Abstract
The purpose of this study is to discuss some problems which arise in heat or mass transfer in complex channels, with special reference to the spacer-filled channels adopted in membrane processes. Among the issues addressed are the consistent definition of local and mean heat or mass transfer coefficients; the influence of the wall boundary conditions; the influence of one-side versus two-side heat/mass transfer. Most of the results discussed were obtained by finite volume CFD simulations concerning heat transfer in Membrane Distillation or mass transfer in Electrodialysis and Reverse Electrodialysis, but many of the conclusions apply also to different processes involving geometrically complex channels
Highlights
The conclusions reached were based on many years’ experience with Membrane Distillation and Reverse Electrodialysis, but many of them can be extended to other membrane processes as well
The results of the study can be summarized as follows: - In regard to the definition of local heat or mass transfer coefficients, the definition based on the local heat / mass flux and on the difference between the local temperature or concentration at the wall and the corresponding bulk values, i.e. h=q”w/(Tb-Tw) or k=jw/(Cb-Cw), remains the most commonly adopted, but we have shown that these quantities may become singular or negative at some points of the wall in complex geometries
For this and other reasons, the definition of the average coefficients as surface averages h(1)= h, k(1)= k should be avoided, and the alternative averages h(2)= q”w /(Tb- Tw ), k(2)= jw /(Cb- Cw ), which do not suffer from singularities, are preferable both in computational and in experimental work. - In regard to the choice of temperature or concentration boundary conditions at the walls representing the fluid-membrane interfaces in CFD simulations, we have shown that it may have a significant impact on the computed Nusselt or Sherwood numbers, especially in laminar flow
Summary
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