| The atomization of fuel spray in internal combustion(IC)engines has a significant effect on the combustion of fuel/air mixture in cylinder,and then influences the power performance,fuel consumption,and exhaust emission.Flash boiling is an effective method to improve the atomization of fuel spray and to facilitate the formation of fuel/air mixture.Deep understanding the characteristics and mechanism of flash boiling are essential for its application in the IC engines.In the present thesis,the breakup induced by flash boiling was investigated systematically from the application level of fuel spray to the physical level of a single droplet.An optical method was proposed to measure the droplet size in flash boiling spray,and the effect of flash boiling on the atomization of spray was investigated based on this method.In addition,in order to understand the underlying physics of breakup induced by flash boiling,the breakup of single droplets was investigated for water-in-oil emulsifed fuels and ethanol-diesel fuels.Extended Glare Point Velocimetry and Sizing(EGPVS)was developed to measure the sizes of droplets in the flash boiling spray based on the traditional GPVS method.The measurement accuracy of this method was calibrated using standard particles.The results show that the measurement error of the method is less than 4%.Based on the proposed method,the effect of flash boiling on the atomization of fuel spray was studied.The results show that flash boiling can effectively reduce the average diameter of droplets in spray.As flash boiling occurs in the spray,the ratio of large droplets in the spray decreases rapidly,the distribution of droplet diameter becomes narrower,and the size distribution of droplets in the spray is uniform.The size distribution of droplets is mainly determined by the superheat degree,while the fuel type has little influence on it.In order to understand the mechanism of breakup induced by flash boiling,the breakup characteristics of single droplets for water-in-oil(W/O)emulsified fuels were studied.It is found that there are two different breakup modes as the droplet is heated:one is puffing with some small droplet ejected from the droplet surface;the other is micro-explosion with droplet exploded abruptly.Based on the breakup strength,the micro-explosion can be further divided into catastrophic micro-explosion and local micro-explosion.The catastrophic micro-explosion is more violent than local microexplosion and than puffing.As the water concentration is in the range of 20?30vol%,the surfactant concentration is less than 3vol%,and the heating temperature is in the range of 623?773K,the droplet has the most breakup strength.In this range,the dispersed water droplets in oil can coalesce into one single water droplet during the heating,and then vaporizes explosively at a high temperature to induce a catastrophic micro-explosion.Beyond this range,the breakup strength of droplet is weakened.In order to reveal the mechanism of micro-explosion,the oil/water interfacial tension was investigated,and its effect on the phase separation and micro-explosion was analyzed.It is found that phase separation happened during the heating is caused by the increased water/oil interfacial tension after the deactivation of surfactant at water/oil interface.In addition,the elevated interfacial tension can also improve the superheat limit of the emulsified fuel that can make droplet break up at high temperatures.The deactivation of surfactant is a prerequisite for the occurrence of catastrophic micro-explosion.Catastrophic micro-explosion can be controlled to occur by adjusting the surfactant concentration in emulsified fuels.A model for W/O emulsified fuel was developed for droplet heating and evaporation based on the experimental heating behaviors.In this model,the diffusion and evaporation of dispersed water droplets in oil and coalescence of dispersed water droplets after deactivation of surfactant were considered.The model was validated against experimental data under different conditions.The predicted results show that the model can well predict the droplet heating and evaporation for W/O emulsified fuel.Based on the proposed model,the water left in the droplet and the temperature distribution inside the droplet just before the breakup were analyzed.It is found that the surfactant concentration has a remarkable influence on the water left in the droplet just before breakup at a low heating temperature,and the small size of dispersed water droplet has a remarkable influence on the water left in the droplet just before breakup.Heating temperature influences the temperature distribution inside the droplet,and then the breakup mode and strength.Considering that the ethanol-diesel fuel is more stable than the W/O emulsified fuel,the breakup characteristics of single droplets for ethanol-propanol-diesel(EPD)homogeneous fuels and ethanol-biodiesel-diesel(EBD)micro-emulsified fuels were also studied.The results show that the fuel structure and concentration of volatile components in fuels influence the breakup mode and strength.For EPD homogeneous fuels,with the increase of ethanol-propanol concentration in fuel,the breakup mode of the droplet is changed from puffing to micro-explosion.However,for EBD microemulsified fuel,with the increase of ethanol-biodiesel concentration in fuel,the breakup mode of the droplet is changed from micro-explosion to puffing.The mechanism of micro-explosion of these two different types of fuels are different: for homogeneous fuels,micro-explosion is induced by intense bubble nucleation,growth and breakup;for micro-emulsified fuels,micro-explosion is caused by rapid vaporization of ethanol droplets after phase separation that is similar to the W/O emulsified fuel. |
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