The Effect of Pressure on Ammonia/Hydrogen/Nitrogen Premixed Flames in Intense Sheared Turbulence*

The combustion of ammonia/hydrogen/nitrogen blends presents an attractive carbon-free alternative to conventional natural gas firing in gas turbines. However, challenges exist, such as NOx formation and thermo-diffusively unstable lean combustion behavior, both of which are not well understood yet, especially at elevated pressure. We present results from Direct Numerical Simulations (DNS) of lean premixed ammonia/hydrogen/nitrogen-air flames in a temporally-evolving shear layer configuration at atmospheric and elevated pressure (10 atm). These DNS show fundamentally different turbulent combustion behavior at 1 and 10 atm, with the case at elevated pressure exhibiting stronger effects of preferential diffusion and enhanced thermo-diffusively unstable behavior. The DNS results are compared to results from simulations of canonical cases with reduced complexity to shed light on the change of thermo-diffusive instabilities with pressure. The DNS results are analyzed in terms of general burning behavior, including global fuel consumption speed and flame surface area statistics. Changes in global burning behavior observed at 1 and 10 atm are discussed in relation to preferential and differential diffusion effects, intrinsic flame instabilities and the underlying sheared turbulence encountered by the flame brush.