Stabilization and emissions characteristics of CH4–H2 blends in a premixed jet stabilized combustor

Abstract

Low emissions and fuel flexibility are two important criteria required for gas turbine combustors to facilitate the energy transition to low-carbon fuels for propulsion and power applications. A jet-stabilized combustor, having both these characteristics, was operated with CH₄–H₂ fuel mixtures with H₂ varying from 0 to 100 % and with varying equivalence ratios ($\varphi$). Comprehensive measurements were carried out of the velocity field using Particle Image Velocimetry (PIV), temperature and gas composition by traversing probes in the chamber, and flame topology using chemiluminescence imaging. The flow field in this combustor consists of a jet that undergoes recirculation, generating Central and Peripheral Recirculation Zones (CRZ and PRZ). The recirculation ratio in the PRZ is found to be twice that of the CRZ. Increasing H₂ % for the same $\varphi$ leads to higher NO_x. Ultra-low $\varphi$ flames could be stabilized only at H${}_2\ge$50 %, which in turn leads to low NO_x due to low adiabatic flame temperatures. The combination of temperature, gas composition (CO/NO), and chemiluminescence images is used to identify the extent and location of the reaction zone. Distributed reaction zones, stabilizing at around 30 % of the length of the chamber, are achieved at lean conditions, whereas an increase in H₂ % makes the reaction zone more compact and shifts upstream towards the burner head. Flame kernels are extracted from the instantaneous chemiluminescence images, and probability distribution functions for their aspect ratio and axial location are constructed. It is seen that reducing $\varphi$ leads to low aspect ratio kernels that tend to occur further downstream, whereas increasing H₂ % leads to higher aspect ratio kernels, stabilizing upstream. These flame kernel statistics are also used to identify ignition modes (autoignition/flame propagation) for varying fuel H₂ % and inlet $\varphi$ based on a hypothesis of flame stabilization mechanisms.