Computational Investigation of Backward-Facing Step Flow Preceding a Porous Medium

Abstract

Optimal performance of air filters and heat exchangers requires uniform inlet flow, but flow separation produces nonuniformity. The backward-facing step flow has a separation resembling those found in industrial flows. Flow resistance of the devices is a parameter which alters upstream pressure gradients, thereby affecting separation and device performance. Air filters often are modeled as porous media using an extended Darcy Law. The present work applied Computational Fluid Dynamics (CFD) to examine the changes in the step flow resulting from the resistance of a downstream air filter. Computations were performed for a backward-facing step with a 2:1 expansion ratio for a case without a filter (reattachment at ∼6 step heights) and for filters located 4.25 and 6.75 step heights downstream. FLUENT commercial CFD software was used and results were compared to many no-filter case results in the literature and our own experimental studies for the step with downstream filters. The simulations were performed for Reynolds numbers based on approach channel mean velocity and hydraulic diameter of 2000, 3750, 6550 and 10000. The different turbulence models available in FLUENT were evaluated and the Realizable k-ε model was used for the final computations. Grid independence studies were conducted. The effects of different values of the filter modeling permeability, inertial constant and thickness also were investigated for Re = 10000 with the filter at 4.25 step heights. It was found that the computational results did not compare well to no-filter cases or the experiments with filters at the lower Reynolds numbers. It is believed that the turbulence models were unsuitable for these flows at transitional Reynolds numbers. Good agreement for no-filter results and for the experiments with filters was observed for Re = 10,000. The CFD model seems to capture the physics of the separation better at the higher Reynolds numbers. The CFD velocity profiles at Re = 10,000 with the filters agree with those of the experiments. When the filter is placed at 4.25 step heights, the flow reattaches upstream of the filter with a reduction in recirculation area. When the filter is at 6.75 step heights, the separated flow tends to reattach and the opposite side tends to separate. At Re = 10,000 and the filter at 4.25 step heights, the variations of porous medium permeability, inertial constant and the filter thickness have negligible effects on the recirculation region over the parameter ranges considered.