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

Saturated flow boiling and convective condensation experiments for oil-free Refrigerant-134a have been carried out with two microfin tubes as well as with a smooth one. All the tubes have the same outer diameter of 9.52 mm; they are horizontally operated and are heated/cooled by a water stream. Both the microfin tubes are characterized by sharp fins (apex angle of 40°) alternating with two different heights whereas the fin number is different, namely, 54 and 82, respectively. Evaporation tests are carried out at a nominal temperature of 5°C, for a mass flux ranging from about 100 to 340 kg/(m2s), inlet quality between 0.25 and 0.70, and quality change varying from 0.10 to 0.70, whereas for the condensation tests the nominal temperature is 35°C, the mass flux varies between 100 and 440 kg/(m2s), the inlet quality ranges from 0.75 to 0.10 and the quality change from 0.10 to 0.70. The paper reports on average heat transfer coefficient and pressure drop. Furthermore, a visual inspection was carried out to characterize how fins affect the flow pattern transitions. Possible modifications of some flow pattern maps, developed for horizontal smooth tubes in evaporation and condensation, are suggested to extend their use to microfin tubes. Finally, the paper presents comparisons between experimental data and estimates obtained by recent correlations specifically proposed for these tubes.