Large-eddy simulation analysis of the influence of the needle lift on the cavitation in diesel injector nozzles

The cavitation phenomenon has a strong influence on the internal flow and spray development in diesel injector nozzles. Despite its importance, there are many aspects which still remain unclear, especially for partial needle lifts when the injector is in the opening and closing phases. For that reason, the current paper is focused on the influence of the needle lift on the internal flow in a diesel nozzle. This study was carried out with three-dimensional simulations at a high injection pressure (160 MPa) using a homogeneous equilibrium model implemented in OpenFOAM to model the cavitation phenomenon. The nozzle was simulated with large-eddy simulation methods at six different needle lifts (10 mm, 30 mm, 50 mm, 75 mm, 100 mm and 250 mm), providing relevant information about the evolution of the internal flow, the turbulence development (the vorticity, the turbulence–cavitation interaction and the turbulent structures) and the flow characteristics in the nozzle outlet (the mass flow, the momentum flux and the effective velocity) with the needle position.Desantes Fernández, JM.; Salvador Rubio, FJ.; Carreres Talens, M.; Martínez López, J. (2015). Large-eddy simulation analysis of the influence of the needle lift on the cavitation in diesel injector nozzles. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering. 229(4):407-423. doi:10.1177/0954407014542627S4074232294Faeth, G. ., Hsiang, L.-P., & Wu, P.-K. (1995). Structure and breakup properties of sprays. International Journal of Multiphase Flow, 21, 99-127. doi:10.1016/0301-9322(95)00059-7Park, S. H., Suh, H. K., & Lee, C. S. (2009). Effect of Bioethanol−Biodiesel Blending Ratio on Fuel Spray Behavior and Atomization Characteristics. Energy & Fuels, 23(8), 4092-4098. doi:10.1021/ef900068aPAYRI, R., GARCIA, J., SALVADOR, F., & GIMENO, J. (2005). Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics. Fuel, 84(5), 551-561. doi:10.1016/j.fuel.2004.10.009Suh, H. K., & Lee, C. S. (2008). Effect of cavitation in nozzle orifice on the diesel fuel atomization characteristics. International Journal of Heat and Fluid Flow, 29(4), 1001-1009. doi:10.1016/j.ijheatfluidflow.2008.03.014Payri, R., Salvador, F. J., Gimeno, J., & de la Morena, J. (2009). Effects of nozzle geometry on direct injection diesel engine combustion process. Applied Thermal Engineering, 29(10), 2051-2060. doi:10.1016/j.applthermaleng.2008.10.009Park, S. H., Kim, S. H., & Lee, C. S. (2009). Mixing Stability and Spray Behavior Characteristics of Diesel−Ethanol−Methyl Ester Blended Fuels in a Common-Rail Diesel Injection System. Energy & Fuels, 23(10), 5228-5235. doi:10.1021/ef9004847Desantes, J. M., Payri, R., Salvador, F. J., & Gil, A. (2006). Development and validation of a theoretical model for diesel spray penetration. Fuel, 85(7-8), 910-917. doi:10.1016/j.fuel.2005.10.023Desantes, J. M., Payri, R., Garcia, J. M., & Salvador, F. J. (2007). A contribution to the understanding of isothermal diesel spray dynamics. Fuel, 86(7-8), 1093-1101. doi:10.1016/j.fuel.2006.10.011Badock, C., Wirth, R., Fath, A., & Leipertz, A. (1999). Investigation of cavitation in real size diesel injection nozzles. International Journal of Heat and Fluid Flow, 20(5), 538-544. doi:10.1016/s0142-727x(99)00043-0Som, S., Aggarwal, S. K., El-Hannouny, E. M., & Longman, D. E. (2010). Investigation of Nozzle Flow and Cavitation Characteristics in a Diesel Injector. Journal of Engineering for Gas Turbines and Power, 132(4). doi:10.1115/1.3203146Macian, V., Payri, R., Margot, X., & Salvador, F. J. (2003). A CFD ANALYSIS OF THE INFLUENCE OF DIESEL NOZZLE GEOMETRY ON THE INCEPTION OF CAVITATION. Atomization and Sprays, 13(5-6), 579-604. doi:10.1615/atomizspr.v13.i56.80Alajbegovic, A., Meister, G., Greif, D., & Basara, B. (2002). Three phase cavitating flows in high-pressure swirl injectors. Experimental Thermal and Fluid Science, 26(6-7), 677-681. doi:10.1016/s0894-1777(02)00179-6Unverdi, S. O., & Tryggvason, G. (1992). A front-tracking method for viscous, incompressible, multi-fluid flows. Journal of Computational Physics, 100(1), 25-37. doi:10.1016/0021-9991(92)90307-kBrackbill, J. ., Kothe, D. ., & Zemach, C. (1992). A continuum method for modeling surface tension. Journal of Computational Physics, 100(2), 335-354. doi:10.1016/0021-9991(92)90240-yPlesset M, Devine R. Effect of exposure time on cavitation damage. Report (Office of Naval Research