Turbulent thermal diffusion in a multi-fan turbulence generator with imposed mean temperature gradient

Abstract

We studied experimentally the effect of turbulent thermal diffusion in a multi-fan turbulence generator which produces a nearly homogeneous and isotropic flow with a small mean velocity. Using Particle Image Velocimetry and Image Processing techniques we showed that in a turbulent flow with an imposed mean vertical temperature gradient (stably stratified flow) particles accumulate in the regions with the mean temperature minimum. These experiments detected the effect of turbulent thermal diffusion in a multi-fan turbulence generator for relatively high Reynolds numbers. The experimental results are in compliance with the results of the previous experimental studies of turbulent thermal diffusion in oscillating grids turbulence (Buchholz et al. 2004; Eidelman et al. 2004). We demonstrated that turbulent thermal diffusion is an universal phenomenon. It occurs independently of the method of turbulence generation, and the qualitative behavior of particle spatial distribution in these very different turbulent flows is similar. Competition between turbulent fluxes caused by turbulent thermal diffusion and turbulent diffusion determines the formation of particle inhomogeneities.

Detection of turbulent thermal diffusion of particles in numerical simulations

“…This effect is robust and the results of the numerical simulations are in agreement with theoretical studies [29,38], laboratory experiments [31,33,34] and atmospheric observations [35]. When St > 1, this effect is decreasing with Stokes number.…”

“…The phenomenon of turbulent thermal diffusion has been predicted theoretically in [29] and detected in different laboratory experiments in stably and unstably temperature-stratified turbulence [30][31][32][33][34]. This phenomenon is shown to be important for atmospheric turbulence with temperature inversions [35] and for smallscale particle clustering in temperature-stratified turbulence [34], but it is also expected to be significant for different kinds of heat exchangers, e.g., industrial boilers where Reynolds numbers and temperature gradients are large.…”

Experimental detection of turbulent thermaldiffusion of aerosols in non-isothermal flows

“…(1) is N =N 0 = const and α=0. However, in the experiments with oscillating grids turbulence , the turbulent thermal diffusion ratio varied from α=1.29 to α=1.87 depending on the frequencies of the grid oscillations, and in the experiments with the multi-fan turbulence (Eidelman et al, 2006), α=2.68. The latter value of α is larger than that obtained in the experiments in oscillating grids turbulence, where the Reynolds numbers were smaller than that achieved in the multi-fan turbulence generator.…”

“…The turbulent thermal diffusion ratio, α, was determined in the experiments in oscillating grids turbulence Buchholz et al, 2004) and in multi-fan turbulence (Eidelman et al, 2006). The turbulent thermal diffusion ratio was introduced using the steady-state solution of Eq.…”

“…First experiments in oscillating grids turbulence that demonstrated the phenomenon of turbulent thermal diffusion, were discussed by Eidelman et al (2004) and Buchholz et al (2004). Later, the phenomenon of turbulent thermal diffusion was observed in a multi-fan turbulence generator (Eidelman et al, 2006). Observation time in these devices is almost unlimited due to the small mean velocity of the flow.…”