Influence of thermal buoyancy on heat transfer in spacer-filled channels for Membrane Distillation

Numerical results are discussed for the flow in a horizontal plane channel filled with a novel sphere-rod spacer and exchanging heat from both the top and the bottom sides. Direct Numerical Simulations (DNS) are compared with RANS ones based on different turbulence models in the Reynolds number range 100∼2000. Preliminary comparisons for non-buoyant flow show that models using wall functions perform poorly, grossly overpredicting Nusselt numbers, while ω-based models resolving the viscous-conductive sublayer all yield satisfactory results. In the presence of buoyancy, simulations using either DNS or the k-ω model yield a thermal asymmetry between top and bottom wall, confirmed by experiments and related to the stable or unstable thermal stratification occurring in the lower and upper layers of the channel. The asymmetry, large at low Re, becomes negligible for Re≥1000. The Spalart-Allmaras model yields satisfactory results in the absence of buoyancy but grossly overpredicts Nu in buoyant flows.