Analysis Of Cavitation Flow,Cavitation Risk And Atomization Of Diesel Engine Nozzles With Different Structural Parameters

In recent years,the national requirements for the power performance and emission performance of diesel engines have become increasingly stringent,the electronic control technology of diesel engines continues to improve,the injection pressure continues to increase,the injector nozzle cavitation flow characteristics,the risk of cavitation corrosion and spray characteristics have been widely concerned.When high-speed fuel flows into the nozzle hole,cavitation is generated by the influence of the nozzle hole structure,and the repeated generation and collapse of cavitation bubbles continuously impact the wall surface,which will lead to the spalling of the wall material inside the nozzle hole,and in serious cases,the failure of the injector.Therefore,numerical simulations are used to study the effects of cavitation flow and cavitation characteristics inside the nozzle for different structures of nozzles and different injection pressures,to analyze the morphology of atomization outside the spray hole,and to provide a more systematic and practical reference theoretical basis for subsequent developers in designing the structure of the nozzle.First,according to the theories of research scholars for bubble collapse,Matlab software was used to fit an equation for the relationship between microjet velocity and pressure generated by cavitation bubble collapse at different distances from the near wall,and an empirical equation was derived for cavitation bubble impingement on the wall at different distances from the near wall under different cylindrical nozzle structures to relate the cavitation flow characteristics and cavitation risk in the nozzle.The Rrs cavitation risk model validated by other researchers was imported into Fluent,and the results were compared with those of the new cavitation risk model to conclude that the new model has some reliability in predicting the cavitation risk region.Secondly,modeling and meshing of nozzles with different structures and simulating the change of oil film during the lifting and lowering of the needle valve,compiling modified empirical equations in the UDF module of Fluent software,establishing a complete cavitation flow and cavitation risk model,studying the aspect ratio of the spray hole,the volume of the pressure chamber,the number of spray holes,double-layer spray holes versus single-layer spray holes,and the injection pressure as a single variable to simulate The cavitation flow and cavitation characteristics in the spray hole under needle valve motion.It was found that the impact pressure caused by the collapse of the cavitation bubble micro jet and the pulse pressure caused by the shock wave are of the same order of magnitude,so they cannot be ignored when conducting the study.In the spray hole diameter of the same and increasing length,that is,the length to diameter ratio increases,the development of cavitation flow in the spray hole is inhibited,the wall is subject to lower impact pressure;in the spray hole length remains the same and the increase in diameter,that is,the length to diameter ratio decreases,the cavitation generated in the spray hole area in the reduction,the impact pressure of cavitation bubble collapse on the wall increases.Consider different types of pressure chamber volume,no pressure chamber nozzle cavitation flow phenomenon growth is the most obvious and the impact pressure on the inner wall surface is the largest.With the increase in injection pressure,the degree of cavitation in the spray hole is increasing,the impact pressure on the wall increases significantly.With the increase in the number of holes,the cavitation phenomenon occurs in a wider area coverage,the impact pressure on the wall of the spray hole increases.Single and double spray holes compared to the two,the impact pressure on the wall of the nozzle is not a big difference in the size of the impact pressure;in the needle valve rise stage cavitation risk areas appear in different locations.Finally,The fluid domain of the spray field is established outside the spray hole,and the parameters of the spray hole outlet obtained from the cavitation simulation are used as the initial setting conditions for the simulation of the spray atomization characteristics to obtain the spray morphology of the spray atomization under different structures and at different pressures.The results show that when the pressure at the inlet of the nozzle hole is higher,i.e.,the pressure of fuel injection is higher,the spray cone angle is also larger.In the case of constant import and export pressure,when the diameter of the spray hole increases,the atomization effect at the outlet of the spray hole deteriorates,and the spray cone angle decreases.When the injector nozzle pressure chamber structure is different,no pressure chamber nozzle pressure chamber volume is smaller,which makes the nozzle internal cyclonic flow strength is reduced,so that the spray hole spray atomization taper is reduced.