Verification Of The Stability Condition Of EMMS Model By Direct Numerical Simulation

Gas-solid fluidization systems are typically non-liner and non-equilibrium systems,which may have complex spatio-temporal multi-scale structures,therefore multi-scale methods are necessary to study these systems.The energy-minimization multi-scale(EMMS)model has been widely used in gas-solid fluidization systems in which mesoscale structures and stability conditions play important roles.As the stability condition is closely related to the energy consumptions of the system,in order to explore the mechanism of the stability condition and its quantitative relationship with energy consumptions,this work uses the direct numerical simulation(DNS)based on immersed boundary method to analyze the heterogeneous structures and verify the stability condition in the EMMS model,viz.,the mass specific energy consumption for suspending and transporting the solids,Nst tends to be a minimum.Compared with the earlier simulation work based on pseudo-particle modeling,this work achieves the following progresses.First,the simulation based on DNS can retain the characteristics of the conservation equations and have higher accuracy,in this way we could carry out the energy consumption statistics in a stricter sense and simulate gas-solid fluidization systems with real physical properties.On the other hand,the acceleration effect is included in the equations of the force balance and the stability condition and different initial distributions of particles are considered to make the conclusion more comprehensive and persuasive,which also result in the more precise statistics of the energy consumptions.The main contents of this thesis are as follows:Chapter 1 introduces the gas-solid fluidization systems,reviews the research results of experiments,empirical formulas and numerical simulation methods.The merit and demerit of three common numerical simulations including two-fluid model,Eulerian-Lagrangian method and DNS are illustrated in order to explain why we choose DNS to study the gas-solid fluidization.Chapter 2 introduces the DNS method used in this paper in detail,including the solver for the continuous fluid,particle movement process and particle-fluid coupling.The reliability of the DNS solver is verified by comparing the simulated results with the data in literature for flow past a single particle and a periodic array of particles.Chapter 3 considers the influence of acceleration in the statistics of Ust in the EMMS model.The gas-solid fluidization systems with different initial distributions and different inlet fluid velocities are simulated with the DNS method.The results show that Ust evolves to be a minimum,as well as Nst,then the stability condition of the EMMS model is verified.Chapter 4 summarizes the main results in this thesis and gives a perspective for the future work.The verification work in this thesis still uses the indirect method which counts Ust rather than Nst.In the future,as the energy consumptions in the EMMS model correspond better with those in the dynamic simulation,it is promising to verify the stability condition in the EMMS model by counting Nst directly with the DNS method,thus revealing the relationship between mesoscale structures and energy consumptions in-depth,which deserves long-term efforts towards this research direction.