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National Aeronautics and Space Administration

Lewis Research Center

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January 1992

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A02

10

Effect of Radiation on Convection at Moderate Temperatures

Henry C. deGroh III and Mohammad Kassemi

Prepared for the 30th Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics and Astronautics, Reno, Nevada,

January 6–9, 1992. Henry C. deGroh III, NASA Lewis Research Center, Mohammad Kassemi, Ohio Aerospace Institute, 2001 Aerospace Parkway,

Brook Park, Ohio 44142 and NASA Resident Research Associate at Lewis Research Center. Responsible person, Henry C. deGroh III, (216) 433–5025.

Radiation; Convection; Crystal growth; Physical vapor transport

The top-heated enclosure is of considerable importance in many engineering situations especially in crystal growth from

vapor where it is used to minimize convection. In this work we present a combined numerical and experimental investigation of radiation induced convection to show that the convective stability of the top-heated enclosure is disrupted by

heat transfer conditions at the wall. When the enclosure is not insulated the thermal stratification of the fluid is modified

by convective and radiative losses to the surrounding environment. This results in a double annular cell flow which

when cut by the laser sheet shows a four-vortex pattern with a weak annular cell at the bottom and a large counterrotating annular cell at the top. When the enclosure is insulated the convective stability of the fluid is again disrupted;

this time as a result of radiative heat transfer between the enclosing surfaces which drives two annular flow cells of

relatively equal size. Comparison between model and experiment shows that radiation effects are important even at

temperature levels as low as 300 °C and if these effects are not included, numerical predictions can be far removed from

reality.