FUNDAMENTAL STUFY OF A SMALL-SCALE HEAT PUMP

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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.