleccionados, a saber WALE, QR y VMS. Los números de Reynolds establecidos para el estudio son 3000, 10000 y 30000. Algunos de los principales resultados que se incluyen son el coeficiente de presión alrededor los cilindros, el número de Nusselt promedio en las paredes y la vorticidad del flujo. La parte final de la tesis se dedica a estudiar el flujo tridimensional y los parámetros de transferencia de calor encontrados en un intercambiador de calor de tipo aleta-tubo utilizado para la refrigeración doméstica en equipos de 'no-escarcha'. Las implementaciones del código y el postproceso numéricos se validan en un caso muy similar para un intercambiador de calor con dos filas de tubos a bajos Reynolds para el cual se dispone de datos experimentales. El siguiente análisis que se presenta es una configuración típica para evaporadores 'no-escarcha' con paso de aleta doble (para el que se tiene muy poca información numérica en la literatura). Se considera el acoplamiento conjugado de la transferencia de calor convectiva entre fluido y sólido y conductiva dentro de la aleta. La influencia de algunos parámetros geométricos y de régimen de flujo se analizan con propósitos de diseño. En conclusión, las contribuciones generales de esta tesis junto con el código computacional ya existente, ha demostrado ser capaz de realizar con éxito simulaciones tridimensionales para predecir las características del flujo y los mecanismos responsables de la transferencia de calor en intercambiadores de calor de tipo aleta-tubo.

  • Access to the full text
    Limits of the Oberbeck¿Boussinesq approximation in a tall differentially heated cavity filled with water  Open access

     Kizildag, Deniz; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio; Lehmkuhl Barba, Oriol
    International journal of heat and mass transfer
    Date of publication: 2014-01-01
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    The present work assesses the limits of the Oberbeck¿Boussinesq (OB) approximation for the resolution of turbulent fluid flow and heat transfer inside a tall differentially heated cavity of aspect ratio G = 6.67 filled with water (Pr = 3.27, Ra = 2.12e11). The cavity models the integrated solar collector-storage element installed on an advanced façade. The implications of the Oberbeck¿Boussinesq approximation is submitted to investigation by means of direct numerical simulations (DNS) carried out for a wide range of temperature differences. Non-Oberbeck¿Boussinesq (NOB) effects are found to be relevant, especially beyond the temperature difference of 30 °C, in the estimation of heat transfer, stratification, and flow configuration.

    The present work assesses the limits of the Oberbeck–Boussinesq (OB) approximation for the resolution of turbulent fluid flow and heat transfer inside a tall differentially heated cavity of aspect ratio G = 6.67 filled with water (Pr = 3.27, Ra = 2.12e11). The cavity models the integrated solar collector-storage element installed on an advanced façade. The implications of the Oberbeck–Boussinesq approximation is submitted to investigation by means of direct numerical simulations (DNS) carried out for a wide range of temperature differences. Non-Oberbeck–Boussinesq (NOB) effects are found to be relevant, especially beyond the temperature difference of 30 °C, in the estimation of heat transfer, stratification, and flow configuration.

  • Numerical simulation of multiphase immiscible flow on unstructured meshes

     Jofre Cruanyes, Lluís
    Defense's date: 2014-07-25
    Department of Heat Engines, Universitat Politècnica de Catalunya
    Theses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    Aquesta tesi té com a objectiu desenvolupar una base per a la simulació numèrica de fluids multi-fase immiscibles. Aquesta estratègia, encara que limitada per la potència computacional dels computadors actuals, és potencialment molt important, ja que la majoria de la fenomenologia d'aquests fluids sovint passa en escales temporals i especials on les tècniques experimentals no poden ser utilitzades. En particular, aquest treball es centra en desenvolupar discretitzacions numèriques aptes per a malles no-estructurades en tres dimensions (3-D). En detall, el primer capítol delimita els casos multifásics considerats al cas en que els components són fluids immiscibles. En particular, la tesi es centra en aquells casos en que dos o més fluids diferents són separats per interfases, i per tant, corresponentment anomenats fluxos separats. A més a més, un cop el tipus de flux es determinat, el capítol introdueix les característiques físiques i els models disponibles per predir el seu comportament, així com també la formulació matemàtica i les tècniques numèriques desenvolupades en aquesta tesi.El segon capítol introdueix i analitza un nou mètode ¿Volume-of-Fluid¿ (VOF) apte per a capturar interfases en malles Cartesianes i no-estructurades 3-D. El mètode reconstrueix les interfases com aproximacions ¿piecewise planar approximations¿ (PLIC) de primer i segon ordre, i advecciona els volums amb un algoritme geomètric ¿unsplit Lagrangian-Eulerian¿ (LE) basat en construïr els poliedres a partir de les velocitats dels vèrtexs de les celdes. D'aquesta manera, les situacions de sobre-solapament entre poliedres són minimitzades.Complementant el capítol anterior, el tercer proposa una estratègia de paral·lelització pel mètode VOF. L'obstacle principal és que els costos computacionals estan concentrats en les celdes de l'interfase entre fluids. En conseqüència, si la interfase no està ben distribuïda, les estratègies de ¿domain decomposition¿ (DD) resulten en distribucions de càrrega desequilibrades. Per tant, la nova estratègia està basada en un procés de balanceig de càrrega complementària a la DD. La seva eficiència en paral·lel ha sigut analitzada utilitzant fins a 1024 CPU-cores, i els resultats obtinguts mostren uns guanys respecte l'estratègia DD de fins a 12x, depenent del tamany de la interfase i de la distribució inicial.El quart capítol descriu la discretització de les equacions de Navier-Stokes per a una sola fase, per després estendre-ho al cas multi-fase. Una de les característiques més importants dels esquemes de discretització, a part de la precisió, és la seva capacitat per conservar discretament l'energia cinètica, específicament en el cas de fluxos turbulents. Per tant, aquest capítol analitza la precisió i propietats de conservació de dos esquemes de malla diferents: ¿collocated¿ i ¿staggered¿.L'extensió dels esquemes de malla aptes per els casos de una sola fase als casos multi-fase es desenvolupa en el cinquè capítol. En particular, així com en el cas de la simulació de la turbulència les tècniques numèriques han evolucionat per a preservar discretament massa, moment i energia cinètica, els esquemes de malla per a la discretització de fluxos multi-fase han evolucionat per millorar la seva estabilitat i robustesa. Per lo tant, aquest capítol presenta i analitza dos discretitzacions de malla ¿collocated¿ i ¿staggered¿ particulars, aptes per simular fluxos multi-fase, que afavoreixen la conservació discreta de massa, moment i energia cinètica.Finalment, el capítol sis simula numèricament la inestabilitat de Richtmyer-Meshkov (RM) de dos fluids immiscibles i incompressibles. Aquest capítol es una prova general dels mètodes numèrics desenvolupats al llarg de la tesi. En particular, la inestabilitat ha sigut simulada mitjançant un mètode VOF i un esquema de malla ¿staggered¿. Els resultats numèrics corresponents han demostrat la capacitat del sistema discret en obtenir bons resultats per la inestabilitat RM.

