Ambient Pressure On In-nozzle Flow In The Process Of Stopping Injection

The internal flow field of nozzle could effect injection parameters.In recent years,some studies have found that there are residual bubble inside the nozzle after spray which from outside gas.For the actual working process of injector,these residual bubble were from the high temperature gas in cylinder.The high temperature gas may be cause coking in the hole.For the further understanding of internal flow field of nozzle,The article has emphatically studied the influence of ambient pressure on air ingestion in the process of oil stopping by the method of visualization experiment and numerical simulation.The paper has designed a constant volume chamber and built a visualization experiment platform which can provide a stable environment pressure.In order to ensure the safety of the experimental process,the test of bearing and sealing capacity must needed implement first.Then in order to make the spray environment is more close to the actual injection nozzle,the transparent nozzle which made according to the actual size was used.The nanosecond light source platform can frozen the high speed flow field effectively and then shoot by shadow method.The results by different back pressure conditions have found that,the cavitation phenomenon was not found in stable injection process but quickly produced when the valve was closing,The process of cavitation bubble collapse accompany with the air ingested phenomenon.The higher back pressure,the amount of cavitation formation is less when the valve is closed and the ingested air volume is also less.The VOF method and the overset grid technology were used to simulate transient state flow in the spray orifice and the sac.The simulation purpose was quantitative analysis some parameters,such as the cavitation volume inside nozzle,ingested air volume,needle valve sealing surface and the flow velocity in nozzle.The results show that the residual bubbles near the nozzle exit will make the initial spray form a double mushroom shape.The velocity at sac inlet decreased rapidly which caused by setting off process was the mainly factor that create cavitation in nozzle;The time of cavitation nascent and the mass flow decreases rapidly at the needle sealing surface is agree,and the time could not be influenced by back pressure and injection pressure.Cavitation collapse process is caused by outside pressure transfer to the hole and made the cavitation collapse phenomenon also transmit to the sac from the orifice.The sac pressure reaches the maximum value when the cavitation collapse completely.The cavitation collapse process inside nozzle and air ingested process is a complementary process in space.The vacated volume because of the cavitation collapse was filled by ingested air,the volume of air ingested is substantially equal to the volume of cavitation collapse.When the back pressure is increasing,the volume of cavitation and ingested air is reduced,the law is consistent with the test results.The duration of cavitation and amplitude of cavitation duration both decreases with the increasing of back pressure.But the cavitation and ingested air volume would raise with the increasing of injection pressure,as well as the duration of the cavitation.The dimensionless cavitation number C_N which express the cavitation degree was derived after simplifying the math model,The cavitation number would increased when the back pressure was raised as well as the decrease of injection pressure and fuel volume modulus.The greater of the cavitation number,the easier of cavitation occurrence,the more volume of ingested air at the oil stopping process.Consider experiment and simulation results synthetically,the cavitation number C_N can reflect the cavitation degree of difficulty in a certain extent.