Thermophoretic Effects on Particles in Counterflow Laminar Diffusion Flames

Abstract

Thermophoresis, meaning particle drift down a local gas temperature gradient, is now known to be important to many combustion-related technologies. Until now, however, no direct experimental determinations of primary and aggregated particle thermophoretic diffusivities, α_T ^D, in high temperature combustion environments have been reported. To perform such measurements, we selected a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment. We established a CH₄/ O₂Inert opposed jet diffusion flame in which the gaseous fuel/oxygen ratio, and the diluent flow rates were adjusted to obtain a flat, stable flame, approximately coincident with the gas stagnation plane (GSP). Particles fed to or formed on either or both sides of the GSP move toward this plane until the local axial velocity is exactly counterbalanced by the thermophoretic velocity. As a result of this dynamic "equilibrium" condition, a particle stagnation plane (PSP) is established on one or both sides of the GSP,resulting in the formation of a readily observable "dust-free" zone. Dramatic confirmation of this phenomenon is offered by using laser-sheet visualization of the region, which reveals a thick dark zone, the dust-free volume, that contrasts with the bright particle-laden regions. This "phase separation" phenomenon allowed us to determine TiO₂ particle thermophoretic diffusivities by: i) measuring the temperature field using fine thermocouples; ii) measuring the thickness of the dark zone (i.e. PSP-positions) using laser light scattering; and iii) measuring/computing the axial gaseous convective velocity at the particle stagnation plane(s). Experiments and calculations indicate quantitative agreement between these measurements and kinetic theory predictions for isolated spheres at Kn_p≫ I in the case of CH₄/O₂N₂ diffusion flames. Replacement of N₂ with He as diluent resulted in a much thicker and more readily measurable particle-free layer, but yielded only qualitative agreement with the theory, because of uncertainties in the gas composition in the flame, as well as possible contributions from simultaneous diffusiophoretic mechanisms. In some 'sooting' diffusion flames, and in 'synthesis' flames in which particles are the desired products, both laminar and turbulent, it is shown that thermophoresis can influence particle residence times in the decisive region where nucleation, growth, coagulation, sintering and oxidation occur, as well as particle temperatures, which influence particle morphology and radiative heat loads. We briefly discuss the non-premixed combustion conditions under which these thennophoretically-induced effects are likely to be appreciable.