Effect of Flow Direction of Heating Medium on Boiling Heat Transfer Characteristics in a Single-Channel Plate-Fin Heat Exchanger

This study deals with evaporative heat transfer characteristics of vertically upward flows in single-channel plate-fin heat exchangers. R-134a was used as the refrigerant. The refrigerant channel was sandwiched by two water channels with the same channel shape. The refrigerant was heated by hot water from the both side. In this experiment, pressure of the working fluid was maintained at 0.665 MPa (saturation temperature is 25 °C)．Subcooled liquid with the subcooling of 5 and 10 K, or wet vapor with the quality of 0.1 was supplied to the evaporator. Two kinds of offset fin, Fin A and B, whose fin pitch were 1.95 and 3.175 mm, respectively, were used. The channel hydraulic diameter with Fin A and Fin B were 1.78 and 2,89 mm, respectively. Effects of the fin pitch and flow direction of the heating water, such as counter and parallel flow arrangements, on the heat transfer rate, pressure drop, and wall temperature distribution on the outside of the water channel were examined. The wall temperature was visualized by an IR camera. As the results, it was shown that the effects of the flow direction of water and the fin pitch on the heat transfer rate were a little due to a dryout in the refrigerant flows. However, a large difference in the wall temperature distribution between the counter and parallel flow was observed. The difference might be caused by the phase distribution of refrigerant, and lead to the increase in the pressure drop for the parallel flow. For the parallel flow arrangement, the temperature on the right side with the water inlet port was quite higher than the right side. The temperature distribution was kept over the heat exchanger. The tendency was weaker for Fin B with the larger fin pitch due to phase separation caused by gravity