Transient radiative heat transfer in a suspension of ceria particles undergoing non-stoichiometric reduction

Abstract

Transient radiative heat transfer is analyzed numerically in a directly-irradiated plane-parallel medium containing a suspension of ceria particles undergoing non-stoichiometric thermal reduction. The micrometer-sized ceria particles are assumed to be homogenous, non-grey, absorbing, emitting, and anisotropically scattering, while the overall medium is of non-uniform temperature and composition. The unsteady mass and energy conservation equations are solved using the finite-volume method and the explicit Euler time-integration scheme. Radiative transport is modeled using the energy-portioning Monte Carlo ray-tracing method. The radiative properties are obtained using the Mie theory. The influence of selected model parameters is investigated. In all cases, the time to reach steady-state temperature is shorter than that to reach the equilibrium non-stoichiometry, indicating that the reduction reaction is limited by chemical kinetics, rather than by heat transfer. Increasing particle volume fraction and decreasing particle diameter both increase the optical thickness of the particle suspension, resulting in increasing peak temperature and temperature non-uniformity at steady state. For 5 μ-dia. particles under 1000-sun irradiation, the peak temperature at steady state ranges from 1860 K for a particle volume fraction of fv = 10-6 to 2200 K for f = 10-3; the temperature non-uniformity ranges from 20 to 1200 K. For a fixed volume fraction of fV = 10-6, decreasing the particle diameter from 20 to 1 μ increases the peak temperature at steady state from 1700 to 2250 K; the temperature non-uniformity increases from a 10 to 70 K. Both the final non-stoichiometry and the reaction rate are sensitive to temperature changes. For example, a temperature change of 230 K from 1860 to 2090 K nearly doubles the final non-stoichiometry from 0.095 to 0.18 and reduces the time to reach steady state by a factor of 10 from 4 to 0.4 s for the selected set of parameters. The influences of the reaction rate constant and of the morphology-dependent optical properties of the particles on the temperature and non-stoichiometry distributions are discussed.