An Investigation On Particle Motion In The Stratified Turbulence By LES

Two-phase turbulent channel flow with the thermal stratification is investigated by combining large eddy simulation with Lagrangian tracking approach. The three-dimensional Navier-Stokes and energy equations under the Boussinesq ap-proximation are numerically solved using a fractional-step method based on high-order accurate instantaneous schemes. Lagrangian tracking approach based on point-particle model is used to describe the dynamics of particles. Some typical statistical quantities on the turbulent flow and particle movement are obtained and analyzed. In our inves-tigation, the volume fraction of the dispersed phase is small enough such that particle collisions are negligible.To reveal the characteristics of different particles in the stable stratification, we simulate three kinds of particle under the Richardson number 30, the Reynolds number 180 (based on the wall friction velocity), and the Prandtl number 0.71. The simulation results show that the effects of the Stokes number on particles transport in the stable stratification are remarkable due to the strong inertia of the high Stokes number parti-cles. We find the fluid-particle correlation coefficient decreases with the Stokes num-ber increasing. The activity of the particles varies weak in the stable stratification with the vertical fluctuation reduced by the effect of the buoyancy flux. As a result, the higher particle concentrations under the mid Stokes number close to the near-wall re-gion and the core region of the channel.To reveal the effects of thermally stable and unstable stratification on the charac-teristics of turbulent flow and particles transfer, we also simulate 250000 particles sized 50μm under different Richardson number ranging. The simulation results show that the effects of stratification on the particles transport are remarkable since turbulence in-tensities reduce due to stable stratification and enhance due to unstable stratification. We find the anisotropy of the particles velocity is stronger than the fluid velocity in unstably stratified flow. The activity of the particles varies weak with increasing Richardson number and decreasing the vertical fluctuation. As a result, the higher par-ticle concentrations close to the core region of the channel as Richardson number in-creases.Then we consider the influence of dispersed phase on the carrier phase, a point-force model is used in the two-way coupling governing equations, the effect of particles on the fluid was approximated as a point force added into the Navier-Stokes equation. And the calculations under two-way coupling conditions are compared with that by the one-way coupling. The results show that the differences of the streamwise mean velocity between the one-way and two-way coupling model are unconspicuous. But the turbulent intensity and the particle fluctuations become lower than that under the one-way coupling model.Finally, micro-particle concentration evolution and thermophoresis sedimenta-tion in a stably stratified turbulent flow are calculated and analyzed in the chapter 5. The effect of the thermophoresis on the particle sedimentation in the flow with the temperature gradient can not be ignored. The thermophoretic force enhances the par-ticle deposition in the direction opposite to the temperature gradient. The thermopho-resis still makes the streaky structure to disappear or disorder gradually in the cold and hot sublayer region, respectively. For the different thermal stratification, we find the vertical fluctuation of micro-particles decreases with the Richardson number in-creasing under the effect of the buoyancy flux. The magnitudes of the thermophoretic force in the viscous sublayer decrease as the stability increases. So the particle depo-sition in the direction opposite to the temperature gradient weakens. Furthermore, the expression of the particle deposition rate that depends on the particle relaxation time scale and the Richardson number is proposed by a least-square fitting method.