On the role of trancritical evaporation in controlling the transition from two-phase to single-phase mixing

The dynamics of near-critical single droplets allows to investigate the transition from two-phase to single-phase mixing under well-defined conditions, devoid of the additional complications due to drop-drop interactions and combustion. Recently, an empirical regime map was proposed to predict the evolution of microscopic transcritical droplets. The experiments show that classical evaporation remains the controlling mechanism over a wide range of supercritical ambient pressures and temperatures with respect to the critical point of the evaporating fluid. Moreover, the onset ambient pressure for the transition to single-phase mixing varies inversely with temperature. To explain this trend, the behavior of a single droplet at near-critical conditions is investigated theoretically by means of a Langmuir-type evaporation model, originally proposed by Young. The model incorporates a modified boundary condition due to the inclusion of gas kinetic effects close to the vapor-liquid interface. This advanced evaporation model is employed to reproduce analytically the above-mentioned regime map, showing a good agreement with experimental findings. The analysis also revealed that the onset of the single-phase mixing regime is associated to the quenching of the evaporation process. The latter is caused by the decrease of the evaporation coefficients, which control the mass transfer rate across the Knudsen layer. The resulting reduction in evaporative cooling leads to the rapid heating of the liquid droplet and to the disintegration of the material interface at the critical temperature.