## 3 Results and Discussions
### Suppression of NO\({}_{x}\) Emissions  
This study examined the NO\({}_{x}\) emission characteristics under different jet-in-crossflow conditions. Before the introduction of the stratified Ar/He jet-in-crossflow, the NO\({}_{x}\) concentration in the flue gas is 25.6 ppm. After jetting, the NO\({}_{x}\) emissions concentration in the outlet flue gas gradually decreases (Figure 9). A higher Ar/He stratified flow rate results in a lower NO\({}_{x}\) concentration. At an Ar/He volume ratio of 0%/100%, the NO\({}_{x}\) concentration gradually decreases from 25.6 to 14.6 ppm. Moreover, the Ar/He volume ratio significantly affects the NO\({}_{x}\) concentration. With a jet flow rate of 7 L/min, an increase in the Ar/He volume ratio from 0%/100% to 100%/0% reduces the NO\({}_{x}\) concentration from 14.6 to 13.1 ppm. This indicates that a larger jet flow rate and a higher inert gas content in the stratified jet lead to lower NO\({}_{x}\) emissions during combustion. This is due to the stratified Ar/He jet-in-crossflow, which alters the flame structure, resulting in a more uniform flame  
Figure 8: Phase difference and nonlinear analysis of thermoacoustic oscillation signals before and after the introduction of the stratified Ar/He jet-in-crossflow. (**a**) limit cycles; (**b**) bi-stable; (**c**) chaos (**d**) intermittent beating. temperature field [15]. Additionally, the low-temperature stratified jet effectively eliminates local hotspots in the flame, further reducing NOx emissions. To further elucidate the evolution mechanism of NOx emissions, this study examined the transient distribution characteristics of amino-group (NH2*) free radicals in flames at a constant flow rate of 5 L/min. As the Ar/He volume ratio gradually increases from 0%/100% to 100%/0%, the NH2* free radical concentration gradually increases, corresponding to a decrease in NOx concentration (Figure 10). The injection flow rate is maintained at 5.0 L/min. The inert gas atmosphere improves the preheating intensity, enhances fuel modification, and reduces the unit volume, providing key technologies for enhanced combustion regulation and achieving ultra-low NOx emissions. The stratified jet creates a reducing atmosphere in the combustion chamber, which facilitates the conversion of NO to N2. The stratified Ar/He jet-in-crosflow reduces the temperature level in the chamber, leading to a decrease in NOx emissions. Inert gases do not participate in chemical reactions but can alter the flame structure and temperature distribution, which affects NOx generation [31]. To further analyze the effect of the stratified Ar/He jet-in-crosflow on NOx emissions, the temperature field distribution was measured before the jet introduction and under different jet flow conditions. A comparison of Figure 11a,b indicates that the Ar/He jet significantly reduces high-temperature areas. However, the peak temperature of the flame remains significantly unchanged, and the high-temperature zone is mainly concentrated in the root area of the flame. A comparison of Figure 11b,c reveals that as the argon gas content gradually increases, the peak flame temperature significantly decreases, leading to a uniform flame temperature field. This is due to the increase in gas density, which enhances the combustion efficiency, reduces high-temperature spots, and improves the  
Figure 10: Distribution of NH2* radicals in flame under stratified Ar/He jet-in-crosflow conditions with the injection flow rate set to 5.0 L/min, and the Ar/He volume ratios range from 0%/100% to 100%/0%. (**a**) no jets; (**b**) Ar-0% + He-100%; (**c**) Ar-25% + He-75%; (**d**) Ar-50% + He-50%; (**e**) Ar-75% + He-25%; (**f**) Ar-100% + He-0%. Figure 9: NOx emissions in the stratified Ar/He jet-in-crosflow conditions. temperature field distribution. The stratified jet significantly alters the flame structure, and this structural change affects the NO\({}_{\rm x}\) concentration (Figures 5, 10 and 11). The Ar/He density significantly affects the flame length (Figure 11).