Transport phenomena in high-temperature multiphase-flow solar thermochemical reactors

Abstract

High-temperature solar thermochemical technologies utilise the entire spectrum of solar radiation to drive endothermic chemical reactions, promising an energy-efficient and environment-friendly pathway to a wide variety of industrial processes. In these solar thermochemical processes, chemically reactive and radiatively participating multiphase flows in various regimes are frequently encountered. Numerical modelling of multiphase flows assists the design and optimisation of solar thermochemical systems by reducing the need for costly and time-consuming experimental testing. However, the complex interplay between reaction kinetics, mass transfer, momentum transfer and conductive, convective and radiative heat transfer in the flows has made the numerical simulation challenging. By developing numerical models for high-temperature reactive solid-gas flows and applying them to selected solar thermochemical processes, this thesis presents two main contributions to the research field of solar thermochemistry: (i) design and optimisation of an indirectly-irradiated packed-bed solar thermochemical reactor and (ii) maintenance of a windowed directly-irradiated solar thermochemical reactor.

Numerical models for multiphase flows of different complexity and purposes can be classified into two groups based on the mathematical formulation: the computational fluid dynamics model and the empirical model. A computational fluid dynamics model describes the fluid flows using conservation equations of mass, momentum and energy, while an empirical model is based on correlations derived from empirical data. In this work, the author formulates the Kunii-Levenspiel two-phase model, a transient three-dimensional Eulerian-Eulerian model, and a single-phase computational fluid dynamics model to simulate the heat and mass transfer in high-temperature reactive solid-gas flows in diverse regimes. The Arrhenius equation is used if any heterogeneous reaction is to be coupled to the solid-gas flow. The kinetic theory of granular flow is used to simulate the momentum and energy transfer in the dispersed solid phase. The radiative exchange between surfaces and the radiative transfer in particulate media are simulated using the Monte Carlo ray-tracing method and the discrete ordinates method, respectively. The formulated equations for the computational fluid dynamics and empirical models are applied to two solar thermochemical reactors: (i) an indirectly-irradiated reactor for thermal reduction of metal oxide particles and (ii) a directly-irradiated impinging-jet reactor for effective removal of particulate contaminants. The former reactor performs the solar-driven endothermic reduction step of a two-step metal-oxide-based redox cycle. The latter is a conceptual design exploring a novel method for aerodynamic-aided window protection.

Firstly, the Kunii-Levenspiel two-phase model is used to evaluate the performance of an indirectly-irradiated reactor for Mn2O3 reduction at 1800 K. Then, a transient three-dimensional Eulerian computational fluid dynamics model is applied for the reactor performance evaluation, affording rich details in the heat and mass transfer process. A reactor prototype is tested under simulated high-flux solar irradiation emitted from the high-flux solar simulator for model validation. Finally, a transient two-dimensional single-phase model for dilute solid-gas flows is developed to explore the transport of small particulate contaminants (Stokes number << 1) in windowed directly-irradiated solar reactors. Such understanding is of practical significance for protecting the window, a crucial optical component affecting the overall energy conversion efficiency of a solar reactor, from being contaminated by fine particles generated from chemical reactors and physical attrition. Show more