Contract Nonr-220(28)), California Institute of Technology, Pasadena, California, USA, 1965.Chen, Y., & Heister, S. D. (1996). MODELING CAVITATING FLOWS IN DIESEL INJECTORS. Atomization and Sprays, 6(6), 709-726. doi:10.1615/atomizspr.v6.i6.50Vortmann, C., Schnerr, G. H., & Seelecke, S. (2003). Thermodynamic modeling and simulation of cavitating nozzle flow. International Journal of Heat and Fluid Flow, 24(5), 774-783. doi:10.1016/s0142-727x(03)00003-1Echouchene, F., Belmabrouk, H., Le Penven, L., & Buffat, M. (2011). Numerical simulation of wall roughness effects in cavitating flow. International Journal of Heat and Fluid Flow, 32(5), 1068-1075. doi:10.1016/j.ijheatfluidflow.2011.05.010Salvador, F. J., Romero, J.-V., Roselló, M.-D., & Martínez-López, J. (2010). Validation of a code for modeling cavitation phenomena in Diesel injector nozzles. Mathematical and Computer Modelling, 52(7-8), 1123-1132. doi:10.1016/j.mcm.2010.02.027Salvador, F. J., Hoyas, S., Novella, R., & Martínez-López, J. (2011). Numerical simulation and extended validation of two-phase compressible flow in diesel injector nozzles. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 225(4), 545-563. doi:10.1177/09544070jauto1569Payri, F., Payri, R., Salvador, F. J., & Martínez-López, J. (2012). A contribution to the understanding of cavitation effects in Diesel injector nozzles through a combined experimental and computational investigation. Computers & Fluids, 58, 88-101. doi:10.1016/j.compfluid.2012.01.005Salvador, F. J., Martínez-López, J., Caballer, M., & De Alfonso, C. (2013). Study of the influence of the needle lift on the internal flow and cavitation phenomenon in diesel injector nozzles by CFD using RANS methods. Energy Conversion and Management, 66, 246-256. doi:10.1016/j.enconman.2012.10.011Salvador, F. J., Martínez-López, J., Romero, J.-V., & Roselló, M.-D. (2013). Computational study of the cavitation phenomenon and its interaction with the turbulence developed in diesel injector nozzles by Large Eddy Simulation (LES). Mathematical and Computer Modelling, 57(7-8), 1656-1662. doi:10.1016/j.mcm.2011.10.050Piomelli, U. (1999). Large-eddy simulation: achievements and challenges. Progress in Aerospace Sciences, 35(4), 335-362. doi:10.1016/s0376-0421(98)00014-1Launder, B. E., & Spalding, D. B. (1974). The numerical computation of turbulent flows. Computer Methods in Applied Mechanics and Engineering, 3(2), 269-289. doi:10.1016/0045-7825(74)90029-2Payri, F., Bermúdez, V., Payri, R., & Salvador, F. J. (2004). The influence of cavitation on the internal flow and the spray characteristics in diesel injection nozzles. Fuel, 83(4-5), 419-431. doi:10.1016/j.fuel.2003.09.010Payri, R., Salvador, F. J., Gimeno, J., & de la Morena, J. (2009). Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber. International Journal of Heat and Fluid Flow, 30(4), 768-777. doi:10.1016/j.ijheatfluidflow.2009.03.011Martínez López, J. (s. f.). Estudio computacional de la influencia del levantamiento de aguja sobre el flujo interno y el fenómeno de la cavitación en toberas de inyección diésel. doi:10.4995/thesis/10251/29291Tabor, G. R., & Baba-Ahmadi, M. H. (2010). Inlet conditions for large eddy simulation: A review. Computers & Fluids, 39(4), 553-567. doi:10.1016/j.compfluid.2009.10.007Payri, R., Gimeno, J., Marti-Aldaravi, P., & Bracho, G. (2013). Study of the influence of the inlet boundary conditions in a LES simulation of internal flow in a diesel injector. Mathematical and Computer Modelling, 57(7-8), 1709-1715. doi:10.1016/j.mcm.2011.11.019de Villiers E. The potential of large eddy simulation for the modeling of wall bounded flows. PhD Thesis, Imperial College of Science, Technology and Medicine, London, UK, 2006.Lee, J. W., Min, K. D., Kang, K. Y., Bae, C. S., Giannadakis, E., Gavaises, M., & Arcoumanis, C. (2006). Effect of piezo-driven and solenoid-driven needle opening of common-rail diesel injectors on internal nozzle flow and spray development. International Journal of Engine Research, 7(6), 489-502. doi:10.1243/14680874jer00806Desantes, J. M., Payri, R., Salvador, F. J., & De la Morena, J. (2010). Influence of cavitation phenomenon on primary break-up and spray behavior at stationary conditions. Fuel, 89(10), 3033-3041. doi:10.1016/j.fuel.2010.06.004Lesieur, M., Métais, O., & Comte, P. (2005). Large-Eddy Simulations of Turbulence. doi:10.1017/cbo9780511755507Sagaut, P. (2001). Large Eddy Simulation for Incompressible Flows. Scientific Computation. doi:10.1007/978-3-662-04416-

