Nowadays, hydropower plays an essential role in the energy market. Due to their fast response and regulation capacity, hydraulic turbines operate at off-design conditions with a high number of starts and stops. In this situation, dynamic loads and stresses over the structure are high, registering some failures over time, especially in the runner. Therefore, it is important to know the dynamic response of the runner while in operation, i.e., the natural frequencies, damping and mode shapes, in order to avoid resonance and fatigue problems. Detecting the natural frequencies of hydraulic turbine runners while in operation is challenging, because they are inaccessible structures strongly affected by their confinement in water. Strain gauges are used to measure the stresses of hydraulic turbine runners in operation during commissioning. However, in this paper, the feasibility of using them to detect the natural frequencies of hydraulic turbines runners while in operation is studied. For this purpose, a large Francis turbine runner (444 MW) was instrumented with several strain gauges at different positions. First, a complete experimental strain modal testing (SMT) of the runner in air was performed using the strain gauges and accelerometers. Then, the natural frequencies of the runner were estimated during operation by means of analyzing accurately transient events or rough operating conditions.
Hydropower plays a key role in the actual energy market due to its fast response and regulation capacity. In that way, hydraulic turbines are increasingly demanded to work at off-design conditions, where complex flow patterns and cavitation appear, especially in Francis turbines. The draft tube cavitation surge is a hydraulic phenomenon that appears in Francis turbines below and above its Best Efficiency Point (BEP). It is a low frequency phenomenon consisting of a vortex rope in the runner outlet and draft tube, which can become unstable when its frequency coincides with a natural frequency of the hydraulic circuit. At this situation, the output power can significantly swing, endangering the electrical grid stability. This study is focused on the detection of these instabilities in Francis turbines and their relationship with the output power swings. To do so, extensive experimental tests for different operating conditions have been carried out in a large prototype Francis turbine (444 MW of rated power) within the frame of the European Project Hyperbole (FP7-ENERGY-2013-1). Several sensors have been installed in the hydraulic circuit (pressure sensors in the draft tube, spiral casing, and penstock), in the rotating and static structures (vibration sensors, proximity probes, and strain gauges in the runner and in the shaft), as well as in the electrical side (output power, intensity, and voltage). Moreover, a numerical Finite Element Method (FEM) has been also used to relate the hydraulic excitation with the output power swing.
The global trend on energy integration and building efficiency is making both researchers
and building developers look for technical solutions to use facade surfaces for energy harvesting.
In this work, the assessment of the thermal performance of a double-skin facade (DSF) with a venetian
blind-type of structure used as a solar thermal collector by means of computational fluid dynamics
(CFD) is presented. A Venetian blind collector would allow for heat rejection/energy harvesting and
exterior views simultaneously and can be easily integrated into the DSF aesthetical design. For the
purposes of this study, the modeled facades (south, west, and east-oriented) were set to be located in
Barcelona (Spain), where large solar gains are a constant condition throughout the year, and such
large semi-transparent envelopes lead to interior over-heating in buildings, even during the winter.
For the studied facades, both the reductions in radiative heat gains entering the building and the heat
recovery in the Venetian blind collector were evaluated for a yearlong operation.
