EXPERIMENTAL INVESTIGATION OF THERMAL DIFFUSION EFFECTS IN LAMINAR AND TURBULENT SHEAR FLOW.
MASSACHUSETTS INST OF TECH CAMBRIDGE AEROPHYSICS LAB
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An experimental investigation was conducted to determine the order of magnitude of thermal diffusion across a laminar and a turbulent shear layer. A short length of a cooled free jet was passed through stationary gas and subsequently recaptured into a continuous circulating system. Various mixtures of helium and nitrogen were investigated. With temperatures of 78K in the jet and 310K in the surrounding chamber, steady-state helium concentrations in the laminar jet were as much as 7 per cent smaller than in the surroundings. The experimental results are in good agreement with a simplified analysis. With a turbulent sheat layer between the jet and surroundings, the helium concentration inside the jet increases to within 0.1 per cent higher than the chamber level. The thermal diffusion ratio i.e. thermal to mass concentration diffusion coefficients in the turbulent shear layer was thus at least two orders of magnitude smaller than in the laminar case and of opposite sign. It is suggested that similar separation effects are to be expected for other steady flows with closed streamlines, such as base flows and flows past cavities. Author