Comparative study on the pool boiling and falling film evaporation of refrigerant R134a outside the same tubes

Pool boiling and falling film evaporation of R134a outside a typical reentrant enhanced tube was investigated with an experimental approach. Experimental data from literature with other refrigerants were also compared. The saturate temperature was 11℃. It was found that the overall heat transfer coefficients for the enhanced tube was up to 3 times higher than smooth tube for the water velocity from 0.8 to 3.5m/s. Falling film evaporation heat transfer coefficients increased by a factor of 2.5 to 4. The pool boiling yielded a slightly higher heat transfer coefficient at higher heat flux for the enhanced tube. The dependence of falling film evaporation heat transfer coefficient on the heat flux was also not different from pool boiling for the same tube. Although the bubbles in pool boiling were pushed up by buoyancy and in falling film evaporating were driven by the flow of films, the transmission of energy were both dominated by the phase change heat transfer. The contributions of forced convection to falling film evaporation heat transfer coefficient seems to be weak compared with phase change heat transfer.