Egusquiza, M.; Egusquiza, E.; Valentin, D.; Valero, M.; Presas, A. Engineering failure analysis Vol. 81, p. 234-244 DOI: 10.1016/j.engfailanal.2017.06.048 Data de publicació: 2017-11 Article en revista
In this paper, an uncommon failure of a Pelton turbine has been analyzed. After the monitoring system detected a sudden increase in the vibration levels, the turbine was inspected. The inspection showed that a fragment of one bucket broke off during operation. Moreover there were several buckets with cracks, always located in the same side of the buckets. An analysis of the detached fragment revealed a fatigue problem.After the damage was found, the vibration signatures measured by the monitoring system before damage, with damage and after repair, were analyzed. Before damage occurred, an excessive axial vibration and the excitation of several natural frequencies of the turbine were detected in the measured vibration.In order to identify the origin of the problem the first task was to analyze the dynamic response of the turbine. A numerical model of the runner using the finite element method (FEM) was done. Experimental research using modal analysis techniques (EMA) was also carried out in the turbine runner. The results of the numerical model were compared with the experimental results obtained. With the validated numerical model natural frequencies and mode-shapes were determined and studied.The next step was to determine experimentally the influence of the mounting conditions on the runner dynamics and the transmissibility of the runner vibrations to the machine bearings where the monitoring sensors are located.From the results of this study it was concluded that the natural frequencies excited during machine operation had axial mode shapes indicating that axial forces were applied to the runner. In a Pelton turbine, this can only be produced by a misaligned jet.To determine the influence of a misaligned jet on the bucket stresses, the dynamic behavior of the runner was performed. The dynamic force of the water jet was applied to the runner bucket. The results showed that with a misaligned jet the dynamic stress distribution increases in one side of the bucket with a maximum stress located where the cracks appeared.
Presas, A.; Valentin, D.; Egusquiza, M.; Bossio, M.; Egusquiza, E.; Valero, M. International Symposium on Sensor Science p. 822- DOI: 10.3390/proceedings1080822 Data de presentació: 2017-09 Presentació treball a congrés
Nowadays, hydropower plants are of paramount importance for the integration of intermittent renewable energy sources in the power grid. In order to match the energy generated and consumed, Large Hydraulic Turbines have to work at off-design conditions, which may lead to dangerous unstable operating points involving the hydraulic, mechanical and electrical system. Under these conditions, the stability of the power grid and the safety of the powerplant itself can be compromised. For many Francis Turbines, one of these critical points, that usually limits the maximum output power, is the full load instability. Therefore, these machines usually work far away from this unstable point, reducing the effective operating range of the unit. In order to extend the operating range of the machine, working closer to this point with a reasonable safety margin, it is of paramount importance to monitor and to control relevant parameters of the unit, which have to be obtained with an accurate sensor acquisition strategy. In the frame of a large EU Project, field tests in a large Francis Turbine located in Canada (rated power 444 MW) have been performed. Many different sensors were used to monitor several working parameters of the unit for all its operating range. Particularly for these tests, more than 80 signals, including ten types of different sensors and several operating signals that define the operating point of the unit, were simultaneously acquired. The present study focuses on the optimization of the acquisition strategy, which includes type, number, location, acquisition frequency of the sensors and corresponding signal analysis to detect the full load instability and to prevent the unit from reaching this point. In this way, the operating limits of the unit can be more accurately defined and therefore the effective operating range increased.
Valentin, D.; Presas, A.; Bossio, M.; Egusquiza, M.; Egusquiza, E.; Valero, M. International Symposium on Sensor Science p. 821- DOI: 10.3390/proceedings1080821 Data de presentació: 2017-09 Presentació treball a congrés
Nowadays, hydropower plays an essential role in the energy market. Due to its fast response and regulation capacity, hydraulic turbines operate at off-design conditions with a high number of start and stops. In this situation, dynamic loads and stresses over the structure are high, documenting some failures over time, especially in the runner. Therefore, it is important to know the dynamic response of the runner under operation, i.e., natural frequencies, damping and mode-shapes, in order to avoid resonance and fatigue problems. The detection of natural frequencies of hydraulic turbine runners in operation is challenging because they are inaccessible structures strongly affected by the confinement in water. Strain gauges are used to calculate stresses of hydraulic turbine runners under operation. However, in this paper, the feasibility to use them to detect natural frequencies of hydraulic turbine runners under operation is studied. For this purpose, a large Francis turbine runner (444 MW) was instrumented with several strain gauges at different positions. First, a complete Experimental Modal Analysis (EMA) of the runner in air was performed using the strain gauges. Then, the natural frequencies of the runner were estimated during operation by means of analyzing accurately transient events or rough operating conditions.
