Statistical Analysis On Particles In Gas-solid Two-phase Flow Using Fully-resolved DNS And Research About Grid Resolution

Gas-solid two-phase flow is widely existed in nature and the industrial production processes.It is very important to study the mechanism of the interaction between particles and fluid by the numerical simulation,as well as the heat and mass transfer processes during gas-solid transportation.Fully resolved direct numerical simulation(FR-DNS)method considers the finite volume of the particles and does not use any closed models for interphase interactions.The interaction forces are solved by integrating the shear stress on the surface of particles,which can be used not only to study microscopic mechanism of the two-phase flow,but also to sum up the laws and models of the interaction between two phases through statistical analysis of the data.Further,these laws and models are able to be used to improve the traditional two-fluid method(TFM)or discrete element method(DEM).Thus FR-DNS method has been paid more and more attention.However,even using the embedded Euler mesh algorithm,the huge grid consumption and computational cost still can't be neglected.The contents of this thesis can be divided into the following three parts.Firstly,fully-resolved DNSs of three systems under the condition of a laboratory,scale gas-solid bubbling bed which contains respectively one particle,two particles and 9240 particles are carried out by using the multi-direct forcing scheme with the immersed boundary method,and statistical analysis is performed on samples on time series.Multimodal distribution is showed on the horizontal and vertical movement of particles in the multiple-particle system in contrast to other two systems,which could be caused by particles' movement in the gas-solid bubbling bed.The peak values coincide with the bubble formation,rising,cracking and the particles falling after bubbles burst.However,quantitative statistics can't be continued because of its huge computational cost,it is needed to develop a new method to reduce the resolution of IBMs.Secondly,the sharp interface IBMs are compared with the traditional IBM,especially the dependence on the grid resolution.It is found that the equivalent diameter is closer to the actual size of particles using the sharp interface IBMs in the simulation process,so that sharp interface IBMs are less dependent on the mesh resolution than the non-sharp surface IB methods.At the same time,it is found that the best retraction distance of Lagrangian nodes is not easy to be determined.Finally,based on the "HD forcing strategy" IBM,this paper proposes a method to determine the most suitable retraction distance of Lagrangian nodes.The method can be applied to simulate flow past a fixed spherical(Reynolds number is from 30 to 400)and sedimentation of one particle with coarse grids(d/h = 10,15).It is shown that this method is more effective for the calculation of the static and moving particles with coarse grids.