NOx production in a canonical Micromix hydrogen flame

A direct numerical simulation (DNS) of two reactive hydrogen jets in an air crossflow at representative gas turbine conditions is performed. The thermochemical state nominally corresponds to a non-autoignitive, partially premixed turbulent flame. The analysis focuses on the instantaneous and conditional mean flame structures, and the NO${}_x$ production mechanism. The results show that the flame along the jet centerline plane features two branches, one stabilized on the leeward side and a second lifted above the jet trajectory. The former is located close to the jet exit where the advective velocity is low due to the recirculation zone and the boundary layer. The hot products of the leeward flame are transported downstream and interact with the windward non-premixed flame branch. An analysis of the flame index indicates that both, non-premixed and premixed, flames coexist and undergo strong interactions. Through reaction pathway analyses, it is demonstrated that the production of NO${}_x$ over the whole domain proceeds mainly through the thermal (Zel’dovich) route; this is the primary pathway in near-stoichiometric regions, while the N₂O and NNH routes are locally dominant in lean and rich premixed regions, respectively. Moreover, a post-flame ($T>1850$ K) residence time is used to track the time spent by fluid parcels in regions where thermal-NO prevails. This reveals that large quantities of NO produced through the thermal route near stoichiometry are transported in rich zones, resulting in a strong departure from 1D laminar reference cases.