Heat-transfer characteristics of the two-phase-change process between a microencapsulated phase-change-material-slurry droplet and air: A numerical study

Abstract

A concept is proposed for spray-heat exchangers with the aim of addressing the mismatch of heat supply-demand within and outside a water droplet. The concept involves using a microencapsulated phase-change-material (MPCM) slurry as the working medium, enabling two-phase-change heat transfer with air. The temperature of the water droplet rapidly decreases owing to continuous evaporation, leading to a rapid decrease in its heat-transfer capacity. The phase change of the MPCM particles not only provides sufficient heat for evaporation but also maintains a high surface temperature, thus sustaining effective evaporation. A numerical model was developed to theoretically investigate the effect of the MPCM phase change on external convection. The heat-mass transfer of a droplet with a radius of 0.25 mm composed of water and 20 vol% MPCM was simulated. The results indicate that the MPCM phase change effectively reduces the temperature decrease rate. However, when the overall surface temperature of the MPCM droplet exceeds that of water, the total heat-transfer coefficient is not necessarily higher. Further local analysis reveals that maintaining both an overall high surface temperature and a reasonable local temperature distribution is crucial to achieve a higher total heat-transfer coefficient. The MPCM phase change significantly influences the local surface-temperature distribution.