Study Of The Evaporation Combustion And Micro-explosion Characteristics Of Diesel Fuel Blends

The massive application of diesel fuel has caused energy shortage and environmental pollution while promoting rapid economic and social development,and the use of clean and efficient diesel alternative fuels has become a trend in energy development.Ethanol,as a very promising carbon-neutral fuel,is considered as an ideal fuel alternative when blended with diesel.However,the addition of ethanol and the surfactant(tetrahydrofuran)complicates the physical properties of diesel fuel blends,increasing the fire hazard during their processing,use,and storage and transportation.This paper presents an experimental and numerical study of the evaporative combustion characteristics of diesel fuel blends using a single droplet study method to understand the influence of the correlation between the physical properties of diesel fuel blends and their evaporative combustion characteristics.In this paper,the evaporative and micro-explosion characteristics of diesel fuel blends were firstly studied and discussed by constructing an experimental system for single droplet evaporation under convective environment,using biodiesel as a control group,and analyzing the evaporative,blowing and micro-explosion characteristics of fuel blend droplets under different ambient temperatures,ethanol contents and surfactants.The results showed that due to the low boiling point of ethanol,it would preferentially vaporize and nucleate inside the droplet,which would trigger the fluctuating evaporation of the droplet and the puffing or micro-explosion of the droplet under the action of pressure and surface tension inside the droplet.Ambient temperature has a catalytic effect on the droplet internal fluctuations,for the droplet with tetrahydrofuran as surfactant(DTE30),when the ambient temperature increases from 573 K to 623 K,673 K and 723 K,the micro-explosion intensity increases by 15.2%,25.4%and 56.8%,respectively.Compared with the DBE droplets with biodiesel as surfactant,the DTE droplets exhibited more significant fluctuation phenomenon.Based on the nucleation and fragmentation behavior of bubbles inside the droplet,two fluctuation modes are proposed,i.e.,edge mode and inner mode.The tetrahydrofuran in DTE droplets has lower viscosity and surface tension,resulting in a greater tendency to evaporate microbursts in edge mode,while the evaporation of DBE droplets is more inclined to inner mode,which causes higher micro-explosion intensity in DBE droplets than in DTE droplets.The calculation of the multi-component droplet evaporation model shows that although tetrahydrofuran has a lower boiling point,its mass decay rate inside the droplet is still lower than that of ethanol,which is related to the molecular polarity of the different components;in the model calculation,the activity coefficient of ethanol is 3.04,which is much larger than that of tetrahydrofuran at 1.01,which leads to a stronger volatility of ethanol.The combustion and micro-explosion characteristics of diesel blend droplets,including ignition delay time,combustion duration and burning rate,were then carried out under auto-ignition and forced ignition conditions.The effects of different droplet particle sizes on the combustion and diffusion processes of diesel fuel were first investigated.Experiments showed that larger droplets require higher ignition energy to promote droplet temperature rise and evaporation,thus causing longer droplet ignition delay times,which is consistent with the predictions of theoretical models.The physical evaporation phase of the droplet is shortened under the forced heating method using an electric heater,when the ignition delay time of the droplet is smaller,but the combustion duration is longer.The increase in the initial droplet diameter results in a lower specific surface area,slower evaporation of the droplet by heat,and lower burning rate.The droplet in the heating furnace’s homogeneous heating environment will first go through a fluctuating evaporation phase,with puffing and micro-explosion,forming a combustible vapor mixture around the droplet,and after reaching the lower limit of combustion concentration,the droplet ignites to form premixed combustion,followed by a transition to diffusion combustion.Droplets in the forced ignition heating mode will quickly enter the diffusion combustion,and due to uneven heating of the temperature field,droplet microburst combustion more intense,the emergence of sputtering flame.When the droplet ethanol content of more than 30%of the case,compared to the heating furnace auto-ignition conditions,forced ignition of droplet micro-explosion intensity is greater.The addition of ethanol will also promote the combustion process of mixed droplets,however,with the increase in ethanol content,ethanol’s higher enthalpy of evaporation and suppression of droplet temperature rise will dominate the droplet mass transfer heat transfer process,the experimental results show that the ignition delay of diesel mixed droplets with the increase in ethanol concentration shows a trend of first falling and then rising,for both auto-ignition and ignition conditions of the turning point corresponding to the ethanol concentration of 20%and 50%.Finally,increasing the volume ratio of tetrahydrofuran,the surfactant in the diesel fuel mixture droplet,the ignition delay time of the droplet is generally higher,which is related to the larger evaporation enthalpy of tetrahydrofuran and causes the increased evaporative heat loss of the droplet,and the measured droplet temperature value verifies this trend.Also,an increase in tetrahydrofuran content significantly increases the burning rate of mixed droplets.