中文摘要：

利用界面结构效应实现蒸汽滴状冷凝对能源动力系统的高效化和集成化具有至关重要的作用.深入认识液滴动态特性对传热性能的影响有利于探究低压蒸汽冷凝中传热控制机理和研发高效紧凑的传热强化技术.本文系统地研究了蒸汽滴状冷凝中液滴脱落特性,提出了低压蒸汽冷凝液滴脱落滞后效应.随着蒸汽压力降低,冷凝液滴黏度增加,引起自身脉动特性下降,接触线移动受阻,接触角滞后现象明显,导致脱落尺寸明显增加而脱落速度大大降低.与常压蒸汽冷凝中液滴的快速启动连续下落不同,低压蒸汽冷凝中液滴长至脱落尺寸后表现出了＂阶跃式＂下落的现象,即液滴脱落滞后效应.在液滴整个生命周期中,低过冷度时液滴生长速度控制着传热过程,而高过冷度时液滴脱落滞后延缓了表面更新,降低了有效换热面积,成为了控制传热过程的因素.通过液滴脱落效应解释了传热通量随过冷度非线性增加现象的内在机理,明晰了蒸汽冷凝传热控制机制,为低压蒸汽冷凝传热强化指明了方向.

英文摘要：

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.