Abstract
Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO2 and H2O. They feature dual-scale porosity with mm-size pores for effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography. Total and open porosity, pore size distribution, mean pore diameter, and specific surface area are extracted from the computer tomography (CT) scans. The 3D digital geometry is then applied in direct pore level simulations (DPLS) of Fourier’s law within the solid and the fluid phases for the accurate determination of the effective thermal conductivity at each porosity scale and combined, and for fluid-to-solid thermal conductivity from 10−5 to 1. Results are compared to predictions by analytical models for structures with a wide range of porosities 0.09–0.9 in both the strut’s µm-scale and bulk’s mm-scale. The morphological properties and effective thermal conductivity determined in this work serve as an input to volume-averaged models for the design and optimization of solar chemical reactors.
Highlights
Foam-type reticulated porous ceramics (RPC) structures are applied in a broad range of physical processes requiring enhanced heat and mass transfer [1,2]
We investigate the effect that the dual-scale porosity has on the morphological properties and on the conduction heat transfer across the RPC, and further compare the results to predictions by analytical models for structures with a wide range of porosities in both the strut’s μm-scale and bulk’s mm-scale
This anisotropic region results from the 2-step coating applied during the fabrication process, and it is not considered in the determination of morphological properties and effective thermal conductivity within isotropic regions
Summary
Foam-type reticulated porous ceramics (RPC) structures are applied in a broad range of physical processes requiring enhanced heat and mass transfer [1,2]. Various porous structures made of ceria have been investigated for enhanced reaction rates [18,19,20], including structures with submicron-sized interconnected pores, but these are problematic to retain because of partial sintering at elevated temperatures [19] Their high optical thickness inhibits penetration of concentrated solar radiation, resulting in non-uniform heating and temperature distributions [14]. Since resolving the solar reactor at the pore scale would require tremendous computational demand, volume-averaging theory is often applied for solving the mass, energy, and momentum conservation equations using effective heat and mass transport properties [22,23,24,25] These can be determined accurately by direct pore-level simulations (DPLS) using the detailed 3D digital geometry of the structure obtained by computer tomography (CT) [26,27,28]. Transfer across the RPC, and further compare the results to predictions by analytical models for structures with a wide range of porosities in both the strut’s μm-scale and bulk’s mm-scale
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