  • Access to the full text
    Wind speed effect on the flow field and heat transfer around a parabolic trough solar collector  Open access

     Amine Hachicha, Ahmed; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    Applied energy
    Date of publication: 2014-10-01
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    Parabolic trough solar collectors are currently one of the most mature and prominent solar technology for the production of electricity. These systems are usually located in an open terrain where strong winds may be found, and could affect their stability and optical performance, as well as the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterise the wind loads and heat transfer coefficients. After the study carried out by the authors in an earlier work (Hachicha et al. 2013) at ReW1=3.9e5, computations are performed at a higher Reynolds number of ReW2=1e6, and for various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow are assessed. The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross-flow. The unsteady flow is studied for three pitch angles: 0; 45 and 90 and different structures and recirculation regions are identified. A spectral analysis around the parabola and its receiver is also carried out in order to detect the most relevant frequencies related to the vortex shedding mechanism which affects the stability of the collector.

    Parabolic trough solar collectors are currently one of the most mature and prominent solar technology for the production of electricity. These systems are usually located in an open terrain where strong winds may be found, and could affect their stability and optical performance, as well as the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterise the wind loads and heat transfer coefficients. After the study carried out by the authors in an earlier work (Hachicha et al. 2013) at ReW1=3.9e5, computations are performed at a higher Reynolds number of ReW2=1e6, and for various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow are assessed. The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross-flow. The unsteady flow is studied for three pitch angles: 0 ; 45 and 90 and different structures and recirculation regions are identified. A spectral analysis around the parabola and its receiver is also carried out in order to detect the most relevant frequencies related to the vortex shedding mechanism which affects the stability of the collector.

  • Access to the full text
    Flow and turbulent structures around simplified car models  Open access

     Aljure Osorio, David E.; Lehmkuhl Barba, Oriol; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2014-06-13
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    External car aerodynamics study has great importance in overall car efficiency and ride stability, being a key element in successful automotive design. The flow over car geometries shows three dimensional and unsteady turbulent characteristics. Additionally, vortex shedding, flow reattachment and recirculation bubbles are also found around the bluff body. These phenomena greatly influence the lift and drag coefficients, which are fundamental for ride stability and energy efficiency, respectively. The aim of the present study is focused on the assessment of different LES models (e.g. VMS or SIGMA models), as well as to show their capabilities of capturing the large scale turbulent flow structures in car-like bodies using relative coarse grids. In order to achieve these objectives, the flow around two model car geometries, the Ahmed and the Asmo cars, is simulated. These generic bluff bodies reproduce the basic fluid dynamics features of real cars. First, the flow over both geometries is studied and compared against experimental results to validate the numerical results. Then, different LES models are used to study the flow in detail and compare the structures found in both geometries.

    External car aerodynamics study has great importance in overall car efficiency and ride stability, being a key element in successful automotive design. The flow over car geometries shows three dimensional and unsteady turbulent characteristics. Additionally, vortex shedding, flow reattachment and recirculation bubbles are also found around the bluff body. These phenomena greatly influence the lift and drag coefficients, which are fundamental for ride stability and energy efficiency, respectively. The aim of the present study is focused on the assessment of different LES models (e.g. VMS or SIGMA models), as well as to show their capabilities of capturing the large scale turbulent flow structures in car-like bodies using relative coarse grids. In order to achieve these objectives, the flow around two model car geometries, the Ahmed and the Asmo cars, is simulated. These generic bluff bodies reproduce the basic fluid dynamics features of real cars. First, the flow over both geometries is studied and compared against experimental results to validate the numerical results. Then, different LES models are used to study the flow in detail and compare the structures found in both geometries.

  • Access to the full text
    Large eddy and direct numerical simulations of a turbulent water-filled differentially heated cavity of aspect ratio 5  Open access

     Kizildag, Deniz; Trias Miquel, Francesc Xavier; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    International journal of heat and mass transfer
    Date of publication: 2014-10-01
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    Natural convection in a differentially heated cavity is characterized by different phenomena such as laminar to turbulent flow transition in the boundary layer, turbulent mixing, and thermal stratification in the core of the cavity. In order to predict the thermal and fluid dynamic behavior of the flow in these cavities, the location of transition to turbulence should be accurately determined. In this work, the performance of three subgrid-scale (SGS) models is submitted to investigation in a water-filled cavity of aspect ratio 5 at Rayleigh number Ra=3e11. To do so, the models are compared with the solution obtained by means of direct numerical simulation. The models tested are: (i) the wall-adapting local-eddy viscosity (WALE) model, (ii) the QR model, (iii) the WALE model within a variational multiscale framework (VMS-WALE). It has been shown that the VMS-WALE and WALE models perform better in estimating the location of transition to turbulence, and thus their overall behavior is more accurate than the QR model. The results have also revealed that the use of SGS models is justified in this flow as the transition location and consequently the flow structure cannot be captured properly if no model is used for the tested spatial resolution.

    Natural convection in a differentially heated cavity is characterized by different phenomena such as laminar to turbulent flow transition in the boundary layer, turbulent mixing, and thermal stratification in the core of the cavity. In order to predict the thermal and fluid dynamic behavior of the flow in these cavities, the location of transition to turbulence should be accurately determined. In this work, the performance of three subgrid-scale (SGS) models is submitted to investigation in a water-filled cavity of aspect ratio 5 at Rayleigh number Ra=3e11. To do so, the models are compared with the solution obtained by means of direct numerical simulation. The models tested are: (i) the wall-adapting local-eddy viscosity (WALE) model, (ii) the QR model, (iii) the WALE model within a variational multiscale framework (VMS-WALE). It has been shown that the VMS-WALE and WALE models perform better in estimating the location of transition to turbulence, and thus their overall behavior is more accurate than the QR model. The results have also revealed that the use of SGS models is justified in this flow as the transition location and consequently the flow structure cannot be captured properly if no model is used for the tested spatial resolution.

  • Numerical modeling and experimental validation of encapsulated PCM thermal energy storage tanks for concentrated solar power plants

     Galione Klot, Pedro Andres; Perez Segarra, Carlos David; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio
    Eurotherm Seminar
    Presentation's date: 2014-05-29
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    A numerical simulation model of thermal energy storage systems with encapsulated PCM is presented. In previous papers (Galione et al., 2011), the performance of cylindrical containers filled with PCM spheres and liquid fluids occupying the rest of the space was studied. Different charging and discharging processes were not only numerically analyzed, but also experimentally tested. Based on these numerical simulation tools for predicting the thermal and fluid dynamic behavior of these systems, new PCM tanks built in different solid filled materials and encapsulated PCMs combined into multi-layer storage with molted salt as heat transfer fluid are here presented. A virtual prototype proposed is numerically analyzed, while performance behavior is presented under different devices (charging and discharging conditions, PCMs temperatures, and multilayer configurations). The efficiency of these new configurations is also compared against conventional thermocline, showing the better option depending on cases and/or plants proposed.

  • A simple approach to discretize the viscous term with spatially varying (eddy-)viscosity

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
    Journal of computational physics
    Date of publication: 2013-11-15
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    A simple approach to discretize the viscous dissipation term in the incompressible Navier¿Stokes equations with spatially varying viscosity is presented. Unlike the case where the viscosity remains constant, its discretization may be quite cumbersome especially for high-order staggered formulations. To circumvent this problem, we propose an alternative form of the viscous term whose discretization is straightforward. Notice that this approach is also suitable for eddy-viscosity models for Large-Eddy Simulation. Moreover, since it is based on already available operators, it can be easily implemented on any structured or unstructured code. The (supra)convergence of the method is numerically shown on both a fourth-order Cartesian staggered and an unstructured collocated formulation.

