Thermophoresis in the cryogenic temperature range

Abstract

A comprehensive study was made of thermophoretic deposition in the cryogenic temperature range as well as in the high-temperature range. Emphasis was given to the behavior in the cryogenic temperature range. Both theoretical and experimental methodologies were deployed. Monodisperse submicron solid NaCl aerosols were studied in an annular flow with imposed thermal gradients between the two cylindrical surfaces. The overall formulations consist of the momentum and energy equations for the gas phase and the general dynamic equation (GDE) for the particle phase. The principal mechanisms for aerosol transport in the GDE are convection, Brownian diffusion, and thermophoresis. The solution was derived based on an implicit finite difference approach. Experimental quantification of thermophoretic deposition was carried out in a thermal cell that was made of two concentric cylinders. Measurements were taken by employing a differential mobility analyzer and a condensation particle counter. Comparisons between experiments and computational simulations reveal that the widely accepted thermophoresis theories predict the thermophoretic deposition with reasonable accuracy in the high-temperature range. However, in the cryogenic temperature range,thermophoresis does not seem to be predicted satisfactorily by the presently available theories. Based on the present investigations, a new modified thermophoresis model is proposed, which displays broad agreement with experimental observations. (C) 2000 Elsevier Science Ltd. All rights reserved.