Visual study of flow patterns during evaporation and condensation of R-600a inside horizontal smooth and helically dimpled tubes

Abstract

In this paper, flow patterns and their transitions for the refrigerant R-600a during flow boiling and condensation inside a helically dimpled tube and a smooth tube were observed and analysed. The inner surface of the helically dimpled tube was enhanced by a modified pattern consisting of both shallow and deep protrusions.For evaporation, the experiments were performed for refrigerant mass velocities in a range of 155kg/m2s to 467kg/m2s, all at an average saturation temperature of 56.5°C with the vapour qualities up to 0.8. Stratified-wavy, intermittent, and annular flows were observed for the smooth tube; for the dimpled tube, the stratified-wavy flow was not seen.For condensation, all tests were conducted at vapour qualities up to 0.8, and average saturation temperatures ranging between 38°C and 42°C. The refrigerant mass fluxes varied in the range of 114–368kg/m2s. Annular, intermittent, and stratified-wavy flows were recognized for the plain tube, but there was no stratified-wavy flow in the flow pattern visualization of the dimpled tube.The investigation clearly shows that the dimples in both evaporation and condensation have a significant impact on the two-phase flow pattern. Inside the helically dimpled tube, the transition from intermittent to annular (or vice versa) occurred at a lower vapour quality value than for the smooth tube.