Numerical investigation of transient hole-to-hole variation in cavitation regimes inside a multi-hole diesel nozzle

Dynamics of in-nozzle flow are known to have crucial impacts on the spray structure, and thus hole-to-hole variation in transient cavitating flow is strongly suspected of being a source of undesirable spray asymmetries. This study focuses on effects of the needle motion on evolution of in-nozzle cavitation regimes. For verifying the numerical models, simulation results obtained using the Winklhofer throttle "U" and the real-size two-hole optical nozzle were validated against available experimental data of mass flow rate at the nozzle exit and evolution of geometrical/string cavitation regions. Two different patterns of string cavitation defined as "needle" and "hole connecting" string cavitation were obtained in the optical experiments. Then the transient flow characteristics inside a five-hole injector nozzle were studied under needle lift and off-axis motion using the validated models. Results show that the "needle" string cavitation originated from the needle tip and/or hole exit and then developed into the orifice at lower needle lifts, and the "hole connecting" ones connecting orifices through the sac were tended to form at higher needle lifts with weaker intensity and shorter duration. Meanwhile, both "needle" and "hole connecting" string cavitation strongly depended on the structure and intensity of the in-nozzle vortex flow. Furthermore, both geometrical and string cavitation were tended to occur at lower needle lifts within the simulated five-hole injector, and all of the single-phase flow, string cavitation flow and geometrical cavitation flow regimes can simultaneously form in the hole 1, 2 and 3 respectively due to the needle lift and radial displacement.