Visualization Experimental Study Of Diesel Spray Characteristics Under Ultrahigh Injection Pressure

With the increasing prominence of environmental problems and the gradual depletion of fossil fuel,the energy conservation and emission reduction of power machinery has become a hot issue in current research.The diffusion combustion mode of direct injection diesel engine makes its combustion quality heavily depend on the atomization quality of fuel spray.Studies have shown that increasing the injection pressure can effectively improve the combustion process of diesel spray and reduce the emission of harmful substances such as soot.However,there are relatively few experimental studies on the characteristics of diesel spray under ultrahigh injection pressure at present,and theoretical studies lack corresponding experimental data for verification.This paper carried out experimental studies on macroscopic and near-field spray characteristics under 180 MPa conventional injection pressure,220 MPa high injection pressure and 270 MPa ultra-high injection pressure,aiming to explore the characteristics of diesel atomization process during ultra-high pressure injection,analyze the applicability of existing atomization mechanism in ultra-high pressure injection,and at the same time provide more data support for theoretical and simulation research.Based on Delphi 300 MPa high pressure common rail injection system,Zeuch method is used to measure the fuel injection rate.The macroscopic and near field characteristics of fuel atomization process were visually observed by Schlieren method and background light microscopic imaging method respectively.The characteristic parameters of spray were extracted by image processing,and the influence of environmental conditions on fuel atomization process under ultra-high injection pressure was analyzed.The main conclusions are as follows:For the traditional needle valve electromagnetic injector,the velocity of fuel flow at the nozzle outlet is proportional to the square root of pressure drop from rail to ambient gas.The ultra-high injection pressure has no effect on the flow coefficient of the nozzle,and the increase of the nozzle diameter will lead to the increase of the nozzle’s flow coefficient.The response time of needle valve decreases with the increase of injection pressure.The setting time of needle valve lengthens with the increase of injection pressure and shortens with the increase of nozzle diameter.With the increase of injection pressure,the penetration ability of spray increases significantly,and the axial penetration distance in ambient gas gets longer.The influence of injection pressure on spray cone Angle is coupled with the effect of ambient gas property,showing different trends at different ambient densities.Higher injection pressure enhanced the air entrainment of fuel spray and effectively reduced the average equivalent ratio of spray at the later stage of injection.With the increase of ambient density,the spray head has stronger radial diffusion with the increase of resistance,so the penetration distance of spray decreases but the spray cone Angle increases.The average equivalent ratio of spray is subject to the coupling effect of injection pressure and ambient density.Under different injection pressure,the influence of ambient density on the average equivalent ratio of spray shows different rules.The increase of nozzle diameter will slow down the response speed at the initial stage of injection,but will increase the penetration distance at the later stage.In the steady injection stage,the spray cone Angle increases with the increase of the nozzle diameter.Finally,the prediction ability of existing empirical formulas for macroscopic spray parameters is analyzed by comparing with experimental data.The increase of injection pressure will increase the penetration velocity of spray in the initial injection stage,improving atomization of near-field spray,and the fuel break-up at the end of injection stage is also more sufficient.The near-field cone Angle increases with the increase of injection pressure,and the length of stable liquid column decreased.The higher ambient density makes the spray break more sufficiently and the fuel droplets distribute more evenly along the spray radial direction.The near-field spray cone Angle increases with the increase of ambient density.When the injection pressure is 50 MPa and the ambient density increased from 29.44kg/m3 to 67.93kg/m3,the length of the spray stable liquid column decreased by more than 60%.The response speed of fuel jet is faster and the length of stable liquid column is obviously shorter for a smaller orifice.The spray head with large nozzle diameter breaks first under the disturbance of ambient gas,and the fuel jet breaks more rapidly at the end stage of injection.The near-field spray cone Angle decreases with the increase of nozzle diameter.When the injection pressure is low,the greater the diameter of the nozzle is,the longer the length of the liquid column is.When the injection pressure is very high,the length of the stabilizer column of the near-field spray decreases with the increase of the nozzle diameter.