Study On A General Gas-solid Drag Model Based On Generalized Elastic Modulus

The gas-solid two-phase flow system in a circulating fluidized bed is characterized by the dilute phase composed of gas and dispersed particles and the dense phase composed of gas and cluster,it is a typical non-homogeneous nonlinear system,and the presence of cluster and it's a series of dynamic processes of generation,growth,and rupture have a significant impact on the flow and heat transfer process of fluidized bed.Therefore,it is of great significance to deeply understand and study the two-phase flow mechanism for its industrial application.In the current gas-solid two-phase flow numerical simulation study,the Euler-Euler two-fluid model is widely used in industrial-scale simulation studies because of its high computational efficiency and better grid adaptability.However,due to its assumption of particle continuity,the gas-solid interphase drag model is used to describe the gas-solid interaction and momentum exchange.The accuracy of the results depends to a large extent on the accuracy of the phase-to-phase drag model.A large number of studies have shown that particles in high-mass flow rate gas-solid two-phase flow systems tend to agglomerate to resist gas entrainment.The presence of such non-homogeneous structures will have a greater impact on gas-solid inter-phase drag.The multi-scale energy dissipation minimum theory developed in recent years has closed the equations by multi-scale partitioning of non-uniform systems and adopting stable extreme conditions where the multi-scale energy consumption tends to be minimum to solve the gas-solid drag coefficient.It can effectively describe the typical characteristics of non-homogeneous drag and become the hotspot of studying non-homogeneous drag.Based on the two-fluid model and the multi-scale energy dissipation minimization theory,this paper considers the impact of inter-particle collisions at high mass flow rates,two different empirical formulas of the generalized modulus of elasticity are selected.The momentum transfer effect due to inter-particle collisions is characterized by the introduction of the solid-phase pressure term described by the soild-phase generalized elastic modulus function.And the introduction of dilute and dense phase particle collisional dissipation terms in the multi-scale energy dissipation analysis to consider the energy dissipation effect due to inelastic collision between particles,eventually established a general gas-solid drag model based on generalized elastic modulus.By using the established drag model,the flow characteristics in the riser are simulated.By comparing with the experiment and the Gidaspow drag model,the results better reflect the S-type distribution along the axial direction of the tube and the non-uniform characteristics along the radial direction,verifying the accuracy of the established model.At the same time,the instantaneous characteristics and the time average state parameters of the agglomerate structure are obtained.The relationship between the diameter of the cluster and the concentration of the local particles is revealed.The higher or lower concentration of the particles will inhibit the increase of the cluster diameter,and the difference of the empirical correlations between the two different generalized solid modulus of elastic modulus is compared in detail,the drag model 1 more accurately describes the phenomena of increased particle collision due to the increase of particle concentration and the enhancement of the cluster effect in the flow field,which is reflected by the increase of particle collision energy consumption.Secondly,through the construction of a two-dimensional complete circulating fluidized bed,the flow and cluster characteristics under different working conditions are simulated and analyzed.By comparing with the experimental values,the results show that the diameter of the cluster and the generating frequency in the three conditions show a gradual increase in the low edge wall of the central region,and with the mass flow rate increasing the diameter of the cluster and the frequency of formation.In addition,When 277 /()SG(28)kg m s,the predicted value is closest to the experimental value.At the same time,through the analysis of multi scale energy consumption,the formation mechanism of the cluster structure under the action of different control mechanisms in the grid is revealed.Finally,using the fixed wall temperature condition analysis to study the heat transfer characteristics of the tube considering the cluster structure,the distribution of the temperature field inside the tube using the drag force model 1 and the Gidaspow drag model is simulated and compared.The results show that the temperature field distribution obtained from the drag force model 1 considering the cluster structure is more uniform in both radial and axial directions than the Gidaspow model.And the intense movement of agglomerates at the wall accelerates the renewal of adherent high-temperature particles and surrounding cryogenic particles,and maintains a large heat-transfer temperature difference between the particles and the wall surface.