Characterization of Dynamics of Binary Gas-Solid Flows in a Pseudo-3D Fluidized Bed: Measurements and CFD Simulations
8th, May 2019, Wednesday, 4:30pm, ChE Seminar hall
Abstract Several industrial processes employing gas-solid fluidized beds often involve a mixture of particles differing in size, density, and shape (e.g., coal and inerts, coal and ash, biomass and inert used in gasification process). The difference in the physical properties (size, density, and shape) of these particles give rise to non-uniform mixing and segregation which leads to the formation of hot-spots and agglomerates (e.g. clinker formation), thus deteriorating the performance of the gasifier. Therefore, the performance of these industrial processes is influenced by the local mixing of the gas and solid phases, which is primarily governed by the local fluidization/bubbling behaviour. Therefore, it is important to develop Computational Fluid Dynamics (CFD) models capable of predicting the bubbling characteristics, mixing and segregation phenomena of binary gas-solid flow accurately. As the flow is inherently unsteady the CFD models should be validated with temporally- and spatially-resolved measurements. Such models can then be extended to predict the local temperature and concentration distributions in gasifiers and eventually to optimize the performance of such fluidized bed gasifiers/reactors. Therefore, the objective of the present work is to characterize the dynamics of the binary gas-solid flow of particles differing in density and shape using temporally- and spatially-resolved phase hold-up measurements and to develop experimentally validated CFD model to simulate the binary gas-solid flow under cold-flow conditions.
The experiments are performed in a pseudo-3D (rectangular) fluidized bed using High- Speed Imaging technique. The instantaneous and local phase hold up measurements are obtained by Digital Image Analysis. The effects of the bed compositions and superficial gas velocities on the dynamics of fluidization of unary and binary gas-solid flow with particles differing only in density (of the same size) are investigated. With the increase in the mass fraction of the lesser dense particles in the binary mixture, the bubbling frequency and the bubble size were found to increase. Due to this, a higher recirculation of the particles around the bubble and in the bubble wake is observed leading to a higher mixing index in the case of binary gas-solid flow dominated by lesser dense particles. The detailed analysis of the dynamics of mixing and segregation will be presented in the seminar. These experimental measurements help to understand the segregation and mixing phenomena of binary gas-solid flow and provide the database for the validation of CFD simulations.
Further to predict the mixing and segregation rates and bubbling characteristics (bubble size and frequency), Eulerian simulations of unary and binary gas-solid flows are performed. The effect of several closure models for gas-solid drag and solid shear stresses (solid pressure) on the bubble size distribution are investigated. From the Two-Fluid Eulerian (TFM) simulations of the gas-solid flow of two unary systems, it was found that the gas-solid drag has a significant influence on the predictions of the solid dispersion height and bubble size distribution than solid pressure model. However, reasonable differences were found in the predicted bubble size distribution at higher velocity. Therefore, further investigations of the gas-solid drag model are being carried out. The detailed analysis of the contribution of different closure models on the predictions of the bubbling characteristics, mixing and segregation phenomena will give a comprehensive understanding of the effect of different forces on the dynamics of the binary gas-solid flow.