Experimental investigation on thermal performance of multiport minichannel flattened tube thermosyphon heat exchanger

This study investigates the air-cooled thermosyphon heat exchanger having multiport minichannel flattened tubes. The louver fin was incorporated in the air-side on the condenser to improve the heat transfer performance. The test was conducted under a heat load of 110 – 330 W, a filling ratio of 20 – 60 %, an air velocity of 0.5 – 4.0 m/s, and an ambient temperature of 25 – 26 °C. R-134a was used as a working fluid. The Geyser boiling phenomenon and confinement effect were examined using existing flow regime maps. In the meantime, the flow behavior inside the thermosyphon was examined using pressure fluctuation. The key parameters that affected the thermal performance of the thermosyphon were investigated. It is found that slug/plug and Geyser flow are observed by using the existing flow regime map. Moreover, Geyser boiling flow is detected under the test conditions by monitoring pressure fluctuation. However, the intensity of Geyser flow decreases as heat load and air frontal velocity increase, while the filling ratio shows the opposite trend. The confinement effect was evaluated by using the confinement number (Co). In this study, Co was about 0.54–0.58. However, a smaller hydraulic diameter plays a crucial role in the geyser boiling flow and confinement effect. The filling ratio, heat load, and air velocity significantly influenced the thermal performance of the thermosyphon. The best performance was obtained from a filling ratio of 30 % which yields a minimum overall thermal resistance and the evaporator surface temperature of 0.052 °C/W and 33.7 °C, respectively.