(264a) Numerical Simulation of Dense Gas-Solid Flow in a Loop Seal Based on an Improved EMMS Bubbling Model

A loop seal, as one of the most widely used non-mechanical valves in circulating fluidized beds (CFB), serves as a recycle device allowing the transfer of the solids from the high pressure standpipe to the low pressure riser, but preventing the inverse flow of the gas from the riser bottom to the standpipe. A typical loop seal consists of a supply chamber and a recycle chamber as well as a bottom connection opening or slit in between. In general, the supply chamber is operated in the minimum fluidization state, while the recycle chamber is fluidized at low gas velocities and run in the bubbling fluidization regime.

Traditional two-fluid-model (TFM) approach, as the most frequently used simulation technique for gas-solid flow, generally assumes homogeneous conditions inside a control volume, hence giving rise to the calculation deviation of interphase drag force and further to the numerical inaccuracy of multi-phase simulation. Thereby, so-called energy-minimization multi-scale (EMMS) scheme was used to simulate local heterogeneous characteristics in the CFB system with a loop seal, in which a structure-dependent drag coefficient is calculated from the EMMS model and then integrated into the TFM approach by utilizing a user defined function (UDF). However, in the loop seal the particles represent the continuous dense emulsion phase with the formation of discrete bubble-like voids, so the EMMS scheme is still open to discussion since it is based on the concept of particle clusters.

In low-velocity fluidization, gas bubbles generally begin to form as a discrete phase. A gas-solid bubbling reactor can thus be resolved into three sub-systems including the dense emulsion phase, the dilute bubble phase and the inter-phase in between. By considering both the normal pressure stress and tangential drag force between the emulsion and bubble phases, the authors proposed a stability condition for gas-solid bubbling fluidization and formulated an EMMS bubbling model. This model has been proved to be able to reasonably simulate dense gas-solid flow without introducing bubble-specific empirical correlations.

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 TFM approach to simulate the loop seal for a CFB system. In bubbling fluidization this index is dependent on superficial gas velocity, so a specific drag correction scheme based on the regional division of computational domain is proposed to allow the application of various heterogeneity index correlations to different regions of the loop seal, since superficial gas velocity may differ much from the recycle to supply chamber because of the variation of superficial gas velocity itself as well as the resultant effect of the circulated solids on the actual gas flow in the loop seal. According to the uncertainty of gas flow pattern in the loop seal, several typical cases of drag correction are comparatively investigated by applying various gas velocities and/or drag models in the recycle and supply chambers. The simulation results indicate that the drag coefficients in the recycle and supply chambers should be calculated from the EMMS bubbling drag model at their respective superficial gas velocities. By incorporating with the TFM approach, the preceding optimal scheme is proved to be able to reasonably predict the parametric effects on gas inverse flow in the loop seal with an inclined internal baffle.