In-tube Condensation and Evaporation Heat Transfer Coefficients in Four Enhanced Surface Tubes: An Experimental Investigation

Abstract

An experimental investigation was conducted to evaluate heat transfer performance of several enhanced surface tubes during in-tube evaporation and condensation of R410A; results were then compared to the results of a smooth tube. Tubes considered in this evaluation included: smooth; herringbone and helix micro groove; herringbone-dimple and hydrophobic; all the tubes evaluated have the same external diameter of 12.7 mm. Experimental condensation and evaporation results were acquired at saturation temperatures of 318 K and 279 K. The mass velocities varied in the range of 40 – 230 kg m-2s-1; vapor quality decreased from 0.8 to 0.2 for condensation and increased from 0.2 to 0.8 for evaporation. Moreover, the heat fluxes increased with mass velocity. Condensation heat transfer coefficients are enhanced by 40 % to 73 %, with the dimpled herringbone grooved tube (EHT-HB/D) exhibiting the highest heat transfer coefficient among the five tested tubes. In addition to producing the condensate drainage effects, the herringbone grooves can help lift the accumulated condensate up along the circumference; dimples produce condensate turbulence and droplet entrainment. For the evaporation, the hydrophobic-herringbone tube (EHT-HB/HY) provides the best thermal performance; its heat transfer coefficients are 4 - 46 % larger than those of the smooth tube. This enhancement may be attributed to the expanded heat transfer area and the increased nucleation sites.