Experimental Comparison of R245fa Falling Film Evaporation in Two Doubly-enhanced Tubes

The heat transfer performance of two types of doubly enhanced tubes was experimentally studied using R245fa after the horizontal single-tube falling film evaporating test bench was built. The Wilson–Gnielinski graphic method was proposed to obtain the surface heat transfer coefficient of the tubes. The effect of the wall structure on the heat transfer performance was analyzed. The heat transfer correlations inside and outside the tubes were provided, and a fitting optimization scheme was proposed. The results showed that the enhancement rates of in-tube and out-tube heat transfer of the Y tube were 2.12–2.94 and 2.27–5.54, respectively. The internal and external enhancement rates of the T tube were 2.48–2.98 and 2.58–3.00, respectively. The out-tube heat transfer performance of the Y tube and the in-tube heat transfer performance of T tube were better. The heat transfer performance of the Y tube can be reinforced by optimizing its in-tube structure. The best spraying density of the Y tube [0.14–0.18 kg/(m?s)] was higher than that of the T tube [approximately 0.10 kg/(m?s)]. The surface heat transfer coefficient of the two tubes first increased and then decreased with the increase in the heat flux. However, the surface heat transfer coefficient of the Y tube changed more rapidly, which indicated that the thermosyphon boiling phenomenon caused by the smaller fin pitch of the Y tube accelerated the evaporation of refrigerant. The surface heat transfer coefficient of the two tubes increased with the increase in the evaporation temperature.