Bossio, M.; Valentin, D.; Presas, A.; Ramos, D.; Egusquiza, E.; Valero, M.; Egusquiza, M. Journal of fluids and structures Vol. 73, p. 53-69 DOI: 10.1016/j.jfluidstructs.2017.05.008 Data de publicació: 2017-06-20 Article en revista
The dynamic response of disks has been deeply studied in the last years given that their dynamic characteristics present similarities with more complex disk-like structures used in real engineering applications, such as hydraulic turbine runners. Because of disk-like structures could present fatigue damage or critical failures as a result of resonance conditions, it is of paramount importance to determine their natural frequencies.
The dynamic response of disk-like structures is heavily affected by the added mass effect when they are surrounded by a heavy fluid. This added mass is greatly affected by the proximity of walls. Furthermore, the surrounding fluid cavity has its own natural frequencies and mode shapes, called acoustic natural frequencies and acoustic mode-shapes. All studies of submerged and confined disks have been carried out considering that the acoustic natural frequencies of the surrounding fluid cavity are much higher than the natural frequencies of the disk, so they do not affect each other. However, in some cases the acoustic natural frequencies are close to the natural frequencies of the submerged structure, which can be affected considerably. This case has not been deeply discussed yet.
In this paper, the influence of the acoustic natural frequencies of a cylindrical fluid cavity on the natural frequencies of a disk has been analysed numerically. First, the effect of the added mass of the fluid has been estimated when the acoustic natural frequencies of the fluid cavity are much higher than the natural frequencies of the disk. For this case, different geometrical and material parameters have been considered. Then, the parameters that affect the acoustical natural frequencies of the fluid cavity have been identified. Finally, the case with acoustic natural frequencies close to the structural natural frequencies is studied in detail and the affectation between both is discussed. All the results presented in this paper have been dimensionless in order to be used for a wide range of disk-like structures.
Therefore, with this study it is possible to identify for which conditions the dynamic response of a generic disk-like structure will be affected by the acoustic natural frequencies of its surrounding fluid cylindrical cavity
Valentin, D.; Presas, A.; Egusquiza, E.; Valero, M.; Egusquiza, M. Journal of Vibration and Acoustics, Transactions of the ASME Vol. 139, num. 2, p. 1-11 DOI: 10.1115/1.4035105 Data de publicació: 2017-04-01 Article en revista
Determining the dynamic response of submerged and confined disklike structures is of interest in engineering applications, such as in hydraulic turbine runners. This dynamic response is heavily affected by the added mass and damping as well as the proximity of solid boundaries. These solid boundaries are normally considered as completely rigid in theoretical or numerical calculations, however, this assumption is not always valid. Some hydraulic turbines have noncompletely stiff casings, which can modify the dynamic response of the runner itself, affecting specially its natural frequencies and damping behavior. To determine the influence of noncompletely rigid nearby surfaces in the dynamic behavior of a submerged structure, an experimental test rig has been constructed. This test rig is based on a disk attached to a shaft and confined in a tank covered with two different casings with different mass and stiffness. For both covers and different disk to cover distances, natural frequencies and damping ratios of the disk have been obtained experimentally. Accelerometers installed on the disk and covers as well as pressure sensors are used for this purpose. Results obtained for all the cases are discussed in detail and compared with a simplified theoretical model.