Computational study of the cavitation phenomenon and its interaction with the turbulence developed in diesel injector nozzles by Large Eddy Simulation (LES)

Firstly, the code has been validated at real operating diesel engine conditions with experimental data in terms of mass flow, momentum flux and effective velocity, showing that the model is able to predict with a high level of confidence the behavior of the internal flow at cavitating conditions. Once validated, the code has allowed to study in depth the turbulence developed in the discharge orifices and its interaction with cavitation phenomenon. (C) 2011 Elsevier Ltd. All rights reserved.This work was partly sponsored by "Vicerrectorado de Investigacion, Desarrollo e Innovacion'' of the "Universitat Politecnica de Valencia'' in the frame of the project "Estudio numerico de la cavitacion en toberas de inyeccion Diesel mediante Grid Computing (Cavigrid)'', reference No. 2597 and by "Ministerio de Ciencia e Innovacion'' in the frame of the project "Estudio teorico-experimental sobre la influencia del tipo de combustible en los procesos de atomizacion y evaporacion del chorro Diesel (PROFUEL)'', reference TRA2011-26293. This support is gratefully acknowledged by the authors.Salvador Rubio, FJ.; Martínez López, J.; Romero Bauset, JV.; Roselló Ferragud, MD. (2013). Computational study of the cavitation phenomenon and its interaction with the turbulence developed in diesel injector nozzles by Large Eddy Simulation (LES). Mathematical and Computer Modelling. 57(7-8):1656-1662. https://doi.org/10.1016/j.mcm.2011.10.050S16561662577-

Study of the influence of the inlet boundary conditions in a LES simulation of internal flow in a diesel injector

In this paper the study of the behavior of the fuel flow through the injector nozzle using CFD tools is presented. Large Eddy Simulation will be used to model the internal flow turbulence in a Diesel fuel injector with velocities over 500 m/s. More specifically, the influence of boundary conditions applied to the model will be studied. The article analyzes the influence of the inlet boundary condition upon activation and maintenance of turbulent flow during the calculation. Carefully assessing which inlet boundary condition is more trustworthy in reality, for this the outlet velocity, pressure, turbulence and level of stabilization will be studied.This work has been funded by UNIVERSIDAD POLITECNICA DE VALENCIA from Spain, in the framework of the project "ESTUDIO DE LA INFLUENCIA DEL LEVANTAMIENTO DE AGUJA EN EL PROCESO DE INYECCION DIESEL'', Reference No. PAID-06-10-2362.Payri Marín, R.; Gimeno García, J.; Marti Aldaravi, P.; Bracho León, GC. (2013). Study of the influence of the inlet boundary conditions in a LES simulation of internal flow in a diesel injector. Mathematical and Computer Modelling. 57(7-8):1709-1715. https://doi.org/10.1016/j.mcm.2011.11.019S17091715577-

Fluid structure interactions within a common rail diesel injector.

