  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 $N_2$, $O_2$, $CO_2$ and $H_2O$ 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 [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. 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 regions near to the
active 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.
