Measurements and LES computations of a turbulent particle-laden flow inside a cubical differentially heated cavity

Publication date: August 2018
Source:Atmospheric Environment, Volume 186
Author(s): H. Kim, A. Dehbi, J. Kalilainen
The estimation of particle removal rates in closed atmospheric environments such as rooms or airplanes is of great interest because it is directly relevant to air quality and disease transmission. In this work we investigate experimentally and numerically the depletion of micron-sized particles inside a turbulent, naturally driven enclosure flow. The enclosure is a cubical cavity with side 0.7 m in which two opposite vertical walls are held at different temperatures while other walls are adiabatic. The transport of monodisperse SiO2 particles with diameter 0.5, 1.0, and 2.5 μm was studied at Rayleigh numbers varying in the range of 3 × 10⁸ to 10⁹, corresponding to temperature differentials ΔT from 10 to 40 K. The decay time constant of the airborne particle concentration was calculated from discrete measurements performed with an Electrical Low-Pressure Impactor. It is found that the temperature difference greatly influences the deposition of particles with sizes equal to or less than 1 μm in diameter. For this particle range, thermophoresis becomes important and hence the larger the ΔT, the larger the deposition, a significant part of which taking place on the cold wall by the thermophoretic effect. As a result, the decay time constant of submicron particles is up to 5 times smaller than predicted by the stirred settling model, depending on the imposed ΔT. For the largest particles with diameter 2.5 μm, the influence of thermophoresis and hence ΔT is negligible, and particle removal rates are in line with the stirred settling model predictions, with mostly gravity dominated deposition on the bottom wall. A Large Eddy Simulation of the tests was performed using an Euler-Lagrange approach, and predictions of the decay time constant are within less than 20% of the measured values.

Graphical abstract

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