Marangoni condensation of steam-ethanol mixtures on a horizontal low-finned tube

Careful heat-transfer measurements have been conducted for condensation of steam-ethanol mixtures in vertical downflow over a horizontal, water-cooled, low-finned copper tube. Care was taken to avoid error due to the presence of air in the vapor. The tube had diameter at the fin root 12.7 mm and rectangular section fins with height 1.6 mm, thickness 0.5 mm and space between fins 1.0 mm. Tests were conducted at pressures of atmospheric, 55 kPa and 14 kPa. Concentrations of ethanol by mass in the boiler when cold prior to start up were 0.025%, 0.05%, 0.1%, 0.5% and 1.0%. The highest vapor velocity at approach to the tube was 7.5 m/s at atmospheric pressure and 15.0 m/s at vapor pressure 14 kPa. Effects of ethanol concentration on both retention angle and heat transfer were measured. The retention angle was strongly dependent on the vapor velocity and ethanol concentration which affected the condensation rate, composition and temperature of the condensate at the interface and consequently the surface tension of condensate. The results are compared with data for pure steam on the same finned tube and Marangoni condensation on a smooth tube under the same conditions. Similarly, results for condensate retention of water on finned tube are compared with earlier data and theoretical model. Vapor-side, heat-transfer coefficients were obtained by subtraction of coolant side and test tube wall thermal resistances from overall measurements.