Modelling of cavitation in nozzles for diesel injection applications

Abstract

Extreme low pressure regions develop in the high pressure direct injection fuel flow inside the fuel injector holes, compelling the liquid fuel to transform to vapour phase in the form of vapour cavities or bubbles, a phenomenon known as cavitation. The cavitation phenomenon determines the quality of primary atomization and hence a ffects the performance of direct injection diesel or gasoline engines. A cavitation model, coupled with the mixture multiphase approach and RNG k-e turbulence model, has been developed and implemented in this study for analysing cavitation. The cavitation model has been implemented in ANSYS Fluent platform. The model predictions have been compared with results from experimental works available in the literature. A good agreement of the model predictions has been observed. Comparisons of the model with other cavitation models (Schnerr & Sauer and Zwart-Gerber-Belamri) available in ANSYS Fluent have been carried out with both mixture and Eulerian-Eulerian multiphase approaches. The overall performance of the proposed model in comparison with other models has been observed to be more eff ective.

The model has been further applied to diesel vs. biodiesel cavitation as biofuels are the greener alternatives of conventional fossil fuels in recent times. Additionally eff ects of property di erences between diesel and biodiesel, inlet pressure fluctuations have been investigated. Liquid phase viscosity has been observed to be the determining parameter amongst all the properties for cavitation characteristics. The present study has also assessed the relevance of following factors for the case of cavitation in diesel injectors : a) compressibility, b) stress of a flowing liquid, c) wall roughness and d) turbulence. The two phase flow passes through the nozzle at very high velocities and hence can no longer be considered incompressible. Stress can aff ect the inception of cavitation as the liquid under considerable stress can fail and then rupture to form cavities. In the real nozzles at microscopic levels there are always some non-uniformities or crevices that can aggravate cavitation and hence its importance should be assessed. The flow passage inside the injector is small enough to have high enough Reynolds number to get a turbulent flow. Moreover the turbulent fluctuations can cause drastic drop in the local pressure, even though the mean thermodynamic pressure is higher than the saturation pressure, causing unexpected cavitation. Parametric studies indicate that the compressibility becomes important at high pressure diff erences and e ffects of stress and turbulent pressure fluctuations are not significant for cavitation in diesel injectors. The eff ect of the inlet pressure fluctuation has also been assessed for diesel and biodiesel. Diesel appears to be more susceptible to pressure fluctuations compared to biodiesel due to the di fference in the viscosity.

The developed cavitation model has been fi nally implemented to simulate cavitation in the complex geometry of a real fuel injector along with needle movements. Diesel vs. biodiesel cavitation has also been studied in the complex geometry to understand the e ffects of needle movements.