LES Simulation And Experiment Study Of Cloud Cavitation In Diesel Nozzles And It’s Effect On Near-flied Spray

Cavitating flow in a diesel nozzle is an important factor for the improvement of the fuel-air mixing, combustion and the performance of emission under high injection pressure conditions. Although spray is affected by aerodynamic interaction, different cavitation structures(such as sheet cavitation, cloud cavitation and vortex cavitation)and turbulent flow inside a diesel nozzle play a critical role in primary breakup and all subsequent development processes. Therefore, it is very necessary for us to investigate the mechanism of the complete primary injection process including the internal cavitating flow and external primary jet break up. It has important theoretic signification.In this paper, in order to investigate the periodic cloud cavitation shedding, a flow visualization experiment system with a transparent scaled-up nozzle was setup.Moreove Large Eddy Simulation(LES) simulation combined with visualization experiments were carried out to study cloud caviation and primary jet break.Numerical and experimental images provide a better insight into this physical mechanism. The results show that cavitation inception is formed at the edge of inlet and the recirculation zone which below the shear layer. Then the attached cavitation grows and shedding of cloud cavitation occurs due to the re-entrant jet located between cavity and wall. Moreover, the dynamic mechanism causing shedding of cloud cavitation is a combination of shedding vortex and pressure fluctuations. Cavitation shedding frequency can also be measured by experiment and numerical simulation. As a result,inception of cavitation, re-entrant jet and cavitation cloud shedding are accurately predicted by LES in accordance with the quantitative images observed in the experiment.Information on internal flow and jet break up structures near nozzle the exit are described at early and quasi-steady stages of injection based on LES simulation. The results show that added a bubble inside nozzles makes a double- mushroom structure at the start of the spray which is different without a remnant bubble. The jet surface becomes unstable so surface wave and ligaments are created which are effected by bothcavitating flow and aerodynamic interaction. Besides that, air can suck into the nozzle orifices which may be responsible for the existence of hydraulic flip in an actual diesel nozzle. Through the in depth analysis of spray atomization condition, made a further confirmed near the exit of nozzles has a unperturbed liquid core and its length decreased with the increasing injection pressure.