title: Modeling and simulation of near-wall combustion of renewable fuels
Creators: name: Luo, Yujuan
Publisher: name: UNSPECIFIED
Year: 2023

description: The downsizing concepts in the design of modern combustion engines and gas turbines result in an increased surface-to-volume ratio in combustion chambers, which amplifies the influence of flame-wall interaction (FWI). Due to the low temperature in the near-wall region, the flame structure may be significantly altered and quenching is likely to occur, which leads to lower combustion efficiency and increased pollutant formation. Therefore, near-wall combustion is substantially different from unconfined combustion. Furthermore, the coexistence of heat loss and other phenomena such as differential diffusion effects, mixing-induced mixture fraction variation, and turbulence brings additional complexity. In order to gain a deep understanding of the unique characteristics caused by FWI, combustion processes in a small vicinity of the wall need to be investigated in detail. Besides experimental measurements, numerical simulations have become an increasingly powerful tool for such purposes. Although fully resolved simulations with detailed kinetics provide the most accurate results and are useful for comprehensive analyses, they are usually adopted in simple configurations. For complicated industrial applications, manifold-based reduced kinetic models represent an ideal alternative that is able to reduce the computational cost significantly while maintaining high accuracy. Based on the above background, this thesis mainly contributes to the development and application of manifold-based reduced kinetic models for near-wall combustion. Specifically, this thesis deals with (1) analyses of the fundamentals of the FWI processes, providing guidance for modeling, (2) the development of manifold-based reduced kinetic models with different levels of complexity, and (3) the application of various manifold-based reduced kinetic models. Different scenarios where near-wall combustion plays a role are investigated numerically. Starting from the simplest canonical configuration for FWI, one-dimensional laminar premixed head-on quenching (HOQ) is simulated with detailed kinetics and transport. Unlike previous studies that focused only on unstrained conditions, the relevance of the underlying flow is taken into consideration in the present study. The focus is the analysis of the influence of the strain rate and finding suitable parameters that can characterize the strain rate both for propagating and quenching flamelet manifold. As for the two-dimensional laminar side-wall quenching (SWQ) setup, efforts are put on the development of reduced kinetics based on Reaction-Diffusion Manifolds (REDIM) generated and formulated within the framework of generalized coordinates. Compared to the manifold-based reduced kinetic models in thermokinetic coordinates that were adopted in all previous related studies, this kind of new model presents advantages in dealing with non-invariant manifolds. Different models are proposed for scenarios where strong heat losses exist without and with prominent differential diffusion effects, respectively. Model validation is carried out on configurations where experimental data is available. As an extension, an on the fly technique is further developed to reduce the dependence of the model accuracy on the a-priori identification of the system. While it was shown for laminar FWI that the choice of the manifold is crucial in the fully coupled simulations, this has not been investigated for turbulent FWI yet. To fill the gap, three different flamelet manifolds: Flame-Generated Manifold (FGM), the Quenching Flamelet-Generated Manifold (QFM), and the Quenching Flamelet-Generated Manifold with Exhaust Gas Recirculation (QFM-EGR), are coupled to the Large-Eddy Simulations (LES) of turbulent SWQ of a premixed CH4-air flame. The performance of the manifolds is assessed by comparison with the flame-resolved simulation, and the advantages and disadvantages of each model are identified and discussed. Being a carbon-free energy carrier, hydrogen is a promising substitute for fossil fuels. To enable its application in combustion chambers, detailed investigations on the FWI of H2-enriched fuels are necessary. As a starting point, a new flamelet model extended from the FGM used in turbulent SWQ is proposed to simultaneously include multiple complex physics that may appear in realistic configurations, such as differential diffusion effects, heat loss, and mixing of different streams. The new model is validated for a CH4-H2-air Bunsen flame by comparing the results with those from the detailed kinetic simulation and experiments.