DNS Study On The Flow And Heat Transfer In Liquid-Solid Two-Phase Systems

Multiphase reactors are widely used in chemical industry.Understanding their multiscale transport phenomena is an important foundation for their rational design,scale-up,and process intensification.From the level of continuous medium,this kind of understanding needs to go deep into the scale of basic units such as particles,droplets,bubbles,and dissipative eddies,etc.,which challenges both high-resolution measurement and simulation.This thesis develops direct numerical simulation methods on heterogeneous parallel computers for liquid-solid two-phase systems by coupling the finite volume method and immersed boundary methods,based on which several complex processes involving hydrodynamics and heat transfer have been simulated successfully.Based on this study,the mechanisms behind these complex phenomena was revealed.The main contents of this thesis include:·Developing a strategy for coupling different immersed boundary methods to simulate both static and moving boundaries efficiently and accurately;Coupling particle tracking method and finite volume method based on staggered grid with explicit 3rd order Runge-Kutta scheme for temporal evolution and 2nd order spatial discretization scheme,which provides a powerful tool for high-resolution simulation of the transport phenomena in liquid-solid two-phase systems.·Investigating the local and global hydrodynamic characteristics in a 4-baffled cylindrical stirred tank with a 6-pitched blade-turbine-downflow(PBTD)impeller.With solid volume fraction at 0.1,impeller Reynolds number at 7031 and 4676,and particle-fluid density ratio at 1.5,2.161,and 3.0,analysis shows suppressed turbulence and intensified global liquid circulation in the bulk region due to the presence of particles;whereas quasi-steady and unsteady drags are controlled by the relative turbulence intensity and the turbulence dissipation rate,respectively.The drag correlations thus developed agree well with the simulated data.· Investigating the effect of particle shape on the fluid-wall and interphase heat transfer to further characterize local liquid-particle transport.Under the constant heat-flux boundary condition for temperature,with solid volume fraction at 0.1and 0.3 and bulk Reynolds number at 3800 and 5600,it has been found that addition of neutrally buoyant particles reduces the average Nusselt number.However,the Nusselt number in the near-wall region is enhanced by spheres and prolates due to their intensification of the streamwise vortices.Moreover,according to the developed correlations,at solid volume fraction below 0.3,the Nusselt number of both prolates and oblates is higher than spheres.When oblates align their large cross-sections streamwisely,with the Reynolds number increases to 200,oblates catch up prolates‘ Nusselt number from smaller values due to the dominating blocking and mixing mechanisms that suppress and intensify heat transfer at low and high Reynolds numbers,respectively.The findings above are helpful to modulate and intensify the transport process in liquid-solid two-phase systems.However,more extensive and in-depth researches are required in the future to cover a more comprehensive parameter space.