Evaporation Flow Patterns and Heat Transfer in Copper and Stainless Steel Three-Dimensional Dimpled Tubes

An experimental study evaluating tube side evaporation (using R410A refrigerant) in smooth and enhanced tubes was performed. Enhanced heat transfer tubes made of copper and stainless steel were studied visually and experimentally; with a nominal outer diameter of 12.7 mm and inner diameter of 11.5 mm; tube length of 1.85 m; with an enhanced surface that was composed of dimples and petal arrays. Experiments were carried out at a saturated temperature of 6°C; for a vapor quality range from 0.1 to 0.9; with mass velocities fixed at 100, 150 and 200 kg m−2 s−1. Flow patterns observed in this study included stratified flow, stratified-wave flow, annular flow and dry flow. These patterns, in addition to vapor quality and mass flux all influence the flow patterns/heat transfer, which is discussed in detail. The enhanced heat transfer structure produces the most violent disturbance and the lowest transition vapor quality between the stratified-wave flow and annular flow; additionally, the heat transfer enhancement ratio was 1.27–1.96 while the pressure drop during boiling was 11–36% higher when compared to the smooth tube. Influence of the thermal conductivity of the enhanced tubes on the flow pattern/ evaporation characteristics is also discussed; copper enhanced tube produced 17-53% larger heat transfer coefficient than stainless steel enhanced tube. The flow pattern maps suitable for enhanced tubes were modified and new heat transfer correlations were established.