The internal flow of a high-pressure diesel injector is simulated numerically to investigate the complex transient flow structures and the unsteady forces imparted to the injector needle that result from the asymmetric flow fields developed during operation. The gas-liquid two phase flow is simulated using a mixture model with the cavitation numerically modeled using the Zwart-Gerber-Belamri model. Both the k-ε model and the detached eddy simulation (DES) model are used, and the numerical results are compared. This dissertation looks at the internal flow of a generic injector at different lifts and characterizes the flow parameters at high lift and low lifts. This paper shows that the DES model captures the important unsteady flow features missed by the k-ε model. A DES simulation of a dual gain orifice injector is performed and the impact of a unique vortical structure that is generated by the gain orifices on the flow characteristics is discussed. The fluid-structure interactions of an injector at hover are simulated and the behavior of this injector and the impact of the resulting lateral bending motion of the needle is discussed. This paper identifies the geometric feature that creates the asymmetrical flow that leads to the bending motion. In the final portion of this dissertation the fluid-structure interactions are simulated over the entire injection cycle. This dissertation discusses how the bending motion of the needle is initiated and develops over the injection cycle and discusses the impact of this motion on the fuel quantity injected and the vapor formed during operation by comparing the FSI simulation to a simulation where the lateral motion is artificially limited

Reproduction of the cavitating flows patterns in several nozzles geometries by using calibrated turbulence and cavitation models

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.Publicado en: Mecánica Computacional vol. XXXV, no. 15Facultad de Ingenierí

VOF Simulation of The Cavitating Flow in High Pressure GDI Injectors

The paper describes the development in the OpenFOAM ® technology of a dynamic multiphase Volume-of-Fluid (VoF) solver, supporting mesh handling with topological changes, that has been used for the study of the physics of the primary jet breakup and of the flow disturbance induced by the nozzle geometry during the injector opening event in high-pressure Gasoline Direct Injection (GDI) engines. Turbulence modeling based on a scale-resolving approach has been applied, while phase change of fuel is accounted by means of a cavitation model that has been coupled with the VOF solver. Simulations have been carried out on a 6-hole prototype injector, especially developed for investigations in the framework of the collaborative project FUI MAGIE and provided by Continental Automotive SAS. Special attention has been paid to the domain decomposition strategy and to the code development of the solver, to ensure good load balancing and to minimize inter-processor communication, to achieve good performance and also high scalability on large computing clusters

Reproduction of the cavitating flows patterns in several nozzles geometries by using calibrated turbulence and cavitation models

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.Publicado en: Mecánica Computacional vol. XXXV, no. 15Facultad de Ingenierí

Computational Study of the Injection Process in Gasoline Direct Injection (GDI) Engines

