Experimental Study On Spray Characteristics Of Small Fuel Flow Rate Pressure-swirl Nozzle Under Ultra-high Ambient Pressure

Stirling engine is a type of external heat engine with high efficiency and low emission,its combustion efficiency and emission characteristics are greatly influenced by the injection and the atomization of fuel.Pressure-swirl nozzle can form proper spray under atmospheric pressure,while its spray quality is strongly restricted by higher ambient pressure and smaller fuel flow rate.Experimental study of spray characteristics under high ambient pressure and small fuel flow rate is thus important for the improvement of spray quality under the corresponding working conditions.In this work,an ultra-high pressure fuel spray test bench was built and the experimental study of the macroscopic characteristics of spray was done on it with high speed photography.The main contents of this thesis are as follows:(1)First,a systematical study about the effect of ambient pressure,fuel flow rate and nozzle parameters on spray pattern was performed.Results show that:1.Spray pattern is mainly influenced by ambient pressure.An increase of ambient pressure causes a closure of spray cone,the spray pattern changes from hollow cone to solid tulip-shaped;2.Fuel flow rate has little influence on spray pattern under low ambient pressure.When the ambient pressure is higher,an increase of fuel flow rate could cause a closure of spray,droplets’ distribution near the central axis and a reduced spatial homogeneity;3.Nozzles with larger nominal spray cone or larger nominal fuel flow rate could form a wider spray cone with better spatial uniformity,and a morphology closer to hollow cone.(2)Then,spray cone angle was chosen as the key macroscopic characteristics to describe the spray quality.A systematical study on the effect of those three factors on spray cone angle was performed.Results show that:1.Spray cone angle is mainly affected by ambient pressure.A phenomenon of decreasing spray angle is observed with an increase of ambient pressure.This effect is initially dramatic,then weakened as the ambient pressure continues to grow;2.The effect of fuel flow rate on spray cone angle is negligible under low ambient pressure.With medium or high ambient pressure,the increase of fuel flow rate causes a decrease of spray cone angle and this effect is weakened as the fuel flow rate continues to grow;3.Nozzles with larger nominal spray cone or larger nominal fuel flow rate have a larger critical flow rate under high ambient pressure and a better adaptation of high ambient pressure at large spray flow working conditions.(3)Finally,the critical atomization fuel flow rate was defined and the effect of ambient pressure,nozzle parameters and fuel temperature on critical atomization fuel flow rate was studied.Results show that:1.Ambient pressure has an essential influence on critical atomization fuel flow rate.The critical value varies monotonously with the increase of ambient pressure.The effect is negligible when the ambient pressure is higher than a certain threshold value which is identical for nozzles with various design parameters.2.Nozzles with larger nominal spray cone angle have a smaller critical atomization fuel flow rate;3.The rise of fuel temperature causes a decrease of kinetic viscosity,the atomization process is thus facilitated and the critical atomization fuel flow rate decreases obviously.This research provides a helpful reference and support for improving engine’s combustion and emission performances with high ambient pressure and small fuel flow rate,which has a great theoretical and engineer application values.