Experimental study on the spray characteristics discharging from elliptical diesel nozzle at typical diesel engine conditions

摘要

The elliptical orifice diesel nozzle has potential beneficial for improving spray and air-fuel mixing quality, which has great impact on combustion and emission performances for diesel engine. This paper presents experimental studies on the macro spray characteristics and the mechanism of axis-switching for elliptical and circular orifice diesel nozzles with same cross section area, with biodiesel fuel by using shadowgraph technique, under various injection pressures (50 MPa, 70 MPa, 90 MPa) and backpressures (1MPa, 2 MPa, 3 MPa). Two CCD cameras were adopted to acquire the macro spray images at the minor and major view planes simultaneously. Macroscopic spray characteristics from different view planes of spray were processed by MATLAB. The results indicated that the spray width of the elliptical spray was greater than that of the circular spray in all the view planes. Elliptical orifice spray exhibited axis-switching phenomenon during the injection process even at high injection pressure and backpressure. The increasing of backpressure enhances the aerodynamic effects, resulting in suppressing axis-switching of elliptical spray. However, increasing the injection pressure is beneficial for improving the axisswitching frequency, because the higher spray inertia resulting from higher injection pressure can cause greater spray surface-instabilities, which are conductive to enhance the axis-switching. Moreover, the spray width at the downstream increases as the backpressure increases, and for all geometrical orifices exhibited greater spray width under the higher injection pressure. In addition, the spray tip penetration of circular orifice was longer than that of elliptical orifice during the whole injection process. This is due to the larger spray surface of elliptical spray resulting in larger aerodynamic drag. Furthermore, the spray cone angle of the circular orifice is consistently smaller than that of elliptical orifice under typical diesel engine conditions. Because the axisswitching phenomenon of elliptical spray is helpful to improve the air entrainment ratio and thus resulting in larger spray cone angle. Finally, the present experiment results indicate that elliptical sprays are conductive to increase air entrainment ratio and air-fuel mixing quality as compared to circular sprays, due to the axisswitching phenomenon, larger spray cone angle and width.