Transport Phenomena in Turbulent Falling Films

Abstract

This study was concerned with transport phenomena in turbulent falling films, with negligible interfacial shear. Emphasis was on the development of general expressions for eddy diffusivities, which could then be utilized to predict transport coefficients for various momentum and heatand mass transfer processes. Experiments were pursued to obtain new data for heat transfer during film evaporation, a phenomenon that involves significant boundary layer resistance at both the wall and the free interface. These new data extended the existing database by an order of magnitude in the Prandtl number. When predictions of the new model are assessed against both new and existing experimental information, good agreement was found for film thickness, sensible heat transfer at the wall, mass transfer at the free surface, and film evaporation.

A Distributed-Parameter Model of Black Liquor Falling Film Evaporators. Part I. Modeling of a Single Plate

“…Numerous studies have been conducted on the fluid dynamics of falling films, starting with the classic Nusselt theory for laminar, flat falling films. However, real films are wavy laminar or turbulent in most industrial applications . Thus, it is not surprising that Nusselt's theory is unable to predict important parameters such as the heat-transfer coefficient.…”

“…The early models assume that a turbulent falling film consists of a boundary layer at the wall and a turbulent layer. More recent acceptable theories hypothesize that these films consist of a boundary layer at the wall, a turbulent core, and a second boundary layer at the free surface . The presence of the additional boundary layer might explain the overprediction of the models by Chun and Seban 6 and Limberg, because their models do not account for the damping of turbulence at the free surface.…”

“…Alhusseini and Chen 5 compared four models in terms of the presence of the boundary layer at the free surface and the eddy viscosity in the film . They provided a model for the eddy diffusivity of momentum given by where …”

“…However, real films are wavy laminar or turbulent in most industrial applications. 5 Thus, it is not surprising that Nusselt's theory is unable to predict important parameters such as the heat-transfer coefficient. The modeling presented in this paper follows the concepts and models presented by Alhusseini and Chen.…”

“…The modeling presented in this paper follows the concepts and models presented by Alhusseini and Chen. 5 The surface waves in the wavy laminar regime carry a significant part of the mass flow, and they change the nature of the flow, for instance, causing local recirculation. In the turbulent regime, the nature of the entire flow changes, and eddy momentum transport becomes significant.…”