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 degrees) alternating with two different heights whereas the fin number is different (54 and 82). For evaporation (5 degrees C nominal temperature) test conditions were set as: mass flux 100 divided by 340 kg (m(2) s)(-1), inlet quality 0.25 divided by 0.70, quality change 0.10 divided by 0.70; for condensation (35 degrees C nominal temperature): mass flux 100 divided by 440 kg (m(2) s)(-1), inlet quality 0.75 divided by 0.10, quality change 0.10 divided by 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. (C) 2012 Elsevier Ltd and IIR. All rights reserved.