G.I. Palchonok

Gas and solids flow in circulating fluidized beds : discussion on turbulence

This work discusses the presence of gas-phase turbulence in the flow taking place in the circulating fluidized-bed riser. The experimental basis consists of measurements of pressure fluctuations and gas mixing by tracer-gas injection. Several observations, such as a power-law fall-off in the power spectrum of the pressure fluctuations, point at the existence of turbulent structures in the gas flow. The dense bottom bed is of importance because of high local gas velocities through the bubbles passing the bed. The bubbles and consequently the perturbations generated, are unevenly distributed in space and time. Experimental data of gas diffusivity are compared to published data on diffusivity in single-phase turbulent flows.

Estimates of turbulence mechanisms in circulating fluidized bed combustors

Estimates of the characteristic time scales (particle relaxation time, eddy-particle interaction time, interparticle collision time), and the energy dissipation mechanisms are performed together with a turbulent kinetic energy budget, for a simple equilibrium shear flow with conditions similar to those of a circulating fluidized bed (CFB) combustor. Based on dimensional analysis, the influence of five parameters is investigated: the Eulerian integral length scale in the gas phase, the gas-particle density ratio, the mean velocity gradient, the particle diameter, and the particle volume fraction. The results show significant differences between particles of Geldart groups A and B regarding response to gas phase turbulence, transition from dense to dilute suspensions, pressure and temperature dependence, and also energy dissipation mechanisms.