Fully-resolved Direct Numerical Simulation Of Inter-phase Heat Transfer In Gas-solid Two-phase Turbulent Boundary Layer

A ghost-cell based high-order immersed boundary method with special treatment of moving boundary is developed in the present thesis,in order to fully resolved direct numerical simulate(DNS)hydrodynamic and thermal interaction between finite-size particles and the fluid.The newly developed method stands various tests and shows its high efficiency.It also has been proven to achieve high-order accuracy in space,be capable of handling Dirichlet and Neumann boundary conditions consistently,as well as treating problem with thermal effect precisely.By simulating forced convection from a cluster consists of 20 spherical particles,it is found that clustering blocks heat exchange between particles and the fluid,while hydrodynamic behavior of the cluster can be modeled by an isolate particle with the same equivalent diameter.The simulation of a particle settling while cooling shows that heat transfer and flow pattern are remarkably altered by bouyancy induced natural convection,especially when the Richardson number is high.The high-order immersed boundary method is furtherly used to simulate hundreds of freely moving particles hydrodynamically and thermally interacting with homogeneous isotropic turbulence,to investigate influences of particle-to-fluid density ratio,turbulence intensity,dimensionless temperature fluctuation and solid volume fraction,on frictional drag coefficient and inter-phase heat transfer Nusselt number.In contrast to a single stationary particle under turbulence,it is the velocity fluctuation that plays a leading role on instantaneous inter-phase heat transfer in particle-laden flow,while the effect of temperature fluctuation is relatively small.It also turns out that,the smaller the particle-to-fluid density ratio the higher the average Nusselt number,the higher the solid volume fraction the average Nusselt number.Furthermore,by comparing DNS results with predictions from generally available correlations,it turns out that these empirical correlations on drag and heat transfer coefficients validate only when the particle Reynolds number is relatively high.And the DNS research indicates that better estimates can be obtained only if local flow and thermal conditions are also taken into consideration.Finally,non-isothermal turbulent boundary layer with thousands of finite-size spherical particles is direct numerically simulated for the first time,by taking the advantage of the well validated ghost-cell based high-order immersed boundary method.Systematic contrastive studies of the effects of temperature gradient buoyancy and finite-size particles on boundary layer development,profiles of mean and fluctuating velocity and temperature,evolution of characteristic vortex structures and heat transfer process in the turbulent boundary layer are provided.It turns out that both the buoyancy and particles create significant differences in all these respects.Specifically,the local friction coefficient and Stanton number is significantly increased by the buoyancy effect through enhancing the wall-normal motion of the fluid.While finite-size particles introduce abundant disturbance to increase velocity fluctuations over the whole boundary layer,and destroy streak and turbulent coherent structures in it.Meanwhile,as additional heat sources,the particles dramatically promote the development of the thermal boundary layer,as well as stabilizing temperature field near the wall region.