Abstract
Condensation heat transfer performance was studied experimentally in horizontal, stainless-steel (type 304) tubes. This study experimentally studied the effect of hydrophobicity on condensation heat transfer by comparing heat transfer performance of smooth tubes (ST), hydrophobic tubes (HYD), herringbone tubes (HB), and hydrophobic/ herringbone composite tubes (HYD/HB). All tubes had an outer diameter of 12.7 mm; with saturation temperatures of 35 °C and 45 °C; for mass flow rates of 100 kg/(m2·s) and 150 kg/(m2·s); for refrigerant quality that ranged from 0.2 to 0.9. Performance data that varied with mass flow indicate that the HB tube exhibits superior heat transfer performance when compared to the HYD tube. However, for constant mass flow rate and increasing refrigerant quality, the heat transfer coefficient (h) of the HYD tube increases gradually; at a quality of 0.6, the HYD tube outperforms the HB tube. The physical properties of refrigerant R32 are better (promoting higher h values) at a saturation temperature of 35°C than at 45°C; all the tube types show an improvement in h of approximately 15% at 35°C compared to performance at 45°C. Finally, as the mass flow rate increases, the heat transfer coefficient of the HYD tube improves by about 20% compared to a smooth tube. This improvement is due to the hydrophobic structure, which causes droplet condensation on the inner surface of the tube, increasing the droplet detachment rate and enhancing heat transfer. As the mass flow rate increases, the heat transfer coefficient (h) of the HB tube increases, while those of the HYD/HB tube decrease slightly. This can be explained by the hydrophobic structure of the HB tube; this inhibits the induction of droplets by the herringbone fin; however, the hydrophobic structure is not sufficient to balance this weakening. The HYD tube shows an increasing trend for h as the vapor mass flow increases. This occurs when the refrigerant gas phase dominates over the liquid phase; this prevents the formation of a liquid film forming (from the large droplets produced by the hydrophobic structure). As a result, there is a significant increase in h at an average quality of 0.5; for these conditions the HYD tube outperforms the HB tube and the h of the HYD tube is approximately 20% higher than that of the smooth tube.
