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

•Dimples in both evaporation and condensation influence the flow patterns.•Intermittent and annular flows were dominant flow pattern for the dimpled tube.•There was no stratified-wavy flow in the flow pattern map of the dimpled tube.•Transition lines of both dimpled and plain tubes were recognized and compared.•Transitions for the dimpled tube occurred at a lower x than the plain tube. 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.