    A simple approach to discretize the viscous dissipation term in the incompressible Navier–Stokes equations with spatially varying viscosity is presented. Unlike the case where the viscosity remains constant, its discretization may be quite cumbersome especially for high-order staggered formulations. To circumvent this problem, we propose an alternative form of the viscous term whose discretization is straightforward. Notice that this approach is also suitable for eddy-viscosity models for Large-Eddy Simulation. Moreover, since it is based on already available operators, it can be easily implemented on any structured or unstructured code. The (supra)convergence of the method is numerically shown on both a fourth-order Cartesian staggered and an unstructured collocated formulation.

  • DNS and regularization modeling of a turbulent differentially heated cavity of aspect ratio 5

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Perez Segarra, Carlos David; Oliva Llena, Asensio
    International journal of heat and mass transfer
    Date of publication: 2013-01-15
    Journal article

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Access to the full text
    Direct numerical simulation of a NACA0012 in full stall  Open access

     Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Borrell Pol, Ricard
    International journal of heat and fluid flow
    Date of publication: 2013-10-15
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    This work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50,000 (based on the free-stream velocity and the airfoil chord) and angles of attack AOA = 9.25 and AOA = 12 have been carried out. At low-to-moderate Reynolds numbers, NACA0012 exhibits a combination of leading-edge/trailing-edge stall which causes the massive separation of the flow on the suction side of the airfoil. The initially laminar shear layer undergoes transition to turbulence and vortices formed are shed forming a von Kármán like vortex street in the airfoil wake. The main characteristics of this flow together with its main features, including power spectra of a set of selected monitoring probes at different positions on the suction side and in the wake of the airfoil are provided and discussed in detail.

    This work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50,000 (based on the free-stream velocity and the airfoil chord) and angles of attack AOA = 9.25 and AOA = 12 have been carried out. At low-to-moderate Reynolds numbers, NACA0012 exhibits a combination of leading-edge/trailing-edge stall which causes the massive separation of the flow on the suction side of the airfoil. The initially laminar shear layer undergoes transition to turbulence and vortices formed are shed forming a von Kármán like vortex street in the airfoil wake. The main characteristics of this flow together with its main features, including power spectra of a set of selected monitoring probes at different positions on the suction side and in the wake of the airfoil are provided and discussed in detail.

  • Parallel sweep-based preconditioner for the solution of the linear Boltzmann transport equation

     Borrell Pol, Ricard; Colomer Rey, Guillem; Lehmkuhl Barba, Oriol; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2013-12-15
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The Boltzmann transport equation is solved in the context of radiative heat transfer, for an isotropically scattering medium with reflecting boundaries. Under these circumstances, the different ordinates of the angular flux are mutually coupled. We explore here the use of a parallel sweep-based block diagonal preconditioner as a complement of the GMRES solver on the solution of the discretization matrix (which includes all the inter-ordinate couplings). The validity of this approach, when compared to the standard source iteration scheme, is successfully assessed for a significant range of the coupling parameters.

  • Conservation properties of unstructured finite-volume mesh schemes for the Navier-Stokes equations

     Jofre Cruanyes, Lluís; Lehmkuhl Barba, Oriol; Ventosa Molina, Jordi; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Numerical heat transfer. Part B, fundamentals
    Date of publication: 2013-11-09
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The Navier-Stokes equations describe fluid flow by conserving mass and momentum. There are two main mesh discretizations for the computation of these equations, the collocated and staggered schemes. Collocated schemes locate the velocity field at the same grid points as the pressure one, while staggered discretizations locate variables at different points within the mesh. One of the most important characteristic of the discretization schemes, aside from accuracy, is their capacity to discretely conserve kinetic energy, specially when solving turbulent flow. Hence, this work analyzes the accuracy and conservation properties of two particular collocated and staggered schemes by solving various problems.

  • An openCL-based parallel CFD code for simulations on hybrid systems with massively-parallel accelerators

     Gorobets, Andrei; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Procedia engineering
    Date of publication: 2013
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    A parallel finite-volume CFD algorithm for modeling of incompressible flows on hybrid supercomputers is presented. It is based on a symmetry-preserving high-order numerical scheme for structured meshes. A multilevel approach that combines different parallel models is used for large-scale simulations on computing systems with massively-parallel accelerators. MPI is used on the first level within the distributed memory model to couple computing nodes of a supercomputer. On the second level OpenMP is used to engage multiple CPU cores of a computing node. The third level exploits the computing potential of massively-parallel accelerators such as GPU (Graphics Processing Units) of AMD and NVIDIA, or Intel Xeon Phi accelerators of the MIC (Many Integrated Core) architecture. The hardware independent OpenCL standard is used to compute on accelerators of different archi- tectures within a general model for a combination of a central processor and a math co-processor.

  • Direct numerical simulation of incompressible flows on unstructured meshes using hybrid CPU/GPU supercomputers

     Oyarzun Altamirano, Guillermo; Borrell, Ricard; Gorobets, Andrei; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    Procedia engineering
    Date of publication: 2013
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    This paper describes a hybrid MPI-CUDA parallelization strategy for the direct numerical simulation of incompressible flows using unstructured meshes. Our in-house MPI-based unstructured CFD code has been extended in order to increase its performance by means of GPU co-processors. Therefore, the main goal of this work is to take advantage of the current hybrid supercomputers to increase our computing capabilities. CUDA is used to perform the calculations on the GPU devices and MPI to handle the communications between them. The main drawback for the performance is the slowdown produced by the MPI communication episodes. Consequently, overlapping strategies, to hide MPI communication costs under GPU computations, are studied in detail with the aim to achieve scalability when executing the code on multiple nodes.

  • On the large-eddy simulations for the flow around aerodynamic profiles using unstructured grids

     Lehmkuhl Barba, Oriol; Rodriguez Pérez, Ivette Maria; Baez Vidal, Aleix; Oliva Llena, Asensio; Perez Segarra, Carlos David
    Computers and fluids
    Date of publication: 2013-09-15
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    This paper investigates the capabilities of two subgrid-scale (SGS) models suitable for unstructured grids for predicting the complex flow in transitional separated bubbles. The flow over a NACA 0012 airfoil at Reynolds number Re = 5e4 and angles of attack (AOA) AOA = 5° and 8° is here considered. The SGS models investigated are: the wall-adapting eddy viscosity model within a variational multiscale method (VMS-WALE) and the QR eddy-viscosity model. Both are well suited for large-eddy simulations (LES) in complex geometries with unstructured grids. The models are assessed and compared to the results of direct numerical simulations (DNS) on the basis of first and second order statistics. Based on the good results obtained, specially with the VMS-WALE model, challenging simulations at high Reynolds numbers and various AOA are also performed. It has been found that predictions of the lift and drag coefficients agree reasonably well with experimental data.

  • Access to the full text
    Heat transfer analysis and numerical simulation of a parabolic trough solar collector  Open access

     Amine Hachicha, Ahmed; Rodriguez Pérez, Ivette Maria; Capdevila Paramio, Roser; Oliva Llena, Asensio
    Applied energy
    Date of publication: 2013-11-01
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    Parabolic trough solar collector is the most proven industry-scale solar generation technology today available. The thermal performance of such devices is of major interest for optimising the solar field output and increase the efficiency of power plants. In this paper, a detailed numerical heat transfer model based on the finite volume method for these equipment is presented. In the model, the different elements of the receiver are discretised into several segments in both axial and azimuthal directions and energy balances are applied for each control volume. An optical model is also developed for calculating the non-uniform solar flux distribution around the receiver. This model is based on finite volume method and ray trace techniques and takes into account the finite size of the Sun. The solar heat flux is determined as a pre-processing task and coupled to the energy balance model as a boundary condition for the outer surface of the receiver. The set of algebraic equations are solved simultaneously using direct solvers. The model is thoroughly validated with results from the literature. First, the optical model is compared with known analytical solutions. After that, the performance of the overall model is tested against experimental measurements from Sandia National Laboratories and other un-irradiated receivers experiments. In all cases, results obtained shown a good agreement with experimental and analytical results.

