A DNS Study of Aerosol Deposition in a Turbulent Square Duct Flow

Abstract

A particle-laden turbulent flow through a square duct was simulated using a direct numerical solution of the Navier-Stokes equations coupled with Langrangian particle tracking. Computations of particle transport were employed to elucidate the mechanisms by which particles with varying inertia deposit to the walls of a square duct. Gravity was neglected and a one-way coupling was assumed between the particles and the fluid. The computational results demonstrate that, although the aerosol penetration through a square duct is not significantly different than through a circular pipe, there exist differences in the transport and deposition mechanisms. Most notably, the off-axis secondary flows unique to the square duct preferentially deposit higher-inertia particles closer to the corners of the duct. By contrast, the same secondary flows act to suppress the deposition of lower-inertia particles to the duct corners by efficiently transporting them back towards the duct core before deposition can occur.

Particle deposition in turbulent square duct flows

“…For the Re b =10k flow, the largest particles preferentially deposit near the centre of the duct floor at all times, in contrast with the higher Reynolds number flows, and these particles are also never observed to deposit near the side walls. These findings are in line with those of Winkler et al (2006) and Phares and Sharma (2006).…”

“…072-256.32, noting preferential deposition at the wall centre with little in the duct corners. Results for this particular flow are also in line with those of Phares and Sharma (2006). Figure 3(a-c) shows results for particle deposition in the y-direction for the Re b =250k flow.…”

Voronoi analysis of beads suspended in a turbulent square channel flow

“…3) indicates that beads are absent near the side walls/corners corresponding to Yao and Fairweather (2012) who showed that particles having s þ p < 6:43 deposit near the channel bottom center. The latter is likely the result of secondary flows (Phares and Sharma, 2006;Yao and Fairweather, 2012;Rabencov et al, 2014). Note, the corresponding h# d i in the y + -x + plane is uniform (not shown).…”

“…Clustering in the y + -z + plane may be due to bead ''channeling'' caused by coherent hairpin packets (Soldati, 2005;van Hout, 2013;Rabencov et al, 2014) as well as secondary flows (Phares and Sharma, 2006). Hairpin packet length can be estimated by a slightly different Voronoï analysis by collapsing bead positions at consecutive time instants onto a single y + -z + plane.…”

“…As a result, particle accumulation in longitudinal streaks with transverse spacing of $100 wall units was observed (Kaftori et al, 1995). Only a few investigations on particle dispersion have been performed in square duct flows (Phares and Sharma, 2006;Yao and Fairweather, 2010;Rabencov et al, 2014) that are characterized by Prandtl's secondary motions of the 2nd kind. It has recently been shown that these are induced by streamwise, near wall coherent structures (Uhlmann et al, 2007;Pinelli et al, 2010).…”