Evaporation behavior of R-600a considering exergy losses in horizontal smooth and enhanced tubes

In the present study, evaporation heat transfer and frictional pressure drops of refrigerant R-600a (iso-butane) inside a helically dimpled tube and a plain tube of internal diameter 8.3mm were measured and analyzed. All tests were performed at different vapor qualities up to 0.8 and average saturation temperatures ranging between 38 and 42℃. Refrigerant mass fluxes varied in the range of 114-368 kg/m2s. The inner surface of the helically dimpled tube has been designed and reshaped through three-dimensional material surface modifications consists of both shallow and deep protrusions which is placed evenly in helical directions on the tube wall. The experimental results show that the heat transfer coefficients of the dimpled tube are 1.29-2 times larger than a smooth tube with a pressure drop penalty just ranging between 7% and 103% larger than the smooth tube. Moreover, Performance Evaluation Criteria (PEC) dubbed Penalty Factor (PF) and Total Temperature Penalization (TTP) boiling inside a helically dimpled tube and a smooth tube were analyzed which are a convenient way to relate the two penalization components and provides a rational method to compare the exergy losses associated with frictional pressure drop and heat transfer coefficient.