Numerical simulation of loop seal for a circulating fluidized bed based on an improved EMMS bubbling model

• A structure-dependent drag coefficient is calculated from an EMMS bubbling model.
• The drag coefficient is integrated into the TFM approach to simulate a loop seal.
• The EMMS bubbling scheme reproduces heterogeneous structures in the loop seal.
• The drag correction is based on the regional division of computational domain.
• Parametric effects on gas inverse flow in the loop seal are simulated successfully.

A typical loop seal consists of a supply chamber and a recycle chamber as well as a bottom connection slit in between. The supply chamber is generally operated in the minimum fluidization state, while the recycle chamber in the bubbling fluidization regime. The energy-minimization multi-scale (EMMS) model, as generally used to calculate interphase drag coefficient in previous simulations of the loop seal, is still open to discussion since it is based on the concept of particle clusters. So an EMMS bubbling model was developed to model dense gas–solid flow by taking into account the interphase interaction between the emulsion and bubble phases. In this article, a heterogeneity index is calculated from an improved EMMS bubbling model to measure the interphase drag coefficient and further integrated into the two-fluid-model (TFM) approach to simulate the loop seal for a circulating fluidized bed (CFB) system. This index is dependent on superficial gas velocity in bubbling fluidization, so a region-specific drag correction scheme is proposed to allow the application of various heterogeneity index correlations to different zones of the loop seal, since superficial gas velocity may differ much from the recycle to supply chamber. Several typical cases of drag correction are comparatively investigated by applying various gas velocities and/or drag models in the recycle and supply chambers, and the optimal one is proven to be able to reasonably simulate the parametric effects on gas inverse flow in the loop seal.

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