An experimental study of heat transfer by dropwise condensation

Abstract Measurements of heat flux and steam-side temperature difference during dropwise condensation on plane vertical surfaces are reported. These results are considered to have enhanced precision and, in particular their relation to earlier work supports the view that the effects of “non-condensables” have been avoided. Thermocouples, accurately located and spaced through copper plates served to measure the “mean” surface temperature at a known point on the condensing surface (by extrapolation) and the heat flux (from the temperature gradient). Measurements were made at depths of 1 in (25.4 mm), 1.2 in (28.4 mm) and 4 in (101.6 mm) from the top of the condensing surface. The heat flux used ranged from 0.3 to 1.8 MW/m 2 (i.e. 100000 to 570 000 Btu/ft 2 h or 260 000 to 1550 000 kcal/m 2 h). The pressure was approximately 1.04 bar. Four different promoters were used. The results obtained were very consistent and were reproducible on different days. The steam-side heat-transfer coefficient was found to increase with heat flux over the above range, the maximum coefficient being about 0.3 MW/m 2 degC (i.e. 53 000 Btu/ft 2 h degF or 260 000 kcal/m 2 h degC). No evidence of dependence on plate height was found. Differences between promoters were clearly established.