The Expermental Study On The Unsteady Cavitation In Diesel Injector Nozzles

Cavitation in diesel nozzle orifice significantly influences the quality of fuel atomization. In the past cavitation in nozzles in the steady or quasi-steady states is mostly investigated. In the present work, based on the hydrodynamics similarity principle, a large-scale transparent nozzle unsteady state test system is designed. By using high speed photography, the effects of several parameters including dimensionless parameters, jet parameters and structural parameters on the unsteady state cavitation in VCO-type nozzles are investigated, which are widely used at present.The results reveal that cavitation in the unsteady state is quite different from that in the steady state. As the dimensionless cavitation number and Reynolds number increase, the cavitation will enhance. But the effect of cavitation number is more significant. In terms of the jet parameters, the cavitation changes with the pressure oscillation caused by the moving needle, but the change of cavitation lags behind the pressure oscillation in time. The high frequency pressure oscillation plays a positive part in the cavitation. The rise of inlet pressure or decline of outlet pressure in nozzles will enhance the cavitation. The velocity of needle is higher, the pressure oscillation is severer and the change of cavitation is more observable. In terms of structural parameters, when fixing the length-diameter ratio of the orifice, the diameter is smaller, the cavitation is easier to happen, the jet pressure causing the super cavitation is lower and the region of cavitation changes more greatly during the moving process of the needle. To increase the number of nozzle orifice makes the pressure decline severely. The changes of cavitation in different orifices are out-of-step for each other because the needle is bias during the moving process. Cavitation luminescence will appear in the orifice in specific nozzles only when the pressure is higher than certain critical value. The fluorescent is blue. The intensity and the region of luminescence basically coincide with the cavitation.The study on cavitation in nozzles is very important to improve and to perfect the atomization theory in diesel engines. The changing regularity and factors affected the unsteady state cavitation in large-scale nozzles are derived. It is useful in practice.