Modeling of water droplet in super-cooling water evaporative system for ice slurry production

Ice storage technology is considered as one of the most promising options to achieve the so-called "peak load leveling of power system" and to relieve the contradiction between the supply and demand of peak power currently encountered in China. Among the ice-producing methods being developed around the world, the evaporative supercooling water ice-making one is a new and efficient way for ice slurry production of large scale without ice blockage. Therefore, it is of significant importance to investigate the heat and mass transfer characteristics during the cooling and crystallization process of water droplets in the evaporative supercooling water ice-making chamber to the development of practical ice-making system. In order to analyze the heat and mass transfer characteristics of water droplets in an evaporative super-cooling water system for ice slurry production, we proposed a mathematical model for the cooling and crystallization process of a single water droplet falling in the evaporation chamber with large space, which comprehensively took into account the three different zones of entire liquid phase, solid-liquid interphase and entire solid phase during the cooling and crystallization process of a water droplet. The developed mathematical model was then validated by use of the theoretical and experimental results presented in the available literature and satisfactory agreement was achieved in between the model simulation results and the research ones reported in the literature, indicating the correctness of the mathematical model. The parameter variations of the water droplet with changes in some of the key system operating variables, such as the inlet size and temperature of water droplet, the flow rate and relative humidity of the cold air, were numerically determined by solving the developed mathematical model. Effects of the inlet size and temperature of water droplet, inlet temperature, flow rate and relative humidity of the cold air in the evaporation chamber on the entire cooling and crystallization process were analyzed in detail. The results indicated that the cold air temperature, relative humidity and flow rate had significant effect on the cooling and crystallization process of water droplets. And change in the air temperature had the most obvious impact on the cooling rate of water droplets. Both of the air temperature and relative humidity determined the minimum temperature (i.e., the wet-bulb temperature) that the droplets can reach in the evaporative supercooling water chamber for ice slurry production. Air velocity was one of the key factors influencing the ice crystal production. It can be concluded from the obtained simulation results that decrease in the inlet diameter of water droplets entering the evaporative supercooling chamber was helpful to enhance the speed of evaporative and crystallization process of water droplets. As a result, it was recommended to incorporate water droplets with diameters as small as possible in practical applications. In order to reduce the energy consumption for ice-making, it was important to employ a higher air temperature in meeting the wet-bulb temperature requirement. While increasing the cold air speed can improve the ice slurry output of ice-making system, it may result in increase in the carrying volume of water droplets. Therefore, the cold air speed should be determined according to the corresponding diameters of water droplets entering the supercooling evaporative chamber. The conclusions obtained in this paper were of significance to the efficient structural design and practical operation of the super-cooling water evaporative system for ice slurry production of large scale. © 2016, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.