Investigations of non-gray/gray radiative heat transfer effect on natural convection in tall cavities at low operating temperature

The present work numerically investigates the influence of radiation on the natural convection in differentially heated slender and square-base tall cavities at low operating temperature range of 288.1-307.7 K. The atmospheric air which has the composition of N-2, O-2, CO2 and H2O in the molar mass proportion of 75.964%, 21%, 0.036% and 3%, respectively, has been considered as the working fluid in the cavities. The non-gray and gray Planck mean absorption coefficients for whole spectrum of the air at the atmospheric pressure and average temperature of the cavity have been calculated from HITEMP-2010 database by Line-By-Line (LBL) approach. The flow and heat transfer simulations for combined radiation and natural convection have been performed for four different scenarios, i.e., pure convection, radiation in transparent, gray, and non-gray radiation medium. First, the numerical results are compared with the experimental results of Betts and Bukhari (Betts and Bokhari, 2000 [1]) for the slender cavity case, subsequently a comprehensive analysis of the fluid flow and heat transfer are presented to demonstrate the influence of the radiation on natural convection inside both the cavities. These study have been further extended for the pure CO2 medium inside the square cavity. The flow structure has been visualized by Q-criterion - a vortex identification technique. The results reveal that the fluid flow and thermal characteristics change significantly at the top and the bottom of both the cavities with the inclusion of radiation. Moreover, the radiation in gray medium has significant effect on these characteristics. The fluid flow and heat transfer are only happening in a narrow region near to the active and the top and the bottom walls. The radiative flux is in the same order of the conductive flux in these cavities and these fluxes are almost same in all four scenarios of the radiation modeling for air as working fluid. The gray case of CO2 medium enhances the conductive heat flux and reduces the radiative heat flux by almost 20% and 25%, respectively.