(207b) The Effect of Boundary Slip and Reynolds Number on Gas-Solid Drag on Multiple Particles

When particles, such as sand grains, are entrained in a fluid with nonzero relative velocity, the drag force on a given particle is dependent upon the local particle concentration. This phenomenon is well-studied for the case of very small Knudsen numbers (Kn < 0.001), when no-slip boundary conditions are valid at the particle surface. Here, Kn is defined as the ratio of the mean free path of the gas molecules to the particle diameter. As the gas becomes rarefied and/or particles are very fine, the drag force becomes a function of Kn, and the effect of particle concentration on the drag force has not been studied. The goal of this study is to investigate the drag force on particle arrays at moderate Knudsen and low-to-moderate Reynolds numbers (0.01 < Kn < 0.1; 0 < Re < 10) via lattice Boltzmann simulations. Characterization of the drag force in this regime is important for applications such as control of airborne pollutants (e.g. PM 2.5), particle emission in combustion and propulsion, and extraterrestrial dust transport induced by jets from landing spacecraft or storms. Using a first-order slip boundary condition at the particle surface, the drag force is determined for three-dimensional simple cubic, body-centered cubic, and face-centered cubic cells. For the case of moderate Knudsen and very low Reynolds numbers, the drag force is a only a function of solids fraction and Knudsen number. However at moderate Reynolds numbers, the drag force is also a quadratic function of Re (Re<10), similar to the no-slip case. For the case of no-slip, the drag force increases with solids concentration. With slip, as Knudsen number increases, the drag on multiple particles decreases and tends towards that of a single particle.