Combustion dynamics of multi-element lean-premixed hydrogen-air flame ensemble

摘要

The crux of the technical issues facing the development of hydrogen gas turbine engines is the problemof how to enable low NOx, low dynamics combustor operations without detrimental flashback events inextremely fast lean-premixed hydrogen flames. While flashback mechanisms and nitrogen oxides emissionshave been investigated extensively for high hydrogen content flames, the self-excited dynamics oflean-premixed pure hydrogen flame ensembles remain unclear. Here we use phase-resolved OH ? chemiluminescenceand OH PLIF imaging in a series of experiments in a multi-element injector configurationconsisting of 293 small-scale injectors with an inner diameter of 3.0 mm. We collect a large experimentaldataset to explore a variety of collective phenomena of the lean-premixed hydrogen-air flame ensemble,and perform systematic investigations of self-induced combustion instabilities. Our observations demonstratethat ultra-compact pure hydrogen flames generate high-amplitude pressure perturbations over abroad range of characteristic frequencies between 400 and 1800 Hz, corresponding to the third to tenthorder eigenmodes. Low-frequency flame dynamics developed under relatively low equivalence ratio conditionsinvolve a complex balance among several coexisting phenomena, including strong vortex interactionsand periodic extinction-reignition processes, giving rise to large-scale asymmetric oscillations of theentire reaction zone. By contrast, the flame surface dynamics at an intermediate frequency of ～600 Hz exhibitprominent symmetric oscillations accompanied by merging and pinch-off of the constituent flames.Unexpectedly, high-frequency instabilities at approximately 1720 Hz (screech tones) are not influencedby such structurally complex flame dynamics, but by exceptionally simple, seemingly linear, flame surfacemotion without sudden flame area annihilation events like those observed for lower frequency cases.Despite the extremely low level of heat release rate fluctuations, on the order of less than 1%, the clusteredpremixed hydrogen flames are capable of producing disproportionately large pressure perturbationsin excess of 12 kPa, originating from the synchronous phase dynamics of acoustic pressure and clusteredflames’ heat release rate oscillations. These findings provide new insight into the driving mechanismsunderlying high-frequency combustion dynamics of densely arranged pure hydrogen-air flames.