To accurately determine the dynamic response of a structure is of relevant interest in many engineering applications. Particularly, it is of paramount importance to determine the Frequency Response Function (FRF) for structures subjected to dynamic loads in order to avoid resonance and fatigue problems that can drastically reduce their useful life. One challenging case is the experimental determination of the FRF of submerged and confined structures, such as hydraulic turbines, which are greatly affected by dynamic problems as reported in many cases in the past. The utilization of classical and calibrated exciters such as instrumented hammers or shakers to determine the FRF in such structures can be very complex due to the confinement of the structure and because their use can disturb the boundary conditions affecting the experimental results. For such cases, Piezoelectric Patches (PZTs), which are very light, thin and small, could be a very good option. Nevertheless, the main drawback of these exciters is that the calibration as dynamic force transducers (relationship voltage/force) has not been successfully obtained in the past. Therefore, in this paper, a method to accurately determine the FRF of submerged and confined structures by using PZTs is developed and validated. The method consists of experimentally determining some characteristic parameters that define the FRF, with an uncalibrated PZT exciting the structure. These parameters, which have been experimentally determined, are then introduced in a validated numerical model of the tested structure. In this way, the FRF of the structure can be estimated with good accuracy. With respect to previous studies, where only the natural frequencies and mode shapes were considered, this paper discuss and experimentally proves the best excitation characteristic to obtain also the damping ratios and proposes a procedure to fully determine the FRF. The method proposed here has been validated for the structure vibrating in air comparing the FRF experimentally obtained with a calibrated exciter (impact Hammer) and the FRF obtained with the described method. Finally, the same methodology has been applied for the structure submerged and close to a rigid wall, where it is extremely important to not modify the boundary conditions for an accurate determination of the FRF. As experimentally shown in this paper, in such cases, the use of PZTs combined with the proposed methodology gives much more accurate estimations of the FRF than other calibrated exciters typically used for the same purpose. Therefore, the validated methodology proposed in this paper can be used to obtain the FRF of a generic submerged and confined structure, without a previous calibration of the PZT.
Presas, A.; Valentin, D.; Egusquiza, E.; Valero, M.; Egusquiza, M.; Bossio, M. International Electronic Conference on Sensors and Applications p. 32- DOI: 10.3390/ecsa-3-E010 Data de presentació: 2016-11 Presentació treball a congrés
Modal Analysis is an experimental technique widely used to determine the dynamic response of structures. One of the most critical part is the selection of the actuator that will excite the tested structure. In many cases, traditional exciters, such as hammers and shakers, have been used for this purpose. Nevertheless, these exciters may have the disadvantage of modifying the modal parameters (as reported in some cases) and they are difficult to be used when the structure is not accessible (confined and/or submerged). For these cases PZT-patches, that are very light structures (compared to the tested structure), have been recently used as exciters. Although, in the analyzed studies the natural frequencies of the structure have been determined using PZTs, the rest of parameters that determine the FRF (Frequency response Function) have been not obtained. This could be, because the calibration of PZTs as dynamic force transducers is a complicated task and not an information given by the manufacturers, as in other exciters used for the same purpose. This paper analyzes experimentally and analytically the use of PZT-patches as exciters for modal analysis. For this purpose, a tested structure is excited in different ways with a PZT and its response is compared with a reference case, obtained with a classical exciter. Analyses show how to obtain different modal parameters that determine the FRF of the structure, without previous calibration of the PZT. Finally, and in order to show the potential advantages of these exciters for inaccessible structures, the procedure is repeated for the same structure submerged in water, showing that PZT are much more appropriated exciters in these cases.
Guardo, A.; Egusquiza, M.; Egusquiza, E.; Alavedra, P. Advanced Building Skins: International Conference on Building Envelope Design and Technology p. 227-231 Data de presentació: 2015-11 Presentació treball a congrés
The global trend on energy integration and building efficiency is making both researchers and developers look for technical solutions to use façade surfaces for electricity and/or domestic hot water production. These applications improve the energy performance of the building, but the integration of solar photovoltaic panels or solar thermal collectors into the façade may block visibility to the exterior and may prevent natural light from entering the building, both important comfort factors for building users.
This paper presents the preliminary results on the assessment of the thermal performance of a double-skin facade (DSF) with a venetian blind-type of structure used as a solar thermal collector to heat up a circulating fluid by means of computational fluid dynamics (CFD). This type of heat exchange structure would allow for energy recovery and exterior views simultaneously, and can be easily integrated into the façade aesthetical design. For the purposes of this study, the modeled façade is set to be located in Barcelona (Spain), where large solar gains are a constant condition throughout the year, and such large semi-transparent areas as this type of façades can produce significant over-heating in buildings, even during the winter.
For the studied façade both the reductions in radiative heat gains entering the building and the heat recovery are evaluated for summer meteorological and solar radiation conditions and numerical results obtained are compared with previous results reported by our research group on a similar DSF model without a façade integrated thermal system.