    Parabolic trough solar collector is the most proven industry-scale solar generation technology today available. The thermal performance of such devices is of major interest for optimising the solar field output and increase the efficiency of power plants. In this paper, a detailed numerical heat transfer model based on the finite volume method for these equipment is presented. In the model, the different elements of the receiver are discretised into several segments in both axial and azimuthal directions and energy balances are applied for each control volume. An optical model is also developed for calculating the non-uniform solar flux distribution around the receiver. This model is based on finite volume method and ray trace techniques and takes into account the finite size of the Sun. The solar heat flux is determined as a pre-processing task and coupled to the energy balance model as a boundary condition for the outer surface of the receiver. The set of algebraic equations are solved simultaneously using direct solvers. The model is thoroughly validated with results from the literature. First, the optical model is compared with known analytical solutions. After that, the performance of the overall model is tested against experimental measurements from Sandia National Laboratories and other un-irradiated receivers experiments. In all cases, results obtained shown a good agreement with experimental and analytical results.

  • Flow dynamics in the turbulent wake of a sphere at sub-critical Reynolds numbers

     Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2013-07-10
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    Direct numerical simulations of the flow over a sphere have been performed. The computations have been carried out in the sub-critical regime at Re = 3700 and Re = 10,000 (based on the free-stream velocity and the sphere diameter). A parallel unstructured symmetry-preserving formulation has been used for simulating the flow. Computations have been carried out on unstructured grids obtained by the constant-step rotation about the axis of a two-dimensional grid. With this discretisation, the Poisson equation has been solved by means of a Fourier diagonalization method. Particular attention has been devoted to investigate the shear-layer instabilities and its influence in the vortical structures, as well as the wake configuration. The main features of the flow including power spectra of a set of selected monitoring probes at different positions have been described and discussed in detail. Detailed information about turbulent statistics have also been provided.

  • Access to the full text
    Modular object-oriented methodology for the resolution of molten salt storage tanks for CSP plants  Open access

     Rodriguez Pérez, Ivette Maria; Perez Segarra, Carlos David; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    Applied energy
    Date of publication: 2013-09
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    Two-tank molten salt storages are the most widespread thermal energy storage technology within concentrated solar power plants. In spite of this, there are design aspects such as thermal losses control, optimisation of the storage or how these devices scale up with the increase in power capacity of the plant which still should be considered. In this sense, numerical modelling of these systems can be a powerful tool for reducing their cost. The present work aims at modelling molten salt tanks by proposing a parallel modular object-oriented methodology which considers the different elements of the storage (e.g. tank walls, insulation material, tank foundation, molten salt storage media, etc.) as independent systems. Each of these elements can be solved independently and using different levels of modelling (from global to fully three-dimensional models), while at the same time they are linked to each other through their boundary conditions. The mathematical models used, together with some illustrative examples of the application of the proposed methodology, are presented and discussed in detail.

    Two-tank molten salt storages are the most widespread thermal energy storage technology within concentrated solar power plants. In spite of this, there are design aspects such as thermal losses control, optimisation of the storage or how these devices scale up with the increase in power capacity of the plant which still should be considered. In this sense, numerical modelling of these systems can be a powerful tool for reducing their cost. The present work aims at modelling molten salt tanks by proposing a parallel modular object-oriented methodology which considers the different elements of the storage (e.g. tank walls, insulation material, tank foundation, molten salt storage media, etc.) as independent systems. Each of these elements can be solved independently and using different levels of modelling (from global to fully three-dimensional models), while at the same time they are linked to each other through their boundary conditions. The mathematical models used, together with some illustrative examples of the application of the proposed methodology, are presented and discussed in detail.

  • Numerical modelling of a parabolic trough solar collector  Open access

     Amine Hachicha, Ahmed
    Defense's date: 2013-09-30
    Universitat Politècnica de Catalunya
    Theses

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    Las tecnologías de energía solar concentrada (CSP) están ganando cada vez más interés en la generación de electricidad debido al buen potencial para el escalemento las energías renovables a nivel de utilidad. El colector solar cillindro-parabólico (PTC) es el más económico, probado y avanzado de las diferentes tecnologías CSP. El modelado de estos dispositivos es una parte fundamental en la mejora de su diseño y rendemiento que puede representar un considerable aumento de la eficiencia global de las plantas de energía solar. En el tema demodelado y mejora del rendimiento de los PTC y sus absorbente (HCE), el estudio térmico, óptico y aerodinámico del flujo de fluidos y transferencia de calor es una herramienta importante para la optimización de la producción del parque solar y el incremento del rendimiento de la planta solar . Esta tesis se centra en la implementación de una metodología general para simular el comportamiento térmico, óptico y aerodinámico de PTCs . Se plantea una metodología para el modelado térmico de un PTC, teniendo en cuenta el flujo de calor real no uniforme en la direccion azimutal. Aunque ab initio, se consideró el método de volúmenes finitos ( FVM ) para la solución de la ecuación de transferencia radiativa , este ha sido posteriormente descartado entre otras razones debido al alto coste computacional y la incapacidad del método para tratar el tamaño angular del Sol. Para superar estos asuntos , se ha desarollado un nuevo modelo óptico. El nuevo modelo que esta basdo tanto las técnicas de FVM y de ray tracing, utiliza un enfoque numérico - geométrico para considerar el cono óptico. Se aborda el effecto de differentes factores, tales como ángulo de incidencia , la concentración geométrica y el ángulo de borde en la distribución del flujo de calor solar. La precisión del nuevo modelo se ha verificado y se ha obentido mejores resultados que el Monte Carlo Ray Tracing MCRT. Por otra parte, se ha analizado el comportamiento térmico del PTC teniendo en cuenta la distribución no uniforme de flujo solar en la direccion azimutal. Se ha desarrollado un modelo de rendimiento general basado en un balance energético sobre el HCE. Las pérdidas de calor y el rendimiento térmico se han determinado y validado con respecto a los experimentos de laboratorios de Sandia. Se muestro tambien la similitud entre el perfil de temperatura del absorbedor y del vidrio con la distribución de flujo solar. Además , se ha considerado las pérdidas de calor por convección hacia la atmósfera y el efecto del flujo de viento en la fuerza aerodinámica que actúa sobre la structura del PTC. Para ello se llevan a cabo, simulaciones numéricas detalladas basadas en Large Eddy Simulations (LES) para dos números de Reynolds de Re_W1= 3,6 10^5 y RE_ W2=1 .10^6. Estos numéros corresponden a condiciones similares de trabajo que aquellos encuentrado en las plantas solares de Eurotrough PTC. El estudio ha considerado diferentes ángulos de paso imitantdo las condiciones reales del mecanismo de seguimiento de PTC a lo largo del día . Las cargas aerodinámicas, es decir, los coeficientes de arrastre y elevación, han sido calculados y validados respecto a los mediciones realizadas en túneles de viento . Se muestra la independencia de los coeficientes aerodinámicos respecto a los números de Reynolds en los rangos estudiado. En cuanto a la transferencia de calor por convección alrededor del absorbedor, se han calculado los promediados del número de Nusselt local para los diferentes ángulos de paso y números de Reynolds y se ha analizado el efecto de la parábola en las pérdidas de calor. Por último, pero no menos importante, el análisis detallado de las furezas transitorias que actúan sobre la estructura de PTC han sido conducidas mediante espectro de potencia en varias sondas . El análisis ha detectado un aumento de la inestabilidad al mover el PTC para angulos de paso intermedios.

