The effects of surface roughness on convection with external flow

The receiver external heat transfer geometry for a fixed-mirror, distributed-focus solar power station was simulated with a 3-inch diameter, 12-inch long coil of 0.375-inch diameter stainless steel tubing. The coil was heated electrically and the heat transfer equated to electrical power. Free convection tests were made with the coil axis varied in steps from horizontal to vertical. Forced convection tests were made at four orientations of the coil axis relative to the flow direction: perpendicular, 60, 45 and 30 degrees. Reynolds numbers ranged from 3,000 to 80,000 for the forced convection tests. Temperature differences ranged from 300 to 950 F. Free convection results showed a greater increase of convection with increasing-temperature difference than a smooth cylinder. Forced convection results were similar to those for a smooth cylinder in cross-flow correlations. Contrary to experience with smooth cylinders, convection was increased when the flow had a component along the coil axis and the Reynolds number was greater than 6000.