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The effect of fluid turbulence models on the coarse-grid simulation for the mesoscale gas-solid flow system is not as clear as that for gas-liquid/single flows. In this study, the effect of different turbulence models on predictions of riser-only, turbulent and bubbling flow hydrodynamics is quantified. Then the selected turbulence models are examined by coarse-grid simulations in a full-loop circulating fluidized bed (CFB) with a proposed hybrid drag model which integrates two SGMs and Hulin-Gidaspow model. Results show that the standard k -epsilon and k -omega models fail to capture a correct fluidization pattern in riser. Meanwhile, the RNG and realizable k -epsilon models and SST k -omega model enable satisfactory predictions which are highly comparable to discrete model predictions and experiments over different flow regimes. However, the realizable k -epsilon model produces more pronounced flow fluctuations causing computational instability. Using superior turbulence closures, full-loop simulations with the proposed hybrid model can predict desirable hydrodynamics. Unlabelled Image • A hybrid drag model is proposed for feasible coarse-grid full-loop CFB simulations. • Applicability of different turbulence models over various fluidization regimes. • Validated against the discrete particle model and experiments. • The RNG k -epsilon and SST k -omega are optimal for gas-solid flows. [ABSTRACT FROM AUTHOR]

The gas-solid fluidized bed is a complex multiscale system with diverse mesoscale structures such as bubbles and particle clusters. The Euler-Euler two-fluid model is one of the most popular methods in simulating this kind of system. However, when using coarse grids, the mesoscale structures involved can hardly be characterized. In this study, a new quasi-three-phase method was proposed to properly account for the mesoscale structures by using three phases (bubble phase/solid phase/emulsion phase) to represent the gas-solid fluidized bed, and comparative simulations were performed under different gas velocities. It was found that the simulation could reasonably predict the flow changes from bubbling fluidization to turbulent fluidization. The predicted gas velocity at the flow pattern transition is close to that calculated by the empirical equation concerning the bed pressure fluctuation, and the predicted solids fraction distribution fits well with the experimental data in the literature. [Display omitted] • Both bubbles and particle clusters are considered simultaneously. • Application of three-fluid model in gas-solid two-phase flow is feasible. • Three-fluid model reveals well the flow pattern transition of FBR. • The simulation using the classical drag model can match the experimental data. [ABSTRACT FROM AUTHOR]