Simulation of throttling effect on cavitation for nozzle internal flow

Abstract

The quality of spray atomization governs combustion, power output, fuel economy and emissions of internal combustion engines. Spray atomization is significantly affected by cavitation, and it is closely related to various factors, such as injection conditions, nozzle geometry and throttling effect. In this work, a 5-time up-scaled transparent nozzle was custom-made and utilized for the study of the dynamics and characteristics of the nozzle internal flow with cavitation under various injection pressures and throttling strengths (by varying needle lift). High-speed imaging technique was employed for the nozzle internal flow visualisation. To further investigate the 3D behaviour of cavitation, the simulation of nozzle internal flow was carried out with Fluent 18.0. It was found that cavitation was mainly induced by the throttling effect at the crevice between the needle and needle seat and the redirection of flow at the nozzle inlet. With the rise of the needle lift, initially the fraction of cavitation increased because of increasing hydraulic force, and then as the needle lift increased further, the fraction of cavitation decreased due to the reduced throttling effect. However, when the needle lift kept constant, the cavitation strengthened quickly with the injection pressure and then kept almost constant. Cavitation shedding was observed near the lower wall of the nozzle at the high injection pressure, but not at low injection pressure. In addition, at the low injection pressure, string cavitation was observed due to the throttling effect, originating from the nozzle and extending upstream to the needle.