Experimental Research Of DME Spray And Numerical Simulation Of Droplet Evaporation

The increase of fuel temperature and the decrease of environmental pressure will make it easier for fuel to overheating and flash boiling.The fuel will get into supercritical state when the temperature and pressure of the fuel are higher than the critical value.Both ways can achieve rapid formation of the mixture to improve power,economy and reduce emission of diesel engine.DME is the substitute fuel for diesel which has high cetane number to catch fire easily,and low boiling point and critical point to realize Flash Boiling Spray and Supercritical Spray easily.Besides,The oxygen content of DME can effectively reduce soot emission.Therefore,it is of great significance to study the spray and evaporation process of DME.In this paper,the optical method was used to study the process of subcooled,superheated and supercritical spray on a constant volume combustion vessel.Results showed that the spray penetration distance decreased with the increase of ambient pressure,but the trend is not obvious with the temperature changed.When the temperature is higher,the penetration decreased with the increase of temperature because of the effect of evaporation.The subcooled spray process can be divided into two stage: the ‘first breakup’ stage and the ‘second breakup’ stage,but for the superheated spray,the formation of bubbles will accelerate atomization process,so the spray will transit into the ‘second breakup’ stage quickly.The bending angle of the spray decreased with the increase of degree of superheat.When the fuel temperature is higher than the critical temperature,the bending angle increased with the increase of temperature.In this paper,the evaporation process of DME droplet is simulated by using PR equation.The physical parameters of DME under high temperature and high pressure were corrected.The influence of ambient pressure,temperature and initial droplet temperature on the evaporation process of droplet was studied.The results show that when the ambient pressure is higher than the critical pressure and the ambient temperature is higher than the critical temperature,the droplet surface will get into supercritical state from subcritical state during the evaporation process and the droplet surface will disappear.At the meantime the physical parameters such as the concentration of DME changed continuously in space.The higher ambient pressure and ambient temperature,the faster heat will be conducted,the smaller enthalpy of evaporation,the faster droplet surface temperature will increase,and the shorter life of the droplet.At the initial stage of evaporation,the droplet diameter increased as the droplet density decreased,and the higher ambient pressure and ambient temperature,the more obvious droplet diameter will increase.The initial temperature of droplet only made the process of droplet evaporation advance or postpone.Although the life of droplet decreased with the increased initial temperature of the droplet,but the time of the supercritical diffusion stage was nearly constant.