THERMOPHORETIC DEPOSITION IN FLOW ALONG AN ANNULAR CROSS-SECTION - EXPERIMENT AND SIMULATION

Thermophoretic deposition was investigated theoretically and experimen tally using polydisperse submicron solid glass aerosols in an annular flow with fixed thermal gradients between two cylinders. The governing equations include the momentum and energy equations for the gas phase and the general dynamic equation (GDE) for the particle phase. Aeroso l mechanisms included in the GDE ale convection, Brownian diffusion an d thermophoresis. The solutions were derived based on an implicit fini te difference approach. Simulation results suggest that thermophoretic deposition increases with increasing thermal gradient and deposition distance, hut decreases with increasing particle size and how rate. Ex perimental quantification of thermophoretic deposition was carried out in a prototype thermal cell consisting of two concentric cylinders wi th the capability of imposing a fixed thermal gradient between the cyl inders. The measurements were with polydisperse solid glass aerosol us ing two optical counters. The effect of thermal gradients, flow rates, and cell orientation on thermophoretic deposition was examined. Therm al gradients covered in this study ranged from 60 to 150 K cm(-1). It was shown that thermophoretic deposition increases with increasing the rmal gradient but decreases with increasing flow rate. Measurements wi th a vertical cell were stable for large particles but unstable for sm all particles. Comparison between experiments and simulations showed q ualitative agreement with the theoretical model. The deposition in the vertical mode was substantially higher than that predicted by the mod el particularly at large thermal gradients. This may indicate the onse t of instability. The measurements do not settle the dispute between t he theories proposed by Derjaguin et al. (1976, J. Colloid Interface S ci. 57, 451-461) and Talbot et al. (1980, J. Fluid Mech. 101, 737-758) . However, the difference between theoretically predicted deposition e fficiency is too small in comparison with the magnitude of fluctuation in the aerosol source itself.