Gas-phase transient effects on droplet evaporation and ignition

In order to achieve high efficiency and power density, modern combustors are generally operated at elevated pressures. As the pressure and gas-phase density increase, classical quasi-steady (QS) models of droplet evaporation, ignition, and burning introduce non-negligible errors. In this paper, we extend the QS model to incorporate the gas-phase transient (GT) effect by examining the development of the evaporating boundary layer near the droplet surface. Specifically, the evaporation and ignition of single droplets in an infinite, hot, and oxidizing environment are analyzed. The theoretical result agrees well with the transient numerical simulation for an extensive range of parameters. The proposed theory successfully predicts the GT effect on evaporation lifetime and critical ignition limit of droplets. It is demonstrated that the GT effect increases the droplet evaporation rate while tends to suppress ignition. In comparison with pure evaporation, the ignition of droplets is more severely affected. Additionally, the GT effect increases as the liquid-to-gas density ratio decreases or evaporation intensity increases.