Annular flow configuration with high deposition efficiency for the experimental determination of thermophoretic diffusion coefficients

An experimental study was carried out to produce reliable data for the determination of the thermophoretic diffusion coefficient Kth of suspended oil particles in air, in the transition regime. An original device was used for the thermophoretic deposition efficiency measurement, involving a turbulent flow through a concentric tube annulus, with the inner tube cooled (5 °C) and the outer heated. Experimental parameters varied in particle diameter (0.039–5.13 μm), flow rate (150, 200, and 250 Nl min−1, corresponding to Reynolds number in the range 5000–10 000) and hot wall temperature (65–125 °C). This configuration, based on three controlled temperatures (gas inlet, cold wall, hot wall), the so-called “3T”, permits an overall deposition efficiency enhancement compared to conventional “2T” penetration devices (hot gas flow in a cooled tube). In the 3T configuration, significant thermophoretic deposition efficiencies have been obtained (up to 27%), together with limited gas temperature axial variations, thus permitting a reliable determination of the thermophoretic diffusion coefficient Kth.An analytical model was developed for the prediction of the thermophoretic deposition efficiency, for a given value of the thermophoretic diffusion coefficient Kth. This model has been used, together with our measurement results, to derive the Kth experimental values, for a Knudsen number ranging from 0.01 to 3. These Kth values were compared with evaluations based on various models available in the literature. Although widely used, Talbot's model always provides Kth values higher than our experimental results in the transition regime. The most relevant model appears to be the one proposed by Beresnev and Chernyak, particularly for an energy accommodation slightly lower than one.