Forced Convection Heat Transfer Outside Enhanced Tubes with Different Surface Structures

To promote the development of enhanced tubes in heat exchangers, forced convection experiments were carried out using R410A in the annular region outside enhanced tubes. Three enhanced tubes were investigated by experiments to evaluate the heat transfer performance; with different surface structures of novel fin arrays (C-enhanced tube), newly-developed combined dimples (L-enhanced tube) and sandblasted surface (P-enhanced tube); with the 19.05 mm outer diameter; with mass flow rate range of 40-110 kg/m(2)s. For single-phase and condensation experiments at various saturation temperatures, the C-enhanced tube showed the best heat transfer performance and produced heat transfer coefficients of 4.25 and 3.8 times that of the smooth tube. The results of changing outlet vapor quality (0.3-0.8) of flow boiling experiments indicated that the L-enhanced tube showed the best evaporation characteristics and yielded heat transfer coefficients 2.1-3.9 times that of the smooth tube. Surprisingly, instead of showing the heat transfer performance advantage of the P-enhanced tube, heat transfer deterioration occurred despite numerous micro dimples on the surface. In addition, the pressure drop characteristics were also discussed, and the experimental results were evaluated by empirical correlations.