Browsing by Author "Moll, Flavio"

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    Detailed calibration of eddy viscosity turbulence models for incipient cavitating flow predictions in asymmetrical nozzel of injectors/atomizers
    (2019-11-01) Coussirat, Miguel; Moll, Flavio
    Cavitation in pressure injectors/atomizers strongly affects the liquid/spray jet behavior at its outlet. The type of atomization induced by cavitation allows developing more efficient devices if this cavitation state is controlled. Cavitating flow is related to turbulent and multiphase flows with mass transfer between the liquid and its gaseous phase. It is affected by several factors such as local pressure, local state of the turbulence, non-condensable dissolved gas concentration, nozzle geometry and others. Due to the high speed flow and small spatial and time scales involved, the study of cavitating flows by physical experiments is very expensive. On the other hand, several codes for numerical modeling of cavitating flows have been developed, but turbulent multiphase flow modeling is still a big challenge. Previous works showed that it is possible to capture several of the incipient cavitating flow characteristics performing a careful calibration of the Eddy Viscosity Models in nozzles with symmetrical inlet geometry and with round or square outlet sections. This work extends the study to nozzles with asymmetrical inlet geometry and square outlet section. It was demonstrated in previous works that a careful calibration task should be necessary, because there is a close relation between the cavitation inception/developing condition and the turbulence level in the flow leading to a ‘nonstandard turbulence state’. The spatial distribution and the slow decay of the turbulence level produced by cavitation could be related to some preferred turbulence scales in the process, so cavitating flows should not be modeled as typical turbulence. It is showed that based on the special characteristics of the incipient/slightly developed cavitating flows, a suitable calibration of the turbulence models allows obtaining improved results. These results become competitive when they are compared against ones computed by Large Eddy Simulations which need a lot of computational resources and an appropriate initial solution for running. It was also demonstrated that suppressing by calibration the level of the eddy viscosity in certain zones the vapor fraction predicted rises, provoking the incipient cavitation state in the flow. The obtained conclusions could be useful to improve injectors design using numerical modeling, because the detection of the incipient cavitation flow condition, useful to improve the atomization, could be captured accurately.
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    Numerical study of cavitating flow in asymmetrical nozzles of injectors/atomizer. assessment of calibrated eddy viscosity models in developing cavitation case I
    (2021-11-01) Coussirat, Miguel; Moll, Flavio
    Cavitating flow is related to turbulent and multiphase flows with mass transfer between the liquid and its gaseous phase. Cavitation in pressure injectors/atomizers strongly affects the liquid/spray jet behavior at its outlet. The type of atomization induced by cavitation allows developing more efficient devices if this cavitation state is controlled. Experiments show that an improvement in the quality of the spray is reached when a fully developed cavitation state is present at the nozzle, being this state clearly unsteady due to the presence of the re-entrant jet and the vortex shedding downstream of the cavity. Previous works showed that it is possible to capture several of the incipient cavitating or quasi- steady cavitating flow characteristics performing a careful calibration of the available Eddy Viscosity Models in nozzles with several inlet and outlet geometries. A careful calibration of the selected Eddy Viscosity Model becomes an important task to obtain accurate predictions of the flow both in quasi- steady and unsteady cavitation states. The numerical results discussed in this first part show that it is possible to capture the main cavity features by a steady state cavitation simulation despite the incipient unsteady state of the slightly developed cavitation state. It is demonstrated that the obtained results become competitive when they are compared against ones computed by Large Eddy Simulations which need a lot of computational resources and an appropriate initial solution for running. The possibility to perform unsteady simulations using Eddy Viscosity Models is explored. As a first step, a discussion of the experimental data available for fully developed cavitation cases and the developed simulation strategies to carry out these cases are presented.
