Local flow boiling heat transfer characteristics in three-dimensional enhanced tubes

Experimental study on local flow boiling heat transfer characteristics in two three-dimensional dimple-grooved tubes (2EHT) and an equivalent smooth tube was performed. All the three test copper tubes have the same inner diameter of 11.07 mm and outer diameter of 12.7 mm. The working fluid was the near-azeotropic mixture, R410A. All test runs were conducted in a 2 m long horizontal tube-in-tube heat exchanger. Constant evaporation temperature was maintained at 10 °C when heat flux ranged from 32.6 kW/m2 to 37 kW/m2. Refrigerant quality varied from inlet 0.1 to outlet 0.9 and mass flux was controlled from 70 kg/(m2 s) to 150 kg/(m2 s). The enhanced surface areas of two 2EHT tubes are 1.02 and 1.03 times of the smooth tube, respectively. The local HTC results of saturate flow boiling were presented and compared with the existing correlations. Wall temperature was measured and critical heat flux effect on the tube-side evaporation was discussed. Local wall temperature results showed that the 2EHT tubes produce a better saturate evaporation at a relatively lower wall superheat. HTC increases with increasing heat flux when heat flux is smaller than CHF and decreases when heat flux is larger than CHF. Based on the local thermal properties, a new flow boiling heat transfer correlation for these 2EHT tubes is derived. The new correlation can predict all the experimental heat transfer coefficients within an error band of ±30% and 96.89% of test data within an error band of ±20%.