Thermophoretic transport and deposition of sub-micron particles suspended in gas flows.

摘要

Thermophoretic transport of small particles in gas flows has many scientific and engineering applications, but has not been studied widely and is not well understood. Thermophoretic forces arise in the presence of temperature differences, which drive particles from hotter to colder regions of flows and may lead to deposition on surfaces, which may degrade heat transfer. Previous studies have shown that, in flows with sub-micron particles, and temperature gradients of the order of 10 K/cm, thermophoresis can be a dominant particle transport mechanism. In the research described in this proposal, the governing equations for mass, momentum, energy and species have been formulated and approximate boundary conditions for particulate transport have been proposed. A new series solution has been obtained for the particle concentration field in steady laminar tube flow, the results of which are consistent with particle deposition experiments. The effects of the tube entrance zone and of gas compressibility have been studied using computational fluid dynamics, and also compare well with experimental observations. In the case of steady turbulent duct flows, approximations based on existing direct numerical simulation results lead to a simple 1D model for the deposition efficiency of sub-micron particles that compares well with results of several experimental studies.;There are also many engineering applications in which thermophoretic transport of particles takes place in unsteady pulsating flows, though there have been no previous studies of these problems. In the second part of this thesis, the effect of oscillating flows on thermophoretically driven mass transfer is investigated. It is found that unsteadiness has little or no effect on thermophoretic transport when the direction of flow oscillation is normal to the direction of heat transfer. However, when the directions of flow oscillation and heat transfer are aligned, flow oscillation can lead to significant enhancements in both heat transfer and thermophoretic mass transfer. In the particular problem of oscillating slug flow with an axial temperature gradient, it is found that the mass transfer is enhanced by up to 3 orders of magnitude over its steady rate. Variation of the frequency of oscillation reveals a tuning effect whereby a particular oscillation frequency maximizes the effective thermophoretic diffusivity. In the case of a considerable convective velocity in the direction normal to heat transfer-such as a porous channel flow with a pulsating vertical component of velocity, it is seen that thermal disturbances travel quickly in the longitudinal direction. Thus, in order to attain a tuning effect, a very high pulsating frequency would have to be imposed in the vertical direction, which would require high velocities that would surpass laminar thresholds and is impractical in most circumstances. In many industrial applications, the effect of unsteadiness on heat transfer and thermophoretic mass transfer is negligible. However, significant effects of mass-transfer enhancement could theoretically be observed in a few specialized devices such as conductive heat exchangers, if the heat transfer and flow oscillation periods coincide. This enhancement is a kind of thermal resonance which can theoretically occur when heat transfer takes place slowly, but it is a specialized effect and depends on the characteristics of the heat/mass transfer device and the frequencies of flow and thermal oscillation.