[ES] La creciente preocupación por los problemas medioambientales, la disponibilidad de combustibles fósiles unido a la gran demanda de vehículos, han llevado a los gobiernos a regular las emisiones emitidas a la atmósfera. Existen propuestas de adoptar fuentes de energía renovables. Sin embargo, la sustitución de los combustibles derivados del petróleo no será fácil, rápida o rentable, y el transporte propulsado por motores de combustión interna (ICE) seguirá destacando en los próximos años. La eficiencia de la combustión y el rendimiento del motor están influenciados por el complejo proceso de inyección. La inyección directa de gasolina (GDI) aumenta el ahorro de combustible y cumple los requisitos de emisiones contaminantes, aunque queda potencial por descubrir. Por ello, ha sido objeto de estudio en los últimos años y, en consecuencia, de la presente Tesis. Este trabajo tiene como motivación mejorar el entendimiento en el campo del GDI. La compleja naturaleza transitoria del proceso de inyección hace que el estudio experimental sea un desafío. La Mecánica de Fluidos Computacional (CFD) surge como una potente alternativa a los experimentos y ha sido adoptada para esta investigación. Bajo este contexto, el objetivo de la presente Tesis es desarrollar una metodología predictiva para la caracterización hidráulica del inyector, capaz de ser aplicada a las actuales y futuras generaciones de inyectores GDI, independientemente de las características del inyector y del software de estudio. Una vez validada, el objetivo posterior es utilizar los resultados para analizar el comportamiento del chorro. Este enfoque busca seguir los pasos de la comunidad científica sustituyendo la práctica experimental. La validación de la metodología se lleva a cabo mediante su aplicación en dos inyectores GDI solenoides multi-orificio diferentes. Además, se han utilizado dos códigos CFD comerciales: CONVERGE y StarCCM+. La metodología predictiva se centra en el estudio del flujo interno y el campo cercano para caracterizar hidráulicamente el inyector. El problema a tratar se define como un sistema multifásico en un marco Euleriano y considerando un único fluido. El tratamiento del flujo multifásico se realiza mediante el enfoque Volume-of-Fluid (VOF). Además, se emplea el Homogeneous Relaxation Model (HRM) para considerar el intercambio de masa entre las fases líquida y vapor debido a cavitación y flash boiling. La turbulencia se ha tratado a partir de los enfoques Reynolds-Averaged Navier-Stokes (RANS) y Large Eddy Simulations (LES). Por otro lado, en cuanto al estudio del flujo externo, se ha adoptado el Discrete Droplet Model (DDM). La atomización y el chorro están influenciados por la geometría de la tobera, por lo que la estrategia de acoplamiento del flujo interno y externo complementa los análisis. Se han adoptado enfoques de acoplamiento unidireccional y mapeado, utilizando como parámetros de entrada los datos de flujo interno de la validada metodología. Esta Tesis aporta una nueva y valiosa metodología predictiva con una elevada precisión a la hora de caracterizar el proceso de inyección en comparativa con datos experimentales. Por otro lado, es directamente trasferible a distintos códigos de cálculo así como aplicable a inyectores con características dispares sin perjudicar las exigencias del modelo. La correcta caracterización del flujo interno ha permitido emplear los datos obtenidos para analizar el comportamiento del chorro eliminando la necesidad de usar datos experimentales. Los resultados obtenidos capturan el comportamiento macroscópico del chorro con una precisión comparable a los experimentos. Aunque todavía hay muchos retos que afrontar, la presente Tesis supone un gran avance en el campo del GDI. El remarcable progreso se debe al desarrollo y uso de una metodología totalmente predictiva, que permite prescindir de la mayoría de los experimentos para contribuir a una mayor y más amplia visión de la física del proceso de inyección.[CA] La creixent preocupació pels problemes ambientals, la limitada disponibilitat de combustibles fòssils, acompanyat a la gran demanda de vehicles, ha portat el govern a regular els nivells d'emissions emesos a l'atmosfera. Existeixen propostes d'adoptar fonts d'energia renovables. Tanmateix, la substitució dels combustibles líquids derivats del petroli no es durà a terme de forma fàcil, ràpida o rentable, i el transport propulsat per motors de combustió interna (ICE) continuarà destacant en els pròxims anys. L'eficiència de la combustió i el rendiment del motor són fortament influenciats pel complex procés d'injecció. La injecció directa de gasolina (GDI) augmenta l'estalvi de combustible i complix amb els requisits d'emissions, encara que queda molt potencial per descobrir. Per això, aquest ha sigut objecte d'investigació en els últims anys i, com a conseqüència, d'aquesta Tesi. Aquest treball té com a motivació millorar l'enteniment en el camp del GDI. La complexa natura transitòria de la injecció fa que l'estudi experimental siga força complex. La Mecànica de Fluids Computacional (CFD) sorgeix com una potent alternativa als experiments, i ha sigut adoptada per aquesta investigació. Baix aquest mateix context, es proposa com a objectiu principal d'aquesta Tesi el desenvolupament d'una metodologia predictiva per a la caracterització hidràulica de l'injector, capaç de ser aplicada a les actuals i futures generacions d'injectors GDI (independentment de les característiques de l'injector i del software d'estudi). Una vegada validada, el posterior objectiu és analitzar el comportament de l'esprai. Aquest enfocament busca seguir els passos de la comunitat científica substituint la pràctica experimental. La validació de la metodologia ha sigut duta a terme mitjançant la seva aplicació en dos injectors GDI solenoides multi-orifici. A més, s'han utilitzat dos software CFD comercials: CONVERGE i StarCCM+. La metodologia predictiva se centra en l'estudi del flux intern i el camp proper per tal de caracteritzar hidràulicament l'injector. El problema a tractar es defineix en base a un sistema multi-fàsic en un marc Eulerià i considerant un únic fluid. El tractament del fluid multi-fàsic es realitza mitjançant l'aproximació Volume-of-Fluid (VOF). A més, s'utilitza el Homogeneous Relaxation Model (HRM) per tal de considerar l'intercambi de massa entre les fases líquida