Effects of Vapor Velocity and Pressure on Marangoni Condensation of Steam-Ethanol Mixtures on a Horizontal Tube

Careful heat-transfer measurements have been conducted for condensation of steam-ethanol mixtures flowing vertically downward over a horizontal, water-cooled tube at pressures ranging from around atmospheric down to 14 kPa. Care was taken to avoid error due to the presence of air in the vapor. The surface temperature was accurately measured by embedded thermocouples. The maximum vapor velocity obtainable was limited by the maximum electrical power input to the boiler. At atmospheric pressure this was 7.5 m/s while at the lowest pressure a velocity of 15.0 m/s could be achieved. 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%. Tests were conducted for a range of coolant flow rates. Enhancement of the heat-transfer coefficient over pure steam values was found by a factor up to around 5, showing that the decrease in thermal resistance of the condensate due to Marangoni condensation outweighed diffusion resistance in the vapor. The best performing compositions (in the liquid when cold) depended on vapor velocity but were in the range 0.025-0.1% ethanol in all cases. For the atmospheric pressure tests the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 15 K, increased from around 55 kW/m(2) K to around 110 kW/m(2) K as the vapor velocity increased from around 0.8 to 7.5 m/s. For a pressure of 14 kPa the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 9 K, increased from around 70 kW/m(2) K to around 90 kW/m(2) K as the vapor velocity increased from around 5.0 to 15.0 m/s. Photographs showing the appearance of Marangoni condensation on the tube surface under different conditions are included in the paper.