Simulation Study Of Pressure Boundary Conditions On Cavitating Flow Characteristics In Diesel Injectors

In recent years,air pollution keeps intensifying,to reduce motor vehicle exhaust emissions has become an urgent issues,which needs to be solved.The key to reduce the emission of motor vehicle exhaust is to improve atomization characteristics of the injector.The fuel injector is the core component of the fuel injection system.The fuel flow characteristics in the injection hole have a special effect on the atomization.In the running process of engine,the pressure conditions in the injection hole are complicated and changeable,and it is difficult to explore the fuel flow characteristics in the nozzle hole by using the experimental method.Combined with the experimental and three-dimensional simulation methods,this paper studied the fuel flow characteristics in nozzle caused by pressure boundary.Collectively,we get the following conclusions:(1)fixed inlet pressure,changing outlet back pressure of the nozzle,cavitation can be divided into three stages:no cavitation period,cavitation developing period and super cavitation period.During no cavitation period,with the reduction of the back pressure,the fuel flow rate increases and the flow coefficient keeps constant.At cavitation developing period,with the decrease of the back pressure,cavitation occurs and cavity is formed at the upper of nozzle.The further reduction of the backpressure,cavity constantly flows to the downstream of orifice.At this stage,the fuel flow rate continuously increases and the flow coefficient does not change.In the super cavitation period,the flow in nozzle becomes a blocking flow.At the same time,the back pressure has an effect on the fuel quality,which keeps the fuel mass flow stable and the flow coefficient decreases.(2)fixed outlet back pressure,with the increases of inlet pressure,fuel flow coefficient can be divided into three stages in nozzle.The first stage: no cavitation inside the nozzle,flow coefficient is stable,cavitation without being affected by the cavitation number;The second stage: the flow coefficient is affected by the cavitation number and the shrinkage coefficient,the flow coefficient decreases sharply;the third stage: the flow rate is not affected by the cavitation number,the flow coefficient is stable,the cavitation is independent of the cavitation number;The coefficient is independent of the shrinkage coefficient,only with the cavitation number,with the inl et pressure increases,the flow coefficient tends to be stable.In our study,the relationship between the cavitation flow characteristics and the fuel injection quantity is revealed,and the influence of cavitation characteristics should be taken into account when the fuel quantity of the injector is matched.