The evaporation heat transfer characteristics of R32 in an annular area outside a stainless steel sandblasted tube

dimple tube

dimple/sandblasted tube

and smooth tube with an outer diameter of 19.05 mm were investigated

and the effects of the mass flux [50–140 kg/(m2?s)]

vapor quality (0.2–0.8)

and saturation temperature (279–288 K) on the heat transfer coefficient were analyzed. The results show that the surface heat transfer coefficient is highest for the dimple/sandblasted tube

followed by the dimple tube

and lowest for the smooth tube. The surface heat transfer coefficient and frictional pressure drop are positively correlated with the mass flux and negatively correlated with the saturation temperature. The average vapor quality at high mass flux has a significant effect on the surface heat transfer coefficient. The enhanced heat transfer effect was quantified by introducing the enhancement factor ηEF and the performance evaluation factor ηPEF. The dimple/sandblasted tube combined the advantages of sandblasted and dimple surfaces

thus exhibiting the best evaporation heat transfer performance with the highest ηEF and ηPEF values of 2.84 and 2.31

respectively. The composite treatment of sandblasted and dimple heat transfer tube surfaces increases the heat transfer area and the number of vaporization cores

improving the turbulence intensity

such that the liquid film is stretched and thinned to promote evaporation outside the tube.