Session: 04-17 Hydrogen Emissions II

Submission Number: 175739

Effect of Flame Stabilization Regime on the Nox Emissions of a Hydrogen Multi Jet in Swirled Crossflow Burner 

Hydrogen is a promising carbon-free energy carrier for gas turbine applications. However, its inherently high flame temperature and fast flame speed necessitate non-premixed injection schemes, which can lead to elevated NOx emissions. The incorporated injection schemes therefore need to offer fast mixing, stable flames and detachment of the flame from solid components. To achieve stable flames, low thermal loads and low NOx emissions, swirling flows have to be used in an adequate amount. Swirl is known to improve mixing and allow the stabilization of shorter recirculation-based flames, and thereby reduce residence times in hot regions, ultimately lowering NOx emissions. The burner under investigation employs a multi hydrogen jet in crossflow configuration, enabling rapid mixing through strong interactions between air and hydrogen while maintaining the reactant streams separate. Additionally, varying intensities of crossflow swirl are applied to study the interplay between flame stabilization regimes and NOx emissions.

Simultaneous OH- and acetone-PLIF diagnostics were employed to characterize three distinct flame stabilization modes. At high equivalence ratios, the burner stabilizes anchored jet flames. With increasing crossflow swirl or decreasing equivalence ratio, the flame transitions to an anchored M-shaped configuration, consisting of an outer diffusion-based reaction zone in the shear layer of hydrogen and air and a counter flow diffusions flame within the inner recirculation zone (IRZ). At sufficiently high crossflow velocities, the flame jets detach from the injector, stabilizing as a partially premixed M-shaped flame consisting of an edge flame stabilized in the shear layer of hydrogen and air downstream of the injection and a similar counterflow diffusion flame as for the M-shaped flame.

NOx emissions and combustor exit temperatures were measured via FTIR spectrometry for varied thermal powers between 3–17 kW, equivalence ratios of 0.1–0.9, and swirl numbers up to 1.79. The exit temperatures have shown to be insensitive towards changes in the swirl intensity, which allows the identification of the changes in stabilization regimes as the primary reasons for changes of the emission behaviour. The results confirm that decreasing equivalence ratio and increasing swirl intensity both reduce NOx emissions. A distinct reduction in NOx is observed when transitioning from jet flames to M-shaped flames, attributed to the formation of the IRZ, while flame lifting shows negligible impact. The effective swirl number proposed in a prior work proved to be a suitable parameter for describing the combined influence of equivalence ratio and swirl on NOx formation, allowing the definition of an optimal operational window characterized by low emissions and stable operation in the lifted partially premixed regime [1]. Finally, the applicability of the scaling law proposed by Magnes et al., which relates NOx emissions to residence time and adiabatic equilibrium temperature, was assessed. While valid under constant swirl conditions, the scaling law shifts towards lower emissions with increasing crossflow swirl [2]. To increase the predictive accuracy, an empirical correlation incorporating swirl effects as well as a flame-surface-based approach were evaluated, demonstrating improved agreement with experimental data.

[1] Koch et Al.; Flame characteristics and lift-off dependencies of flames stabilized on a multi hydrogen jet in swirled crossflow burner; Proceedings of the Combustion Institute Vol. 41

[2] Magnes et Al.; Impact of preheating on flame stabilization and NOx emissions from a dual swirl hydrogen injector, Proceedings of ASME Turbo Expo 2023

Presenting Author: Lars Koch Institute of Technical Combustion

Presenting Author Biography: Functional developer - Engine Control Unit - FEV Norddeutschland (04/2020-04/2021)
Team Lead - Functional Development - Thermal Management - FEV Norddeutschland (04/2021-10/2021)
Research Assistant - Institute of Technical Combustion - Leibniz University Hannover (10/2021- present)

Authors:

Lars Koch Institute of Technical Combustion
Friedrich Dinkelacker Institute of Technical Combustion