(232b) Gas-Solid Two-Phase Turbulent Flow in a Circulating Fluidized Bed Riser: an Experimental and Numerical Study

Gas-solid fluidization using circulating fluidized beds has found widespread application in industry, e.g. in fluid catalytic cracking (FCC) and CFB combustors. However, the behaviour of the complex flow structures emerging in this type of turbulent two-phase flow is still not fully understood, despite their importance on the overall reactor performance. In this work the hydrodynamics of gas-solid two-phase turbulent flow in a riser has been studied by detailed experiments using a non-intrusive optical measurement technique and detailed simulations with a 3D discrete particle model (DPM) in a well-defined geometry. In a pseudo two-dimensional circulating fluidized bed constructed of lexane (0.05 m x 1.5 m x 0.015 m) spherical glass beads (2500 kg/m3) with an average diameter of 335 micron (250-420 micron) were fluidized with air. Snaphots of the flow were taken by a high-speed CCD camera showing the formation, growth, motion and break-up of clusters. Using a particle image velocimetry (PIV) technique (following Bokkers et al., 2003) the mean velocity and the RMS velocity of the particles were obtained (see Figure 1). The influence of the superficial gas velocity and the solids flux on the flow patterns at different axial positions has been investigated. From the experimental results it could be concluded that from 0.8 m above the bottom distributor plate the core-annulus flow structure has become fully developed. Furthermore, the gas-solid two-phase flow in the riser has been simulated with a 3D, hard sphere, discrete particle model, applying a large eddy simulation (LES) to account for the influence of gas-phase turbulence using the recently proposed SGS model by Vreman (2004). Simulations were performed either with a fixed solids influx at the bottom, which allowed investigation of the developing flow regime, or with a fixed overall solids fraction and applying periodic boundary conditions, which is used to study the fully developed flow regime. The movement and patterns of the clusters and the corresponding mean and RMS particle velocities obtained from the simulations for different superficial gas velocities and solids fluxes were compared with the experimental findings showing good resemblance. It was found that the superficial gas velocity has a strong influence on the axial solids velocity, but only a minor influence on the extent of solids downflow. The solids flux has a large effect on the extent of solids downflow, but only little influence on the lateral profile of the RMS axial solids velocity.

Key Words: Circulating fluidized bed riser, PIV, discrete particle model, turbulence.

References: Bokkers, G.A., Van Sint Annaland, M. and Kuipers, J.A.M. (2004). Mixing and segregation in a bi-dispersed gas-solid fluidized bed: a numerical and experimental study, Powder Tech., 140, 176-186

Vreman, A.W. (2004). An eddy-viscosity subgrid-scale model for turbulent shear flow: Algebraic theory and applications. Phys. Fluids, 16, 3670-3681