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    Numerical study of cavitating flow in asymmetrical nozzles of injectors/atomizers. application to fully developed cavitation cases II
    (2021-11-01) Coussirat, Miguel; Moll, Flavio
    In the first part of this work a broad discussion related to steady and unsteady state cavitation cases and the calibration of turbulence models in steady state cases simulations was presented. In this second part, a numerical analysis of the unsteady flow behavior in nozzles of injectors/atomizers is presented. Previous works showed that it is possible to capture several of the incipient cavitating flow characteristics performing a careful calibration of the Eddy Viscosity Models in nozzles. This work extends the numerical study for these nozzles in cases where the fully developed cavitation state appears. Again, a careful calibration of the selected Eddy Viscosity Models becomes an important task to obtain accurate predictions of the flow. The results obtained show that it is possible to capture the main cavity features and the characteristic frequencies of the flow unsteadiness. The obtained conclusions could be useful to improve injectors design using numerical modeling, because the detection of the typical frequencies related to the flow unsteadiness could be useful to detect possible coupling between some of these frequencies and one of the natural vibration modes of the nozzle leading to a possible undesired fluid- structure interaction.
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    Prediction of the characteristic pressure pulsations in a reversible pump-turbine
    (Universidad Tecnológica Nacional. Faculatd Regional Mendoza, 2023-11-06) Coussirat, Miguel; Moll, Flavio; MartÌnez, Germán
    Reversible Pump- Turbine turbomachines are frequently used in peaking load hydroelectrical power plants. These plants contribute to satisfy the energy demand during peak load periods. Its dynamic behaviour is influenced by the flow structures developed along the distributor and runner vanes. This behaviour depends both on the internal design of the machine and its operating conditions, whether it works as pump or as a turbine. High-amplitude pressure pulsations appear due to the rotor-stator interaction phenomenon, and under certain operating conditions, the frequency of these pulses can seriously affect the structural integrity of the machine. The goal of this study is to assess the dynamic behaviour of a low-specific-speed reversible pump-turbine when working both as a turbine or as a pump. For validation, the obtained numerical results for the pressure pulsation in both operating modes are compared to experimental results measured on a working prototype. These results could be useful to enhance the knowledge concerning the behaviour of the pressure pulsations due to the internal flow dynamics into the machine.
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    Recalibration of eddy viscosity models for numerical simulation of cavitating flow patterns in low pressure nozzle injectors
    (2021-03-01) Coussirat, Miguel; Moll, Flavio
    Cavitation in pressure injectors/atomizers affects the liquid/spray jet behavior at its outlet. The type of atomization induced by cavitation allows developing efficient devices if this cavitation state is controlled. Cavitating flow is related to turbulent and multiphase flows with mass transfer between the liquid and its gaseous phase and which is affected by several factors. Due to the high-speed flow and small spatial and time scales involved, the study of cavitating flows using physical experiments is very expensive. By means of numerical simulations using eddy viscosity models, some of the incipient and slight developed cavitating flow characteristics in nozzles are captured, but the level of the vapor fraction is commonly underestimated. It is evident that a suitable calibration of the turbulence models based on the special characteristics of the incipient/slight developed cavitating flows allows obtaining improved results. This special calibration is necessary due to the close relation between the cavitation inception/developing conditions and the turbulence level in the flow leading to a “nonstandard turbulence state.” So, cavitating flows should not be modeled as a simple turbulent one. It is also demonstrated that the results obtained become competitive compared against the ones computed by large eddy simulations, which need a lot of computational resources and an appropriate initial solution for running. The conclusions obtained can be useful to improve injector designs because the suitable simulation of the incipient cavitation or slight developed cavitation flow conditions can be accurately simulated after calibration.
