An experimental and numerical study of laminar natural convection along vertical rib-roughened surfaces

Air natural convection along a vertical surface periodically roughened with wooden ribs, square in section and either spanwise-elongated or truncated and arranged in a staggered pattern, is investigated at a Rayleigh number of 2 × 107, which corresponds to a stable buoyant air flow. The influence of roughness on local and overall convective heat transfer was analyzed experimentally via schlieren imaging and extensive energy balance calculations and numerically through both conventional and homogenization-based CFD (computational fluid dynamics) simulations. For the considered range of the rib pitch-to-height ratio (from 3.5 to 20), the continuous, transverse elements were generally found to degrade the local and overall convective heat transfer, with the deterioration becoming more pronounced when the ribs are densely packed on the surface. Furthermore, even staggered truncated ribs failed to provide any local/overall enhancement to convective heat transfer. The simulations performed via the homogenization-based treatment, which represents an easier alternative to the standard fine-grained numerical analysis, led to heat transfer trends in line with those obtained by fully resolving simulations and experiments. Both numerical approaches showed that the conjugate heat transfer problem must be tackled in the case of low-thermal-conductivity ribs since the solution, in terms of heat transfer characteristics, is intermediate between the cases of adiabatic and perfectly conducting elements.