Droplet dynamics and heat transfer for dropwise condensation at lower and ultra-lower pressure
Droplet dynamics and heat transfer for dropwise condensation at lower and ultra-lower pressure
复制标题
DOI：
10.1016/j.applthermaleng.2014.09.069
复制
发表时间：
2015-09-05
影响因子：
6.4
通讯作者：
Ma, Xuehu
中科院分区：
文献类型：
--
作者：
Wen, Rongfu;Lan, Zhong;Ma, Xuehu
为了研究低汽压和超低汽压条件下初始液滴粒径分布、稳态液滴粒径分布和热阻分布的瞬态特性，对大气压~ 1.5 kPa范围内的液滴冷凝进行了研究。在瞬态过程中，初始成核液滴满足对数正态分布，然后形成双峰分布，最后呈指数分布。随着蒸汽压力的降低，峰值越小，演化越慢。在稳定凝结阶段，相应的表面覆盖率增加到0.7 ~ 0.8，而稳定凝结阶段对压力的依赖性较强。引入无因次时间，表面覆盖的演变表明，直接生长所消耗的时间随着压力的降低而增加。蒸汽压力对液滴粒径分布的影响表明，低压时液滴分布更分散，偏离尺寸更大，大液滴密度更大，导致有效换热面积减小。通过对不同压力下的热阻分布进行比较，发现在低压下，大液滴产生的热阻比例更大。研究结果有助于澄清液滴生长机制的局限性，并为优化液滴表面形貌以增强低压和超低压下的蒸汽冷凝提供指导。(C) 2014 Elsevier Ltd.版权所有。
To investigate the transient characteristics of initial droplet size distribution, steady droplet size distribution and thermal resistance distribution at lower and ultra-lower steam pressure, dropwise condensation at the pressure range from atmospheric to 1.5 kPa has been studied. During the transient process, the initial nucleated droplets satisfied lognormal distribution, and then a bimodal distribution formed, finally revealed an exponential distribution. The peak value was smaller and the evolution was slower with the reduction of steam pressure. The corresponding surface coverage increased to 0.7-0.8 at the steady condensation which was strongly dependent on the pressure. Introducing a dimensionless time, the surface coverage evolution indicated that the time consumed by direct growth increased as the pressure decreased. The effect of steam pressure on droplet size distribution revealed a more scattered distribution, larger departure size, and denser large droplets at low pressure, resulting in the reduction of the effective heat transfer area. By comparing the thermal resistance distribution at various pressures, it showed that large droplets induced a greater proportion of resistance at low pressure. The findings help clarifying the limitations of droplet growth mechanism and offer guidelines for the optimization of surface morphology to enhance the steam condensation at low and ultra-low pressure. (C) 2014 Elsevier Ltd. All rights reserved.