Experimental and Numerical Determination of Thermal Radiative Properties of ZnO Particulate Media

The radiative characteristics of dependently scattering packed-beds of ZnO particles, applied in the design of high-temperature solar thermochemical reactors, were investigated experimentally. ZnO samples of varying thickness were exposed to a continuous beam of near monochromatic thermal radiation in the $0.5–1μm$ wavelength range. The overall transmitted fraction measured as a function of sample thickness $s$ obeys an exponential trend $exp(−As)$⁠, with the fit parameter $A$ ranging from $4000±100m−1$ at 555 nm to $2100±100m−1$ at $1μm$⁠. In the forward directions, the measured intensity distribution is approximately isotropic, whereas in the backward directions it is well approximated by a Henyey–Greenstein equation with asymmetry factors $g≈−0.4$ at 555 nm and $g≈−0.1$ at $1μm$⁠. A Monte Carlo ray-tracing model of the experimental setup is employed to extract the extinction coefficient and the scattering albedo for the case of a nongray absorbing-scattering medium.