Wall Conditions Effect On Cavitation Flow Characteristics In Micro-Orifice

In recent years,the nozzle of diesel engine is developing towards the trend of micro orifice and high injection pressure,which makes cavitation more easily happen in the nozzle and brings two opposite effects on the actual operation of the nozzle.In the flow of micro-orifice,the wall condition plays an important role,which results in the flow characteristics inside the micro-orifice being different from that in the macro-orifice.Therefore,the CFD numerical simulation method was adopted in this paper to explore the influence of wall conditions on cavitation flow characteristics in micro-jet orifice,and the influence of injection pressure,wall roughness,wall wettability(contact angle)on flow field,flow coefficient,volume fraction of gas-phase diesel oil and other parameters inside the nozzle orifice was analyzed in turn.The numerical simulation results show that the cavitation of the orifice occurred at the injection pressure of 10 MPa and the flow coefficient reached the maximum value of 0.768.With the increase of injection pressure,the cavitation layer develops along the wall first,then along the radial thickening.Correspondingly,the flow coefficient first decreases rapidly with the development of cavitation,then slowly decreases to the semi-cavitation state at 40 MPa,and then becomes stable at 100 MPa.The influence of wall roughness on the cavitation flow in the orifice is more significant than that of the main flow.Due to the turbulence effect of wall roughness,the larger the roughness is,the lower the velocity of orifice outlet section will be,and the greater the turbulence kinetic energy will be.However,due to the reduction of wall velocity,the reduction of pressure drop and the degree of cavitation will be weakened.With the increase of relative roughness,the flow coefficient of the nozzle increases first and then decreases,during which there will be a peak,which corresponds to the relative roughness of the wall surface can significantly increase the flow coefficient of the nozzle,the maximum increase is 11.5%,which plays a very important guiding significance for machining the inner wall surface of the nozzle.The contact Angle also has an obvious influence on the cavitation flow inside the orifice.The larger the contact Angle is,the weaker the cavitation degree is,the lower the volume fraction of gasphase diesel is,and the larger the flow coefficient of the jet orifice is.However,the contact Angle has no obvious influence on the change trend of the flow coefficient of the jet orifice.The wall surface with wettability will have velocity slip,and the velocity increases with the increase of contact Angle.In addition,the super hydrophobic surface(contact Angle is 150 °)can also improve the nozzle exit velocity,which will improve the nozzle flow coefficient,is more advantageous to the oil atomization.