The simulation and experimental validation on gas-solid two phase flow in the riser of a dense fluidized bed

Abstract

Gas-solid flow in dense CFB (circulating fluidized bed)) riser under the operating condition, superficial gas 15.5 m/s and solid flux 140 kg/m2s using Geldart B particles (sand) was investigated by experiments and CFD (computational fluid dynamics) simulation. The overall and local flow characteristics are determined using the axial pressure profiles and solid concentration profiles. The cold experimental results indicate that the axial solid concentration distribution contains a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. The typical core-annulus structure and the back-mixing phenomenon near the wall of the riser can be observed. In addition, owing to the key role of the drag force of gas-solid phase, a revised drag force coefficient, based on the EMMS (energy-minimization multi-scale) model which can depict the heterogeneous character of gas-solid two phase flow, was proposed and coupled into the CFD control equations. In order to find an appropriate drag force model for the simulation of dense CFB riser, not only the revised drag force model but some other kinds of drag force model were used in the CFD. The flow structure, solid concentration, clusters phenomenon, fluctuation of two phases and axial pressure drop were analyzed. By comparing the experiment with the simulation, the results predicted by the EMMS drag model showed a better agreement with the experimental axial average pressure drop and apparent solid volume fraction, which proves that the revised drag force based on the EMMS model is an appropriate model for the dense CFB simulation.

Wavelet enhanced visualization of solids distribution in the top of a CFB

“…As in our case the value 256 corresponds approximately to loosely packed solids concentration of 57%, the contour values of 60-80 indicate the highest local solids concentration of 13.4%, and occasionally 17.8%, respectively in a very small region just below the exit of the riser, albeit the average solids concentration remains relatively low over the cross section. A recent investigation by Wang et al [23] on a dense fluidized bed reactor also indicated a rather high solids concentration in the upper part of the riser, due to the reflux effect, with even higher values on the wall than the ECT reconstructed values, which shows similar features of a CFB riser as used in this study.…”

Flow Phenomena in the Riser of a Circulating Fluidized Bed

“…Circulating fluidized beds (CFB) are applied to a wide area of application, such as powder, combustion, gasification, calcinations and catalytic cracking [1][2][3][4]. With the development of the computer technology, the computational fluid dynamics (CFD) has developed rapidly, which provide low-cost and high efficient design method [5][6][7].…”