Study On The Effects Of Dispersed Phase Size On Evaporation And Combustion Characteristics Of A Single Water Emulsified Diesel Droplet

Water emulsified diesel has a potential to improve engine thermal efficiency and reduce NO_Xand Soot emissions simultaneously.The micro-explosion phenomenon caused by the difference of boiling points of water and diesel is considered as the main reason for that.In recent years,researchers have noticed that the size of dispersed water droplets has an important influence on the micro-explosion characteristics of water emulsified diesel.Therefore,it is of both theoretical and practical significance to study the influence of dispersed phase size on the evaporation and combustion characteristics of a single water emulsified diesel droplet,and to summarize the mechanism of dispersed phase size impacting on the micro-explosion characteristic.In this paper,an ultrasonic emulsification device with adjustable configuration parameters is used to study the influence of configuration parameters on dispersed phase size through a water emulsified diesel preparation experiment based on orthogonal design.The results show that when the log-normal distribution function is used to fit the size distribution of dispersed phase,the correlation coefficient of the fitting curve reaches99.67%.Meanwhile,the corresponding characteristic parameters are given as the fitting maximum dispersed water size and the standard deviation of droplet size distribution.With the increase of ultrasonic configuration parameters,dispersed phase size is decreased.The significance level of each configuration parameter in descending order is ultrasonic power,ultrasonic duration,ultrasonic frequency.Adjusting the configuration parameters according to the results of orthogonal experiment,the water emulsified diesel of five levels of dispersed phase size and three levels of water content used in the research is prepared.The fitting curve shows that the prepared water emulsified diesel with same dispersed phase size but different water content has good consistency and uniformity.Subsequently,the effects of different dispersed phase particle size on the evaporation and combustion characteristics of W20 droplets were studied at ambient temperatures of 773 K and 973 K,respectively.The results show that the evaporation and combustion process of water emulsified diesel droplets show three-stage characteristics,which are divided into transient heating stage,fluctuating evaporation(combustion)stage and steady-state evaporation(combustion)stage.Based on the bubble breakage scale before and after each micro-explosion,this article quantitatively describes the intensity of each micro-explosion.In the fluctuating evaporation(combustion)stage of the droplet,the micro-explosion is divided into three types:strong micro-explosion caused by the breakage of large-scale bubbles inside droplet,weak micro-explosion caused the breakage of bubbles on the droplet surface or the breakage of small-scale bubbles inside the droplet,puffing phenomenon caused by the coordinated evaporation of dispersed water droplets,which are caused by the different coalescence situation of dispersed water droplets.A new condition of micro-explosion is reported in this paper:the breakage of bubbles on droplet surface will cause strong disturbances,thereby destroying the stable state of the droplet,and then instantaneously triggering the fragmentation of other large bubbles inside,which indicates the existence of a continuous micro-explosion characteristic of multiple bubbles.The influence of dispersed phase size on the micro-explosion characteristics exhibits unimodality.When the dispersed phase size increases,the surface tension of the dispersed water droplets decreases,the transport rate increases,the coalescence between water droplets increases,the proportion of transient heating stage shortens,the bubble nucleation and growth rate accelerate,and the micro-explosion effect is promoted.However,it is also easy to form multiple large bubbles inside the droplet during the expansion process,resulting in surface fragmentation,damaging the droplet oil film,causing water vapor to escape in advance and inhibiting the micro-explosion effect.Therefore,in the process of droplet evaporation and combustion,with the increase of the particle size of the dispersed phase,the proportion of the fluctuating evaporation(combustion)stage of the droplet,the maximum micro-explosion intensity,the micro-explosion frequency and the cumulative micro-explosion intensity all show a trend of first increase and then decrease,while the component injection caused by large-scale micro-explosion makes the droplet life show a trend of first decrease and then increase.Finally,the effects of water content and ambient temperature on the evaporation and combustion characteristics of water emulsified diesel with different dispersed phase size are further studied.At 773 K ambient temperature,with the increase of water content,the number of dispersed water droplets increase and the distance between dispersed water droplet decreases,which increases the scale of micro-explosion and the ability of sustained micro-explosion to occur,and the percentage of fluctuating evaporation phase,maximum micro-explosion intensity,strong micro-explosion frequency and cumulative micro-explosion intensity increase,and the droplet lifetime decreases.However,at the ambient temperature of 973 K,the large-scale micro-explosion of W30 droplets resulted in a higher number of component ejections,causing a decrease in the strong micro-explosion frequency and cumulative micro-explosion intensity.For all water contents,the effect of dispersed phase size to promote micro-explosion showed unimodality.As the ambient temperature increases(673 K to 973 K),the component evaporation rate and dispersed water droplet transport rate increase,bubble nucleation and growth rate accelerate,and the maximum micro-explosion intensity and strong micro-explosion frequency of droplets increase,and the droplet lifetime shortens.However,when the ambient temperature increases to 1073 K,droplet combustion greatly increases the nearby temperature,resulting in rapid evaporation of the surface oil film,the maximum micro-explosion strength of droplets and strong micro-explosion frequency reduce.The increase in ambient temperature is not able to increase the ability of the droplet to sustain micro-explosion,so the fluctuating evaporation time and cumulative micro-explosion intensity changes are small.For all ambient temperatures,the effect of dispersed phase particle size to promote micro-explosion shows unimodality.