Analysis of the origin of NOx emissions in non premixed dual swirl hydrogen flames

The mechanisms of NOx formation in swirled hydrogen-air flames are investigated with Large Eddy Simulations (LES). Two operating conditions, A (3 kW) and L (9 kW), are considered. Flame A is M-shaped with a first diffusion-controlled front attached to the hydrogen injector rim and a second flame front burning hydrogen and vitiated recirculating air. Flame L, instead, is lifted and stabilized as a V-flame at the bottom of the central recirculation zone. Flame L also features a diffusion reaction layer in the central recirculation zone with partially premixed wings that develop along the inner shear layer above the injector. It is shown that the central diffusion front is common to both operating points and plays an essential role in the global NOx emissions of the burner for both operating conditions. A comparison between flamelets extracted from 3D simulations and results obtained with one-dimensional strained flames shows that strain plays an essential role in the production rate of NOx for all zones. In both flames, nitrous oxides are mainly produced in the central diffusion front. The high temperature in this front favor NO formation through the thermal NO pathway, which is strongly dominant over the other ones. Nevertheless, the central diffusion front of the lifted flame (L) is submitted to a higher strain rate than the attached flame (A), which leads to lower temperature and consequently to a smaller NO production. Interestingly, it is demonstrated that these differences can be predicted using one-dimensional strained diffusion flames. These simulations emphasize the key role of strain rate to control NO emissions in hydrogen non-premixed flames, and show that NO emissions are not necessarily increased by diffusion hydrogen flames attached to the injector lips. Novelty and significance statement NOx emissions are a key factor in designing future hydrogen gas turbines. While it is commonly assumed that diffusion-dominated flames produce high NOx levels, this paper shows that hydrogen swirled burners can achieve low NOx emissions even in a diffusion regime. Strained laminar diffusion hydrogen flames are known to limit NOx production, and this study extends that understanding to turbulent swirled flames. The main source of NO is not the typical diffusion ‘H2/air’ flame (Flame I) near the injector, but rather a secondary ‘H2/lean burnt gas’ diffusion layer (Flame II), stabilized downstream in the central recirculation zone. Simulations indicate that Flame I experiences high strain rates, while Flame II is nearly unstrained. This suggests that low NOx levels can be achieved by controlling the flow and local strain rates so that burning in a diffusion mode might not be a showstopper for future H2 engines.