    Concentrated Solar Power (CSP) technologies are gaining increasing interest in electricity generation due to the good potential for scaling up renewable energy at the utility level. Parabolic trough solar collector (PTC) is economically the most proven and advanced of the various CSP technologies. The modelling of these devices is a key aspect in the improvement of their design and performances which can represent a considerable increase of the overall efficiency of solar power plants. In the subject of modelling and improving the performances of PTCs and their heat collector elements (HCEs), the thermal, optical and aerodynamic study of the fluid flow and heat transfer is a powerful tool for optimising the solar field output and increase the solar plant performance. This thesis is focused on the implementation of a general methodology able to simulate the thermal, optical and aerodynamic behaviour of PTCs. The methodology followed for the thermal modelling of a PTC, taking into account the realistic non-uniform solar heat flux in the azimuthal direction is presented. Although ab initio, the finite volume method (FVM) for solving the radiative transfer equation was considered, it has been later discarded among other reasons due to its high computational cost and the unsuitability of the method for treating the finite angular size of the Sun. To overcome these issues, a new optical model has been proposed. The new model, which is based on both the FVMand ray tracing techniques, uses a numerical-geometrical approach for considering the optic cone. The effect of different factors, such as: incident angle, geometric concentration and rim angle, on the solar heat flux distribution is addressed. The accuracy of the new model is verified and better results than the Monte Carlo Ray Tracing (MCRT) model for the conditions under study are shown. Furthermore, the thermal behaviour of the PTC taking into account the nonuniform distribution of solar flux in the azimuthal direction is analysed. A general performance model based on an energy balance about the HCE is developed. Heat losses and thermal performances are determined and validated with Sandia Laboratories tests. The similarity between the temperature profile of both absorber and glass envelope and the solar flux distribution is also shown. In addition, the convection heat losses to the ambient and the effect of wind flow on the aerodynamic forces acting on the PTC structure are considered. To do this, detailed numerical simulations based on Large Eddy simulations (LES) are carried out. Simulations are performed at two Reynolds numbers of ReW1 = 3.6 × 105 and ReW2 = 1 × 106. These values corresponds to working conditions similar to those encountered in solar power plants for an Eurotrough PTC. The study has also considered different pitch angles mimicking the actual conditions of the PTC tracking mechanism along the day. Aerodynamic loads, i.e. drag and lift coefficients, are calculated and validatedwith measurements performed in wind tunnels. The indepen-dence of the aerodynamic coefficients with Reynolds numbers in the studied range is shown. Regarding the convection heat transfer taking place around the receiver, averaged local Nusselt number for the different pitch angles and Reynolds numbers have been computed and the influence of the parabola in the heat losses has been analysed. Last but not the least, the detailed analysis of the unsteady forces acting on the PTC structure has been conducted by means of the power spectra of several probes. The analysis has led to detect an increase of instabilities when moving the PTC to intermediate pitch angles. At these positions, the shear-layers formed at the sharp corners of the parabola interact shedding vortices with a high level of coherence. The coherent turbulence produces vibrations and stresses on the PTC structure which increase with the Reynolds number and eventually, might lead to structural failure under certain conditions.

  • DRAGON - Understanding the DRAG crisis: ON the flow past a circular cylinder from critical to trans-critical Reynolds numbers

     Oliva Llena, Asensio; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Chiva Segura, Jorge
    Participation in a competitive project

     Share

  • Vortex dynamics and coherent structures in swirling flows (Cont.)

     Oliva Llena, Asensio; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Baez Vidal, Aleix
    Participation in a competitive project

     Share

  • Pla d'Actuació CTTC

     Oliva Llena, Asensio
    Participation in a competitive project

     Share

  • Q-00204

     Oliva Llena, Asensio
    Participation in a competitive project

     Share

  • Access to the full text
    Low-frequency unsteadiness in the vortex formation region of a circular cylinder  Open access

     Lehmkuhl Barba, Oriol; Rodriguez Pérez, Ivette Maria; Borrell Pol, Ricard; Oliva Llena, Asensio
    Physics of fluids
    Date of publication: 2013-08-23
    Journal article

    Read the abstract Read the abstract Access to the full text Access to the full text Open in new window  Share Reference managers Reference managers Open in new window

    The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = UrefD/¿) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of fm = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.

    The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = U ref D/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of f m = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.

    Electronic version of an article published as "Physics of fluids", vol. 25, no 8, 2013. DOI: http://dx.doi.org/10.1063/1.4818641.

  • A parallel MPI+OpenMP+OpenCL algorithm for hybrid supercomputations of incompressible flows

     Gorobets, Andrei; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2013-12
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The work is devoted to the development of efficient parallel algorithms for large-scale simulations of incompressible flows on hybrid supercomputers based on massively-parallel accelerators. The governing equations are discretized using a high-order finite-volume scheme for Cartesian staggered meshes with the only restriction that, at least, one direction is periodic. Its ¿classical¿ MPI + OpenMP parallel implementation for CPUs was designed to scale till 100,000 CPU cores. The new hybrid algorithm is developed on a base of a multi-level parallel model that exploits several layers of parallelism of a modern hybrid supercomputer. In this model, MPI and OpenMP are used on the first two levels to couple nodes of a supercomputer and to engage its CPU cores. Then, computing accelerators are further used by means of the hardware independent OpenCL computing standard. In this way, the implementation is adapted to a general computing model with central processors and math co-processors. In this paper the work is focused on adapting the basic operations of the algorithm to architectures of Graphics Processing Units (GPU) without considering the multi-GPU communication scheme. Technology of porting the code to OpenCL is described, certain optimization approaches are presented and relevant performance results obtaining up to 80¿90 GFLOPS on a GPU accelerator are demonstrated. Moreover, the experience with different GPU architectures is summarized and a comparison based on the particular application is given for AMD and NVIDIA GPUs as well as for CUDA and OpenCL frameworks.

  • Improved semi-analytical method for air curtains prediction

     Giraldez Garcia, Hector; Perez Segarra, Carlos David; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    Energy and buildings
    Date of publication: 2013-11
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The present study is devoted to the analysis and prediction of the efficiency of air curtains. The attention is focused on improving existing semi-analytical methods with the information from CFD simulations and experimental measurements. The goal is to obtain an accurate simplified model which describes the three-dimensional behaviour of the air jet without requiring large time consuming calculations. The interaction of the air curtain with other agents, e.g. pedestrians and flying insects, is also revised. The model is validated against both experimental data and advanced LES calculations carried out by the authors. Furthermore, a parametric study shows the air curtain behaviour for different configurations.

    The present study is devoted to the analysis and prediction of the efficiency of air curtains. The attention is focused on improving existing semi-analytical methods with the information from CFD simulations and experimental measurements. The goal is to obtain an accurate simplified model which describes the three-dimensional behaviour of the air jet without requiring large time consuming calculations. The interaction of the air curtain with other agents, e.g. pedestrians and flying insects, is also revised. The model is validated against both experimental data and advanced LES calculations carried out by the authors. Furthermore, a parametric study shows the air curtain behaviour for different configurations.

