Graduate Thesis Or Dissertation
 

The effect of geometric orientation and random wind conditions on flat plate convection rates

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  • An experimental investigation has been performed to determine the forced convection heat transfer due to three-dimensional air flow over a flat plate as a function of angle of attack and free stream velocity. The experimental method employed a transient thermal technique and placed the convecting surface next to the exhaust port of an open circuit wind tunnel. The ratio of convecting surface width to wind tunnel nozzle width was 0.7, but insulating surfaces extended beyond the nozzle width. The flows evaluated ranged in Reynolds number from 32,000 to 140,000 and from zero to 90-degree angles of attack. The data indicated the probable existence of a turbulent boundary layer at zero and 45-degree angles of attack. Data collected for angles of attack of 30, 60, and 90 degrees demonstrate mostly laminar boundary layer flows and indicate a dependence of the Colburn i-factor upon angle of attack. The j-factor was determined to be proportional to the Reynolds number to the minus one-half power, as in previous work, and the multiplier varied between 0.876, as in a Pohlhausen equation corrected for unheated starting length, to 1.114 as predicted in wedge flow measurements performed by Eckert. The results are different from those of Sparrow and Tien, which indicate a single multiplier of 0.931 will provide solutions for all angles of attack between 25 and 90 degrees. The flow in this investigation is largely twodimensional due to the physical arrangement of the apparatus, whereas the flow used by Sparrow and Tien was primarily three-dimensional. An analytical method was also developed which accounted for the random nature of the natural wind velocity distribution in the calculation of the time average convection film coefficient. The method uses the average velocity and the standard deviation of the velocity to provide an evaluation of the time average convection film coefficient. This was accomplished by transforming the defining convection relationship into a second order constant coefficient polynomial over a fixed range of velocities. The method was demonstrated on a statistical record of wind velocities measured on November 29 and 30, 1978 at the Oregon State University campus. The error created by using only the average velocity in calculating the time average convection film coefficient was determined to be 3.4%. The evaluation concludes that with present uncertainties in the convection film coefficient, the error introduced by ignoring the random nature of the wind is insignificant.
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