Heat transfer and pressure drop during condensation of R134a inside micro-fin tubes of new design.

This paper reports on average heat transfer coefficients and pressure drops of oil-free R134a in flow-boiling and convective-condensation inside microfin tubes with a new cross-section profile. Data obtained for a smooth tube are also reported for comparison. All tubes have the same outside diameter of 9.52 mm; they are horizontally operated and are heated/cooled by a water stream. Both 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 flow rate ranging from about 100 to 340 kg/(m2s), inlet quality between 0.25 and 0.7, and quality change varying from 0.1 to 0.7, whereas for the condensation tests the nominal temperature is 35 °C, the mass flow rate varies between 100 and 440 kg/(m2s), the inlet quality ranges from 0.75 to 0.1 and the quality change from 0.1 to 0.7. Finally, the paper presents comparisons between experimental data and estimates obtained by recent correlations specifically proposed for these tubes.