Investigation On Ultra-high Injection Pressure Of Diesel Nozzle With Grooved Cone Needle

With the increasing injection pressure of the common rail system of diesel fuel,most of the world’s injector companies have introduced fuel injection systems with 200 MPa injection pressure or even higher.The increase of injection pressure strengthens the turbulent intensity of the jet flow in the injector,which leads to the enhancement of the unsteady characteristics of the cavitation process in the injector and the more significant cavitation effect.Due to the size limitation of the injector and the difficulty of controlling the ultra-high injection pressure,it is difficult to carry out the experimental study on the internal fuel cavitation characteristics of the injector under ultra-high injection pressure.It is not clear yet that how the highly complex turbulent conditions and the injector structure can influence fuel cavitation initiation.Therefore,numerical study on the cavitation characteristics of injector flow under ultra-high pressure injection conditions is carried out by means of computational fluid dynamics,which provides theoretical support for the design and development of a new type of high pressure common rail electronic controlled injector.Firstly,for the internal flow and cavitation characteristics of injectors under high to ultra-high pressure injection conditions,considering the gas-liquid two-phase compressibility of diesel fuel with pressure variation,a three dimensional transient cavitation numerical calculation model of mixed two-phase flow in injectors is established based on Reynolds Average Navier-Stokes(RANS)method.The cavitation image data captured by visual observation experiment of the transparent injector prototype is obtained.The feasibility and accuracy of the cavitation model of ultra-high pressure injector are verified.The results show that the multi-phase flow cavitation model of ultra-high pressure fuel injection is in good agreement with the experimental data,and can capture the spatial location and morphological characteristics of cavitation generation and development.Secondly,the flow and cavitation characteristics of diesel engine injectors under high to ultra-high injection pressure in steady state conditions are studied.The effects of high to ultra-high injection pressure on average fuel flow rate,mass flow rate and vapor volume fraction in injectors of common and grooved nozzles were investigated.By comparing the spatial distribution of pressure field,density field and velocity field between ordinary injectors and grooved injectors,the effects of grooved cone valves on the flow and cavitation characteristics of injectors under the conditions of high to ultra-high-pressure injection were discussed.The results show that the groove structure transfers the bottom vortex zone of the pressure chamber to the vicinity of the injector entrance near the lower wall,which makes the fuel flow velocity distribution inside the injector relatively uniform,reduces the volume fraction of cavitation in the injector,improves the stability of jet breakage,and effectively reduces the degree of cavitation erosion inside the injector.Finally,the transient flow and cavitation characteristics of the diesel injector under ultra-high injection pressure are studied.The coupling effect of cone valve motion law and groove position on fuel flow in the injector under transient conditions is explored.The effects of groove distribution,including head position,tail position and parallel groove distribution,on internal flow of the injector and cavitation characteristics under ultra-high fuel injection pressure were studied during the whole fuel injection period.The results show that the vapor volume fraction of the nozzle presents four stages over injection time.The distribution of groove directly determines whether fuel is fully developed through groove,the height of fuel turning at the end of groove and the position of vortex zone at the front area of nozzle entrance.The relative position of groove and nozzle and its coupling with needle valve motion law are the key factors affecting flow and cavitation characteristics of the diesel injector.