Numerical Investigation of Reheat Hydrogen Flames in the Sequential-Combustion Stage of a Heavy-Duty Gas Turbine

Abstract Recent theoretical studies and experimental evidence suggest that turbulent burning-rate augmentation, flame instabilities and NOx emissions, notoriously characterizing fuel-lean hydrogen premixed combustion, are significantly mitigated at reheat combustion conditions. This is due to the favorable effects of high reactants temperature in reducing the strength of thermo-diffusive instabilities that occur in hydrogen premixed combustion with augmented severity for increasing pressure and flame temperature. In this context, Ansaldo’s Constant Pressure Sequential Combustion (CPSC) system appears as an attractive approach to enable hydrogen firing of gas turbines that target high flame temperatures to retain high cycle efficiency. The present numerical modelling effort represents the first attempt to perform high-resolution Large-Eddy Simulation (LES), featuring detailed chemical kinetics and a fully compressible representation of the reactive flow, of hydrogen reheat combustion in a full-scale industrial combustor geometry with realistic geometrical features. Building upon earlier numerical modelling efforts that were limited to generic and geometrically simplified configurations with idealized reactants mixing conditions (GT2022-83218) 1, the ability of the turbulent combustion model to predict injection of the hydrogen fuel, mixing with the vitiated oxidizer stream and spontaneous ignition of the reactants mixture at the expected stabilization location is verified. Low and high flame-temperature conditions for 100% hydrogen-firing of the engine are simulated confirming that the numerical results are in accordance with the expected flame stabilization behavior observed in test-rig experiments. Furthermore, an analysis of the hydrogen premixed flame structure at reheat combustion conditions is provided highlighting the differences observed at various locations within the combustion chamber.