  • Numerical simulation of wind flow around a parabolic trough solar collector

     Amine Hachicha, Ahmed; Rodriguez Pérez, Ivette Maria; Castro Gonzalez, Jesus; Oliva Llena, Asensio
    Applied energy
    Date of publication: 2013-07-01
    Journal article

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The use of parabolic trough solar technology in solar power plants has been increased in recent years. Such devices are located in open terrain and can be the subject of strong winds. As a result, the stability of these devices to track accurately the sun and the convection heat transfer from the receiver tube could be affected. In this paper, a detailed numerical aerodynamic and heat transfer model based on Large Eddy Simulations (LES) modelling for these equipments is presented. First, the model is verified on a circular cylinder in a cross-flow. The drag forces and the heat transfer coefficients are then validated with available experimental measurements. After that, simulations are performed on an Eurotrough solar collector to study the fluid flow and heat transfer around the solar collector and its receiver. Computations are carried out for a Reynolds number of ReW = 3.6 × 105 (based on the aperture) and for various pitch angles (¿ = 0°, 45°, 90°, 135°, 180°, 270°). The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. Instantaneous velocity field is also studied and compared to aerodynamic coefficients for different pitch angles. The time-averaged flow is characterised by the formation of several recirculation regions around the solar collector and the receiver tube depending on the pitch angle. The study also presents a comparative study of the heat transfer coefficients around the heat collector element with the circular cylinder in a cross-flow and the effect of the pitch angle on the Nusselt number.

    The use of parabolic trough solar technology in solar power plants has been increased in recent years. Such devices are located in open terrain and can be the subject of strong winds. As a result, the stability of these devices to track accurately the sun and the convection heat transfer from the receiver tube could be affected. In this paper, a detailed numerical aerodynamic and heat transfer model based on Large Eddy Simulations (LES) modelling for these equipments is presented. First, the model is verified on a circular cylinder in a cross-flow. The drag forces and the heat transfer coefficients are then validated with available experimental measurements. After that, simulations are performed on an Eurotrough solar collector to study the fluid flow and heat transfer around the solar collector and its receiver. Computations are carried out for a Reynolds number of Re W = 3.6 x 10(5) (based on the aperture) and for various pitch angles (h=0,45,90, 135, 80, 270). The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. Instantaneous velocity field is also studied and compared to aerodynamic coefficients for different pitch angles. The time-averaged flow is characterised by the formation of several recirculation regions around the solar collector and the receiver tube depending on the pitch angle. The study also presents a comparative study of the heat transfer coefficients around the heat collector element with the circular cylinder in a cross-flow and the effect of the pitch angle on the Nusselt number.

  • Flow past a NACA0012 airfoil: from laminar separation bubbles to fully stalled regime

     Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, Asensio
    ERCOFTAC Workshop on Direct and Large-Eddy Simulation
    Presentation's date: 2013-04-04
    Presentation of work at congresses

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Direct and large-eddy simulation of non-oberbeck-boussinesq effects in a turbulent differentially heated cavity

     Kizildag, Deniz; Trias Miquel, Francesc Xavier; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    ERCOFTAC Workshop on Direct and Large-Eddy Simulation
    Presentation's date: 2013-04-04
    Presentation of work at congresses

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Progress on eddy-viscosity models for LES: new differential operators and discretization methods

     Trias Miquel, Francesc Xavier; Verstappen, Roel; Gorobets, Andrei; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2013-09-04
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers. Therefore, dynamically less complex mathematical formulations are necessary for coarse-grain simulations. Eddy-viscosity models for Large-Eddy Simulation (LES) is an example thereof: they rely on differential operators that should be able to capture well different flow configurations (laminar and 2D flows, near-wall behavior, transitional regime...). In the present work, several differential operators are derived from the criterion that vortex-stretching mechanism must stop at the smallest grid scale. Moreover, since the discretization errors may play a n important role a novel approach to discretize the viscous term with spatially varying eddy-viscosity is used. It is based on basic operators; therefore, the implementation is straightforward even for staggered formulations.

  • Large eddy simulation model assessment of the turbulent flow through dynamic compressor valves

     Estruch Perez, Olga; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio; Perez Segarra, Carlos David
    International Conference on Compressors and Coolants
    Presentation's date: 2013-09-03
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    The present paper attempts the dynamic simulation of the fluid flow through the valve reed taking into account valve movement due to piston displacement. This work widens previous studies based on numerical experiments with static geometry and constant boundary conditions. Hence, in this work attends the newly in-house implemented CFD and moving mesh coupled code TermoFluids. The CFD solver consists of a three-dimensional explicit finite volume fractional-step algorithm formulated in a second-order, conservative and collocated unstructured grid arrangement. Large eddy simulation is performed to solve the turbulent flow, using the subgrid scale WALE model. A radial basis function interpolation procedure is used to dynamically move the mesh according with the displacement of the valve. A simplified geometry of an axial hole plus a radial diffuser with a piston based inlet condition is considered. The valve dynamics is assumed to be given by a law according modal analysis of valve reed.

  • Large eddy simulation of hydrogen autoignition in a preheated turbulent co-flow

     Muela Castro, Jordi; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Ventosa Molina, Jordi
    Mediterranean Combustion Symposium
    Presentation's date: 2013-09
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    The autoignition process of a hydrogen jet into a preheated turbulent air stream is numerically studied. A Progress-variable (PV) model with the turbulence-chemistry interactions modelled using a Presumed Conditional Moment (PCM) closure has been used. Furthermore, the same case is studied using a Finite Rates model without closure for the reaction rate. The PV-PCM model reproduces satisfactorily the physical behaviour found in the experiments, although the model tends to underpredict the autoignition length. The results of the Finite Rates model also capture accurately the autoignition phenomenology observed experimentally and the autoignition lengths are closer to those obtained in the experiment.

  • Large eddy simulation of a turbulent jet diffusion flame using unstructured meshes

     Muela Castro, Jordi; Ventosa Molina, Jordi; Lehmkuhl Barba, Oriol; Perez Segarra, Carlos David; Oliva Llena, Asensio
    Mediterranean Combustion Symposium
    Presentation's date: 2013-09
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    In this work an hydrogen enriched methane flame in a non-premixed configuration is studied, which corresponds to the DLR flame A. Large Eddy Simulation (LES) will be used to numerically analyse the case. Unstructured meshes are used and coupled with conservative discretisations of the differential operators. Chemical kinetics are modelled using the Flamelet/Progress-Variable model, taking into account differential diffusion effects. Computed first and second moments of the transported variables are shown to be in agreement with the experimental data.

  • Large eddy simulation of a turbulent jet diffusion flame using the Flamelet-Progress Variable model

     Lehmkuhl Barba, Oriol; Ventosa Molina, Jordi; Perez Segarra, Carlos David; Oliva Llena, Asensio
    European Combustion Meeting
    Presentation's date: 2013-06
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    In this work an hydrogen enriched methane flame in a non-premixed configuration is studied, which corresponds to the DLR flame A. Large Eddy Simulation (LES) will be used to numerically analyse the case. Unstructured meshes are used and coupled with conservative discretisations of the differential operators. Chemical kinetics are modelled using the Flamelet/Progress-Variable model, taking into account differential diffusion effects. Computed first and second moments of the transported variables are shown to be in agreement with the experimental data.

