Enhancement of condensation heat transfer for steam at near atmospheric pressure

The present research is an experimental study of the impact of various surface treatments on the film condensation heat transfer of steam at near atmospheric pressure. Test surfaces of solid copper blocks 25 mm wide and in lengths of 25, 50, and 75 mm were tested. The surface treatments included a variety of roughened surfaces, porous aluminum foams, and rectangular plate fins. The impact of adding surfactant to the steam source was also compared for these various surface treatments. Finally, some conditions were repeated under flowing vapor conditions.; All of the surface roughness treatments examined degraded condensation heat transfer, indicating that increased viscous losses tended to thicken the condensate film. Similarly, aluminum foams decreased heat transfer for all the porosities and thicknesses evaluated. When condensate was not retained by the fin geometry, the enhanced condensation heat transfer was well modeled by applying the Nusselt-Rohsenow correlation to a fin conduction analysis. When surface tension was sufficient to hold up condensate film, the heat transfer enhancement diminished because of thickened condensate film and inhibited vapor flow between the fins.; With the addition of surfactant, the heat transfer for polished and finned surfaces was enhanced due to reduced condensate retention and possibly to secondary Marangoni flow of the condensate. For the roughened surfaces examined, viscous losses in the condensate film dominated the flow and no enhancement was observed with the addition of surfactant.; Vapor flow further augmented the condensation heat transfer. At the velocities studied, thinning of the condensate film by the interfacial shear was insufficient to explain the improved heat transfer and is likely the result of further reduction of the retained lip of condensate on the condenser trailing edge. Vapor flow was seen to significantly improve the heat transfer in the finned condenser with narrow fin spacing, likely because the vapor flow significantly broke up the condensate retention between the fins. Similarly the addition of surfactant during vapor flow showed further enhancement because of further reductions in condensate hold up.