Abstract:

Frost build-up on the heat exchanger results in an increase in heat transfer resistance and pressure drop, leading to additional energy consumption and operational costs of heat pump systems. Superhydrophobic surface has been shown to retard frosting significantly due to its efficient removal of condensed droplets prior to freezing via coalescence induced droplet jumping. Recently, heat exchangers and fin pitch are designed elaborately to balance heat transfer and pressure drop. Exploring the effect of fin pitch and surfaces wettabilities on frosting rates on surfaces is crucial when designing high efficient heat exchangers. In this paper, frosting dynamics between two parallel heat exchangers with different wettabilities were studied experimentally. Frost growth was continually recorded through high-resolution optical camera from the side and top of the gap between the parallel fins. Average frost thickness was calculated visually by high resolution images and MATLAB image processing techniques. Test experiments were conducted for hydrophilic, superhydrophilic and superhydrophobic surfaces under different frosting conditions with fin pitch of 2, 4, 6, and 8 mm. Results indicated that frost process could be divided into two different regimes, called rapidly frosting and slowly frosting regime. Frost growth rates were distinctly higher under higher relative humidity and lower surface temperature. In addition, frost growth rates on superhydrophobic surface was lower than the hydrophilic and superhydrophilic surfaces significantly. This work here not only help to optimize the design of high efficient coated heat exchangers of heat pump systems, but also provide insights into understanding the complex thermodynamics theory of governing the condensation frosting process on surfaces.

Key words: heat exchanger, frost, thermodynamics, superhydrophobic, dynamics, interface

摘要：

搭建了竖直翅片间霜层生长可视化实验平台，分别制备了铝基亲水、超亲水和超疏水平板换热器，研究了翅片间距、表面润湿性能和环境温湿度等对竖直翅片间霜层生长动态特性的影响。实验结果表明，翅片间霜层的生长过程以临界间距（约1 mm）为界可分为线性快速增长阶段和缓慢增长阶段；相同工况下，亲水表面、超亲水表面和超疏水表面翅片间结霜持续时间分别为177、387和482 min。亲水表面霜层的增长速度分别为超亲水表面和超疏水表面的2倍和3倍。较低的表面温度和较高的环境湿度由于相变过饱和度的增加而提升了霜层增长速度；此外，翅片间霜层积聚过程中霜层密度随时间变化先增长，之后霜层密度增速放缓甚至不再增长直至结霜过程结束。

关键词: 换热器, 结霜, 热力学, 超疏水, 动态学, 界面