i vapor degut als fenòmens de cavitació i flash boiling. La turbulència s'ha tractac a través dels enfocaments Reynolds-Averaged Navier-Stokes (RANS) i Large Eddy Simulations (LES). Pel que fa a l'estudi del fluix extern, s'ha adoptat el Discrete Droplet Model (DDM). Sent conscients que el comportament l'atomització i l'esprai estan influenciats per la geometria de la tovera, l'estratègia d'acoblament del flux intern i extern complementa les anàlisis. S'han adoptat els enfocaments d'acoblament unidireccional i mapejat, utilitzant com a paràmetres d'entrada les dades del flux intern obtingudes amb la validada metodologia. Aquesta Tesi aporta una nova i valuosa metodologia predictiva amb una elevada precisió a l'hora de caracteritzar el procés d'injecció en comparativa amb dades experimentals. És directament transferible a diversos codis de càlcul així com aplicable a injectors amb característiques dispars sense perjudicar les exigències del model. La correcta caracterització del flux intern ha permès utilitzar les dades obtingudes per tal d'analitzar el comportament de l'esprai, eliminant la necessitat d'emprar dades experimentals. Els resultats obtinguts d'aquest estudi capturen el comportament macroscòpic de l'esprai amb una precisió comparable als experiments. Encara que queden molts reptes per afrontar, aquesta Tesi aporta un important avanç al camp del GDI. La ruptura prové del desenvolupament i ús d'una metodologia completament predictiva, que substitueix els experiments requerits i així contribueix a una millor i més ampla visió de la física del procés d'injecció.[EN] Concerns about climate change, availability of fuel resources and the high demand for vehicles, have led governments to regulate the level of pollution emitted by engines into the atmosphere. There is a strong desire to adopt renewable and sustainable energy sources. However, the substitution of liquid fuels derived from petroleum will not emerge easily, quickly or economically, and Internal Combustion Engines (ICE) will continue to excel for the next few years. Combustion efficiency and engine performance are strongly influenced by the complex fuel injection process. Gasoline Direct Injection (GDI) strategies increase fuel economy and meet emission requirements, although many challenges remain, which has therefore been one of the main research objectives in recent years and of this Thesis. The present research aims to provide a better understanding in the field of GDI. The transient and complex nature of the injection process makes the experimental study of GDI quite challenging. Therefore, Computational Fluid Dynamics (CFD) emerges as a powerful alternative adopted for this research. In this context, the main objective of the present Thesis is to develop a predictive methodology capable of being applied to current and future generations of GDI injectors, regardless of the injector features and the software employed, for the hydraulic characterization of the injector. Once validated, the subsequent goal is to employ the obtained results to analyze the behavior of the spray downstream of the injector. The approach attempts to follow the footsteps of the research community to avoid experimental practice. The predictive methodology has been validated through its application to two multi-hole solenoid GDI injectors with different features. In addition, the mentioned methodology has been evaluated using diverse commercial software: CONVERGE and StarCCM+. The methodology focuses on the study of the internal and near-field flow to hydraulically characterize the injector. So the problem to be addressed is a multi-phase system, performed in an Eulerian framework, modeled through a single-fluid approach. The multi-phase flow is treated by means of the Volume-of-Fluid (VOF) approach. Homogeneous Relaxation Model (HRM) is employed to consider the mass exchange between liquid and vapor fuel phases, due to cavitation and flash boiling. The turbulence treatment has been performed from both Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES) approaches. Regarding the external flow study, the Discrete Droplet Model (DDM) has been adopted. In addition, being aware that atomization and spray behavior is greatly influenced by the nozzle geometry, the coupling strategy of the internal and external flow complements the analyses. One-way coupling and mapping approaches have been adopted, using as input parameters the internal flow data obtained from the already validated methodology. Accordingly, this Thesis provides a new and valuable predictive methodology, which has demonstrated a high accuracy in characterizing the flow behavior during the injection process through comparison with experimental data. It has also proven to be directly transferable to different CFD software and applicable to injectors with dissimilar characteristics without compromising the requirements of the model. The correct internal flow characterization has made it possible to employ the obtained data to analyze the spray patterns, which eliminates the need to consider experimental data. The outcomes of this study macroscopically capture the jet behavior with an accuracy comparable to experiments under different operating conditions. Although there are still many challenges to face, the present Thesis brings a breakthrough in the field of GDI. The quantum leap arises from the development and use of a fully predictive methodology, allowing to avoid most experiments to contribute to a greater and broader vision of the injection process physics.María Martínez García has been founded through a grant from the Government of Generalitat Valenciana with reference ACIF/2018/118 and financial support from the European Union. These same institutions, Government of Generalitat Valenciana and the European Union, supported through a grant for pre-doctoral stays out of the Comunitat Valenciana with reference BEFPI/2020/057 the research carried out during the stay at Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, ETH Zurich, Switzerland. Special gratitude from the author to both institutions, Government of Generalitat Valenciana and the European Union, for making this dream possibleMartínez García, M. (2022). Computational Study of the Injection Process in Gasoline Direct Injection (GDI) Engines [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/185180TESI

