变工况条件下微通道蒸发器换热特性实验研究

摘要

In automotive air conditioning applications,parallel flow microchannel evaporators have become increasingly attractive owing to their compactness and high thermal performance per unit weight.However,a misdistribution of the refrigerant among the parallel microchannel tubes deteriorates the heat transfer performance of a microchannel evaporator and limites extensive promotion.Therefore,it is important to study the heat transfer characteristics of microchannel evaporator and how to improve the uniformity of refrigerant distribution.In this paper,an automotive air-conditioning test bench with R134a as refrigerant was set up.First,the heat transfer and exergy loss of the double-row and four-pass microchannel evaporator were analyzed.Then the surface of the evaporator is photographed by infrared camera Distribution image.As the evaporator inlet temperature increased from 21 ℃ to 42 ℃,the cooling capacity increased from 2.37 kW to 4.19 kW,while the evaporator exergy loss increased first and then decreased.And the exergy loss of the evaporator first increases and reaches a peak of 0.21 kW as the inlet air reaches 27 ℃,then decreases to a minimum value of 0.16 kW as the inlet air reaches 42 ℃.The σ value which represents the surface temperature distribution uniformity of the evaporator increases from 2.5 to 19.5 as the inlet air increases from 21 ℃ to 27 ℃.When the inlet air is 42 ℃,the value of σ decreases to 1.8.The surface temperature of the evaporator is the most uneven when the inlet air temperature is 27 ℃,and is most uniform when the inlet air temperature is 42 ℃.Therefore,the performance of the evaporator can be improved effectively at a relatively high air inlet temperature,and the rate of exergy loss and the value of σ decrease by 26.1％ and 91.0％,respectively.Moreover,properly increase the compressor speed can effectively improve the surface temperature distribution uniformity of the evaporator.%微通道蒸发器由于紧凑、换热效果好等优点越来越多应用于汽车空调当中,但存在制冷剂分配不均导致换热效果衰减等问题而限制了大量推广,因此研究微通道蒸发器换热特性及如何改善其制冷剂分布均匀性显得重要.本文搭建了以R134a为制冷剂的汽车空调实验测试台,分析了双排四流程微通道蒸发器的换热量及(火用)损,利用红外热像仪拍摄蒸发器表面得到表面温度分布图像.随着蒸发器进风温度由21℃升高到42℃,制冷量由2.37 kW增加到4.19 kW,而蒸发器(火用)损先增加后减小,并在进风温度为27℃与进风温度为42℃时达到最大值与最小值,分别为0.21 kW与0.16 kW.表征蒸发器表面温度分布均匀性的σ值随进风温度先由2.5增至19.5然后降至1.8,即蒸发器表面温度在进风温度为27℃时分布最不均匀,而在进风温度为42℃时分布最均匀.结果表明:较高的蒸发器进风温度能有效改善蒸发器换热性能,(火用)损及σ值可分别减小26.1％与91.0％.通过实验发现,适当提高压缩机转速能有效改善蒸发器表面温度分布的均匀性.