    In this work an hydrogen enriched methane flame in a non-premixed configuration is studied, which corresponds to the DLR flame A. Large Eddy Simulation (LES) will be used to numerically analyse the case. Unstructured meshes are used and coupled with conservative discretisations of the differential operators. Chemical kinetics are modelled using the Flamelet/Progress-Variable model, taking into account differential diffusion effects. Computed first and second moments of the transported variables are shown to be in agreement with the experimental data.

  • Transient and dynamic numerical simulation of the fluid flow through valves based on large eddy simulation models

     Estruch Perez, Olga; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio; Perez Segarra, Carlos David
    International Conference on Compressors and Their Systems
    Presentation's date: 2013-09-09
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The present paper attempts the dynamic simulation of the fluid flow through valve reed using the in-house implemented CFD and moving mesh coupled code TermoFluids. The CFD solver is based on a parallel, second-order, conservative and unstructured finite volume discretization. Large eddy simulation is performed to solve the turbulent flow, using the subgrid scale WALE model. The moving mesh technique uses RBF interpolation. As a preliminary approach, a simplified geometry of an axial hole plus a radial diffuser with a piston based inlet condition is considered. The valve dynamics is modelled by a specific law according modal analysis of valve reed. © The author(s) and/or their employer(s), 2013.

  • Numerical simulation tools for energy efficiency in buildings. HAM transfer in facades coupled with CFD models for air distribution

     Damle, Rashmin Mohan; Lehmkuhl Barba, Oriol; Rigola Serrano, Joaquim; Oliva Llena, Asensio
    REHVA World Congress
    Presentation's date: 2013-06
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    Buildings represent a major part of the world energy requirement. The simulation of combined heat, air and moisture (HAM) and pollutant transfer in this context is important to predict the indoor air quality, along with the thermal comfort. Moreover, it is important to have appropriate levels of indoor humidity along with the room temperature as movement of water vapor through the building envelope causes a lot of harm to the building structure and reduces the quality of its thermal insulation leading to higher energy demand. The knowledge of the peak loads, temperatures, humidity levels can help to optimize the building design and therefore results in energy efficient buildings. In this work a modular object-oriented building simulation tool (NEST), capable of coupling different levels of simulation models, allowing the simulation of heat, air, moisture distribution (multizone model, envelope model, room analysis and HVAC system) is presented. The modular approach gives flexibility of choosing a model for each element and to have different levels of modeling for different elements in the system. Special attention has been focused on: the large eddy simulation turbulence models used for the room air dynamics and pollutants distribution transport and high performance parallel software. Parallelization of the building simulation is necessary if some critical processes/zones need to be modeled with more detail for reducing computational time. The main focus of this article is to couple the HAM models for the building envelope with CFD HT models with heat, moisture and pollutant transfer models for room airflow.

  • Blending regularization and large-eddy simulation. From homogeneous isotropic turbulence to wind farm boundary layers

     Folch Flórez, David; Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2013-08
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The incompressible Navier-Stokes equations form an excellent mathematical model for turbulent flows. However, direct simulations at high Reynolds numbers are not feasible because the convective term produces far too many relevant scales of motion. Therefore, in the foreseeable future numerical simulations of turbulent flows will have to resort to models of the small scales. Large-eddy simulation (LES) and regularization models are examples thereof. In the present work, we propose to combine both approaches. Restoring the Galilean invariance of the regularization method results into an additional hyperviscosity term. This approach provides a natural blending between regularization and LES. The performance of these recent improvements will be assessed through application to homogeneous isotropic turbulence, a turbulent channel flow and a wind-farm turbulent boundary layer.

    The incompressible Navier-Stokes equations form an excellent mathematical model for turbulent flows. However, direct simulations at high Reynolds numbers are not feasible because the convective term produces far too many relevant scales of motion. Therefore, in the foreseeable future numerical simulations of turbulent flows will have to resort to models of the small scales. Large-eddy simulation (LES) and regularization models are examples thereof. In the present work, we propose to combine both approaches. Restoring the Galilean invariance of the regularization method results into an additional hyperviscosity term. This approach provides a natural blending between regularization and LES. The performance of these recent improvements will be assessed through application to homogeneous isotropic turbulence, a turbulent channel flow and a wind-farm turbulent boundary layer.

  • Experimental and numerical investigation of H2O vapor absorption processes in falling film of LiBr aqueous solution in vertical tubes

     Castro Gonzalez, Jesus; Farnos Baulenas, Joan; Garcia Rivera, Eduardo; Oliva Llena, Asensio
    World Conference on Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics
    Presentation's date: 2013-07-20
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    The LiBr-H2O absorption systems are used mainly in large cooling capacity applications (industry, large buildings, etc.), therefore require water from cooling towers to reject heat. However, if middle and low capacity are required (commercial and residential systems), absorption machines should be air-cooled in order to become competitive [1-4]. The absorber represents a major critical component in absorption systems and one of the key issues, in it is the combined heat and mass transfer in the absorption process. For this reason the development of mathematical models for the simulation and experimental data for the validation are always useful tools for the design and improvement of falling film vertical absorbers. A testing device has been designed and built for reproducing absorption phenomena in vertical tubes with the primary objective to obtain experimental data in LiBr-H2O vertical absorbers. The versatility of the experiment allows to obtain a wide range of data.

  • Numerical simulation of heat transfer and fluid flow in flat integrated collector storage system

     Souaihi, Oussama; Kessentini, Hamdi; Oliva Llena, Asensio; Bouden, Chiheb
    International Symposium on Computational and Experimental Investigations on Fluid Dynamics
    Presentation's date: 2013-03-19
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The aim of this work is the numerical simulation of the heat transfer and fluid flow in flat integrated collector storage (ICS) water heater. A general numerical algorithm based on the resolution of the energy balance equations of the different elements of this solar system is developed. In order to evaluate the convective heat transfer coefficients used in the balance equations, a CFD numerical model is carried out for the simulation of the most critical elements of the studied ICS water heater (Air cavity and storage tank). The developed CFD model is a 2D object oriented code applied to structured meshes, which can handle the thermal and fluid dynamic problems in simple geometries. Finite volume second order schemes for spatial discretization and second order explicit time integration were used. The pressure-velocity linkage was solved by means of an explicit fractional step procedure. The numerical code is validated by comparison with benchmark results of closed cavities found in the literature which showed a good agreement. The general model is then used to perform simulations in order to predict the thermal behavior of the ICS system in different weather conditions and different operation modes (stagnation, draining ).

  • Large-eddy simulations of turbulent flow around a wall-mounted cube using an adaptive mesh refinement approach

     Antepara Zambrano, Oscar; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Favre, Federico
    European Turbulence Conference
    Presentation's date: 2013-09-04
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    In the present work two LES models for predicting turbulent flow and an Adaptive Mesh Refinement (AMR) technique are proposed and tested for a fully 3D geometry: turbulent flow around a wall-mounted cube at Reh=7235. The wall-adapting eddy viscosity model within a variational multiscale method (VMS-WALE) and the QR model are tested to predict the flow around the body. The numerical algorithm used to solve the governing equations preserves the symmetry and conservation properties. AMR algorithm is applied to get enough grid-resolution to solve the vortical structures near the body, adapting the mesh according to physics-based refinement criteria. High order conservative schemes are applied in the connection between coarse and fine regions. The numerical results obtained are assessed and compared to the results of the direct numerical simulations (DNS) on the basis of first and second order statistics.

