Heat Transfer Performance of Microfin Tubes and -Three-Dimensional Heat Transfer Tubes

Abstract

Condensation heat transfer characteristics under different working conditions were studied to determine the heat transfer performance in horizontal, smooth and enhanced heat transfer tubes. Condensation heat transfer of these tubes was studied using R410A for a range of mass flow rates from 250 to 450 kg m-2 s-1 and a saturation temperature of 45 ?. Visual flow pattern images and heat transfer performance for flow condensation are presented for the enhanced tubes and compared with that of a smooth tube for the same conditions. The condensation heat transfer coefficient enhancement ratio for the three dimensional tube is in the range 1.15 ~ 2.05, and the range of the microfin tube is 1.18 ~ 1.69. Additionally, it was found that the thermal conductivity of the smooth tube has a slight influence on its heat transfer performance, while a greater enhancement was found in the enhanced tubes. Stratified wavy flow (SW), intermittent flow (I), semi-annular flow (SA) and annular flow (A) were observed for the range of conditions investigated. The heat transfer coefficient increased with an increase in mass velocity; as the mass flow rate increases, the turbulence of the liquid flow increases and the liquid film becomes thinner. This reduces thermal resistance and enhances heat transfer. Heat transfer performance for low mass velocities rise slowly; however the improvement of heat transfer performance at high mass flux rates is larger than that at low mass flux rates. Finally, better heat transfer performance is found in high thermal conductivity tubes with smaller diameters.