Static configuration of liquid films on horizontal tubes with low radial fins: implications for condensation heat transfer

Abstract

Abstract The configuration of a liquid film retained by surface tension forces on horizontal low-finned tubes has been analysed. It has been shown that liquid is retained on the upper parts of the tube surface previously regarded as 'unflooded'. The meniscus in a radial plane has been shown to approximate to a circular arc, with radius dependent only on the distance from the bottom of the tube, fluid properties and gravity. Four ‘flooding’ conditions are identified for trapezoidal-section fins and the positions around a tube at which these occur have been determined. Experimental measurements, for condensation of three fluids on tubes with rectangular-section fins, suggest that maximum heat-transfer en­hancement occurs when the spacing between the fins (the only geometric variable in these tests) is such as to maximize the ‘unblanked’ (by retained liquid under static conditions) finned tube surface area. The ‘unblanked’ or ‘active’ area is increased by using a radiused fillet at the fin root rather than a sharp corner. This might also be expected to give a corresponding increase in heat-transfer enhancement during condensation.