Droplet and retained condensate effects on local convective heat transfer for annularly-finned tubes in cross flow

In many heat exchanger applications droplets or retained condensate, acting essentially as surface irregularities, have an influence on the local and spatially-averaged heat transfer behavior. Previous studies have reported an enhanced performance under these circumstances, but the mechanism and true impact of the enhancement remain unclear for finned-tube applications. From mass transfer experiments with simulated condensate, we have inferred the surface irregularity impact on heat transfer and fin efficiency. An enhancement in spatially-averaged behavior as high as 33% over dry fin behavior was found, and true fin efficiencies were found to be as much as 13% below the Gardner (uniform heat transfer coefficient) fin efficiencies. Flow and heat transfer appear to be significantly affected by small horseshoe vortices associated with the droplets or surface irregularities.