(499e) Numerical Heat Transfer Simulation and Optimization of Indirectly Heated Porous Foams in Thermochemical Hydrogen Generation

Solar-driven high temperature thermochemical fuel production has been seen as a potential solution towards the decarbonization of various energy intensive industries such as automotive iron and steel, and aviation industries. One of the major approaches towards sustainable fuel production is utilizing the solar heat flux to drive a high temperature redox reaction of metal oxides for hydrogen production. In recent times, reticulated porous foams with varying pore size and geometry have been investigated for the effective heat transfer and improved solar to fuel efficiency. Heat transfer is one factor that governs how much of the sample undergoes redox cycling, and therefore has a significant impact on the overall performance of hydrogen-producing reactors. Porosity in foam samples improves radiative absorption within the volume, resulting in an increased heated zone and uniform temperature gradient. However, parameters such as the size of pores, overall porosity and topology of such samples need to be optimized to maximize the benefits offered by foam structure. In this study, we are carrying out a parametric analysis on the heat transfer characteristics of porous foam samples of varying geometry and boundary conditions using Discrete Ordinate (DO) radiation model.

The model considers all three types of heat transfer, as well as necessary reactor components like the foam sample, work tube, and insulator. The performance of the modeling is evaluated based on the minimization of the temperature gradient (both axial and radial) in the foams as it can severely affect reduction rates because of the inherent thermal resistance. DO model uses the radiative transfer equation (RTE) considering absorbance, emittance, and scattering for a user defined finite number of discrete polar and azimuthal angles. Unlike the typically used Monte Carlo ray tracing method, this method does not use ray tracing and is computationally inexpensive and fast, especially if multiple geometric models are being evaluated. The model offers flexibility in the choice of 3D geometric models i.e., whether a 3D computed tomography scan of a fabricated foam or a CAD model. It is therefore possible to investigate how foam designs affect its thermal performance, which could guide future advancements in structural design. Additionally, the heating source used in the modelling is indirect type i.e., using heat transfer fluid (HTF) such as steam from a solar receiver to drive the reaction rather than concentrated solar flux as is commonly observed in ray tracing models. The thermal effects of endothermic reduction and exothermic oxidation reactions are represented as heat sinks and heat sources, respectively based on the oxygen non-stoichiometry and partial molar enthalpy. Other parameters that influence temperature distribution in foam have been investigated, including the temperature and velocity of process gas and HTF, as well as physical characteristics of outer insulator. The simulation data is verified with experimental results in literature and a description of the solution dependence on angular discretization, as well as on boundary conditions have also been presented.