| Faced with the dual problems of energy crisis and emission pollution,the internal combustion engine industry urgently needs to make breakthroughs in energy conservation and emission reduction.The adoption of high-pressure common rail fuel injection systems and the development of new alternative fuels have become the focus of current internal combustion engine technology research.With the increase of injection pressure and the use of low-carbon fuel methanol,the cavitation problem caused by rapid depressurization in the nozzle cannot be ignored.The cavitation flow will not only affect the flow performance and fuel injection stability in the nozzle,but also has an important impact on the subsequent jet breaking and spray atomization.Therefore,the research on the evolution mechanism of cavitation flow in the nozzle and the cavitation flow characteristics of methanol fuel is of great significance to the development of the reliability,efficiency and cleanliness of the fuel injection system.In this paper,based on the principle of flow similarity,a three-dimensional magnified nozzle model is designed,and a visualization experiment is carried out.The evolution law and transient development characteristics of cavitation in the nozzle are obtained by a high-speed camera,and the frequency of cavitation shedding is obtained by the proper orthogonal decomposition method.The influence of cavitation characteristic length on cavitation shedding frequency was further explored.Subsequently,a three-dimensional numerical simulation model of the cavitation flow in the nozzle was established,and the verification of the CFD simulation model was completed by comparing the experimental data.Cavitation characteristics and flow characteristics of methanol under different inlet pressures,different working temperatures and different nozzle length diameter ratios were investigated by using the verified numerical model.Finally,in order to obtain the mechanism and influencing factors of string cavitation,the second-order invariant velocity tensor(Q criterion)with a nozzle model capable of generating vortex motion was used to explore the effect of different needle valve lift,inlet pressure and fuel temperature on the effect of string cavitation.The main conclusions show that:(1)Affected by the re-entrant jet,periodic cloud cavitation shedding occurs in the nozzle.The frequency of cloud cavitation shedding is closely related to the characteristic length of cavitation in the nozzle.Under the smaller cavitation characteristic length(L_C<0.35),the frequency of cavitation shedding is higher(about4500Hz),while under the larger characteristic length of cavitation(L_C>0.35),the frequency of cavitation shedding is low(about 2500 Hz).(2)The cavitation intensity of methanol in the nozzle increases with the increase of temperature and inlet pressure;cavitation is the main reason for the decrease of the flow coefficient in the nozzle:when cavitation is initiated,the flow coefficient begins to decline,and when super-cavitation occurs,the flow coefficient drops sharply.With the increase of temperature,the critical pressure required for cavitation initiation and super-cavitation decreases.With the increase of length diameter ratios,the critical pressure required for the super-cavitation increases,and the flow coefficient changes more smoothly at lower length diameter ratios.(3)The vortex movement from the upstream inlet to the nozzle is the main reason for the generation of string cavitation in the nozzle.As the lift of the needle valve decreases,the vortex strength in the nozzle increases,and the string cavitation only occurs in the lower needle valve.The string strength increases with the inlet pressure,and the fuel temperature also increases the vortex cavitation strength,but the increase is modest. |
