Numerical study of thermal wall protection from a hot air by the evaporation of a binary liquid film

Abstract

In this paper, a numerical study is performed to investigate the influence of adding glycol or alcohol to water on the thermal protection of a channel wall from hot air by evaporating this binary liquid film. The coupled governing equations in both phases with the boundary and interfacial conditions are solved using a finite difference numerical scheme. The effectiveness of adding a fraction of glycol or ethanol to water for the thermal wall protection is analysed. The influence of the film composition, the liquid mass flow, the velocity and the gas flow temperature at the inlet, on the intensity of heat and mass transfer is discussed. The results indicate that with a lower inlet liquid flow, the mixture ethanol-water presents the best solution for the thermal wall protection compared to ethylene glycol-water mixture. However, for higher inlet liquid flow rate, the two mixtures have the same trends for a water amount higher than 60%. The presence of ethylene glycol in the mixture reduces the heat transfer by latent mode. Consequently, the decrease of the wall temperature is mainly due to the sensible heat flux. The ethylene glycol-water mixture offers the best wall protection from the hot air stream. Accordingly, the most important factor for the wall protection is the thickness of the film that acts as an insulator. The pure water presents a better thermal protection but a very bad conservation of the liquid film. Furthermore, the heat transfer by sensible heat exchange contributes efficiently in wall protection than heat transfer by latent mode.