Flow patterns, heat transfer and pressure drop for evaporation and condensation of R134A in microfin tubes

Experiments on flow boiling and convective condensation for oil-free R134a were performed with three 9.52 mm outer diameter tubes (two microfined and one smooth) horizontally operated and heated or cooled by a water stream. Microfin tubes have sharp fins (apex angle 40°) alternating with two different heights whereas the fin number is different (54 and 82). For evaporation (5 °C nominal temperature) test conditions were set as: mass flux 100 ÷ 340 kg (m2 s)−1, inlet quality 0.25 ÷ 0.70, quality change 0.10 ÷ 0.70; for condensation (35 °C nominal temperature): mass flux 100 ÷ 440 kg (m2 s)−1, inlet quality 0.75 ÷ 0.10, quality change 0.10 ÷ 0.70. Average heat transfer coefficients and pressure drop were measured. Furthermore, a visual inspection was performed to characterize how fins affect the flow pattern transitions. Finally, experimental data and prediction of well-known correlations proposed for these tubes were compared.

► Experiments on evaporation and condensation of R134a in horizontal microfin tubes.
► Microfin tubes show annular flow in a wider range of conditions than the smooth tube.
► Microfins shift the dryout to higher values of vapor quality.
► Increase in fin number does not necessarily enhance heat transfer.
► Correlations exhibit contradictory results for different microfin tubes.