Mixed Free and Forced Convection Cooling in a Long Slender Tube with Inlet Effects
Final rept. Feb-Jul 1974
ARNOLD ENGINEERING DEVELOPMENT CENTER ARNOLD AFB TN
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Results are presented concerning an experimental study of mixed free and forced convection cooling of a long slender tube where the inlet is a high- speed jet which severely disrupts the normally expected boundary layer or free convection development. The similarity parameter GRRe to the 2nd power ranged from 10 to 100, which would suggest free convection dominance were it not for the inlet jet. In contrast to most heat-transfer experiments, the tube was insulated from its external environment and its temperature was allowed to vary axially, circumferentially, and temporally in order to study the gradients induced in the tube by the free-forced convection process. The resulting rate of cooling of the tube was measured. Timewise and spatial temperature variation of both the tube and cooling air are presented for various pressure levels up to 28 atmospheres at nominally constant flow rates of 0.3 to 0.4 lbsec. Heat-transfer coefficients derived from these data are found to be two to five times greater than anything expected from the classical Nusselt number relations. A physical model based on buoyant effects opposed by a convected decaying jet turbulence is correlated with the experimental results.
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