Investigating the Influence of Evaporation and Mixing Regimes on Droplet Evaporation Rate at Elevated Pressure

Abstract Understanding the evaporation of spray droplets under high-pressure conditions holds significant relevance across various applications, particularly in combustion systems such as rocket combustors, gas turbine engines, and compression-ignition engines. Operating at pressures above the fuel’s critical point, these systems undergo a transcritical change of state where the injected fuel transitions from a liquid-like to a gas-like state. This complex process involves diffusive evaporation and mixing, challenging the classical perspective of droplet evaporation governed solely by surface area. To shed light on the evaporation characteristics of fuel droplets under such transcritical conditions, we conducted experimental measurements in well-controlled, high-pressure, and high-temperature inert gaseous environments. High-resolution imaging experiments at high repetition rates enabled us to detect and characterize fuel droplets as function of time, until complete evaporation. By comparing the droplet population decays with time, which was expressed as a droplet population lifetime metric, the results confirm that higher temperatures led to faster evaporation. Using the droplet lifetimes, the evaporation rate constant was estimated, showing an expected increase with temperature. However, deviations from the expected linear behavior were observed, particularly in cases where the droplet evaporation regime was identified as diffusive, with evaporation rate constant estimates up to nearly 70% higher than predicted by established correlations such as the d2-law of evaporation. Our findings bridge a significant gap in understanding droplet evaporation under extreme conditions, revealing that the evaporation rate is strongly influenced by the evaporation regime.