Condensation heat transfer and pressure drop characteristics of R-134a flowing through dimpled tubes with different helical and dimpled pitches

The heat transfer enhancement and pressure loss penalty of condensing refrigerant R-134a in dimpled tubes with different helical and dimpled pitches are experimentally investigated. The test section is a 1500-mm-long tube-in-tube heat exchanger placed horizontally. The inside and outside diameters of the inner tubes are 8.1 and 9.52 mm, respectively. One smooth tube and five dimpled tubes with different helical and dimpled pitches are tested. The experimental runs are done at saturated condensing temperatures ranging from 40 to 50 °C. The mass fluxes and heat fluxes vary from 300 to 500 kg/m2 s and from 10 to 20 kW/m2, respectively. The results show that the dimpled tubes yield higher heat transfer enhancement and pressure drop penalty compared to the smooth tube under the same operating condition. By decreasing helical and dimpled pitches, both the heat transfer coefficient and frictional pressure drop increase significantly. The maximum heat transfer coefficient and frictional pressure drop are achieved through the tube with the lowest helical and dimpled pitches (p = 5.08 mm, z = 3.24 mm) up to 88% and 630% higher than those of the smooth tube, respectively. The new correlations for predicting the Nusselt number and friction factor of R-134a condensing in dimpled tubes with different helical and dimpled pitches are proposed.