Highly Strained Lean Premixed Hydrogen Flames

Abstract

In the urgent need to decarbonise energy systems, hydrogen combustion is set to play a key role in hard-to-electrify sectors such as aviation, power generation, and heavy industry. However, the practical deployment of hydrogen combustion faces critical challenges, including high reactivity, high NOx emissions, and thermodiffusive instabilities, which compromise flame stability and control. While hydrogen premixed flames show a distinctive response to strain compared to other fuels, the fundamental effects of strain on hydrogen flame dynamics and emissions remain poorly understood. Furthermore, lean premixed hydrogen flames feature differential and preferential diffusion effects which lead to the onset of thermodiffusive instabilities. These instabilities, in turn, interact with turbulence and strain. Existing computational fluid dynamics (CFD) models struggle to accurately and affordably predict these distinctive features, thereby limiting the development of safe and low-emissions hydrogen combustion devices in industrial design frameworks. Addressing these gaps is essential for advancing hydrogen combustion technologies, particularly within the aviation and transportation sectors where they are still at a low Technology Readiness Level (TRL).

This thesis aims to contribute to the development of more accurate and affordable CFD tabulated-chemistry large eddy simulation (LES) models of lean premixed hydrogen flames subjected to intensive strain, thereby advancing the capabilities to optimally design hydrogen combustor leveraging strained regimes. First, the fundamental hydrogen flame response to strain is investigated extensively from the point of view of emissions, flame structure, and flame stability through high-fidelity detailed chemistry simulations in simplified laminar settings. Hence, with the help of the insights gathered in the previous phase, novel tabulated chemistry modelling approaches are proposed for LES of strained and turbulent hydrogen flames. The proposed models are tested a priori at unfiltered and filtered grids in a turbulent counterflow setup, where strain is established both by shear-driven turbulence and by the configuration....