Numerical investigation of chemiluminescence and acoustic radiation in hydrogen flames

Hydrogen combustion is a promising solution to reduce carbon emissions in gas turbines for aircraft propulsion and power generation. However, the markedly different physical and chemical properties of hydrogen compared to conventional hydrocarbon fuels pose significant modeling and technological challenges. To improve the efficiency and reduce nitrogen oxide emissions, gas turbines are commonly operated under lean premixed conditions, which, however, promote thermoacoustic instabilities. Moreover, for hydrogen flames, this regime is characterized by a sub-unity Lewis number, promoting strong thermodiffusive effects and intrinsic flame instabilities. To characterize thermoacoustic instabilities, it is fundamental to capture the unsteady heat release rate (HRR). Its direct experimental measurement, however, is impractical, hence chemiluminescence of excited radicals is often used as a surrogate. Moreover, thermoacoustic instabilities can be triggered by combustion noise, which has become an increasingly critical concern also due to stricter noise regulations. This dissertation addresses these challenges, focusing on chemical modeling, applicability of chemiluminescence as a HRR surrogate, and combustion noise generation, through high-fidelity numerical simulations. Indeed, Computational Fluid Dynamics is a fundamental tool for investigating hydrogen flames, yet most of the existing numerical approaches require adaptation. First, a global one-step reaction mechanism is developed for premixed hydrogen-air flames. The reduced scheme is designed to reproduce the main laminar flame properties over a wide range of pressures, unburned gas temperatures, equivalence ratios, and stretch conditions. An explicit analytical dependence of the reaction rate parameters on pressure and equivalence ratio is introduced, together with a correction for the thermal flame thickness. The model is validated in canonical laminar flame configurations, providing sufficiently accurate results, while yielding a significant improvement in computational efficiency. Second, the adequacy of the excited hydroxyl radical OH* as a HRR marker is investigated for laminar and turbulent hydrogen-air flames. In laminar premixed hydrogen flames, a systematic spatial shift between the OH* and HRR peaks is observed, attributed to the specific chemistry of hydrogen oxidation, and to the role of the H-radical in the OH* formation, which enhances post-flame OH* production. The effects of pressure, unburned gas temperature, and flame stretch are analyzed, showing that stretch and intrinsic flame instabilities can significantly degrade the OH*-HRR correlation in premixed flames, while OH* remains a reliable HRR marker for stretched hydrogen-air diffusion flames. These conclusions are supported by three-dimensional simulations of turbulent diffusion and partially premixed flames stabilized in the HYLON burner, highlighting the importance of explicitly accounting for OH* kinetics in numerical simulations when interpreting experimental chemiluminescence measurements. Finally, combustion noise generation is studied in laminar and turbulent premixed hydrogen-air flames using Direct Numerical Simulations. In laminar acoustically forced M-shaped jet flames, numerical simulations, validated by experimental measurements, show that thermodiffusive effects modify the HRR distribution and flame dynamics, reducing the contribution of flame annihilation events to the overall acoustic radiation. At higher frequencies, resonance with intrinsic flame dynamics triggers thermodiffusive instabilities, leading to enhanced local HRR unsteadiness. Comparisons between simulations performed with detailed and reduced chemical schemes reveal that the latter captures the main trends, but overestimates noise at high forcing frequencies due to an exaggerated stretch response. The reduced scheme is further exploited to isolate Lewis number effects by artificially modifying the transport properties of species, while preserving the main laminar flame properties. Removing thermodiffusive effects promotes flame annihilation, similarly to hydrocarbon flames, but a separate analysis of differential and preferential diffusion reveals an antagonistic interaction of the two, with preferential diffusion mitigating the impact of differential diffusion in thermodiffusively unstable lean premixed hydrogen flames. In turbulent premixed slot jet flames, thermodiffusive effects strongly influence HRR fluctuations and flame surface dynamics, leading to increased low-frequency acoustic radiation and to a steeper high-frequency spectral decay compared to thermodiffusively stable methane flames. A theoretical framework extending the classical flamelet theory to thermodiffusively unstable flames is proposed, linking combustion noise to flame surface fluctuations. In addition, instabilities at the shear layer between hydrogen combustion products and ambient air are enhanced, thereby promoting localized density gradients and finer scale structures that affect the acoustic emission at low frequencies. These results suggest that hydrogen differential diffusion impacts both direct and indirect combustion noise generation mechanisms. By varying the Karlovitz number, it is shown that increasing turbulence intensity suppresses thermodiffusive instabilities and shifts the acoustic emission toward higher frequencies, reducing the contribution of thermodiffusive effects on direct combustion noise. Additional simulations of the low Karlovitz case using reduced chemistry confirm that, as in the laminar M-shaped flame, the global mechanism captures the main features of the flame, albeit overestimating its response to stretch in the highly corrugated regions and the high-frequency acoustic radiation. Imposing unity Lewis numbers significantly attenuates the acoustic radiation, confirming the noise-amplifying role of thermodiffusive effects in lean premixed hydrogen flames.