Experimental investigation of combustion dynamics and NOx/CO emissions from densely distributed lean-premixed multinozzle CH4/C3H8/H2/air flames

Abstract

Lean-premixed fuel-flexible combustion technology capable of operating on a wide range of fuels, including high or pure hydrogen, is expected to play a crucial role in reducing nitrogen oxides and carbon dioxide emissions from heavy-duty gas turbine engines, ultimately in achieving energy system decarbonization. However, fuel-flexible operations, particularly with high hydrogen content fuels, require a fundamental, drastic change from conventional nozzle architecture. The use of a number of small-scale multitube injectors – also referred to as micromix nozzles – is indispensable to reduce the likelihood of flashback events in ultra-fast premixed hydrogen flames. At present, the vast majority of what is known about the fundamental mechanisms of combustion dynamics and emissions stem from intensive research in large-scale swirl-stabilized flames. The behavior of small-scale multinozzle flames governed by isolated or collective movements of constituent flames remains, however, largely unexplored. To address these questions, we use densely distributed sixty-injector mesoscale flames as a platform to explore the influence of a full range of CH4/C3H8/H2 fuel blends – including all possible combinations of a blend of two different fuels (CH4 + C3H8, CH4 + H2, C3H8 + H2) and three different fuels (CH4 + C3H8 + H2), as well as single-component fuels (CH4, C3H8, H2) – on combustion dynamics and exhaust emissions under a constant adiabatic flame temperature condition. Here, we show that while the nitrogen oxide emissions are almost unaffected by the fuel compositions considered here, the variation of fuel composition has a significant impact on self-excited instabilities, which tend toward higher frequency oscillations with increasing hydrogen concentration. From phase-resolved OH PLIF measurements for a 50/50 mixture of CH4 and H2, we identify the creation, evolution, and annihilation of an array of coherent vortical structures as the mechanism responsible for sound generation and flame surface modulations without strong interactions between adjacent flames. In contrast, the behavior of the 50% C3H8 + 50% H2 case is dictated primarily by periodic merging and separation of neighboring reactant jets, leading to large-scale asymmetric oscillations in the transverse direction. Given the substantial change in transport properties, effective Lewis number-related interpretation provides a reasonable explanation for the different behaviors of a cluster of small-scale flames.