Condensation heat transfer of R22 outside horizontal doubly-enhanced tubes

An experimental study on R22 film condensation on single horizontal tubes (one smooth tube and two doubly enhanced ones) was conducted at the saturation temperatures of 32°C, 35°C and 38°C respectively. A modified Wilson plot was used to obtain the water-side and vapor-side heat transfer coefficients. The results indicated that the experimental condensation heat transfer coefficients agreed with the Nusselt theory with ±10%. The water side heat transfer coefficients of tube No.1 and tube No.2 were 2,78 and 2.99 times of that of smooth tube, respectively. At the same heat flux the condensation heat transfer coefficients of tube No.1 and tube No.2 were 8.2-18.0 times and 8.1-12.4 times of the smooth tube, respectively. Thermal resistance analysis showed that the relative percentage of vapor-side thermal resistance increased with water velocity. When the-thermal resistance values of the two sides were equal, the water velocity of tube No.1 was 1.25 m·s^-1 (Re=2.41×10⁴) and that of No.2 was 0.82 m· s^-1(Re=1.54×10⁴). For all cases, the wall thermal resistance was less than 4 percentage. The reasons accounting for the superiority of tube No.1 were analyzed.