Numerical Studies on Hydrodynamics in Various Circulating Fluidization Systems
Abstract
Various circulating fluidized bed (CFB) systems including gas-solid fluidization, liquid-solid fluidization, and gas-liquid-solid three-phase fluidization are numerically studied. With a comprehensive knowledge from the experiments, improved computational fluid dynamic (CFD) models are developed for detailed investigations on a wide operating range in the gas-solid CFB (GSCFB) system. The CFD model developed is also extended to study two new types of fluidized beds, an inverse liquid-solid circulating fluidized bed (ILSCFB) and a bubble induced fluidized bed (BIFB), as a supplement to the experimental work.
Flow structures and transitions from low-density operations to high-density operations in both GSCFB riser and downer are characterized based on numerical results and validated by experimental data. Correlations on the overall bed density in the GSCFB riser and downer under different operating conditions are developed respectively. The solid inlet geometry is found to have profound impacts on the flow structure in the GSCFB riser, which leads to the modifications on the inlet boundary conditions in the CFD model.
A cluster-driven drag model, which includes the information of clusters, is proposed for the simulation of the GSCFB riser. With more realistic physical meanings of the gas-solid interactions provided, a good agreement with the experimental results is also achieved. The cluster effects on the flow development and solids distribution are discussed based on the numerical results.
The CFD approach is also extended to study an ILSCFB system where light particles are used and validated by experimental results. The flow structures from the CFD simulations in the ILSCFB riser and downer are compared. CFD results show that the flow structure in the ILSCFB is more uniform compared with the GSCFB system. Numerical results also show that the binary particle system in the ILSCFB shares many similarities with the single-particle system.
A three-phase Eulerian-Eulerian CFD model is developed and validated by the experimental results for a newly invented BIFB. Three flow regimes and the corresponding transition gas velocities in the BIFB are defined based on the experimental and numerical results. Effects form the particle density, solids loading, and superficial gas velocity are also studied.