Combined inertial and thermophoretic effects on particle deposition rates in highly loaded dusty-gas systems

Abstract

Little is yet known (theoretically or experimentally) about the simultaneous effects of particle inertia, particle thermophoresis and high mass loading on the important engineering problem of predicting deposition rates from flowing “dusty” gases. For this reason, we investigate the motion of particles present at nonnegligible mass loading in a flowing nonisothermal gaseous medium and their deposition on strongly cooled or heated sold objects by examining the instructive case of steady axisymmetric “dusty-gas” flow between two infinite disks: an inlet (porous) disk and the impermeable “target” disk—a flow not unlike that encountered in recent seeded-flame experiments. Since this stagnation flow/geometry admits interesting self-similar solutions at all Reynolds numbers, we are able to predict laminar flow mass-, momentum- and energy-transfer rate coefficients over a wide range of particle mass loadings, dimensionless particle relaxation times (Stokes numbers), dimensionless thermophoretic diffusivities, and gas Reynolds numbers. As a by-product, we illustrate the accuracy and possible improvement of our previous “diffusion model” for tightly coupled dusty-gas systems. Moreover, we report new results illustrating the dependence of the important “critical” Strokes number (for incipient particle impaction) on the particle mass loading and the wall/gas temperature ratio for dust-laden gas motion towards “overheated” solid surfaces. The present formulation and resulting transport coefficients should not only be useful in explaining/predicting recent deposition rate trends in “seeded”-flame experiments, but also highly mass loaded systems of technological interest, such as the deposition of opto-electronic materials by jet impingement, and fouling layers from ashladen fossil fuel combustion products.