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.

The main contributions of this study are, to provide deeper insights into the design space of a hydrogen micromix injection system via numerical simulations. The geometrical design of the micromix combustion system mainly consists of the sizing of the air gates, and the hydrogen injector orifices; as well as, the offset distance between air gate and hydrogen injection, the mixing distance and the injector to injector spacing. This paper first presents results of the numerical simulation of four designs, down-selected from a series of combinations of aforementioned design parameters, including cases with low and high momentum flux ratio, weak and strong flame-flame interaction.

It was discovered that the hydrogen/air mixing characteristics, and the flame to flame interactions, are the main factors influencing the combustor gas temperatures, flame lengths and the corresponding NOx production. The current study then focused on the effect of air gate geometry on the mixing characteristics, flame shape and temperature. The momentum flux ratio was kept constant throughout this investigation by keeping the air gate area constant. Variations of the original baseline air gate design were studied, followed by a study on various other novel air gate geometries, including circular, semicircular and elliptical shapes.

It is concluded that higher NOx could be produced at lower global equivalence ratio due to strong flame-flame interactions at momentum flux ratios with high hydrogen jet penetration. When the momentum flux ratio between the hydrogen jet and air flow remains unchanged, by varying the actual distribution of the air flow, the hydrogen distribution and the mixing process can be significantly influenced, resulting in different shapes of the flame and temperature distribution. Among those designs, the elliptical-shaped air gate yields the lowest temperature and consequently the lowest NOx in the combustion zone.

Keywords: Hydrogen; Micromix; Combustion; Injector Design; Low emissions.


Numerical Investigation Into the Impact of Injector Geometrical Design Parameters on Hydrogen Micromix Combustion Characteristics

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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