Reproduction of the cavitating flows patterns in several nozzles geometries by using calibrated turbulence and cavitation models

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.Publicado en: Mecánica Computacional vol. XXXV, no. 15Facultad de Ingenierí

Effect of Diesel injection pressures up to 450MPa on in-nozzle flow using realistic multicomponent surrogates

Investigations with 300MPa injection pressures show significant soot reduction, but the effect of such extremepressures on the in-nozzle flow has not been closely examined. The study of the in-nozzle flow is important becauseit dominates primary break-up characteristics and therefore the combustion efficiency. Moreover, the characteristicpressure drops in Diesel injectors may cause the fuel to cavitate, which leads to enhancements in the nozzleoutlet velocity, the spray cone angle and the fuel atomisation. In this work, the fuel property database is modelledusing the molecular-based PC-SAFT EoS with an eight-components surrogate based on a grade no. 2 Dieselemissions-certification fuel. The composition for the surrogate is (in mole fration): 2.7% n-hexadecane, 20.2% n-octadecane, 29.2% heptamethylnonane, 5.1% n-butylcyclohexane, 5.5% trans-decalin, 7.5% trimethylbenzene and15.4% tetralin. Then, this surrogate is utilised in simulations for a common rail 5-hole tip injector tapered nozzle.The needle is assumed to be still at a lift of 100μm, which is representative of the lift reached during pilot injection.The injector operating pressures start from 180MPa and reach 450MPa. The collector back pressure is 5MPa. Thedensity of the bulk fluid is assumed to vary according to a barotropic-like scheme, following an isentropic expansion.Results show an increase in the mass flow rate, following the square root of the pressure difference law and alsoin the outlet velocity, both as expected. Surprisingly, the cavitation is significantly reduced as the injection pressureincreases. A focused study on this particular phenomenon shows a significant decrease in the Reynolds numberin the sac, therefore the flow is found to be more stable and the pressure drop along the nozzle is smaller. Thereason for the lower Reynolds number is found on the heavy nature of Diesel fuels. While the sac average velocityincreases 15% between an injection pressure of 180MPa and 450MPa, the kinematic viscosity increases close to a70%