Effect of venting the periodic reverse vapor flow on the performance of a microchannel evaporator in air-conditioning systems

This paper proposes a novel solution to reduce the impacts of periodic reverse flow and induced boiling fluctuations on the performance of a microchannel evaporator used in an air-conditioning or other similar refrigeration system. Reverse vapor trapped within the inlet header is continuously vented bypassing microchannels. Simultaneous flow visualizations and measurements quantified the effects of venting the reverse vapor on the evaporator performance. The vapor-liquid interface in the inlet header is elevated above all the microchannel inlets, resulting in more uniform liquid distribution among channels. Evaporator surface temperatures measured by an infrared camera oscillate with reduced amplitude (less than 1 C) compared to the situation without venting, indicating more stable boiling heat transfer and two-phase flow within the microchannels. The evaporator pressure drop is reduced up to about 15% due to removal of the reversed vapor flow. In addition, the reverse vapor flow is characterized for the first time through this method. Both its average flow rate and oscillation amplitude increase with average heat flux, while the oscillation period is reduced. Compared to the total refrigerant flow rate supplied to the evaporator, the average reverse vapor flow is in the range of 2-8% at the conditions explored.