It is requested to develop a small and high performance heat exchanger for small size energy equipments such as fuel cells and CO2 heat pumps, In author’s previous studies, a high pressure resistant microchannel layers stacked heat exchanger has been developed. The heat exchanger is manufactured by diffusion bond technique. It can be used under high pressure condition larger than 15 MPa. Due to the high pressure resistance, the device can be applied for high flow rate condition with boiling and condensation. The objectives of the present study are to estimate the heat transfer performance of the heat exchanger and to investigate the thermal hydraulic behavior in the microchannel. The flow pattern in a glass capillary tube is observed by fabricating visualization system. As the results, it is measured that the present device attained high heat transfer quantity of approximately 7000 W on steam condensation despite the weight is only 230 g. The measurement results clarified that the device achieves very high heat transfer rate of hundreds times larger than that of the existing heat exchanger. Furthermore, visualization experiment with single glass pipe is conducted to clarify the flow condensation behavior in the microchannel. In the experiment, the microchannel of Pyrex glass is surrounded by the subcooling water. The flow patterns can visualized from the side of the microchannel. Flow patterns observations are conducted for various inlet pressure and temperatures of the subcooling water. It is observed that the continuous flow transition from annular and injection flow to slug-bubble flow in the microchannel. The reason of large heat transfer rate per unit volume is discussed as relating to narrow interval of each microchannels and small thermal resistance.

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