Experimental confirmation of radiative heating or cooling of the wall of natural convective thermal boundary layer in a differentially heated cubic cavity

In this study, the impact of surface radiation on the natural convection inside a differentially heated cubic cavity was experimentally evaluated. The thermal boundary layer near the vertical isothermal walls was visualized for cases with two emissivity of 0.1 and 0.94 on the top and bottom walls, using a phase-shifting Mach–Zehnder interferometer. The temperature around the vertical isothermal walls was determined through imaging process. Three-dimensional numerical simulation was also performed using OpenFOAM 2.3.1. The temperature distributions obtained from experiments and numerical simulations were compared. Experimental results indicated that the fluid around the top and bottom walls is cooled and heated, respectively, with high emissivity (ε = 0.94), which was also confirmed by the numerical simulations. This result was explained by the difference in the radiative heat flux on these walls, which originated from the radiation absorption at the wall surfaces. This effect of emissivity affects the development of natural convection along the isothermal vertical walls. Particularly around the upstream region of natural convection, where the thermal boundary layer was relatively thin, the thermal boundary layer thickness is thicker with high emissivity than for those with low emissivity. On the other hand, around the downstream region, this thickness is thinner with high emissivity. These results can contribute to the development of boundary layer control techniques with surface radiation.