Thermophoretic deposition of aerosol particles in turbulent pipe flow

Thermophoretic deposition of aerosol particles was measured in a 0.965 m long, 0.49 cm ID stainless-steel pipe under laminar and turbulent flow conditions. The experiment consisted of heating an aerosol to a specified temperature, and then passing it through an annular water cooled (20°C) heat exchanger to induce thermophoretic deposition onto the pipe walls. Experimental parameters varied were particle diameter (0.1–0.7 μm), flow rate (5, 20 and 35 l min-1), and the gas temperature at the entrance to the cooling section (300–425 K). The flow rates, 5, 20 and 35 l min-1 corresponded to Reynolds numbers of 1379, 5517 and 9656, respectively. Particle materials used were sodium chloride and polystyrene latex spheres. Experimental results showed a complex coupling of the turbulent and thermophoretic deposition mechanisms with turbulent deposition dominating over thermophoresis for the larger particle sizes at 35 l min-1. The thermophoretic deposition component was extracted from the total deposition and was compared with theoretical predictions. A theoretical expression was derived and compared with other expressions from the literature. Experimental deposition efficiencies were found to be between 1.1 and 2.0 times greater than the theoretical predictions. The discrepancy between theory and experiment increased as Reynolds number and particle size were increased. This discrepancy was attributed to factors not included in the theory which may involve deposition enhancement due to turbulent eddy transport or non-uniform particle concentration gradients at the entrance to the pipe cooling section.