Condensing heat transfer performance in small-diameter enhanced tubes of absorption refrigeration system: An experimental investigation

The condensation heat transfer coefficient (HTC) in an absorption refrigeration system can be increased by using a small diameter as well as enhanced tubes. The small diameter tubes can increase the convection effect by decreasing the insulation layer because of having a larger fraction of the developing boundary layer area. For enhanced tubes, at low vapor velocity, surface tension performs a dominant role in decreasing the film resistance by correspondingly adjusting the film layer. The current research uses four types of small-diameter tubes having enhanced surfaces to augment the performance of the condenser. The saturation pressure is increased from 5.75 to 7.06 kPa, and the Nusselt number shows an increasing trend. However, Tube_D, which has grooves as well as corrugation, shows a decreasing trend with the Nusselt number at a higher saturation pressure. The cooling water velocity is varied from 0.77 to 1.55 m/s, and the overall HTC increased almost linearly. The heat flux also shows an increasing trend as the subcooling is changed from 0.4 to 2.6 °C, however, the HTC decreases with the increase in subcooling. In most cases, the enhancement factor decreased at high values of saturation pressure and condensate Reynolds number, except for the dimple corrugated tube (Tube_A). Of all the condensation tubes, Tube_C which is a corrugated tube with notched fins demonstrated the worst heat transfer performance. The current experiments summarize a new empirical correlation, which reasonably predicted the data within a variation of 1%.