Condensation heat transfer characteristics of CO2 in a horizontal smooth- and microfin-tube at high saturation temperatures

The condensation heat transfer characteristics for CO2 at high saturation temperatures in a horizontal smooth- and microfin-tube were investigated by experiment with respect to condensation temperature and mass flux. The test sections consist of 2400 mm length with a horizontal copper tube of 4.6 mm (smooth) and 4.95 mm (microfin) inner diameter. The experiments were conducted at refrigerant mass flux of 400–800 kg/(m2s), and saturation temperature of 20–30 °C. The main experimental results showed that annular flow almost dominated the major of condensation flow in the horizontal smooth- and microfin-tube. The condensation heat transfer coefficients for the smooth- and microfin-tube increase with the decreasing saturation temperature and increasing mass flux. The average heat transfer enhancement factor (EF) for the microfin tube is approximately from 1.006 to 1.48, and penalty factor (PF) varies from 1.14 to 1.23. The experimental data in the smooth- and microfin-tube were compared against previous heat transfer correlations. Most of correlations failed to predict the experimental data. However, the correlations by Kondou and Hrnjak and Cavallini et al. showed relatively good agreement with experimental data in the smooth- and microfin-tube, respectively. But it is necessary to develop accurate and reliable correlation to predict condensation heat transfer coefficient at high saturation temperatures in the horizontal smooth- and microfin- tube.