Abstract
A Diffusion absorption refrigeration (DAR) system is a kind of thermal cycle employing thermal power as the driving force. It works without an external high power consuming compressor, expanded valve and any other working assembling. In the present study, we investigate one of the most important parts of a DAR system, i.e. the gas-assisted evaporator. The working principle of such an evaporator is to reduce the partial pressure of the working fluid by adding a non-condensable gas into the high concentration coolant. Consequently, the working fluid may evaporate at a lower temperature. Under such a situation, the heat transfer in the system may also be affected. In this study, we have designed test sections with single minichannel or dual minichannels with an import hole located near the inlet bottom of channel for the gas-added. The gasoline imported hole makes helium mix with ethanol fluid by a T-junction. The working fluid is 99.8% ethanol and helium. The flow rate for ethanol is 12 ml/min and is 30/80 ml/min for the helium in the single minichannel system. On the other hand, it is 10, 25 and 40 ml/min for ethanol flow and 100, 200 and 300 ml/min for the helium flow in the dual minichannels. The heat transfer and boiling two-phase flow phenomenon under the conditions above-mentioned are thoroughly investigated. The results of the study reveal that the heat flux rises while the flow rate of ethanol or helium or both increase. The best heat transfer enhancement, comparing to that without assisted gas is demonstrated at a wall superheat around -10℃. Moreover, under the condition of a low ethanol flow rate of 10 ml/min and a high helium flow rate of 300 ml/min, the enhancement reach the maximum value of 204% in the present study. The enhancement drop obviously in region with positive wall superheat. Experiment observations indicate that the disturbance made by the collision between two neighboring long slug bubbles significantly increases convection and possible ethanol evaporation at the gas-liquid interface, the major heat transfer enhancing mechanisms and boosts the enhancement. The study also reveals that the heat transfer enhancement is linearly proportional to the gas quality with a higher slope for the cases with high ethanol flow rates of 25 and 40 ml/min than that with a low ethanol flow rate of 10 ml/min.