Structure and nitrogen oxide emissions of confined turbulent hydrogen jet flames

In this work, a database analysis of three-dimensional direct numerical simulations using detailed chemistry is presented considering a turbulent lean premixed hydrogen/air round jet flame at elevated pressure and temperature ($\varphi =0.5,T_u=530\text{K},p=8\mathrm{atm}$), subject to different levels of domain confinement by solid walls. It is shown that for sufficiently small domain sizes, a coherent recirculation zone is present as expected; however, this does not affect the flame structure, consistent with experiments of attached hydrogen flames. Examination of velocity statistics indicates that, while both turbulent kinetic energy and Reynolds shear stresses increase with increasing domain size, these changes occur sufficiently far away from the flame. Each of the flames experiences the same level of mean shear, leading to the same flame structure. Despite identical turbulence-flame interactions between cases, nitrogen oxide (NO) emissions from the flames are observed to be different. For flames with recirculation zones comparable in size to the flame height, the superadiabatic temperatures caused by intrinsic flame instability are retained within the recirculation zone. Often, residence times in the post-flame are too short for locally elevated temperatures to have a significant impact on thermal NO formation. However, when these higher temperatures are coupled with the long fluid residence time in the recirculation zone, despite lower global residence times, this analysis shows that NO emissions from the flame can be enhanced.