Radiative heat transfer in directly-irradiated high-temperature particle-gas flows

Abstract

The understanding of heat transfer problems encompassing radiative heat transfer in participating media is crucial for engineered systems and processes encountered in multiple fields of science and engineering, such as solar particle receivers. A complete heat transfer model involving hydrodynamics of particle-gas two-phase flows, radiation in the particle phase, interfacial convection, and conduction in the gas phase is required to investigate the problems. Simulation work applied to solar particle receivers that considers non-grey radiative transfer in transient particle-gas flows featuring polydisperse particles is not reported. Radiative heat transfer in particle-gas flows is the dominant mechanism in solar particle receiver applications. Knowledge of the radiative properties as input of radiative transfer analysis is critical to allow for accurate predictions. Optical and radiative characterisation of alumina-silica-based and alumina-mullite ceramic materials, which are the particle materials used in solar particle receivers, are performed using different techniques. Ambient-temperature optical properties of alumina-mullite materials are determined using variable angle spectroscopic ellipsometry in the spectral range of 0.193-1.69 um. Another method to identify the ambient-temperature radiative properties is dispersive spectroscopy. Directional-hemispherical reflectance and transmittance of the alumina-silica-based ceramic samples are obtained using dispersive spectroscopy in the spectral range of 0.2-2.5 um. A two-step inverse methodology consisting of an analytical solution based on the modified two-flux approximation and iterative Monte Carlo ray-tracing is developed and applied to infer the radiative properties. Subsequently, high-temperature optical and radiative properties of the materials are determined in the spectral range of 3-10 um as a function of temperature in the range of 150-650 degree Celsius. Spectral, normal emittance of the samples is measured in the spectral range of 3-20 um using Fourier transform infrared spectroscopy. The radiative properties as a function of temperature are obtained by employing a two-step inverse methodology. Radiative heat transfer in particle-gas media consisting of polydisperse particles is studied to investigate the effects of polydispersity. Three models for treating particle properties and thermal conditions are formulated and applied: (i) a novel multi-component radiation model, in which particle groups within discrete size intervals are assigned individual properties and temperatures locally; (ii) a lumped size model, in which integral properties and a single temperature are assigned to the particle phase locally; and (iii) a monodisperse size model, in which properties are evaluated for the Sauter mean diameter of the polydispersion and a single temperature is assigned to the particle phase locally. Non-grey radiative transfer is analysed using the collision-based Monte Carlo ray-tracing method. The multi-component model is developed for the first time to allow for predicting size-dependent radiative and convective heat transfer rates and non-uniform temperatures of particles with significantly increased fidelity. Finally, heat transfer in directly-irradiated high-temperature particle-gas flows laden with polydisperse particles is investigated using a novel three-dimensional computational fluid dynamics model. The model couples particle-gas hydrodynamics of particle-gas flows featuring polydisperse particles, radiative heat transfer in multi-component media, conduction in the gas phase, and interfacial convective heat transfer. The high-fidelity discrete particle dynamics of particle-gas flows is obtained using the multiphase particle-in-cell method. The spectral and temperature dependent optical properties of alumina-silica-based ceramic materials and multi-component radiative transfer model obtained previously are incorporated in the model. Show more