The turbulent premixed bluff body stabilized ammonia/hydrogen/nitrogen/air flame - A fundamental study for gas turbine combustor regarding extinction and molecular transport effects under different NH₃/H₂ ratios
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The gas turbine is a promising energy supply solution for AI data centers, attributed to its advantages regarding stable, high-power capacity, and fast startup. To get rid of the dependence on fossil fuel and mitigate the emission, renewable fuels produced by redundant green electricity, such as ammonia and hydrogen, become candidates of gas turbine fuel options to replace natural gas and other fossil fuels. Partially cracked ammonia has the advantages of both ammonia and hydrogen; thus, it is regarded as a potential fuel to be used in gas turbines. To mimic the flame within the flow field in practical gas turbine combustors, a bluff-body-stabilized flame is selected as the configuration to study the stretch flame behavior in the recirculation zone. Blow-off (extinction) is one of the most important challenges within the combustor, especially the near blow-off conditions are required for controlling emissions. Figuring out the principle of the bluff-body stabilized partially cracked ammonia flame blow-off is the main purpose of this study. Starting from the most simplified flame extinction process, a one-dimensional twin counterflow premixed flame is first studied to exclude the influence of turbulence, three-dimensional flow field, and flame surface wrinkling. By increasing the strain rate, the flame is first enhanced when pushing towards. The final extinction is caused by incomplete combustion when reactants have not enough space and time to react. With a fixed equivalence ratio, a non-monotonic change of the dimensionless extinction strain rate with ammonia cracking ratio is found. By isolating the preferential diffusion effect and non-unity Lewis number effect, the preferential diffusion effect has proved to be the reason for the non-monotonic change. To accurately predict the blow-off of the bluff-body-stabilized partially cracked ammonia flame, a numerical method with Large Eddy Simulation, detailed chemistry, and a conjugate heat transfer model is established. The simulation result is validated by experiment regarding the flow field and flame distribution. With the validated numerical method, the extinction of two ammonia/hydrogen flames is predicted within the experimental error bar, and the blow-off processes are analyzed. For the 70% NH₃ flame, the continuous shear layer flame surface prevents the thermal convection between the hot burnt gas inside the recirculation zone and the cold unburnt. The blow-off of the 70% NH₃ flame starts from the local extinction along the shear layer flame and causes the shear layer flame breakup. With the breakup, the mixing at the shear layer dominated by turbulent vortices cools down the recirculation zone and makes the flame kernels not sustainable, and the flame finally extinguishes. In comparison, the 40% NH₃ flame is more fragmented than the 70% NH₃ flame during the stable stage. Due to the absence of the continuous shear layer flame surface, the blow-off bulk velocity of it is lower than the expected value, which is based on the one-dimensional extinction strain rate ratio of these two flames.
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