Filmwise condensation on a horizontal tube in the presence of forced convection and non-condensing gas
Accurate and repeatable heat-transfer data have been obtained for filmwise condensation from pure vapours (steam and Refrigerant 113) and vapour-gas (steam-air, steam-hydrogen, Refrigerant 1 13-air and Refrigerant 113- hydrogen) mixtures flowing vertically downward over single horizontal tubes. The tube surface could be viewed to ensure that the filmwise mode of condensation prevailed throughout all tests. Two copper tubes having diameters 12.5 mm and 25.25 mm were used. Surface temperatures at four positions were obtained from thermocouples embedded in the tube wall. The heat flux was obtained from coolant measurements which were checked against values obtained by condensate collection. The vapour mass flow rate was obtained from the electrical power input to the boiler. (The mass flow rate determination incorporated a correction for relatively small 'thermal losses' to the environment which were established by preliminary measurements in which all the vapour supplied to the test section was condensed and. collected. Non-condensing gases could be supplied continuously via variable-aperture float-type flowmetera to the boiler. The working length (r11O mm) of the condenser tube was located, centrally in the cylindrical test section (152.4 mm). The vapour Reynolds number (based on the teat section diameter) was generally greater than 2000. The mean vapour approach velocity over the working length was determined on the basis of a 'seventh power profile' in conjunction with the measured flow rate. The approximate ranges of the variables used were:- pressure (4 - 124 kpa), heat flux (12 - 466 kW/m2 ), vapour velocit' (0.3 - 26 n/a), gas mole (mass) fraction (o.i % (0.02 %) - 35 % (32 %)). The vapour-gas combinations were chosen to give a wide range of Schmidt number (about 0.05 - 0.5). For pure vapoura, the results are in overall agreement with earlier data (mostly steam) at moderate approach velocities. While discrepancies exist at higher velocities, both the present and earlier results show satisfactory agreement with theory at low and moderate velocities. The vapour-gas data are in good agreement with the limited earlier measurements (steam-air only) and with theory. In particular, the theoretically predicted Schmidt number dependence was clearly established.