Boiling heat transfer of R134a outside horizontal doubly-enhanced tubes

An experimental study on the performance of boiling heat transfer of R134a outside a single horizontal tube was conducted for four doubly-enhanced tubes (E10, Ell, E12, E15) with different geometries at saturation temperature of 8°C. Experimental results showed that the studied tubes could effectively enhance heat transfer. The overall heat transfer coefficients of the tube E12 were higher than that of the others, and its enhancement factors, the boiling heat transfer coefficient relative to that of Cooper solutions for smooth surface, varied from 2.23 to 2.71, with an average value of 2.54. Because of the different thermodynamic properties, the boiling heat transfer coefficients of R22 were 20%-40% higher than those of R134a. The enhancement factor of boiling heat transfer was independent of refrigerant.