The effect of turbulence on the concentration and velocity fields of microparticles.

摘要

This thesis studies the dynamics of particle-laden turbulent flows. Specifically, it addresses the effect of the turbulence on the concentration field and drift velocity of spherical particles. The coupling between the particle accumulation and the modification of the drift velocity is also investigated. Turbulent flows with and without mean shear are analyzed and the effect of the turbulent length scales on the behaviour of the particles is described. The effect of the density ratio between the disperse and the continuous phase was considered in the two extreme cases of water droplets in air (10 3) and air bubbles in water (10-3).; Experiments were conducted with water droplets in a homogeneous isotropic turbulent air flow, as well as with air bubbles in a turbulent water flow. To investigate the different effects introduced by mean shear, air bubbles were studied in both homogeneous isotropic turbulence and in a turbulent boundary layer. Measurements of the velocity of the particles were obtained by LDV and DPIV techniques. Instantaneous particle concentration maps were determined from flow visualizations. The length scales present in the concentration field were compared to the turbulence scales, in an effort to clarify the role of the different regions of the turbulent spectrum in the accumulation of micro-particles.; In all cases, the particles were found to be strongly concentrated due to the interaction with the turbulence. The characteristic length of the accumulation of particles in homogeneous isotropic turbulence was found to be between 10 and 20 times the Kolmogorov micro-scale of the turbulence. In the turbulent boundary layer, the instantaneous concentration field showed accumulation at a length scale equal to 100 times the viscous scale of the boundary layer, corresponding to the separation between the counter-rotating vortices that are responsible for ejections and sweeps in the turbulent boundary layer. The mean bubble concentration, however, showed regions that scaled with the boundary layer thickness. The drift velocity of the particles due to gravity was found to differ from the values predicted in still fluid. This difference was found to depend on the turbulent intensity and to scale with the particle Stokes number. The settling velocity of droplets was increased by their interaction with the turbulent flow. The rise velocity of the bubbles, on the other hand, was reduced by the turbulence. Only in the region where the mean shear induces accumulation of the bubbles, their rise velocity is enhanced over the value in still fluid.