Two-phase Flow Heat Transfer And Visualization Study Outside The Horizontal Enhanced Tubes

The two-phase heat transfer characteristics of refrigerants R32 and R410 A in the outer annular area of three stainless steel enhanced tubes with an outer diameter of 19.05 mm and one copper enhanced tube with an outer diameter of 9.52 mm were investigated,and the effects of mass fluxes,vapor quality,saturation temperature and refrigerant type on their heat transfer characteristics were analyzed.The three stainless steel heat transfer tubes were sandblasted(E1 tube),dimple(E2 tube)and dimple sandblasted composite surface tube(E3 tube);the copper heat transfer tube was dimple tube(L tube).The heat transfer coefficient and fractional pressure drop of the smooth tube with the same diameter and wall thickness material were selected for comparison with the enhanced tube,and the enhanced heat transfer effect of the enhanced tube was quantified by introducing the enhancement factor EF and the performance evaluation factor PEF.For flow condensation heat transfer,the heat transfer coefficient and frictional pressure drop of the heat transfer tubes increased with increasing mass fluxes and average vapor quality,and increased with decreasing saturation temperature.the E2 tube had the highest heat transfer coefficient,followed by the E3 tube,followed by the E1 tube,and the smooth tube had the lowest.The sandblasted surface of the dimple structure has a negative effect on the heat transfer coefficient,but the E1 tube obtained by sandblasted outer surface of the smooth tube has a positive effect on the condensation heat transfer coefficient.the E2 tube shows the best heat transfer characteristics with the highest EF and PEF of 1.76 and 1.42,respectively.For flow boiling heat transfer,the heat transfer coefficient and frictional pressure drop of the heat transfer tubes increased with increasing mass fluxes and increased with decreasing saturation temperature,and the frictional pressure drop increased with increasing vapor quality,and the vapor quality variation had a greater effect on the heat transfer coefficient under high mass fluxes conditions.Among the four stainless steel heat transfer tubes,the E3 tube combines the advantages of both dimple surface and sandblasted surface to show the highest heat transfer coefficient,with the highest EF and PEF values of 2.84 and 2.31,respectively.The refrigerant flow pattern in the outer annular area of the L and S2 tubes was photographed,and under low vapor quality conditions,the L tube transforms into a slug flow at a lower mass fluxes;under high vapor quality conditions,the L tube transforms into a popping flow at a higher In high vapor quality conditions,the L tube transforms into a annular flow at higher mass fluxes.The dimple structure creates resistance to the circumferential movement of the fluid along the heat transfer tube,which is not conducive to the generation of annular flow.Three condensation heat transfer correlations and four flow boiling heat transfer correlations were used to predict the condensation and flow boiling heat transfer coefficients of the smooth tube,and the condensation and flow boiling correlations with the highest prediction accuracy were selected and corrected to obtain new heat transfer coefficient correlations suitable for predicting the enhanced tubes,and the new condensation and flow boiling correlations could be predicted within ±30% error by95%.The new condensation and flow boiling correlations can predict more than 95% of the two-phase heat transfer coefficient data points of the enhanced tubes within ±30%error.