Direct Numerical Simulation Of Gas-Solid Two-Phase Turbulent Boundary Layer

Particle-laden boundary layer flows are frequently encountered in a wide range of natural physical processes and industrial and environmental applications,such as sand and dust storms,pollutant dispersion,coal combustion,fluidized beds and pneumatic conveyance,etc.Because of the complexity of turbulence and the multi-field,multi-scale and complex strongly nonlinear coupling effects between the turbulent velocity,temperature and particle concentration fields,the systematic researches on particle-laden turbulent boundary layer flows related to the fields of energy and power engineering remain largely undocumented.Therefore,the study of flow and heat transfer in the turbulent boundary layer not only plays an important role in the engineering application area,but is of innovative and scientific significance since it has become a research hot spot and critical issue in the multiphase flow field.Based on the high accuracy algorithm developed by our group and the high-performance parallel computation platform,direct numerical simulations of particle-laden spatially develop-ing flat-plate boundary layers,ranging from the laminar region to the fully developed turbulent region,have been performed firstly with the Eulerian-Lagrangian point-particle approach.The simulation results show that the addition of particles increases the mean streamwise fluid veloc-ity,which in turn leads to a reduction in the boundary layer integral parameters and an increase in the skin-friction drag.In addition,the presence of particles causes a significant delay in the onset of transition.The streamwise turbulence intensity is slightly enhanced in the close vicinity of the wall but damped in the outer layer.The Reynolds stress and the turbulence intensities in the wall-normal and spanwise directions are substantially attenuated across the entire boundary layer,and the levels of attenuation increase monotonically with both particle Stokes number and mass loading.In addition to the viscous dissipation,particle-fluid interactions will produce extra energy dissipation,i.e.particle dissipation,which plays a crucial role in turbulence modulation.Direct numerical simulations of particle-laden flows in a spatially developing turbulent thermal boundary layer have also been performed with realistic time-dependent turbulent inflow boundary conditions.The simulation results indicate that the presence of particles increases the mean streamwise velocity and temperature gradients of the fluid in the near-wall region,which leads to significant increases in the skin-friction drag and heat transfer.The near-wall sweep and ejection motions are suppressed by the particles and thus the Reynolds stress and wall-normal turbulent heat flux are reduced,which gives rise to a reduction in the production of the turbulent kinetic energy and temperature fluctuation.In the particle-laden flows,the vortical structures become less organized,while the coherences of near-wall velocity and temperature streaks are considerably increased.Moreover,the streaks appear wider under the effect of particles.In addi-tion,it is found that the spatial evolution of the particle wall concentration along the streamwise direction is similar to that of the mean skin-friction coefficient.The particle wall concentration is decreased and the coherence of vortical structures is reduced as the flow develops down-stream.It can be inferred that the coherent vortical structures will gradually diminish or even disappear at very high Reynolds number and eventually particles will behave as passive tracers without preferential accumulation.The present study has investigated the coupling mechanisms of the turbulent velocity,tem-perature and particle concentration fields and the numerical results will be expected to provide theoretic guidance for the practical engineering problems,such as drag reduction,heat transfer enhancement,energy conservation and emission reduction.The simulation results can also pro-vide theoretical support and database for the establishment,development and improvement of the complex turbulence models and promote further development of the theory of multiphase flows.