Different computer-based simulation models, able to predict the performance of Reverse ElectroDialysis (RED) systems, are currently used to investigate the potentials of alternative designs, to orient experimental activities and to design/optimize prototypes. The simulation approach described here combines a one-dimensional modelling of a RED stack with a fully three-dimensional finite volume modelling of the electrolyte channels, either planar or equipped with different spacers or profiled membranes. An advanced three-dimensional code was used to provide correlations for the friction coefficient (based on 3-D solutions of the continuity and Navier-Stokes equations) and the Sherwood numbers (based on 3-D solutions of a scalar transport equation), as well as to test simple models for the Ohmic resistances (based on 3-D solutions of a Laplace equation for the electrical potential). These results were integrated with empirical correlations for the transport properties of electrolytes and membranes, and were used as the input for the higher scale model. The overall model was validated by comparison with experimental data obtained in laboratory-scale RED stacks under controlled conditions. This combined approach constitutes a fully predictive, potentially very accurate, and still extremely fast-running, tool for the approximate simulation of all the main variables, suitable for performance prediction and optimization studies

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This study addresses issues which arise in the computational and experimental modelling of flow and heat / mass transfer in membrane distillation and other processes adopting spacer-filled channels the combined effects of the main parameters that characterize the process (notably spacer pitch to channel height ratio l/H, flow attack angle γ and Reynolds number Re) and the applicability of simple correlations; the influence of the spacer's thermal conductivity. In regard to the complex influence of the parameters, Re, l/H and γ were found to interact heavily, making a separate-effect analysis impossible and power-law friction or heat / mass transfer correlations of little use. Thermal conduction in the spacer, even for low-conductivity polymeric spacers (λ≈0.15 Wm -1 K -1 ), was found to be responsible for up to 10% of the total heat transfer;

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Heat transfer in spacer-filled channels of the kind used in Membrane Distillation was studied in the Reynolds number range 100-2000, encompassing both steady laminar and earlyturbulent flow conditions. Experimental data, including distributions of the local heat transfer coefficient h, were obtained by Liquid Crystal Thermography and Digital Image Processing. Alternative turbulence models, both of first order (k-ε, RNG k-ε, k-ω, BSL k-ω, SST k-ω) and of second order (LRR RS, SSG RS, ω RS, BSL RS), were tested for their ability to predict measured distributions and mean values of h. The best agreement with the experimental results was provided by first-order ω-based models able to resolve the viscous/conductive sublayer, while all other models, and particularly ε-based models using wall functions, yielded disappointing predictions.

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