Investigation of heat transfer in spacer-filled channels by experiments and direct numerical simulations

The analysis of flow fields and heat or mass transfer phenomena is of great importance in the optimum design of spacer-filled channel geometries for a variety of membrane-based processes. In the present work, models of spacer-filled channels often adopted in Membrane Distillation are simultaneously investigated by experiments and Computational Fluid Dynamics (CFD). Experiments rely on a non-intrusive technique, based on the use of Thermochromic Liquid Crystals (TLC) and digital image processing, and provide the local distribution of the convective heat transfer coefficient on a thermally active wall. CFD relies on steady-state (laminar flow) simulations in the lower end of the Reynolds number range investigated and on direct numerical simulations in the upper end of this range. This latter is a region of great practical interest for real applications, in which the flow is chaotic but not fully turbulent, and neither steady-state simulations nor the use of turbulence models would provide satisfactory predictions. Results are reported and discussed for a specific spacer geometry (overlapped orthogonal cylindrical filaments) and different spacer orientations with respect to the main flow. To the authors' knowledge, this is the first study in which an experimental validation of computational results concerning local heat or mass transfer is performed for spacer-filled channels.