Analysis of the energy-minimization multiscale model with multiobjective optimization

Gas-solid two-phase flow is ubiquitous in nature and many engineering fields,such as chemical engineering,energy,and mining.The closure of its hydrodynamic model is difficult owing to the complex multiscale structure of such flow.To address this problem,the energy-minimization multi-scale(EMMS)model introduces a stability condition that presents a compromise of the different dominant mechanisms involved in the systems,each expressed as an extremum tendency.However,in the physical system,each dominant mechanism should be expressed to a certain extent,and this has been formulated as a multiobjective optimization problem according to the EMMS principle generalized from the EMMS model.The mathematical properties and physical meanings of this multiobjective optimization problem have not yet been explored.This paper presents a numerical solution of this multiobjective optimization problem and discusses the correspondence between the solution characteristics and flow regimes in gas-solid fluidization.This suggests that,while the most probable flow structures may correspond to the stable states predicted by the EMMS model,the noninferior solutions are in qualitative agreement with the observable flow structures under corresponding conditions.This demonstrates that both the dominant mechanisms and stability condition proposed for the EMMS model are physically reasonable and consistent,suggesting a general approach of describing complex systems with multiple dominant mechanisms.

Flow characteristics simulation of spiral coil reactor used in the thermochemical energy storage system

According to environmental and energy issues,renewable energy has been vigorously promoted.Now solar power is widely used in many areas but it is limited by the weather conditions and cannot work continuously.Heat storage is a considerable solution for this problem and thermochemical energy storage is the most promising way because of its great energy density and stability.However,this technology is not mature enough to be applied to the industry.The reactor is an important component in the thermochemical energy storage system where the charging and discharging process happens.In this paper,a spiral coil is proposed and used as a reactor in the thermochemical energy storage system.The advantages of the spiral coil include simple structure,small volume,and so on.To investigate the flow characteristics,the simulation was carried out based on energy-minimization multi-scale model(EMMS)and Eulerian two-phase model.CaCO_(3) particles were chosen as the reactants.Particle distribution was shown in the results.The gas initial velocity was set to 2 m·s^(-1),3 m·s^(-1),and 4 m·s^(-1).When the particles flowed in the coil,gravity,centrifugal force and drag force influenced their flow.With the Reynold numbers increasing,centrifugal and drag force got larger.Accumulation phenomenon existed in the coil and results showed with the gas velocity increasing,accumulation moved from the bottom to the outer wall of the coil.Besides,the accumulation phenomenon was stabilized whenφ>720°.Also due to the centrifugal force,a secondary flow formed,which means solid particles moved from the inside wall to the outside wall.This secondary flow could promote turbulence and mixing of particles and gas.In addition,when the particle volume fraction is reduced from 0.2 to 0.1,the accumulation at the bottom of the coil decreases,and the unevenness of the velocity distribution becomes larger.

A computational particle fluid-dynamics simulation of hydrodynamics in a three-dimensional full-loop circulating fluidized bed: Effects of particle-size distribution

A computational particle fluid-dynamics model coupled with an energy-minimization multi-scale(EMMS)drag model was applied to investigate the influence of particle-size distribution on the hydrodynamics of a three-dimensional full-loop circulating fluidized bed.Different particle systems,including one monodisperse and two polydisperse cases,were investigated.The numerical model was validated by comparing its results with the experimental axial voidage distribution and solid mass flux.The EMMS drag model had a high accuracy in the computational particle fluid-dynamics simulation of the three-dimensional full-loop circulating fluidized bed.The total number of parcels in the system(Np)influenced the axial voidage distribution in the riser,especially at the lower part of the riser.Additional numerical simulation results showed that axial segregation by size was predicted in the two polydisperse cases and the segregation size increased with an increase in the number of size classes.The axial voidage distribution at the lower portion of the riser was significantly influenced by particle-size distribution.However,radial segregation could only be correctly predicted in the upper region of the riser in the polydisperse case of three solid species.

CFD simulation of the effects of a suspension section on the hydrodynamics of a circulating fluidized bed

Gas-solid two-phase flow in a circulating fluidized bed (CFB) is affected by operating conditions (e.g., superficial gas velocity, solids inventory), material properties and geometric factors, such as the entry and exit configuration. In particular, the suspension section, which is located between the riser bottom and the solids recycle inlet, affects the hydrodynamics in the riser significantly. However, the suspension section has received less attention compared with other geometric factors. Most computational fluid dynamics (CFD) simulations, especially two-dimensional simulations do not take this factor into account. We performed three-dimensional, full-loop CFD simulations with a drag coefficient that was determined by the energy-minimization multi-scale model, and investigated the flow behavior of two CFBs with different suspension-section lengths. The simulation resuits revealed that the axial profiles of voidage in the riser with a longer suspension section are more likely S-shaped, whereas those with shorter suspension sections decay exponentially. The dependences of solids flux on solids inventory differ in the two CFBs. A shorter suspension section may result in a smooth transition from dilute to dense transport without intermediate accumulative choki-ng, whereas a Ion ger suspe nsion section may lead to a choking transition. These simulation results are qualitatively consistent with the flow behaviors described in literature.