Surface tension effects in condensation heat transfer : condensation on wire-wrapped tubes and Marangoni condensation of mixtures
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Enhancement of condensation heat transfer by wrapping of fine WIres on a condenser tube and Marangoni condensation of binary mixtures have been studied. For wire-wrapped tubes enhancement is due to modification of the profile of the condensate surface which leads to axially-directed pressure gradients and local thinning of the condensate film. Approximate theories do not agree well with limited available data prior to the present work. A systematic experimental investigation has been conducted using three fluids with widely different properties. Five wire diameters and a range of winding pitch have been used. Maximum heat-transfer enhancement ratios of 3.7, 2.2 and 2.3 for R-I13, ethylene glycol and steam respectively were obtained. The effect of inundation for steam condensation on wire-wrapped tubes has also been investigated. Extensive data exist for Marangoni condensation of steam-ethanol mixtures on small plane ve.rtical surfaces. Here the practically more relevant case C?f a horizontal tube has been studied. Apparent differences between the vertical plate and horizontal tube data are shown to be due to circumferential variation of tube surface temperature. Enhancement ratios up to around 3.7 have been obtained with as little as 0.05% mass fraction of ethanol in the boiler feed. For wire-wrapped tube and Marangoni condensation, a copper condenser tube (outside diameter 12.2 mm) fitted with four embedded wall thermocouples was cooled internally by water using a wide range of flow rates. The coolant temperature rise was measured to within 0.01 K using a ten-junction thermopile while the coolant temperature rise ranges were 0.11 to 0.77 K, 0.89 to 9.28 K and 1.00 to 6.98 K for the wire-wrap tests with R-I13, ethylene glycol and steam respectively and 1.24 to 29.1 K for Marangoni condensation. The effect on the boiler performance for water-ethanol mixtures has also been investigated.
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