低温高湿环境中冷面结霜的实验和模拟研究

发布时间：2019-03-06 08:43

【摘要】：结霜是湿冷环境中的一种常见现象。换热器表面结霜会造成热阻增加、空气流道堵塞等危害。对冷面上结霜规律和表面特性对结霜影响的认识,以及对霜层生长过程的准确模拟,对于探索合适的抑霜手段和低温高湿工况下运行的换热器的设计具有指导意义。本文实验和数值模拟研究了冷面上霜层生长过程,数值模拟研究了翅片管换热器表面霜层分布,主要工作及结果如下:对低温高湿环境中亲水表面、裸铝表面和疏水表面上结霜过程进行实验研究。实验条件为:冷面温度-20~-5oC,湿空气温度-5~2oC,相对湿度85%,湿空气流速0.31~0.92 m/s。结果表明:冷面温度越低,裸铝表面上冷凝水珠冻结越早,冻结时水珠越小,而且霜晶沿垂直冷面方向生长越快;冷面温度越低、湿空气流速越快,霜层生长越快;疏水表面具有延缓冷凝水珠冻结和减慢霜层高度增长速率的作用,但随冷面温度降低,延缓效果减弱;用于结霜/融霜循环中的表面,较好的排液效果可以有效减少再结霜量。对水平冷面上霜层生长过程进行了数值模拟。结果显示:霜层平均高度和密度均随时间增长,冷面前沿处水蒸气浓度高,相转移速率快,霜密度较大;霜层内部热传递以导热为主,霜层外以对流换热为主,霜层界面处温度曲线出现拐点。模拟结果与本文实验结果的对比显示:霜层平均高度90%以上的数据在-15%到+40%的偏差范围内,霜层质量所有数据点在-30%到+20%的偏差范围内。对翅片管换热器三维模型表面霜层生长过程进行了数值模拟。结果显示:在各基管后都存在一个背风无霜的区域,换热器翅片上霜层分布的模拟结果与实验结果吻合较好;在本文的模拟工况下,结霜后湿空气流过换热器的压降约为结霜前的2倍,且与实验结果吻合较好。上述结果表明霜层生长模型可以用于翅片管换热器表面霜层分布及湿空气流动阻力的预测。
[Abstract]:Frosting is a common phenomenon in wet and cold environments. Frosting on the surface of heat exchanger will result in the increase of thermal resistance and blockage of air passage. The understanding of the effect of frosting law and surface characteristics on the cold surface and the accurate simulation of frost growth process are of guiding significance for exploring suitable frost suppression methods and the design of heat exchangers operating under low temperature and high humidity conditions. In this paper, the growth process of frost layer on the cold surface is studied experimentally and numerically, and the frost layer distribution on the surface of finned tube heat exchanger is numerically simulated. The main work and results are as follows: for the hydrophilic surface in low temperature and high humidity environment, The frosting process on bare aluminum surface and hydrophobic surface was studied experimentally. The experimental conditions are as follows: cold surface temperature-20? 5oC, wet air temperature-5? 2oC, relative humidity 85%, wet air flow rate 0.31? The results show that the lower the cold surface temperature is, the earlier the condensation water droplets on the bare aluminum surface freeze, the smaller the water droplets are during freezing, and the faster the frost crystal grows along the vertical cold plane direction, the lower the cold surface temperature, the faster the velocity of wet air and the faster the frost layer grows, and the lower the cold surface temperature is, the faster the wet air velocity is. The hydrophobic surface has the function of delaying the freezing of condensate droplets and slowing down the growth rate of frost height, but with the decrease of cold surface temperature, the delay effect weakens, and for the surface of frosting / defrosting cycle, the better efflux effect can effectively reduce the amount of refrosting. The growth process of frost layer on horizontal cold surface was numerically simulated. The results show that the average height and density of frost layer increase with time, the water vapor concentration at the front of the cold surface is high, the phase transfer rate is fast, and the frost density is higher. Heat transfer inside frost layer is mainly heat conduction, convection heat transfer is main outside frost layer, temperature curve at frost layer interface appears inflection point. The comparison between the simulation results and the experimental results shows that the average height of frost is more than 90% of the data in the range of-15% to 40% deviation, the frost quality of all data points in the range of-30% to 20% deviation. The growth process of frost layer on the surface of finned tube heat exchanger was numerically simulated. The results show that there is a frost-free zone behind each base tube, and the simulated results of frost layer distribution on the fin of heat exchanger are in good agreement with the experimental results. Under the simulated conditions, the pressure drop of wet air flowing through the heat exchanger after frosting is about twice as much as that before frosting, and it is in good agreement with the experimental results. The above results indicate that the frost growth model can be used to predict the frost layer distribution and wet air flow resistance on the surface of finned tube heat exchangers.
【学位授予单位】：清华大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TK172

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