Experimental study of turbulent transport of nanoparticles in convective turbulence

Abstract

We perform an experimental study of transport of nanoparticles in convective turbulence with the Rayleigh number ∼108 in the air flow. We measure a temperature field in many locations by a temperature probe equipped with 11 E-thermocouples. Nanoparticles of the size ∼70 nm in diameter are produced by an advanced electrospray aerosol generator. To determine the number density of nanoparticles, we use a condensation particle counter. The joint action of turbulent effects and the large-scale circulations in convective turbulence, which are important in the core flow, and molecular effects, which are essential near the boundaries of the chamber, results in an effective accumulation of nanoparticles at the cold wall of the chamber. The turbulent effects are characterized by turbulent diffusion and turbulent thermal diffusion of nanoparticles, while the molecular effects are described by the Brownian diffusion and thermophoresis, as well as the adhesion of nanoparticles at the cold wall of the chamber. In different experiments in convective turbulence in a chamber with the temperature difference ΔT between the bottom and top walls varying between ΔT=29 and ΔT=61 K, we find that the mean number density of nanoparticles decreases exponentially in time. For instance, the characteristic decay time of the mean number density of nanoparticles varies from 12.8 min for ΔT=61 K to 24 min for ΔT=29 K. For better understanding of experimental results, we perform one-dimensional mean-field numerical simulations of the evolution of the mean number density of nanoparticles for conditions pertinent to the laboratory experiments. The obtained numerical results are in agreement with the experimental results.