helix angle, micro-fin, visualization, R410A, evaporation
This paper presents the effect of the helix angle on flow boiling in micro-finned tubes supported by a novel visualization in diabatic conditions. Such visualization helps better understanding of the mechanism of boiling and heat transfer enhancement. In most of the literature, the flow regime of flow boiling in the micro-finned evaporator tube is visualized by the sight glass without internal fin geometry and at the exit of the test section. In order to better understand effect of micro-fins on flow regime in the visualization test section, transparent micro-finned tubes with different helix angles placed in outer glass tube with annular coolant flow were used to visualize flow during evaporation. R410A flow boiling experiments are conducted at 10 °C saturation temperature in smooth tube and micro-finned tubes of 0° and 18° helix angle. The visualization in flow boiling is presented. The results showed that the annular flow pattern in the helical micro-finned tube occurred at lower vapor quality than the smooth. This early transition was attributed to the capillary force from spiral grooves, which enabled the liquid to reach the top of the tube easily. For the axial micro-finned tube, the flow patterns were not significantly different from the smooth tube, but the liquid refrigerant was easily trapped in the grooves especially in the region of stratified-wavy or slug flow.