Choosing structure-dependent drag coefficient in modeling gas-solid two-phase flow

Abstract

Experiments have shown that various structures (e.g., the streamer, cluster, void and bubble) can be clearly observed in gas–solid fluidized beds. These structures affect the overall behavior of flow, mass/heat transfer and reactions significantly and should be accounted for in simulations of gas–solid flow. For the purpose of riser flow simulation, the structure-based drag model has already been proposed based on the description of meso-scale clusters, and incorporated into the framework of a two-fluid model to calculate the effective drag force. However, there is still no clear agreement about the characterization of clusters, such as the cluster shape, size and orientation. In contrast, it is easier to describe and measure the characteristic size of visible bubbles. Here, we attempt to upgrade the bubble-based EMMS model which was established for bubbling fluidized beds originally, and then extend it to simulations of heterogeneous gas–solid flows in CFB risers. Numerical analysis on this new model reveals that it is capable of predicting two turning points, where the flow state is transforming rapidly from uniform to heterogeneous distribution. The predicted voidage in the dense phase is qualitatively consistent with experimental data in the literature. To evaluate this new drag, finally, CFB riser simulations have been carried out by coupling of the two-fluid model and the bubble-based EMMS drag. The results show good agreement with experimental data.