Abstract
The use of highly reactive hydrogen-rich fuels in lean premixed combustion systems strongly affects the operability of stationary gas turbines (GT) resulting in higher autoignition and flashback risks. The present study investigates the autoignition behavior and ignition kernel evolution of hydrogen–nitrogen fuel mixtures in an inline co-flow injector configuration at relevant reheat combustor operating conditions. High-speed luminosity and particle image velocimetry (PIV) measurements in an optically accessible reheat combustor are employed. Autoignition and flame stabilization limits strongly depend on temperatures of vitiated air and carrier preheating. Higher hydrogen content significantly promotes the formation and development of different types of autoignition kernels: More autoignition kernels evolve with higher hydrogen content showing the promoting effect of equivalence ratio on local ignition events. Autoignition kernels develop downstream a certain distance from the injector, indicating the influence of ignition delay on kernel development. The development of autoignition kernels is linked to the shear layer development derived from global experimental conditions.
Highlights
Hydrogen-rich fuels can play a major role in achieving de-carbonisation targets in power generation systems with minimal CO2-emissions and minimal environmental impact as requested in the COP 21 goals
The ”subsequent” kernels form either at the same minimum hydrogen volume fraction, but after the ”first” kernel or at higher hydrogen contents at constant global mixing section parameters. They might be influenced by a change in local conditions (T, u, Φ) due to preceding kernels or due to the higher hydrogen mass flow injected into the mixing section or both. ”First” and ”subsequent” kernels are both convectively transported downstream out of the mixing section
The ”stabilising” kernels are a sub-type of the ”subsequent” kernels since for constant TMS and TC they occur at hydrogen volume fractions much higher than the autoignition limit (XH2,stab > XH2,min) and initiate a non-interrupted sequence of autoignition kernels which lead to a stable flame in the mixing section
Summary
Hydrogen-rich fuels can play a major role in achieving de-carbonisation targets in power generation systems with minimal CO2-emissions and minimal environmental impact as requested in the COP 21 goals. In this context, ensuring operational reliability remains one of the key aspects of GT combustor design Due to their higher reactivity, hydrogen-rich fuels pose significant challenges to the gas turbine design in terms of avoiding autoignition and flashback leading to flame stabilisation in combustor components, e.g. the premixing section, not designed to sustain higher thermal loads. Conditions favouring autoignition at the ”most reactive mixture fraction” are present for low scalar dissipation rates and a low mixture fraction gradient, as it is the case in vortex cores [2, 5], where fluid patches of high temperature and fluid patches of sufficient oxidiser or fuel content, respectively, are brought together by the strong mixing field In those vortex cores the residence time of a sufficiently pre-mixed fluid parcel can be high enough and above the ignition delay time, which is another pre-requisite for the development of autoignition kernels. The third requirement for the development of autoignition kernels, a sufficiently high local mixture temperature above the autoignition temperature of the local mixture, is a result of the local mixing process and strongly influenced by the turbulence-chemistry-interaction
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