Experimental and numerical investigations of multi-element lean-premixed hydrogen flame dynamics

Abstract

Hydrogen-fired gas turbine combustion technology is expected to play a key role in accelerating large-scale energy system decarbonization. However, the high reactivity of hydrogen poses significant challenges in designing safe and reliable gas turbine combustion systems. These challenges include increased NOx emissions, potential risks of detrimental flashback events, and modified thermoacoustic instabilities. All of these aspects are addressed in this doctoral dissertation. The present work aims to investigate combustion instabilities in lean-premixed hydrogen flames through both experimental and numerical approaches. To address flashback issues in ultrafast lean-premixed hydrogen flames, a multi-element nozzle array is employed, consisting of 293 small-scale injectors, each with a 3.0 mm inner diameter. Various measurement techniques, including high-speed OH$^*$ chemiluminescence imaging, OH planar laser-induced fluorescence, acoustic pressure measurements, photomultiplier tubes, and hot-wire anemometry, are utilized. Additionally, reduced-order thermoacoustic modeling analysis is applied. For the case of forced flame response, Large Eddy Simulation (LES) is performed using the Ansys Fluent CFD solver under both non-reacting and reacting flow conditions, and the results are then compared against experimental data. The first part of the study investigates self-excited instabilities of lean-premixed pure hydrogen flame ensemble under a broad range of operating conditions. The findings reveal that ultra-compact pure hydrogen flames generate high-amplitude pressure oscillations over a wide range of characteristic frequencies, from 400 to 1800 Hz, corresponding to the third- to tenth-order eigenmodes. Low-frequency flame dynamics exhibit complex interactions between vortices and periodic extinction-reignition processes, leading to large-scale asymmetric oscillations of the entire reaction zone. Intermediate-frequency dynamics display symmetric oscillations with flame merging and pinch-off, without interactions between constituent flames. High-frequency instabilities, surprisingly, are not influenced by structurally complex flame dynamics but instead exhibit a simple back-and-forth motion. This suggests that densely distributed lean-premixed hydrogen-air flames can sustain unstable combustion across a broad spectrum of time scales by modifying the spatiotemporal evolution of the flame ensemble. The second part of the study focuses on understanding the effect of hydrogen content on the thermoacoustic and emission characteristics of multi-element lean-premixed hydrogen/methane/air flames. The results indicate that the system’s response can be classified into several distinctive stages based on their static and dynamic stabilities. These stages include flame blowoff and thermoacoustically stable regions under relatively low hydrogen conditions, low-frequency instabilities at intermediate hydrogen concentration, and the triggering of intense pressure perturbations at approximately 1.7 kHz under high or pure hydrogen conditions. While the flame dynamics subjected to the lower hydrogen concentrations are described by axisymmetric longitudinal motions of parallel flame fronts, the response of higher hydrogen content flames is more pronounced in the transverse direction, accompanied by small-scale vortex roll-up and flame surface annihilation. The longitudinal-to-transverse dynamics plays a mechanistic role in accommodating higher-frequency heat release rate fluctuations, and this newly identified mechanism suggests the possibility of high-frequency transverse modes if such lateral motions are strong enough to induce inter-element flame interactions. Contrary to the substantial differences in thermoacoustic properties for different fuel compositions, the total nitrogen oxides emissions primarily depend on the adiabatic flame temperature. The final part of the study employs an integrative approach, combining loudspeaker-forced direct measurements and LES-based numerical simulations, to explore the dynamic response of multi-element lean-premixed hydrogen flames to harmonic velocity perturbations. The electronically-excited OH intensity distribution, generally assumed to be equivalent to the flame’s heat release rate, shows an anchored conical reaction zone. However, numerical simulations of heat release rate contours reveal the formation of a more concentrated thin annulus region created by preferential diffusion of hydrogen molecules. These results highlight discrepancies between OH intensity distribution and heat release rate contours, suggesting uncertainties in surrogate-dependent flame transfer function evaluations. By employing a reduced-order network modeling framework, the accuracy of predicting self-excited instabilities is tested, and LES-based transfer functions provide more accurate predictions of system stability compared to measurements. This study also identifies moderate transverse oscillation of local energy-concentrated regions as pivotal processes controlling high-frequency hydrogen combustion dynamics. Taken together, the combustion instability phenomenon arising from the coupling between turbulent flames and the acoustic field of the system exhibits significant nonlinear characteristics, emphasizing the importance of acquiring experimental data using a well-designed laboratory setup. The current experimental and numerical investigations provide valuable insights into the dynamics of clustered hydrogen flames under a wide range of operating conditions. These findings are expected to serve as fundamental knowledge for effectively managing high-frequency instabilities in future hydrogen-fired gas turbine engines.