Mass transfer to liquid films on an inclined plane

Abstract

Flow visualisation studies on a small inclined plane show that gravity-controlled roll waves cause mixing of the surface layers of liquid but have little effect on the liquid adjacent to the solid surface. Mass transfer experiments are described in which case β-naphthol and benzoic acid surfaces are dissolved by a water film; in other tests carbon-dioxide-air mixtures are absorbed into the surface of a flowing film. Mass transfer from solid to liquid is adequately predicted by the theory of a smooth laminar film, despite the presence of waves in the real case. Certain anomalies are present in the behaviour of dissolving β-naphthol. In the range of liquid film Reynolds numbers from 20 to 300, mass transfer from gas to liquid is markedly enhanced by waves. For each channel inclination, the mass transfer coefficient reaches a maximum value relative to the predictions of laminar flow theory when the Reynolds number is about 100. The maximum corresponds to a 250 per cent increase of mass transfer coefficient for a vertical surface, a 160 per cent increase for channel inclined at 45°, and a 90 per cent increase for channel inclined at 7°30′. The countercurrent gas flowrate has a moderate effect on mass transfer coefficient in the range of gas Reynolds numbers from 6000 to 32,000. Superimposed vibrations have very little effect on mass transfer to an already-rippling film, though the surface is disturbed by a complex pattern of sinusoidal waves. This supports the contention that the major contribution to mass transfer is from gravity-controlled roll waves and not from the more regular sinusoidal waves.