Two-phase heat transfer of R410A in annuli outside enhanced tubes with micro-fin and dimple

Abstract An experimental study evaluating the two-phase heat transfer using refrigerant R410A in the annuli outside the smooth tube and the enhanced tubes was investigated. Four tubes made of stainless steel including the smooth tube(ST), herringbone tube(HB), dimple tube(DIM), and dimple/herringbone tube(DIM/HB) were studied; with varying surface modification; with an inner diameter of 11.5 mm and an outer diameter of 12.7 mm; with a test length of 2 m. The condensation/evaporation experiments were performed at a saturated temperature of 45/6 °C; for the inlet and outlet vapor quality fixed at 0.8/0.2 and 0.2/0.8; with mass fluxes range from 75 to 225 kg m−2 s−1. The results indicated that: herringbone structure is more effective in condensation, especially at low flow rates, and the dimple structure is more effective in evaporation. The composite surface of the herringbone and dimple exhibited a substantial enhancement of thermal-hydraulic performance in two-phase flow. DIM/HB tube shows the highest performance factor in two-phase heat transfer. Besides, a new improved Nusselt equation for condensation outside the smooth tube is proposed with a prediction error within 5 %. Correlations for predicting the heat transfer coefficient of evaporation outside the smooth tube were also compared and analyzed.