FUNDAMENTAL STUFY OF A SMALL-SCALE HEAT PUMP

换热器的小型化是微型热泵的重要技术之一。为了发展小型热泵，本研究对小型换热器进行了基础性研究，包括小管径换热管内的传热和压力损失特性、T形管内气液两相流动的分布以及小管径换热器的测量和模拟。(1)对于小口径管内的沸腾换热和压降特性，换热管的管径在0.5~3 mm之间变化，并选择HFC134a作为工质。对于直径小于1 mm的管子，在大于0.5的质量范围内，由于干燥，换热系数恶化。当蒸发器入口蒸汽质量小于0.1时，流态呈现间歇性，换热性能显著下降。(2)对三通冲击段气液两相流相分离进行了实验和理论研究。研究发现，如果分支流速不同，则会出现相分离现象。根据最小能量耗散原理，推导出了一个热力学模型。该模型的计算结果与冲击流动的实验行为具有较好的一致性。(3)对细小管换热器的性能进行了实验和理论研究。管子内径分别为1.0 mm、2.1 mm和4.0 mm。计算结果表明，2.0 mm管式换热器的体积可以减少到相同容量的4 mm管式换热器的33%。分流单元内的流量分布对换热有很大影响。

Size reduction of heat exchangers is one of the most important technologies for micro-scale heat pumps. In order to develop small-scale heat pumps, in this study fundamental research of small-scale heat exchangers were executed, which include heat transfer and pressure loss characteristics in small diameter heat transfer tubes, distribution of gas liquid two-phase flow at a T-junction and measurement and simulation of heat exchangers with small diameter tubes.(1) For boiling heat transfer and pressure drop characteristics inside small diameter tubes, the diameter of heat transfer tubes were varied between 0.5 and 3 mm ID, and the HFC134a was selected as a working fluid. For smaller diameter tubes than 1mmID, heat transfer coefficients were deteriorated due to dry out in the quality range higher than 0.5. When the vapor quality at the inlet of the evaporator is less than 0.1, the flow pattern becomes intermittent and heat transfer performance decreases significantly.(2) Phase separation of gas-liquid two-phase flow at an impacting tee was investigated experimentally and theoretically. It was found that if the flow rates in the branches were different, phase separation was observed. Based on the principle of minimum energy dissipation, a thermodynamic model was derived. The results of the model were qualitatively in good agreement with the experimental behavior of impacting flows.(3) The performance of fined small tube heat exchangers was investigated both experimentally and theoretically. The Inner diameters of tubes were 1.0mm, 2.1mm and 4.0mm. Calculation results show that the volume of a 2.0mm tube heat exchanger can be reduced to 33% of that of a 4mm tube heat exchanger with the same capacity. The flow distribution in a branching unit strongly affects the exchanged heat.