Meso-scale Analysis And Improvement Of Fluidized Heterogeneous Drag Model Based On EMMS Theory

In order to achieve the accurate simulation,prediction and process optimization of the circulating fluidized bed by improving the numerical simulation method of large scale fluidization,it is necessary to deeply study the lawofcomplexnon-equilibriummulti-scaleheterogeneousgas-solid fluidization process.Based on the previous study of the drag model of our research group,the influence of meso-scale structure on the heterogeneous drag between the gas and solid phase is further studied,to further improve the calculation accuracy of the QC-EMMS drag model.The model can be used to predict the unknown cases by establishing the relationship between the macroscopic parameters of different operating conditions and the drag reduction of fluidized processes.In this paper,we first develop the 3D simulation of a circulating fluidized bed by QC-EMMS drag model.Based on the accurate simulation,the meso-scale structure characteristics is deeply studied,including the pulsating characteristic,the time characteristic and the scale evolution and so on.Compared with other EMMS based drag models,the accuracy of the QC-EMMS heterogeneous drag model is higher.The key point is that the proposal of cluster solid concentration equation.In this paper,two series of dilute and dense CFB experiments were used to validate our drag model.The time series of local particle concentration pulsation was measured by optical fiber probe.In addition,we develop a mathematical model of the coefficient n of cluster distinguishing.The coefficient n should not be a fixed value,but a single peak distribution with the local concentration.In view of the simulation inaccuracy of the dilute fluidized bed by the QC-EMMS drag model.Through analysis the parameters of the cluster affecting the drag force directly in the EMMS theory,we notice that one of the parameters–the volume fraction of dense phase‘f'is unreasonable in the dilute area,which should tend to 0 instead of 1.Through improve the parameter‘f'to reasonable,the drag model was improved.The improved QC₁.1-EMMS drag model can improve the computational accuracy obviously.Finally,in order to obtain the generalized applicability of the drag model at different fluidized conditions and the prediction of the unknown conditions,the generalization of the heterogeneous drag model was constructed.Based on the analysis of the transition of different flow patterns of fluidized beds and their overall heterogeneity,it is found that the overall heterogeneousity index?in the drag model should show a single peak distribution with the overall particle concentration?_s,bed,bed of the bed.Then,through the accurate simulation of a large number of cases,it is verified that a single peak is true and the peak is located near?_s,bed=0.12.Thus,the generalized mathematical model is constructed.