Experimental study of turbulent thermal diffusion of inertial particles in a convective turbulence forced by oscillating grids

Abstract

We investigate the phenomenon of turbulent thermal diffusion of inertial solid particles in laboratory experiments with convective turbulence forced by one or two oscillating grids in the air. Turbulent thermal diffusion causes a non-diffusive contribution to turbulent flux of particles described in terms of an effective drift velocity directed opposite to the gradient of the mean fluid temperature. For inertial particles, this effective drift velocity depends on the Stokes and Reynolds numbers. In the experiments, fluid velocity and spatial distribution of inertial particles are measured using a Particle Image Velocimetry (PIV) system, and the temperature field is measured in many locations by a temperature probe equipped with 12 thermocouples. Measurements of temperature and particle number density spatial distributions have demonstrated the formation of large-scale clusters of inertial particles in the vicinity of the mean temperature minimum due to turbulent thermal diffusion. In the experiments, the effective drift velocity caused by turbulent thermal diffusion that results in the formation of large-scale clusters of inertial particles (having the diameter $10 μm$) is in 1.5 -- 2.5 times larger than that for noninertial particles (having the diameter $0.7 μm$) depending on the level of turbulence. This is in agreement with the theoretical predictions.