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Hydrogen is one of the most promising fuels to decarbonize energy systems since it has a high specific energy, a carbon-free combustion process, and may be produced sustainably through electrolysis. Direct fuel drop-in replacement cannot be done in traditional lean, premixed combustion burners because of the high reactivity of the hydrogen-air mixture and its inherently unstable nature that might lead to flashback and hardware failure. Consequently, new injection strategies and burner geometries are investigated to mitigate these risks. Here we present hydrogen capabilities of a premix/micromix injector that relies on a two-staged fuel injection strategy to offer wide fuel flexible capability (methane to hydrogen) within a single design. Five injectors are placed in a cross-shaped array to simulate a sector found in multi-element combustion systems. Stability and combustion dynamics maps are obtained for the array and OH planar laser-induced fluorescence (PLIF) provides additional insight into the combustion process of these novel burners when placed in an array. The use of micromixing is shown to drastically improve the acoustics of this geometry and expand the flashback limits compared to the fully premixed configuration. Significant variability in flashback limits are observed for different additively-manufactured injector configurations. Phase-averaged OH-PLIF measurements, obtained by registering the acquisition with the acoustics module, and the 3D reconstruction of a partially-premixed flame highlight the complexity of the stabilization mechanisms for these highly three-dimensional, non-axisymmetric flames that may be subjected to large thermoacoustics. This first investigation into premix/micromix clustered injectors demonstrates the importance of better understanding the impact of flame–flame interaction in multi-element combustion systems with micromixed, or partially-premixed, combustion.

Hydrogen combustion is a promising alternative to fossil fuel combustion in an effort to reduce our carbon footprint. However, hydrogen combustion is prone to thermodiffusive instabilities largely dependent on differential diffusion, a phenomenon that can lead to higher probabilities of flashback in industrial burners, given hydrogen’s high reactivity and diffusivity. This paper evaluates low-swirl flames of methane and air enriched with hydrogen to highlight the onset of differential diffusion. Testing was conducted in a fully controllable swirl burner, where bulk velocity Uav = 13 m/s and swirl number S = 0.6 were kept constant for each hydrogen–methane blend to isolate increases in flame surface area from increases in turbulence intensity. Furthermore, each fuel blend of hydrogen and methane is evaluated at the same laminar flame speed of SLo = 0.267 m/s to isolate flame stretch effects on the turbulent burning rate. Combined hydroxyl (OH) PLIF and stereoscopic PIV at the National Research Council of Canada were used to analyze the OH fluorescence in a 2D-3C velocity field for each flame condition. High-speed PIV at McGill University was used to resolve local flame phenomena, such as local flame displacement velocity and flame stretch rate. Using these techniques, it can be observed that the flame displaces axially in response to turbulent flame speed while exhibiting increases in flamefront wrinkling. This increased corrugation due to flame stretch is highlighted in the PDFs of local curvature and κSf and is further evidenced by a shift towards positive curvatures (κ > 0) for increasing H2 volume fraction. This trend suggests that there is a strong correlation with increases in turbulent burning rate and positive curvature as a result of differential diffusion, but it is not necessarily a control mechanism of the most forward propagating points proposed by the theory of leading points.

Plasma-assisted combustion has the potential to improve flame stability and combustion performance. Among all the different types of plasma source, nanosecond repetitively pulsed (NRP) discharges have received a lot of attention since they can produce nonequilibrium plasma containing excited species, ionized species, and radicals. These species can then accelerate combustion processes by opening new kinetic pathways. NRP discharges can also affect combustion through thermal and hydrodynamic pathways. This study investigates the hydrodynamic effect of NRP discharges produced throughout the surface of a lean premixed methane-air flame at atmospheric pressure. Time-resolved imaging was used to study the effect of the NRP discharges pulse repetition frequency (PRF). The imaging showed that the flame stabilizes further upstream as the PRF increases and that the flame remains steady and does not relax between consecutive discharges. The flame relaxation was also studied using time-resolved imaging after abruptly turning off the NRP discharges. This revealed that the flame takes about 50ms to completely relax to its unactuated position. Particle image velocimetry showed that the NRP discharges substantially reduce the flow velocity in their vicinity, which causes the flame to move upstream and stretch. The induced stretch was found to increase flame speed though nonequidiffusive effects. The experiment was repeated with a lean premixed propane-air flame. In those conditions, the flame speed was unaffected by the NRP discharges, as expected for a mixture that is less sensitive to nonequidiffusive effects. Hence, the hydrodynamic effect of NRP discharges appears to dominate over the thermal and kinetic pathways for the conditions investigated.

As the foundation for detailed hierarchical combustion chemistry models, the accuracy of the hydrogen oxidation mechanism is paramount to achieve truly predictive models. The recent introduction of new chemically termolecular reactions to this system suggests that our understanding of this fundamental chemistry could have initially been plagued with structural inaccuracies that have been carried along in the development of larger thermochemical mechanisms. Consequently, there is a need for additional independent experimental data in various experimental setups to assess the accuracy of model predictions. An atmospheric jet-wall stagnation-flame configuration is used in this work to measure the reactivity of premixed, lean-to-rich hydrogen-air flames diluted with argon. Mixtures with estimated adiabatic flame temperatures of ∼1800K are obtained by argon dilution to maintain the oxygen-nitrogen ratio while allowing for stable, laminar flames over the desired conditions. The accuracy of various thermochemical mechanisms available in the literature is assessed with direct comparisons of the velocity profiles in the stagnation configuration obtained through Particle Tracking Velocimetry to the solution of the quasi–1D flame simulation. Unstretched laminar flame speeds (SLo) are subsequently inferred using a direct comparison technique between the experiments and the simulations at multiple stretch values, similarly to a non-linear extrapolation to unstretched conditions. Significant discrepancies are observed between model predictions and experimental measurements, which supports the current efforts in model development. The inaccuracies in kinetic rates of the hydrogen oxidation model must first be resolved to avoid optimization and validation errors in the hierarchical development of increasingly complex hydrocarbon mechanisms.