Influence of Autoignition Kernel Development on the Flame Stabilisation of Hydrogen-Nitrogen Mixtures in Vitiated Air of High Temperature

Autoignition and flame stabilisation in a gas turbine combustor presents severe challenges for safe and reliable gas turbine operation as soon as they occur in parts of the combustor that are not designed to sustain higher thermal loads. Especially when operating on highly-reactive fuels like hydrogen, higher autoignition and flashback risk associated with these fuels have to be taken into account. In the present study, flame stabilisation initiated by autoignition events is investigated in an optically accessible mixing duct of a generic reheat combustor at typical reheat conditions. The experiments were conducted at pressures of 15 bar, vitiated air temperatures higher than 1100 K and bulk velocity of 200 m/s. The fuel was a hydrogen-nitrogen mixture with up to 70 vol. % hydrogen and was injected by a coflow inline injector along with preheated carrier air of temperatures up to 623 K. The autoignition-driven flame stabilisation process was investigated by recording the luminescence signal with high-speed cameras and by tracking the temporal and spatial development of autoignition kernels in the mixing duct. A detailed and comprehensive data set could be generated providing the basis for an in-depth analysis of the stabilisation process on time scales down to 0.3 milliseconds, which is fast enough to disclose the small timescales at which the autoignition kernels develop in the mixing section. A stabilising sequence was found to lead to the stabilised flames due to a non-interrupted sequence of autoignition kernels. The stabilising sequence was found behave differently in two different temperature regimes where sequence durations and amounts of kernels differed significantly from each other. A state in which the cross section of the mixing section is fully blocked by one or more kernels in vertical direction could be identified for all operating conditions and the development of subsequent autoignition kernels after the section blockage changed clearly once this state was reached.