    In the present work two LES models for predicting turbulent flow and an Adaptive Mesh Refinement (AMR) technique are proposed and tested for a fully 3D geometry: turbulent flow around a wall-mounted cube at Reh=7235. The wall-adapting eddy viscosity model within a variational multiscale method (VMS-WALE) and the QR model are tested to predict the flow around the body. The numerical algorithm used to solve the governing equations preserves the symmetry and conservation properties. AMR algorithm is applied to get enough grid-resolution to solve the vortical structures near the body, adapting the mesh according to physics-based refinement criteria. High order conservative schemes are applied in the connection between coarse and fine regions. The numerical results obtained are assessed and compared to the results of the direct numerical simulations (DNS) on the basis of first and second order statistics.

    Postprint (author’s final draft)

  • An OpenCL-based parallel CFD code for simulations on hybrid systems with massively-parallel accelerators

     Gorobets, Andrei; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2013-05-23
    Presentation of work at congresses

    View View Open in new window  Share Reference managers Reference managers Open in new window

  • Direct numerical simulation of incompressible flows on unstructured meshes using hybrid CPU/GPU supercomputers

     Oyarzun Altamirano, Guillermo; Borrell, Ricard; Gorobets, Andrei; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2013-05-23
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    This paper describes a hybrid MPI-CUDA parallelization strategy for the direct numerical simulation of incompressible flows using unstructured meshes. Our in-house MPI-based unstructured CFD code has been extended in order to increase its performance by means of GPU co-processors. Therefore, the main goal of this work is to take advantage of the current hybrid supercomputers to increase our computing capabilities. CUDA is used to perform the calculations on the GPU devices and MPI to handle the communications between them. The main drawback for the performance is the slowdown produced by the MPI communication episodes. Consequently, overlapping strategies, to hide MPI communication costs under GPU computations, are studied in detail with the aim to achieve scalability when executing the code on multiple nodes.

  • Study of the autoignition of a hydrogen jet in a turbulent co-flow of heated air using LES modelling

     Muela Castro, Jordi; Lehmkuhl Barba, Oriol; Ventosa Molina, Jordi; Oliva Llena, Asensio
    European Combustion Meeting
    Presentation's date: 2013-06
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The autoignition process of a hydrogen jet into a preheated turbulent air stream is numerically studied. A Progress-variable model with the turbulence-chemistry interaction s modelled using a Presumed Conditional Moment (PCM) closure has been used. Furthermore, the same case is studied using a Finite Rates model without closure for the reaction rate. The PV-PCM model reproduces satisfactorily the physical behaviour found in the experiments, although the model tends to underpredict the autoignition length. The results of the Finite Rates also capture accurately the autoignition phenomenology observed experimentally and the autoignition lengths are closer to those obtained in the experiment.

    The autoignition process of a hydrogen jet into a preheated turbulent air stream is numerically studied. A Progress-variable model with the turbulence-chemistry interaction s modelled using a Presumed Conditional Moment (PCM) closure has been used. Furthermore, the same case is studied using a Finite Rates model without closure for the reaction rate. The PV-PCM model reproduces satisfactorily the physical behaviour found in the experiments, although the model tends to underpredict the autoignition length. The results of the Finite Rates also capture accurately the autoignition phenomenology observed experimentally and the autoignition lengths are closer to those obtained in the experiment.

  • On the CFD&HT of the flow around a parabolic trough solar collector under real working conditions

     Amine Hachicha, Ahmed; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    Solar Power and Chemical Energy Systems Conference
    Presentation's date: 2013-09
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    Parabolic trough solar collector is currently one of the most mature and prominent solar applications for production of electricity. These systems are usually located in open terrain where strong winds may occur and affect their stability and optical performance, as well as, the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterize the wind loads and the heat transfer coefficients. Computations are performed for two Reynolds number ReW1=3.9×10^5 and ReW2=1×10^6 and various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow have been assessed. The aerodynamic coefficients are calculated around the solar collectorand validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross flow.

  • Advanced CFD&HT numerical modeling of solar tower receivers

     Colomer Rey, Guillem; Chiva Segura, Jorge; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    Solar Power and Chemical Energy Systems Conference
    Presentation's date: 2013-09-18
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    This paper presents an advanced methodology for the detailed modeling of the heat transfer and fluid dynamics phenomena in solar tower receivers. It has been carried out in the framework of a more ambitious enterprise which aims at modeling all the complex heat transfer and fluid dynamics phenomena present in central solar receivers. The global model is composed of 4 sub-models (heat conduction, two-phase flow, thermal radiation and natural convection) which are described.

  • A parallel object oriented code framework for numerical simulation of reciprocating compressors - introduction of solid parts modeling

     Lopez Mas, Joan; Rigola Serrano, Joaquim; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    International Conference on Compressors and Their Systems
    Presentation's date: 2013-09-09
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    A partitioned coupled approach is employed to modeling a reciprocating compressor in a modular way. The approach allows the implementation of an object oriented parallel code framework for simulation of multiphysics systems in general and hermetic reciprocating compressors in particular. Several works in compressor modeling have been presented before. Those works already addressed the resolution of the fluid flow by using this code framework. Now, a new model for simulation of solid components has been developed. In this way the thermal effect of the solid parts on the working fluid can be considered as well. Some numerical results are presented to show first achievements in this research line. © The author(s) and/or their employer(s), 2013.

  • Direct numerical simulation of viscoplastic-type non-Newtonian fluid flows in stenosed arteries

     Carmona Muñoz, Angel; Lehmkuhl Barba, Oriol; Perez Segarra, Carlos David; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2013-09-01
    Presentation of work at congresses

    Read the abstract Read the abstract View View Open in new window  Share Reference managers Reference managers Open in new window

    The aim of this work is to provide DNS solutions for turbulence flows of viscoplastic-type non-Newtonian fluids and thus contribute to gain insight into the underlying physics of the non-Newtonian turbulent flows. This knowledge may be useful, among many other things, for developing more accurate turbulence models which describe better the implicit physics of this subject. Nevertheless, from our point of view, few DNS solutions of viscoplastic-type non-Newtonian fluid flows have been provided with this objective, despite the growing presence of these kind of fluids in the field of CFD simulations.

    The aim of this work is to provide DNS solutions for turbulence flows of viscoplastic-type non-Newtonian fluids and thus contribute to gain insight into the underlying physics of the non-Newtonian turbulent flows. This knowledge may be useful, among many other things, for developing more accurate turbulence models which describe better the implicit physics of this subject. Nevertheless, from our point of view, few DNS solutions of viscoplastic-type non-Newtonian fluid flows have been provided with this objective, despite the growing presence of these kind of fluids in the field of CFD simulations.

  • Dynamic wall modelling for large-eddy simulation of wind turbine dedicated airfoils

     Calafell Sandiumenge, Joan; Lehmkuhl Barba, Oriol; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2013-09
    Presentation of work at congresses

    Read the abstract Read the abstract  Share Reference managers Reference managers Open in new window

    This work aims at modelling the flow behaviour past airfoils used for wind turbine blades at high Reynolds number and large angles of attack (AoA). A previous work has been carried out on the airfoil profiles of DU-93-W-210, DU-91-W2-250 and FX-77-W-500 with a parallel unstructured symmetry preserving formulation together with wall-adapting Local-eddy viscosity model within a variational multi-scale framework (VMS-WALE) as a subgrid-scale model. However for the FX-77-W-500 profile, a mismatch between experimental results and numerical ones has been observed for the drag coefficient. To overcome this disagreement, a dynamic wall model has been implemented in order to compute accurately the wall shear stress without increasing prohibitively the computational costs.