Publisher Summary This chapter studies fluid–flow characteristics of liquids in layers, with and without a superimposed gas velocity. The types of turbulence in layers need to be investigated and also the nature of a laminar layer containing ripples. As regards the theoretical studies of film flow, it is shown in the chapter that it is possible to predict quite accurately the flow behavior in the smooth laminar flow regime of the film; unfortunately, this flow regime is not of great practical importance. The Kapitsa theory for wavy film flow appears to apply over only a very limited part of the total wavy flow regime. Several new experimental techniques for the study of film flow have been developed. Film flow is a special case of two-phase flow. It takes place along a solid surface of some sort, with only one free surface. The second phase in contact with the free surface of the film may be either a gas or a second liquid, which may be at rest or in motion relative to the solid surface on which the film flows. Film flow is distinguished from other forms of two-phase flow by the presence of large interfaces of basically simple geometrical configuration. Two-phase flows are also often further classified as single-component. The occurrence and applications of film flow in modern technology are numerous and important.

The Flow of Liquids in Thin Films

Mass transfer at rough surfaces

Characteristics of inclined thin films, waviness and the associated mass transfer

Enhancement of mass transfer into a falling laminar liquid film by two-dimensional surface waves—Some experimental observations and modeling

Hydrodynamics and heat transfer of wavy thin film flow

On the basis of fluid dynamic and heat transfer studies on falling-film towers by various investigators, it has been commonly accepted by most workers that the liquid flow is essentially streamline in nature for liquid-film Reynolds numbers under 1,800 to 2,000; conseuquently it would be expected that the rate of physical gas absorption in such liquid films could be predicted directly from a knowledge of molecular diffusion rates.

Measurements of the absorption of pure gases in falling liquid films at low Reynolds numbers substantiated the findings of other investigators that the mass transfer rates were manyfold greater than could have been predicted if molecular diffusion were the only transfer process. Increased interfacial area due to rippling of the liquid films could not account for the large increase in mass transfer rates found, and experiments with the addition of a dye stream to the liquid at the freer interface indicated turbulence.

Dissolution rates of slightly soluble solids coated on the tube wall to liquid films were measured and showed that the liquid film was not in laminar flow even for Reynolds numbers as low as 300.

An explanation is proposed which resolves these apparently conflicting results between momentum and heat and mass transfer, based on the fact that mass transfer measurements provide a more sensitive test for the presence of turbulence than do momentum or heat transfer measurements.

Turbulence in falling liquid films

Plate efficiencies measured on 18-in.-diam. sieve plates are reported for the acetic acid-water system and for the methyl isobutyl ketone-water system at atmospheric pressure. In the former system the major resistance to mass transfer is in the gas phase; liquid-phase resistance to mass transfer is controlling in the latter. Efficiencies are also reported for the aniline-nitrobenzene system (gas-phase resistance controlling) at 5 mm. Hg absolute on 6-in.-diam. sieve plates. Pure gas- and liquid-phase efficiencies for both plate designs were determined by the adiabatic humidification of air and the desorption of oxygen from water into air. Predicted values of plate efficiency for each of the binary systems studied were computed from the pure phase efficiencies according to the procedure outlined by Gerster et al. A comparison between measured and predicted efficiencies is presented as a guide for future research in this field.

Clifford Stirba

Papers

Turbulence in falling liquid films

Measured plate efficiencies and values predicted from single‐phase studies