THE EFFECT OF LIQUID PROPERTIES ON HEAT TRANSFER AND EVAPORATION CHARACTERISTICS OF THE LIQUID FILMS FORMED BY UNSTEADY SPRAY-WALL IMPACTS

Abstract

For intermittent spray-cooling purpose, it is essential to study the unsteady aspects of film evaporation and heat-transfer characteristics. In the present study, total evaporation time and surface temperature variations are investigated for four different liquid films (water, ethanol, n-octane, and n-hexane). The evaporation process is analyzed using a three-dimensional spray-wall impact with Lagrangian wall-film model. The evaporation process occurs in three stages; at the initial moments, most of the heat is used to raise the film temperature, and slight evaporation also exists. The film temperature rises until it reaches the liquid saturation point to evaporate at a constant rate. In the last stage, the evaporation rate decreases with time due to the accumulation of vapor in the bulk flow. The effect of heat flux and initial film thickness on the total evaporation time and the slope of its changes are investigated. The results show that the total evaporation time increases linearly with the initial thickness. Also, the molecular weight and saturation point of liquids are influential parameters after the enthalpy of evaporation. The surface temperature rises to a maximum value before reducing by the film evaporation. The maximum amount of the wall temperature depends on the liquid thermal conductivity and the evaporation rate. Finally, the effect of the initial value of the film temperature is investigated, and a correlation for estimating the total evaporation time is extracted.