Analysis of the Flow in the Nozzle of a Vertical Multi-Hole Diesel Engine Injector

An enlarged transparent model of a six-hole vertical diesel injector has been manufactured in order to allow flow measurements inside the sac volume and the injection holes to be obtained using a combination of laser Doppler velocimetry (LDV) and the refractive index matching technique under steady state conditions. The measurement points were concentrated in the sac volume close to the entrance of the injection holes as well as inside them on a vertical plane passing through the axis of two injection holes for two different needle lifts. The velocity flow field was characterized in terms of the mean velocity and the turbulent intensity. The results revealed that, under certain conditions, cavitation may occur in the recirculation zone formed at the entrance to the hole since the pressure in this region can reach the value of the vapor pressure of the flowing liquid; this was found to strongly depend on the needle lift and eccentricity. Images taken with a CCD and a high-speed camera have identified differences on the onset and the development of the cavitation films as they move towards the exit of the injection holes.

The experimental data obtained during the present investigation were used to validate an existing CFD model which was found to predict reasonably well the flow characteristics under non-cavitating conditions. Following successful validation of the model, parametric studies were performed in order to investigate the effect of the hole inclination angle, needle eccentricity and hole entrance curvature radius on the flow distribution in the sac volume and the injection holes.