Ma Xuehu

Retention effect of droplet departure in dropwise condensation at low steam pressure

Dropwise condensation caused by interfacial effects plays a crucial role in the thermal effciency and integration of power production. It is important to understand the effect of droplet dyanmic characteristics on heat transfer perfromace for the controlling mechanism of dropwise condensation heat transfer at low steam pressure and the development of efficient heat transfer techniques. Based on droplet departure characteristics in dropwise condensation at different steam pressures, the droplet departure retention effect at low pressure was proposed in this paper. Due to the increase of condensate viscosity at low steam pressure, the pulsation of droplet surface weakened and the contact angle hysteresis became apparent. Condensed droplets grew larger and moved more slowly in the departure process at low pressure compared to that at atmospheric pressure. Meanwhile, the droplet moved quickly downward once it grew to the departure size at atmospheric while the droplet exhibited a slow stick-slip motion although it reached the departure size at low pressures, which was called the droplet departure retention effect. In a growth cycle of condensed droplet, the heat transfer process was controlled by the droplet growth rate in small surface subcooling range. However, the droplet departure retention delayed the surface renewal reducing the effective heat transfer area in large surface subcooling range. Furthermore, droplet departure retention was introduced to explain the inherent mechanism of the nonlinear increasement of heat flux varying with surface subcooling. This work is beneficial to clear the controlling mechanisms of dropwise condensation heat transfer and offer a new avenue to further enhance heat transfer of dropwise condensation at low steam pressure.