Effect of flow direction of heating medium on the heat transfer performance of upward evaporating refrigerant flows in a plate-fin heat exchanger was examined using HFC-134a as the refrigerant. The heat exchanger had a single refrigerant channel sandwiched by two water channels. Hot water flew upward or downward to form a parallel or counter flow heat exchange, respectively. To understand the heat flux distribution, temperature distributions on the outside wall of the water channel were visualized by an IR camera. As the results, it was shown that the difference in heat transfer rate between the parallel and counter flow was a little due to the large temperature difference in the heat exchange. The pressure loss of the refrigerant flow was larger for the parallel flow than the counter flow. It could be estimated from the wall temperature distribution that the increase in pressure loss might be caused by inhomogeneous phase distribution of the refrigerant flow.

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