Effect of Nongray Gas Radiation on Thermal Stability in Carbon Dioxide

Abstract

The problem of the onset of thermal instability in a horizontal layer of radiatively participating gas, heated from below and cooled from above, is considered. In the present work, both experimental heat transfer measurements across carbon dioxide and air layers before and after the onset of instability and a theoretical analysis of the coupled conduction-radiation heat transfer in the base state of the quiescent carbon dioxide gas layers, just before the onset of instability, are presented. The heat transfer measurements reveal a 7-20% increase in the critical Rayleigh number in the radiatively participating carbon dioxide vs a similar layer of air. Measurements of heat flux before the onset of the instability confirm, within experimental uncertainty, the predictions of the nongray analysis of the coupled conductive-radiation heat transfer problem. The nongray analysis also indicates that the distortion of the initial basestate temperature profiles as a result of gas radiation in the quiescent carbon dioxide gas layers is extremely small. Finally, a 20% reduction in heat transfer, after the onset of the instability, is observed in the carbon dioxide layer, compared with a similar air layer, which persists up to the highest Rayleigh number tested, Ra = 104.