The literature shows that axial microfins and spokes are promising means for enhancing evaporation of refrigerants in tubes. However, studies are lacking and available ones are generally limited to large diameter tubes. In this study, evaporation heat transfer and pressure drop tests were performed on four tubes—smooth, helical microfin, axial microfin, and spoke—having 7.0 mm outer diameter for the mass flux from 50 to 250 kg/m2s using R-410A. During the test, the heat flux and the saturation temperature were maintained at 3.0 kW/m2 and 8°C. Above 150 kg/m2s, the best heat conductance (hiAi) enhancement was obtained from the axial microfin tube. At lower mass fluxes, the spoke tube yielded the best enhancement. Possible reasoning is provided based on the flow pattern in each tube. As for the pressure drop, the spoke tube yielded significantly higher pressure drop than other tubes. In addition, the pressure drops of the axial microfin tube were higher than those of the helical microfin tube. With proper selection of the fin height, the axial microfin tube may perfrom better than the helical microfin tube except at a low mass flux. At a low mass flux, where stratified flow prevails, the axial microfin is likely to interrupt peripheral propagation of liquid film and deteriorate the heat transfer.
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