Large eddy simulation of super-adiabatic flame temperature in fully premixed hydrogen jet flames

The hot-spot with super-adiabatic flame temperature (SAFT) in fully premixed hydrogen flames can locally increase the thermal NO x formation under fuel-lean conditions. This study investigates the formation and attenuation of SAFT dominated by turbulent mixing in fully premixed H 2 /air piloted turbulent jet flames using large eddy simulation with finite-rate chemistry, mixture-averaged differential diffusion, and Soret transport. Three jet bulk velocities, 50, 100, and 200 m/s with the nominal equivalence ratio of ϕ = 0.4 , are considered, and the simulations are validated against measured velocity fields, turbulence intensities, OH distributions, and conditional statistics of the thermochemical states. The numerical results reproduce the main flow and flame features and predict a distinct SAFT region near the jet-pilot shear layer. At 50 m/s, the maximum temperature reaches approximately 1650 K, well above the corresponding adiabatic flame temperature of 1427 K with ϕ = 0.4 , and remains super-adiabatic until about x / D = 15 . As the jet bulk velocity increases, both the intensity and spatial extent of SAFT decrease; at 200 m/s, the peak temperature is reduced to approximately 1570 K and super-adiabatic hot-spot diminishes in the near field of x / D = 1.5 . Fuel re-stratification exhibits a similar downstream attenuation, indicating progressive homogenization by turbulent mixing. Joint temperature-velocity statistics and flame stretch analysis show that SAFT preferentially occurs in regions with relatively weak turbulent velocity fluctuations, whereas increased jet velocity enhances tangential strain, shortens the flow residence time, and suppresses the differential diffusion induced SAFT. Novelty and significance statement The emergence of super-adiabatic temperatures within hydrogen flames is generally attributed to differential diffusion effects under laminar or weakly turbulent conditions. Nevertheless, it remains poorly understood how the fully developed turbulence inherent to jet flows modulates such super-adiabatic temperature structures after their formation. The present work connects validated large eddy simulation of a fully premixed piloted hydrogen jet to local mixture re-stratification, normalized temperature statistics, and decomposition of flame stretch into tangential-strain and curvature contributions. This combination clarifies the transition from differential-diffusion induced heating near the inlet to turbulence-dominated homogenization downstream, and identifies tangential strain as the principal stretch pathway associated with SAFT attenuation at high Karlovitz numbers. This finding bears close relevance to the design of hydrogen combustors, where local temperature overshoots can affect NO x emissions despite globally lean operating conditions.