Modeling of particle-radiation-interaction for the numerical simulation of coal combustion

• Modellierung der Partikel-Strahlungs-Interaktion für die numerische Simulation der Kohleverbrennung

Gronarz, Tim; Kneer, Reinhold (Thesis advisor); Kabelac, Stephan (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

In the present dissertation, scattering and absorption of thermal radiation by particles in coal combustion scenarios is investigated, modeling approaches are derived and a parameter variation is performed to quantify the influence of the parameters determining heat transfer. In the first section, scattering and absorption properties of coal particles are investigated, making use of the detailed Mie theory which serves as the reference solution throughout the present work. The complex index of refraction which is the material property that determines the interaction of matter with thermal radiation is introduced. Available data on the complex index of refraction for coal and ash particles in literature are compared, discussed investigated regarding their influence on the scattering and absorption properties. Then, modeling approaches are derived that allow to describe scattering and absorption by coal and ash particles in numerical simulations of coal combustion efficiently. This includes a novel approach to describe the variation of scattering and absorption properties of coal particles undergoing burnout based on a shell model. Also, new approximations for the scattering phase function are presented. A very promising approximation for the scattering phase function is provided by the modified Henyey-Greenstein scattering phase function and forward scattering factors calculated from precise Mie theory calculations. Finally, these models are implemented in a program to solve the radiative transport equation numerically. To be applied in a numerical scheme with an angular discretization, the scattering phase functions are integrated over discrete solid angles based on a customized integration procedure which is introduced in this work. The derived models are tested regarding their ability to describe scattering and absorption by particles. Besides the scattering and absorption properties of the particles, all remaining parameters determining radiative heat transfer in coal combustion scenarios are varied and their influence on heat transfer is investigated and the results are discussed. The most important finding for the present work is that with a good approximate scattering phase function, scattering by coal and ash particles can be described very reasonably. Finally, this thesis can be used as a guide on how to treat radiative heat transfer in coal combustion simulations.