Effect of walls with large scale roughness in deposition efficiency for 90-degree square bend configurations

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This paper presents a numerical study of particle deposition in turbulent flow across bends with large scale roughness. The k-ε RNG model with Enhanced Wall Functions (EWF) for wall treatment and a Lagrangian point particle tracking method with a discrete random walk (DRW) model for turbulence dispersion were chosen in order to study the effect of cavities and ribs on the particle deposition efficiency in 90-degree square bends. The study is a complementary approach to previous ones performed for duct and channel flows. Results show that while cavity-type (c-type) and rib-type (r-type) large scale roughness elements in the external wall of the bend have a low effect on particle deposition, there is significant enhancement for cases in which large scale elements are located also at the inner side of the bend inducing high increments of secondary slow and turbulent kinetic energy. This, in addition to the increment of deposition area, generates a global shift of the particle deposition curve, especially for low and intermediate Stokes numbers and cases with high curvature ratio. Our study shows that bends of square cross section, similar to those used in Heating, Ventilation, and Air Conditioning (HVAC) applications but modified with large scale roughness elements in their walls, can be a potential alternative for filtering applications to be considered and studied more in depth in the future.