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

VOF Simulation of The Cavitating Flow in High Pressure GDI Injectors

[EN] 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.Giussani, F.; Montorfano, A.; Piscaglia, F.; Onorati, A.; Helie, J. (2017). VOF Simulation of The Cavitating Flow in High Pressure GDI Injectors. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 1009-1018. https://doi.org/10.4995/ILASS2017.2017.4989OCS1009101

Analysis of the suction chamber of external gear pumps and their influence on cavitation and volumetric efficiency

Hydraulic machines are faced with increasingly severe performance requirements. The need to design smaller and more powerful machines rotating at higher speeds in order to provide increasing efficiencies, has to face a major limitation: cavitation. A two-dimensional numerical approach, by means of Computational Fluid Dynamics (CFD), has been developed for studying the effect of cavitation in the volumetric efficiency of external gear pumps. Several cavitation models and grid deformation algorithms have been studied, and a method for simulating the contact between solid boundaries has been developed. The velocity field in the inlet chamber has also been experimentally measured by means of Time-Resolved Particle Image Velocimetry (TRPIV) and results have been compared to the numerical ones in order to validate the accuracy of the model. Our two-dimensional model is not able to predict the real volumetric efficiency of the pump, since several simplifications are involved in it. Nevertheless, this model shows to be valid to understand the complex flow patterns that take place inside the pump and to study the influence of cavitation on volumetric efficiency. The influence of the rotational speed of the pump has been analyzed, as well as the effect of the geometry of the inlet chamber, the working pressure, the inlet pressure loss factor, and the flow leakage through the radial clearances of the pump between gears and casing.Postprint (published version

Analysis of the suction chamber of external gear pumps and their influence on cavitation and volumetric efficiency

Hydraulic machines are faced with increasingly severe performance requirements. The need to design smaller and more powerful machines rotating at higher speeds in order to provide increasing efficiencies, has to face a major limitation: cavitation. A two-dimensional numerical approach, by means of Computational Fluid Dynamics (CFD), has been developed for studying the effect of cavitation in the volumetric efficiency of external gear pumps. Several cavitation models and grid deformation algorithms have been studied, and a method for simulating the contact between solid boundaries has been developed. The velocity field in the inlet chamber has also been experimentally measured by means of Time-Resolved Particle Image Velocimetry (TRPIV) and results have been compared to the numerical ones in order to validate the accuracy of the model. Our two-dimensional model is not able to predict the real volumetric efficiency of the pump, since several simplifications are involved in it. Nevertheless, this model shows to be valid to understand the complex flow patterns that take place inside the pump and to study the influence of cavitation on volumetric efficiency. The influence of the rotational speed of the pump has been analyzed, as well as the effect of the geometry of the inlet chamber, the working pressure, the inlet pressure loss factor, and the flow leakage through the radial clearances of the pump between gears and casing

Impact of cavitation erosion on nozzle flow characteristics and liquid fuel atomization

Modern Diesel engines with high injection pressures can suffer from cavitation erosion phenomena. Signs of cavitation erosion in automotive components can manifest in high pressure liquid systems components (e.g. injectors, valves and pumps), as well as in the narrow fluid regions next to the cylinder liners on both the water cooling jacket side and the ring assembly side. Special attention must also be given during the design of marine propellers and water turbines, since the performances and the lifespan of these components mainly depends by the appearance of cavitation and, potentially, erosion.Cavitation erosion alters metal devices by changing their geometry from the original design, with consequences on the overall system performances. Since the lifetime of the components can be significantly shortened due to cavitation erosion, numerical and experimental investigations are usually carried out during the design process to evaluate the risk of incurring in cavitation erosion. It is then of crucial importance for the industry to have access to validated numerical models for the prediction of cavitation erosion within the softwares used for the evaluation of new designs. The scope of this work is then to develop a state–of–the–art numerical framework for the prediction of cavitation erosion in a commercially available software. For this reason, liquid compressibility models are implemented in the software with both, analytical formulations and tabular data, commonly used by the industry. The solver capability to correctly resolve pressure wave velocities is proven with simple 1D test cases, comparing the simulation results against analytical solutions. A novel scientific contribution is made by applying the multifluid model to cavitating flows, thus allowing to model slip velocity between the liquid and the vapor phase. The developed numerical framework for the simulation of cavitating flows at erosive conditions is validated against experimental results of simplified geometries and the obtained results about the effect of viscosity variability of commercial diesel showed the importance of fluid properties for the investigation of cavitation erosion. For the first time, pressure peaks related to the collapse of vapor clouds are recorded due to end of injection events and the effect of actual erosion patterns is investigated in terms of internal injector flow and spray. All the developed methods are implemented in a software commercialized by AVL GmbH,therefore of immediate use to engineers for industrial applications

New Advances of Cavitation Instabilities

Cavitation refers to the formation of vapor cavities in a liquid when the local pressure becomes lower than the saturation pressure. In many hydraulic applications, cavitation is considered as a non-desirable phenomenon, as far as it may cause performance degradation, vibration problems, enhance broad-band noise-emission, and eventually trigger erosion. In this Special Issue, recent findings about cavitation instabilities are reported. More precisely, the dynamics of cavitation sheets are explored at very low Reynolds numbers in laminar flows, and in microscale applications. Both experimental and numerical approach are used. For the latter, original methods are assessed, such as smooth particles hydrodynamics or detached eddy simulations coupled to a compressible approach

Novel Blade Design Strategy to Control the Erosion Aggressiveness of Cavitation

With the reduction in size of turbomachinery systems, cavitation aggressiveness is intensified. Erosion, caused by the repeated collapse of gaseous bubbles in proximity to solid surfaces, occurs at rates that dramatically downgrade the life expectancy of rotating parts. As a result, the compacting strategy, meant to reduce cost and improve efficiency, fails for liquid flows. The research undertaken here proposes a novel design method aimed at controlling the erosion aggressiveness of cavitation. The underlying idea is that the cavity closure shock is a determining factor in the intensity of bubble collapse mechanisms: sharp and high amplitude shocks give rise to strong erosion, while low gradient and low amplitude recoveries reduce the erosive intensity. The working hypothesis is tested here, first, by developing a novel inverse design algorithm capable of handling cavitating flow. The code solves the inviscid Euler equations and models blade cavitation using the Tohoku-Ebara barotropic equation of state. Bespoke preconditioning and multigrid procedures are constructed to handle the large amplitudes in flow regime (from hypersonic in the cavity to very low Mach number in the liquid phase). The inverse solver is then used to produce a set of 2D cascade hydrofoil geometries with smoothed shock profiles at cavity closure. The blades are assessed numerically using both steady state and time-resolved approaches. Both hydrodynamic performance, given in terms of swirl, lift and drag, and cavitation dynamics are evaluated. Recently developed erosion prediction methodologies are implemented and demonstrate compelling correlations between the erosion patterns and shock profile. Finally, experimental testing is carried out using a purposefully developed observation platform. The erosive performance of two of the geometries is measured using the paint removal technique. Results reveal a significant improvement in erosive response for the shock smoothed design, thus confirming the numerical findings as well as the validity of the design hypothesis