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    Reproduction of the cavitating flows patterns in several nozzles geometries by using calibrated turbulence and cavitation models
    (2017-11-01) Coussirat, Miguel; Moll, Flavio
    Cavitating flow is a complex phenomenon related with turbulent and multiphase flows with mass transfer between the liquid and gaseous phases. This flow is affected by several factors as surrounding pressure, the local state of the turbulence, the non-condensable dissolved gases concentration and others effects. To study this kind of flow, several numerical models have been developed and they are now available in commercial and in-house software. A numerical model for cavitating flows involves a multiphase model, including both mass transfer and turbulence submodels. Inside of a commercial or an in-house numerical code there are several options and possible combinations of these submodels. A selection of the more suitable combination from this broad offer is a difficult task, involving then a subsequent careful calibration of the models selected, due to the fact that the default values for the calibration parameters that have these submodels, are related to simple flow conditions, i.e., simple geometries and flows without any detachment. Under cavitation conditions, these conditions are not the common situation. This work deals with the enhancement of some previous results obtained that allow to say that it is possible to capture several cavitating flows characteristics, improving a ‘standard’ numerical (i.e., without any calibration) simulation by means of a detailed tuning of the production/dissipation coefficients present in the equations of the Eddy Viscosity Models for turbulence, and other parameters related to the two-phase state of the flow. The numerical results obtained were compared against experimental data for pressure, velocity and the structure of the two-phase cavity. It is demonstrated that a careful calibration of both the turbulence and the cavitation submodels used is of paramount importance, because there is a very close relation between the turbulence state of the flow and the cavitation inception/developing conditions. A suitable calibration work allows also diminish the mesh size, saving a lot of computational resources or the use of more sophisticated strategies for turbulence simulations (e.g., Large Eddy Simulations). Those are very expensive in terms of the necessary computational resources required. A more general conclusions than obtained in previous works are presented, because results for other different nozzles configurations were obtained.
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    Scale adaptive simulations applied to fully cavitating turbulent flow in injector nozzles
    (Universidad Tecnológica Nacional. Faculatd Regional Mendoza, 2022-04-01) Coussirat, Miguel; Moll, Flavio
    The unsteady and turbulent pressure-driven cavitating flow under fully cavitation conditions through a sharp-edged orifice is studied by numerical simulations. Unsteady cavitating flow is a typical flow configuration in fuels injectors and brings a challenge in the numerical modeling of two-phase fluid flows due to the high-pressure gradients involved and the high ratio of liquid and vapor density. Under this flow condition computationally intensive unsteady simulations are necessary to accurately simulate the irregular cyclic process of bubble formation, growth, filling by water jet re-entry and its breakoff. The capabilities of Reynolds Averaged Simulations are assessed to ensure a suitable cavity structure prediction to capture the main shedding frequencies and the vapor fraction variations along the nozzle. This study is focused on the performance of a modified version of the Shear Stress Transport turbulence model, involving a Scale Adaptive Simulations sub-model related to the unsteady turbulent flows modeling. The obtained results show that the proposed option would allow studies of developed cavitating flows by means of an unsteady Reynolds Averaged Simulation, computationally less expensive than the Large Eddy Simulations option, being this last option not completely affordable for simulating turbulent flows in industrial problems nowadays.
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    Simulación numérica de flujos cavitantes en inyectores/atomizadores con asimetría en la dirección del flujo
    (2020-11-01) Coussirat, Miguel; Moll, Flavio
    : El flujo cavitante está relacionado con flujos turbulentos y multifásicos con transferencia de masa entre el líquido y su fase gaseosa. La cavitación en los inyectores/atomizadores de presión afecta fuertemente el comportamiento del chorro de líquido/spray en su salida. El tipo de atomización inducida por cavitación permite desarrollar dispositivos más eficientes si este estado de cavitación está bien controlado. Los experimentos muestran que se alcanza una mejora en la calidad de la pulverización cuando hay un estado de cavitación completamente desarrollado en la boquilla. La cavitación desarrollada formada en la boquilla muestra tanto una forma compleja como un comportamiento no estacionario debido al complejo patrón de flujo asociado. Este trabajo trata del estudio numérico de boquillas con asimetría en la dirección de avance del flujo y secciones rectangulares tanto en entrada y de salida para casos en los que aparece el estado de cavitación completamente desarrollado. Una calibración cuidadosa de los modelos de viscosidad turbulenta seleccionados se convierte en una tarea importante para obtener predicciones de flujo precisas. Los resultados obtenidos muestran que es posible captar las principales características de la cavidad y las frecuencias representativas de las inestabilidades. Las conclusiones obtenidas podrían ser útiles para el diseño de inyectores de combustible ya que la simulación adecuada de la condición de flujo de cavitación desarrollada y las posibles frecuencias de resonancia, útiles para mejorar la atomización del flujo y la integridad del inyector, podrían capturarse con precisión mediante este tipo de modelado numérico.