R410A flow boiling coefficient in horizontal annular channels of enhanced tubes, Part II: Heat transfer

Flow boiling heat transfer coefficient of R410A in horizontal annular channels were evaluated with a 279 K saturation temperature over a mass flux range of 50-80 kg/(m2s) and a vapor quality range of 0.2-0.8. All the four tested annulus have a 19 mm outer diameter of the flow channel containing three types of enhanced tubes (a micro-porous tube, a ring-shaped finned tube, and an independent small-bosses surface tube) and a smooth tube. The micro-porous tube benefitted by its surface shows the highest heat transfer coefficient (the enhancement factor exceeds 2.52) in flow boiling. The smooth tube shows a higher heat transfer coefficient than the independent small bosses tube due to its special surface structure. For changing average vapor quality of outlet and inlet, the heat transfer coefficients of the micro-porous tube and smooth tube increase with the increase of the average vapor quality. Considering the heat transfer coefficient and the data of pressure drop in all of these annular channels reveals that the micro-porous tube can be a great choice for flow boiling on the annular side for the best performance. The results from correlation analysis of smooth tube predict experimental data with a +/- 10% error band. The improved correlations for enhanced tubes were presented, and the experimental values are predicted within a +/- 2% error band.