Reynolds number effects on swirling flows intensity and reattachment point over a backward-facing step geometry using STD k-ε turbulence model

Abstract

Swirling flow caused by sudden expansion is a common phenomena in many engineering application. The flow reattachment which follow after the expansion determines the flow condition, including pressure fluctuation especially when heat transfer mechanism is highly considered. In many cases, this swirling flow oftenly decrease the system performance since fluid convection will decrease and the reattachment point can indicate the length of the swirling flow. At this condition, Reynolds number will affect rettachment length. Due to high resources of experimental method, CFD simulation is choosen for it's flexibility to varied numbers of model and cases. This study aims to numerically investigate reattachment length of swirling flow over expansion zone of BFS at varied Reynolds number with CFD method. The Reynolds number gained from the experimental data. The model is 3 dimensional with Step height (h) = 41mm, up stream height (H) = 81mm, expansion ratio = 1.5, total length (L) = 4050 mm and width = 20h. The mesh is triangular with 90.293 nodes. Reynolds numbers varied to Re = 4290.39; 6673.94; 7614.23; and 9287.29 which gained from experimental data with working fluid is set to single phase air. The flow condition is assumed transient with time step set of 1 and simulation is done with STD k-ε turbulence model. Respectively, reattachment point at Re = 4290.39; Re = 6673.94; Re = 7614.23 and Re = 9287.29 achieved at x/h = 5.75; 7.8; 9.04; 10.7. This phenomena clearly showed that unsteadiness at higher Re contribute to higher reattachment length.