Effect of radiation on convection in a top-heated enclosure

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 counter-rotating 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. Excellent agreement was found between the numerical predictions and the experiment. Comparison between model and experiments show 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.