## 2 Experimental Setup
### Method and Materials  
Thermoacoustic oscillations and NOx emissions are controlled using the Ar/He stratified jet-in-crossflow in this study (Figure 2). The stratified jet-in-crossflow structure consists of five layers, each with a height of 40 mm. A fire observation hole is installed on the wall of the layered structure for flame photography. The layered structure exhibits an inner diameter of 80 mm and an outer diameter of 120 mm. Each layer is equipped with four transverse jet holes, with adjustable diameters (Figure 2). In this study, the diameters of the inert gas jet holes are set to 2.0, 2.5, 3.0, and 3.5 mm to investigate the effect of injection  
Figure 1: Combustion test bench and measurement instruments.  
velocity. The burner features a swirl premixed structure and a bluff body combustion stabilization design (Figure 3). The five-layer jets are positioned to directly interact with the premixed flame at the burner outlet. Figure 3a,b show the schematic of flame shape changes before and after the jets are introduced. The premixed methane and air mixture infiltrates the combustion chamber through the bluff body after flowing through the swirler, which has a swirl angle of 60\({}^{\circ}\). The bluff body outlet has an outer diameter of 12 mm and an inner diameter of 22 mm. During the combustion experiments, the thermal power and equivalence ratio of the burner remain constant. The variations in thermoacoustic oscillations and NO\({}_{x}\) emissions within the combustion chamber were investigated at different flow rates and argon/helium volume ratios. The jet-in-crossflow consists of argon and helium, with jet flow rates set at 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0 L/min. The Ar/He volume ratios are set to 0%/100%, 25%/75%, 50%/50%, 75%/25%, and 100%/0%. The mixed gas density of argon and helium is designed to examine the effect of jet-in-crossflow. The mixed gases exhibit densities of 1.784, 1.383, 0.982, 0.581, and 0.18 kg/m\({}^{3}\). The maximum density of the mixed inert gases is nearly 10 times that of the minimum density. Before the introduction of jet-in-crossflow, the sound pressure amplitude in the combustion chamber is 25.18 Pa. The experimental conditions are summarized in Table 1. In this study, when the burner enters the thermoacoustic oscillation state, firstly, the signal synchronizer is combined to synchronously measure the combustion chamber acoustics, flame heat release rate, and flame images, and then the jet device is turned on to record the flame thermoacoustic oscillation signal and flame shape changes before and after the jet. At the same time, measure the NO\({}_{x}\) emission concentration of the flue gas at the burner outlet before and after the jet. In each measurement, the condition is tested at least five times, and the MSE (mean square error) of all measurement points was below 2.05%, which is less than 5%.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Units** & **Values** \\ \hline Thermal power & kW & 4.0 \\ Equivalence ratio & / & 0.75 \\ Self-excited frequency & Hz & 267 \\ Self-excited amplitude & Pa & 25.18 \\ Jets flow rate & L/min & 1.0–7.0 \\ Injection hole diameter & Mm & 2.00–3.50 \\ Gas densities & kg/m\({}^{3}\) & 1.784, 1.383, 0.982, 0.581, and 0.18 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental conditions of this study.  
Figure 2: Geometric design of the stratified jet-in-crossflow structure.