Ablative heat transfer in a shrinking packed‐bed of ZnO undergoing solar thermal dissociation

Abstract

A transient heat transfer model is formulated for a shrinking packed-bed of reacting ZnO particles exposed to concentrated solar irradiation. The model combines conduction, convection, and radiation heat transfer with simultaneous sintering and reaction kinetics. Validation is accomplished in terms of temperatures and dissociation rates experimentally measured using a solar-driven thermogravimeter with ZnO packed-bed samples subjected to solar flux concentration ratios in the range 1225-2133 suns and surface temperatures in the range 1834-2109 K. Operating conditions are typical of an ablation regime controlled by the rate of radiative heat transfer to the first layers of ZnO undergoing endothermic dissociation.

Modelling of solar thermochemical reaction systems

“…However, while it is a dramatic geometrical approximation, it has been shown to agree well with experimental data when characterising porous ceria for STRS applications (Dombrovsky et al, 2012;Ganesan et al, 2013a). Simulations of effective radiative transfer (isolated from mass, momentum and other forms of energy transfer) motivated by STRS design have been carried out in conjunction with spectrophotometry set-ups to obtain effective radiative properties for packed beds and porous materials made of ceria (Dombrovsky et al, 2012;Ganesan et al, 2013a,b), zinc oxide (Coray et al, 2009;Schunk et al, 2009b), and a mixture of particles (Jäger et al, 2009).…”

“…We will revisit this concept in Section 3.4. Highly dense active materials may not have a significant amount of radiation penetrating the bulk of the solid, allowing for a simple treatment of radiation that only accounts for absorption, reflection and emission from surfaces as found in (Piatkowski and Steinfeld, 2008;Schunk et al, 2009b;2009a;Villafán-Vidales et al, 2015).…”

“…Scattering and absorption behaviour of radiation in participating media is particularly strongly influenced by changing particle size. Studies dealing with this phenomenon include (Schunk et al, 2009a(Schunk et al, , 2009bVillafán-Vidales et al, 2015) for dense, large ZnO samples, (Dombrovsky et al, 2009;Kenarsari and Zheng, 2014) for ZnO and coal particles and (Kenarsari and Zheng, 2014;; Lipiń ski Table 1 Selected modelling studies of thermochemical processes organised by active material and reactor configuration. Material property extraction studies with an emphasis on materials used in STRS have also been included in the right-hand column.…”

“…Enthalpy of reaction can be available experimentally or estimated by thermodynamic database software as demonstrated in (Villafán-Vidales et al, 2011). The temperature dependence of reaction rates is generally handled by assuming an Arrhenius-type law (see Section 3.2): Lipiń ski, 2012, 2011;Haussener et al, 2009;Lipiń ski and Steinfeld, 2004;Müller et al, 2008;Oles and Jackson, 2015;Schunk et al, 2009b;2009a;Villafán-Vidales et al, 2015;Yue and Lipiń ski, 2015a,b,c;Zedtwitz et al, 2007;Zedtwitz and Steinfeld, 2005). Fluid density, specific heat and thermal conductivity can all also be strong functions of temperature, particularly for gasses; inclusion of these effects can be found, for example, in (Kenarsari and Zheng, 2014;Lu et al, 2016;Müller et al, 2008;Wang et al, 2014b).…”

Discrete vs. continuum-scale simulation of radiative transfer in semitransparent two-phase media

“…Examples include high-temperature chemical reactions in packed beds [1][2][3], porous radiant burners [4][5][6], combustion furnaces for solid feedstock [7], high-flux solar receivers [8][9][10], and porous heat exchangers [11]. Traditionally, radiative transfer in such media has been modeled using the equation of radiative transfer with appropriate average radiative properties [12,13].…”