Droplet Size Distribution And Heat Transfer Enhancement For Steam Condensation On Superhydrophobic Surface At Low Pressure

Steam condensation at low pressure has a wide range of applications such as chemical industry, power generation, air conditioning, desalination, vacuum distillation and absorption heat pump. For its excellent heat transfer performance, dropwise condensation becomes a promising way to improve thermal efficiency and even save energy. Since droplets are heat transfer passages for the condensation process, investigations of the influence of droplet jumping on droplet size distribution evolution was systematically conducted in this work, which are of great importance for heat transfer enhancement during the low steam pressure condensation process.The droplet size distribution and evolution during the initial condensation and the steady-state droplet size distribution with a subcooling of 2 ℃ at different steam pressure on superhydrophobic surface were investigated experimentally. The effect of droplet jumping on droplets size distribution and evolution were attained. The results show that the droplet jumping behavior at low steam pressure and low subcooling increases the density of small droplets and reduces the departure size of droplets. And it results in a decrease of average size of droplets on the condensing surface. Meanwhile, the behavior of droplet jump has little effect on droplet size distribution evolution during the initial condensation. During the initial stage of condensation, the evolution of droplet size distribution still satisfies lognormal distribution, then a bimodal distribution, and finally an exponential distribution. It is similar to the situation of higher steam pressure when droplets departure the surface by gravity. The results show that the generation distribution characteristic was apparently occurred with the droplet jumping at steady state. The droplets of different generations perform the self-similarity and generally present exponential distribution. The droplet jumping behavior produce the decrease of average size of droplets, the optimization of the droplet size distribution at steady-state, and the shortening of life cycle of the condensates.A theoretical model was used to calculate and analyze the heat transfer performance of steam condensation at low pressure. The effect of droplet jumping behavior on heat-transfer and heat transfer enhancement mechanism were also attained. As indicated in the results, coalescence-induced droplet jumping at low steam pressure and low subcooling produce the decrease the departure size of droplets and the average size of droplets, the density increase of small droplets, and the shortening of life circle of droplets. The optimization of droplet size distribution through droplet jumping behavior leads to the number increase of the effective heat-transfer channels on the condensing surface. At the same time, the liquid-vapor interface disturbance by coalescence-induced droplet pulsation and jumping behavior can enhance the molecular diffusion at the liquid-vapor interface. It is an important method to keep low subcooling to achieve droplet jumping for heat transfer enhancement of steam condensation at low pressure. The heat flux with the droplet jumping behavior is about three times as the heat flux when droplets depart the surface by gravity at the same condition of condensation on superhydrophobic surface, and it is even higher than the condensation heat flux at atmospheric pressure.Based on the above experimental and theoretical results, an evaporator-condenser of vapor compression distillation system for seawater desalination was designed and its economic performance was also analyzed. The yield of fresh water is 1.8 t/day. The overall heat transfer coefficient is increased by about 37% due to the enhancement of heat transfer coefficient at the condensation side. The energy consumption is reduced by about 60%. Through the economic performance analysis, if the price of pure water is ￥30/t, the cost will be payed back by 12.8 years, instead of the original design that can’t be payed back. The energy cost will be reduced by ￥93000. Consequently, the design system is more economic for the long-term operation.