Experimental and simulation study on the micro-explosion characteristics of ethanol/biodiesel droplets

Abstract

Biodiesel exhibits superior properties such as high cetane number, high flash point, and high lubricity, which enhance its ignition, storage, and usage characteristics. However, its high density and viscosity can impede fuel atomization. This issue can be effectively mitigated by blending biodiesel with alcohol fuels. The micro-explosion characteristics of ethanol–biodiesel mixture droplets were experimentally studied, followed by numerical simulations to explore the key factors influencing bubble growth and rupture during micro-explosion. By combining millimetre-scale experiments with micrometre-scale asymmetric single-bubble simulations, this study quantitatively links bubble eccentricity to rupture delay and breakup intensity, informing secondary-atomisation submodels. Each experimental condition was independently repeated five times, with the results reported as mean ± standard deviation. The results reveal: 1) Ambient temperature altered the rupture frequency, event duration, and overall droplet survival, but the individual micro-explosion descriptors did not exhibit a uniformly monotonic response across the tested conditions. Smaller initial droplet diameters corresponded to longer survival times, in contrast to non-micro-exploding droplets. The ethanol mass fraction affected droplet evaporation non-monotonically, with the shortest survival time observed at 50% among the three tested mass fractions. 2) Ambient temperature, initial droplet diameter, and ethanol mass fraction significantly affected the micro-explosion delay time and intensity, whereas the micro-explosion duration and equivalent evaporation rate exhibited condition-dependent non-monotonic variations. 3) For asymmetric gas–liquid droplets, increasing the bubble-to-ambient pressure difference shortened the rupture delay and increased the breakup intensity, while increasing the initial bubble radius produced the same overall tendency. Greater displacement of the bubble from the droplet centre promoted earlier rupture but reduced the breakup intensity, revealing competing effects of bubble eccentricity on droplet atomization. Under the simulated conditions, the 50% ethanol case produced the shortest rupture delay, whereas the breakup intensity continued to increase towards 60%.