

Article
Aspect ratio effect on particle transport in turbulent duct flows
AbstractThe dynamics of dilute micronsized spherical inertial particles in turbulent duct flows is studied by means of direct numerical simulations of the carrier phase turbulence with oneway coupled Lagrangian particles. The geometries are a square and a rectangular duct with widthtoheight aspect ratio AR of 3 operating at Ret,c = 360 (based on the centerplane friction velocity and duct halfheight). The present study is designed to determine the effect of turbulencedriven secondary motion on the particle dynamics. Our results show that a weak crossflow secondary motion significantly changes the crosssectional map of the particle concentration, mean velocity and fluctuations. As the geometry of the duct is widened from AR = 1 to 3, the secondary vortex on the horizontal wall significantly expands in the spanwise direction, and although the kinetic energy of the secondary flow increases close to the corner, it decays towards the duct centreplane in the AR = 3 case so as the turbulent carrier phase approaches the behavior in spanwiseperiodic channel flows, a fact that significantly affects the particle statistics. In the square duct the particle concentration in the viscous sublayer is maximum at the duct centreplane, whereas the maximum is found closer to the corner, at a distance of z/h ? 1.25 from the centreplane, in the AR = 3 case. Interestingly the centreplane concentration in the rectangular duct is around 3 times lower than that in the square duct. Moreover, a second peak in the accumulation distribution is found right at the corners for both ducts. At this location the concentration increases with particle inertia. The secondary motion changes also the crossstream map of the particle velocities significantly in comparison to the fluid flow statistics. These directly affect the particle velocity fluctuations such that multiple peaks appear near the duct walls for the particle streamwise and wallnormal velocity fluctuations.

