Numerical analysis of flow and mass transfer in humid fractal surfaces

In this paper we investigate the convective mass transfer process between a laminar air flow passing over a humid fractal surface. To determine the mass transfer convection coefficient, h_m, in complex 'wetted' fractal geometries, a simple numerical method is considered to obtain the rate of evaporation for any type of surface. This approach is validated considering two benchmark cases commonly discussed in laminar boundary layer theory, e.g., the flow over a flat plate and the flow over a cylinder. By considering different types of fractal geometries and different air flow speeds, we characterize the effects that such multi-scale fractals have on the convective mass transport driving the surface averaged rate of evaporation, n¯_A. The results show the potential of fractals surfaces to enhance evaporation, where depending on the fractal shape considered, for instance, an increase by more than a factor of 3 in the rate of evaporation was obtained in comparison with a reference case where no fractals structures are imposed.