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 flow. Turbulent thermal diffusion causes a non-diffusive contribution to turbulent flux of particles described in terms of an effective pumping velocity directed opposite to the gradient of the mean fluid temperature. For inertial particles, this effective pumping velocity depends on the Stokes and Reynolds numbers. In the experiments, fluid velocity and spatial distribution of inertial particles are measured using Particle Image Velocimetry 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 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 pumping velocity resulting in formation of large-scale clusters of inertial particles (having the diameter $10 μm$) is in 2.5 times larger than that for non-inertial particles (having the diameter $0.7 μm$). This is in an agreement with the theoretical predictions.

Figures (14)

• Figure 1: Experimental setup with the convective turbulence forced by one oscillating grid (left panel) and by two oscillating grids (right panel): (1) digital CCD camera; (2) rod driven by the speed-controlled motor; (3) oscillating grid; (4) laser light sheet; (5) temperature probe equipped with 12 E - thermocouples; (6) heat exchanger at the top cooled wall of the chamber; (7) heat exchanger at the bottom heated wall of the chamber.
• Figure 2: Distributions of the mean velocity field $\overline{U}$ for convective turbulence forced by one oscillating grid (left panel) and by two oscillating grids (right panel). The coordinates $Y$ and $Z$ are normalized by $L_z=26$ cm. The mean velocity $\overline{U}$ is measured in cm/s.
• Figure 3: Distributions of the mean velocity shear $\overline{S}=\left[(\nabla_y \overline{U}_y)^2 + (\nabla_z \overline{U}_y)^2 + (\nabla_y \overline{U}_z)^2 + (\nabla_z \overline{U}_z)^2\right]^{1/2}$ for convective turbulence forced by one oscillating grid (left panel) and by two oscillating grids (right panel). The streamlines (white) of the mean velocity $\overline{\bm{U}}$ are also superimposed on this distribution. The coordinates $Y$ and $Z$ are normalized by $L_z=26$ cm, The mean velocity shear $\overline{S}$ is measured in s$^{-1}$.
• Figure 4: Distributions of the turbulent velocity $|u^{\rm (rms)}| = \left[\langle u_y^2 \rangle + \langle u_z^2 \rangle\right]^{1/2}$ for convective turbulence forced by one oscillating grid (left panel) and by two oscillating grids (right panel). The streamlines (white) of the mean velocity $\overline{\bm{U}}$ are also superimposed on this distribution. The coordinates $Y$ and $Z$ are normalized by $L_z=26$ cm.
• Figure 5: Distributions of the ratio $u^{\rm (rms)} / |\overline{\bm{U}}|$ for convective turbulence forced by one oscillating grid (left panel) and by two oscillating grids (right panel). The streamlines (white) of the mean velocity $\overline{\bm{U}}$ are also superimposed on this distribution. The coordinates $Y$ and $Z$ are normalized by $L_z=26$ cm.