Falling film boiling and pool boiling on plain circular tubes : influence of surface roughness, surface material and saturation temperature on heat transfer and dryout

Abstract

Falling film evaporators in the refrigeration industry offer a number of advantages over their flooded counterparts such as improved heat transfer and lower refrigerant charge. Existing literature has not characterised the influence of surface roughness and material on the falling film boiling process. The purpose of this study was therefore to experimentally measure the influence of surface material and roughness on the heat transfer of falling film boiling and pool boiling on the outside of horizontal plain tubes. The falling film enhancement factor and total dryout threshold were also measured in the study. The study was conducted on an experimental set-up at saturation temperatures of 5 °C and 25 °C using refrigerant R-134a at heat fluxes from 20 to 90 kW/m2 and film Reynolds numbers from 2000 to 0. The outside of plain copper, stainless steel and mild steel tubes were roughened with various grades of sandpaper to achieve roughnesses between 0.1 and 1.9 μm. The tubes were heated by water and as such Wilson plots were conducted to characterize the internal heat transfer. Increases in surface roughness were found to increase both pool boiling and falling film boiling heat transfer coefficients. Changes in surface material decreased the heat transfer coefficients in line with the decrease of the material’s thermal effusivity. The falling film enhancement factor was found to increases as surface roughness was increased, but changes in material had no discernible effect. Decreases in saturation temperature decreased pool boiling and falling film boiling heat transfer coefficients and weakly increased the falling film enhancement factor. Changes in surface roughness and material had no discernible influence on total dryout threshold.