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.
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.
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.