Experimental evaluation of thermal radiation effects on natural convection with a Rayleigh number of 108-109 by using an interferometer

In this study, the radiation effects on the temperature field and perturbation of the thermal boundary layer of natural convection for gas were evaluated by an interferometer. The natural convection investigated in this study was that of a cavity, which had differentially heated vertical walls, surrounded by adiabatic walls. The target Rayleigh number was of the order of 10(8)-10(9). By using the interferometer adopted in a phase-shifting technique, surface and gas radiation effects were evaluated. To compare the experimental results, a numerical calculation was also conducted using a large-eddy simulation coupled with radiative heat transfer. By varying the emissivity of the adiabatic walls while maintaining the emissivity of the isothermal walls, the surface radiation effect on the natural convection was analyzed. To neglect the gas radiation effect, air was used as the working fluid. When the adiabatic walls were black-body surfaces, the temperature difference around the adiabatic walls was higher than that of the reflective surface as a result of the optical path difference. From this result, the high temperature difference due to the surface radiation effect was confirmed. To investigate the gas radiation effect on natural convection, trifluoromethane gas, which has significant radiative absorption, was used as a working fluid. By evaluating the interference fringe by the interferometer, the wave motion of the thermal boundary layer around the heated and adiabatic walls was visualized. Comparing the numerical calculation results (when varying the calculation conditions) revealed that the wave motion and turbulence boundary layer around the heated and adiabatic walls were caused by the gas radiation effect. (C) 2018 Elsevier Ltd. All rights reserved.