A comprehensive investigation on evaporation of low-carbon alcohol fuel droplets in hot and humid environment: methanol as an example

Methanol and ethanol are promising renewable alternatives to fossil fuels, yet their evaporation processes involve water vapor absorption from environment, altering evaporation behaviors and thus affecting combustion. Focusing on methanol droplets, this study comprehensively investigated the role of ambient water vapor in the evaporation. The effect of ambient humidity on water vapor absorption and evaporation characteristics is quantitatively analyzed, and the temperature dependence is also discussed in detail. Driven by combination of intermolecular interaction and thermodynamic effect, water vapor absorption of methanol droplets leads to a slower two-stage evaporation process. A new method is developed to accurately calculate instantaneous water absorption rate, which first increases rapidly, then gradually decreases and eventually drops to zero in the later phase of stage Ⅰ. With the increase of absolute humidity, water vapor absorption is promoted and the maximum mass of absorbed water exhibits a linear increase. Then the average evaporation rate of stage Ⅰ increases linearly, while that of stage Ⅱ decreases exponentially, leading to an exponential decrease (over 50 %) in the overall evaporation rate of methanol droplets. Higher ambient temperature enhances water absorption rate but reduces mass of absorbed water due to the shortened absorption duration, resulting in a weakened humidity-induced suppression effect on evaporation. The humidity effect is firstly characterized as an exponential correction term of evaporation rate. Based on ambient temperature and humidity, an empirical correlation is established to predict the evaporation of methanol droplets in hot and humid environment.