Light-round ignition dynamics of a hydrogen-fueled annular combustor: Parametric effects and reduced-order modeling

One issue in the operation of annular combustors is to ensure a reliable light-round ignition that will establish flames on all injector units without excessive pressure excursion and in a relatively short period of time. This issue is here examined in the case where the annular system is fed with pure hydrogen by combining experimentation and reduced order modeling. Systematic experiments are carried out in a model scale configuration equipped with multiple injectors, in which pure hydrogen is delivered in cross-flow in a swirling stream of air. In this large set of experiments, ignition initiated by a single spark plug gives rise to a couple of flames traveling in clockwise and counterclockwise directions which at a later stage, propagate head-on and merge. It is found that the duration of this process is much shorter when the combustor is fed with pure H2 than when it is operated with gaseous propane–air mixtures or liquid sprays of heptane or dodecane. Systematic observations of the final stage before flame merging indicates that a layer of fresh reactants is formed between the two flame branches which slows down the flame propagation. This flame deceleration is here documented in the case of hydrogen flames. A reduced order model, that accounts for this final stage, is shown to suitably capture effects of global equivalence ratio and injection velocity on the light-round time delay. Experiments also provide indications on effects of injector swirl number and preheating of the chamber walls. In contrast with previous experiments with hydrocarbon flames, it is found that preheating has only a marginal effect on the light-round time. An examination of pressure records during ignition is finally carried out to quantify the amplitude of the ignition-induced pressure excursion. A novel scaling law is derived to estimate the corresponding pressure peaks and this model is shown to be consistent with experimental data. It is also found that under certain conditions, the ignition is followed by a cyclic regime corresponding to a thermoacoustic oscillation that is shown to be coupled by the first azimuthal mode of the chamber. Novelty and significance statement The novelty of this research lies in the experimental investigation of light-round ignition in an annular combustor fed with pure hydrogen, a case not well documented in the literature. The broad set of experiments reported in this article provides a comprehensive view of the influence of global equivalence ratio, injection velocity, swirl number and wall preheating on the light-round characteristic time in the case of pure H 2 injection and probably constitutes the only data set that fills an identified gap of knowledge. The flames slowdown in their head-on merging is also analyzed for the first time in the hydrogen case. A reduced-order model of the light-round ignition accounting for equivalence ratio and injection velocity from previous work of the same team is here extended to account for this slowdown phenomenon and shown to provide improved estimates of the flame velocity and light-round time delay. The pressure peak induced by the process is analyzed, a novel scaling law is derived to estimate this quantity and is shown to be consistent with experimental data.