Effect of surface roughness on the wetting of a horizontal tube falling-film heat exchanger

In this study the effect of surface roughness on the wetting characteristics of the horizontal tube banks was experimentally investigated. Eight copper tubes (16mm OD and 200mm length) were arranged in a vertical row. A smooth tube and six tubes with different roughness were tested. Additionally, the effect. of tube diameter on the departure-site spacing Was investigated for the tubes with different outer diameters (9.8, 12.7, 16, and 19.05 mm). The tubes were observed visually and photographed to measure the departure-site spacing and the wetted surface area. It was found that the wetting ratio of the tubes was in the range of 0.45-0.96 and closely related to the departure-site spacing. For a given tube diameter the wetting ratio was increased with the decreasing spacing between the departure sites. The wetting ratio of the roughest tube was enhanced up to 1.75 times of the smooth tube. It was also found that the wetting ratio of the tube was decreased as liquid flew down to the lower tubes. The departure-site spacing was decreased as the tube diameter was decreased. A significant enhancement in heat and mass transfer is expected if tubes with sufficient roughness are used in the absorbers and evaporators.