LES modeling of hydrogen combustion in gas turbines: from ignition to flame stabilization

Hydrogen combustion technologies are nowadays studied and developed by many companies and universities since their employment as carbon-free systems is considered a valid alternative to reach net-zero CO2 emission by 2050. However, despite its advantages, the use of hydrogen presents numerous technical challenges that must be considered and solved, especially if it is used for aeronautical purposes. First, hydrogen does not exist in nature, and its large-scale production is a critical issue, as current generation methods involve the emission of CO\textsubscript{2}, which could nullify its benefits. Similarly, storage both on board aircraft and at airports is a problem given the hazardous nature of hydrogen. Finally, hydrogen has peculiar characteristics from a combustion point of view compared to the usual liquid fuels currently used. Focusing on the latter, an indispensable step that every aircraft combustor performs several times a day is the ignition phase. In fact, before entering service, each combustor must pass rigorous certification of the ignition phase, both on the ground and at high altitude. Therefore, the design of a new combustor requires a thorough and dedicated study due to the different characteristics of hydrogen. In this sense, modern high-fidelity Computational Fluid Dynamics (CFD) simulations are a key tool for understanding the physical phenomena and dynamics occurring inside a combustor, where experiments are very often limited due to operational problems. The present work aims to investigate the current standard models used in industry to study turbulent combustion with particular interest in the ignition phase up to the flame stabilization, simulating different conditions under which it can occur and analyzing the dynamics observed inside the injector that govern its success or failure. In the first part of the work, after introducing the models used in the course of this thesis, the impact of the diffusive transport model used is analyzed through laminar Direct Numerical Simulations (DNS), highlighting its effect in the first instants after kernel formation. This analysis, together with other preliminary studies carried out using the Cantera tool, allowed to identify the main objectives that need to be studied and deepened during the investigation of the two test cases examined. The numerical analyses carried out on the test rig at the Technische Universität Berlin (TUB) aim to test different approaches for modeling the turbulence-chemistry interaction on a technically premixed hydrogen flame. After an initial validation of the cold velocity field by means of a mesh sensitivity analysis, the impact on the prediction of the mixing field is assessed against the assumptions underlying the models used. Although small differences appear in areas where combustion does not take place, the correct flame prediction of the Thickened Flame Model (TFM) approach, due to the inclusion of local effects on the flame front, permits the selection of the most accurate model among those tested for the study of lean hydrogen flames. Following the analysis of the first burner, the investigation of the ignition transient of an academic test rig experimentally tested at the Norwegian University of Science and Technology (NTNU) is presented in which detailed measurements throughout the full process for different operating conditions are available. In the first part of the work, the simulation of a case that provides a successful ignition scenario is presented. It is demonstrated that the TFM coupled with an Energy Deposition (ED) strategy allows an accurate prediction of the most significant phases and mechanisms involved in the ignition dynamics, validating the employed numerical strategy. Subsequently, to mimic the effect of the back pressure at the outlet (e.g. a gas turbine nozzle), a perforated plate is introduced at the combustion chamber exit to slightly increase the blockage ratio. Despite the negligible additional back pressure, the dynamics of the system is drastically altered, promoting the flashback occurrence. The driving mechanisms that trigger the flashback and the flame-holding process inside the injector are identified and explained.