The impact of ethanol addition on the droplet combustion mechanism of saturated and unsaturated fatty acid/fatty acid methyl ester molecules

[Display omitted] •Droplet combustion mechanism of saturated and unsaturated FA/FAME-ethanol blends.•Ethanol promotes thermal expansion through the bubble population formation.•Microexplosion is divided into three child droplet ejection mechanisms.•Oleate as base fuel has better thermal absorption ability than laurate.•Ethanol shortens the soot formation period of FA/FAME droplet combustion. The combustion of biodiesel requires complex analysis because it is composed of multicomponent constituents. The use of additives in biodiesel is an option widely used to improve the combustion performance of the fuel blend. The droplet combustion characteristics of single compounds of biodiesel and vegetable oil constituents with ethanol additives were investigated under normal gravity at room temperature and atmospheric pressure. This study reveals the different combustion mechanisms and their interactions with ethanol between laurate and oleate as the base fuel. Ethanol breaks the base fuel into smaller molecular aggregates, reducing the fuel blend density with many larger gaps between the clusters. Thermal expansion and child droplet ejection are key mechanisms that effectively increase the total droplet surface area, causing an increase in the diffusion contact between the fuel vapor and the air. Three mechanisms of child droplet ejection occur during the heating phase until the microexplosive combustion phase. The period of child droplet ejection during the microexplosive combustion phase is determined by the boiling point difference and the solubility level between the base fuel and ethanol. The addition of ethanol increases the burning rate of the fuel mixture with a higher peak temperature in the laurate-ethanol blend compared to the base fuel. Ethanol plays an important role in delaying the start of evaporative cooling with a significant temperature increase rate. This behavior is an effective mechanism to initiate droplet combustion with a short ignition delay time.