Frost Formation on An Air-Cooler and characteristics in Heat Pump System

冷表面结霜是空气冷却器在低温环境下运行的固有特征，从热效率的角度来看也是一种消极现象。为了开发高性能热泵系统，应研究以下几个基本问题：(1)结霜工况下换热器的特性；(2)将换热器作为复合元件的热泵系统的特性。去年主要对第二个问题进行了分析研究，使用了一个简单的结霜模型。本年度将对第一个问题进行实验和理论探讨，作为追求换热器最佳设计和运行条件的基础研究。采用四排翅片管式换热器，研究了结霜对换热器各排传热传质和压力损失的影响。发现霜层的生长取决于排的位置，因为朝向气流方向的水蒸气浓度降低。预测了各排的换热效率和换热器内的压力损失与各排霜层生长的关系。通过预测发现，逆流式换热器的性能优于平行流式换热器，因为逆流式换热器结构更致密，换热器表面霜层沉积均匀。

Frost formation on a cold surface is an inherent feature for air coolers operating under low temperature circumstances, and is also anegative phenomenon from the standpoint of thermal efficiency. In order to develop high-performance heat pump system, the following fundamental problems should be studied: (1) the characteristic of heat exchanger operating with frost formation, (2)the characteristic of heat-pump system, in which a heat exchanger is include as a compo-site element. In last year, analytical studies were performed mainlyfor the second problem by using a simple frost formation model. In this year, the first problem will be discussed experimentally and theoretically as a basic study to pursue the optimum design and operating conditions for heat exchangers. Namely, the effect of frost deposition on heat and mass transfer for each row and pressure loss were investigated using a four rows finned tube type heat exchanger. The growth of a frost layer was found to depend on the location of the row because of the decrease in water vapor concentration toward the direction of air flow. The efficiency of heat transfer for each row and the pressure loss in the heat exchanger were predicted in relation with the growth of a frost layer for each row. By this prediction, it was found that the performance of heat exchanger operating in counter flow type was higher in comparison with that in parallel flow type, because of a denser structure and a uniform deposition of frost layer on the surface of the heat exchanger.