Research On Transient Characteristics Of Cavitation In Nozzle Of Marine Diesel Engine Under High Pressure Injection

With the in-depth research on the optimization of diesel engine performance,the injection pressure of the high-pressure common rail system of the diesel engine has been further increased.Compared with the higher fuel injection pressure of the vehicle engine(up to 240MPa),the relevant research of high-power marine engine is still at a low level.Therefore,the direction of the development of marine engines towards high-pressure fuel injection can be foreseen.Furthermore,high-pressure fuel injection complicates the flow inside the nozzle hole,and the cavitation phenomenon in the nozzle hole caused by high injection pressure and strong turbulence will affect the effective flow area of the fuel in the nozzle hole,thereby affecting the fuel mass flow.The marine engine has a higher circulating fuel supply,and the injector pressure chamber and nozzle hole size are larger in terms of structure.At present,there are relatively few studies on the cavitation characteristics in the nozzle holes of marine diesel engines and the transient mass flow changes in the nozzle holes caused by cavitation.The formation of cavitation under different pressures and orifice structures and the mechanism of its influence on the injection law have not been fully grasped.Therefore,this paper uses the visualization experiment and simulation research methods to study the cavitation characteristics in the nozzle hole of the marine engine fuel injector,so as to provide theoretical support for the development and optimization of the marine engine high pressure common rail electronically controlled fuel injector.Firstly,the experimental research on the cavitation characteristics in the nozzle hole was carried out,and a high-speed photography technology visualization experimental platform based on the shadow method was built to make a prototype transparent nozzle with large aperture and large pressure chamber structure.The visualization experiment results show that with the increase of the injection pressure,the initial time of geometric cavitation in the nozzle hole is earlier,and the cavitation phenomenon in the nozzle hole is more severe.In addition,the larger size of the pressure chamber of the injector in the experiment causes the primary cavitation in the injection hole to appear mostly on the lower wall of the injection hole inlet.The effect of the injection pulse width on the cavitation phenomenon in the injection hole is not obvious in this paper,which is because the maximum needle valve lift under short pulse width is similar to the maximum needle valve lift of the injector.Secondly,based on the CONVERGE software platform,the simulation study of the flow inside the nozzle hole is carried out.The results show that there is a certain degree of randomness in the cavitation phenomenon in the injection hole during the fuel injection process.The increase of the injection pressure makes the cavitation area in the injection hole more unstable,which in turn leads to an increase in the fluctuation of the transient fuel mass flow during the stable injection stage.The COV of the transient mass flow under the injection pressure of 110 MPa,160MPa and 210 MPa in this stage are 2.18%,2.34% and 2.58%,respectively.The increase of environmental back pressure will inhibit the development of cavitation in the injection hole and increase the instability of cavitation in the stable injection stage.The transient mass flow COV in the stable injection stage under the back pressure of0.1MPa,2MPa and 4MPa are 2.18%,2.60%,and 2.89%,respectively.In addition,the increase of back pressure will significantly reduce the fuel injection flow in the opening stage of the needle valve.The fuel mass flow increases significantly with the increase of the nozzle hole diameter(ie,the aspect ratio decreases),but the cavitation phenomenon is more unstable.Finally,a porous simulation model is established for the actual injector.The simulation results show that although the structure of the nozzle holes is centrally symmetric,the cavitation phenomenon of each hole in the actual injection process has a large inconsistency,resulting in the difference of the injection mass flow of each hole.