Integrated particle collision and turbulent diffusion model for dilute gas-solid suspensions

Abstract

A model is presented that predicts detailed flow characteristics of gas-particle suspensions in vertical flow. Particle-particle collisions and particle-turbulence interactions are modeled. The turbulence is represented by energetic eddies of characteristic size and decay time. The particle phase is discretized into multiple size/density fractions to capture the effects of particle size distribution, such as increased collision frequencies for solids of wide or bimodal distribution. Momentum and ‘fluctuating energy’ balances are derived for each particle fraction, and an additional balance gives the modulation of the gas turbulence intensity due to the particles. Several important phenomena are modeled, including the influence of drag on particle trajectories between collisions, and the impact of turbulence scale on particle motion. These dynamic effects often significantly influence suspension flow, but are not generally considered in complex two-phase flow models. The model has been adapted to predict characteristics of vertical suspension upflow for three important industrial applications: dilute pneumatic conveying, fully developed flow in the ‘core’ region of a CFB (circulating fluidized bed), and fully developed flow in the core of a FCC (fluid catalytic cracking) riser. Predicted fluctuating velocities are similar in magnitude to those measured in small-scale tests. Gas turbulence significantly affects the motion of small low density particles such as FCC catalyst, but is of secondary influence for the relatively large particles used in CFB combustors. Reductions in turbulence intensity are predicted with both riser scale-up and increasing temperature. The coefficient of restitution and particle size distribution are predicted to have significant influences in some cases.