PARTICLE DEPOSITION FROM TURBULENT STREAMS BY MEANS OF THERMAL FORCE

Particle deposition from turbulent streams of hot gases was studied to determine what factors control particle collection. Collection efficiency was found as a function of particle size over the range of 0.3 to 1.3 microns. Diffusion was not a contributing factor in particle deposition; the only significant mechanism proved to be thermophoresis. When the mean free path to particle radius ratio, the Knudsen number, increased from small to large values, collection efficiency rose gradually at first and then increased more rapidly at about a Knudsen number of 0.2. This phenomenon is explained by the increasing contribution of the free molecular thermal force as the Knudsen number approaches unity. The effects of flow parameters and size of collector were determined. A mathematical model, based on existing theories of thermophoresis, was developed to predict collection efficiency. Agreement between the model and experiment was good, considering the accuracy with which the slip flow coefficients and constants are known. The transition thermal force is related theoretically to the free molecular thermal force by an exponential term containing the inverse of the Knudsen number and constant, r. Results suggest that r is not a constant but is temperature-dependent, because of its basic relationship to the momentum accommodation coefficient. © 1969, American Chemical Society. All rights reserved.