59842 - Numerical Investigation of Potential Cause of Instabilities in a Hydrogen Micromix Injector Array 

Hydrogen micromix combustion is a promising concept to reduce the environmental impact of both aero and land based gas turbines by delivering carbon-free and ultra-low-NOx combustion without the risk of autoignition or flashback. The ENABLEH2 project aims to demonstrate the feasibility of such a switch to hydrogen for civil aviation, within which the micromix combustion, as a key enabling technology, will be matured to TRL3. The micromix combustor comprises thousands of small diffusion flames where air and fuel are mixed in a cross-flow pattern. This technology is based on the idea of minimizing the scale of mixing to maximize mixing intensity. The high-reactivity and wide flammability limits of hydrogen in a micromix combustor can produce short and low-temperature small diffusion flames in lean overall equivalence ratios.

    There is limited published information on the instabilities of such hydrogen micromix combustors. Diffusion flames are less prone to flashback and autoignition problems than premixed flames and conventional diffusions flames are less prone to combustion dynamics issues. However, with the high laminar flame speed of hydrogen, lean fuel air ratio (FAR) and very compact flames, the risk of combustion dynamics for micromix flames should not be neglected. In addition, the multi-segment array arrangement of the micromix injectors could result in both potential causes and possible solutions to the instabilities within the combustor.

    This paper employs numerical simulations to investigate potential sources of instabilities in micromix flames by modelling an extended array of injectors, represented by either single or multiple injectors with appropriate periodic or symmetry boundary conditions. Analyses represented combustion at elevated pressure and temperature. By changing injector geometrical design parameters, the shape and position of the flames vary, as well as the acoustic behavior. RANS simulations were performed and used to derive the Flame Transfer Function (FTF) of the micromix flames to inform lower order thermoacoustic modelling of micromix combustion. A number of LES simulations were conducted with various injector designs. It was observed that, for certain representative micromix flames, there are persistent high-frequency instabilities due to the interaction of the flames. This phenomenon is not observed when only a single injector is modelled and it is evident that the acoustic waves propagating in the radial direction are a key factor in the instability. When two or more injectors are modelled in the radial direction, high amplitude instabilities are observed suggesting that the effective radial boundary conditions are critical in the evolution of these instabilities.

    It is suggested that the observed high-frequency instabilities are related to aerodynamic jet instabilities enhanced by or coupled with the acoustic feedback. Only transient simulations such as LES will be able to capture such effects and RANS simulations typically used in early stage design will not identify this issue. A real engine combustor will have a much larger number of injectors arranged radially together with real physical boundaries. The impact of real physical boundaries and its implications for real combustors will be investigated further in future studies.