# Prediction Method of Swirling Flame Lean Blowout Based on Flame Image Morphological Features
## 2 Experimental Setup  
The chemiluminescence images of swirling flame used in this paper for the construction of the LBO prediction model were obtained by experiments on the BASIS burner [34; 35] and also used for other LBO prediction models [36]. The scheme of the experimental system is shown in Figure 1. The burner featured a centrally staged structure with a pilot stage and a main stage. The pilot stage comprises an axial swirler with 8 blades of 40\({}^{\circ}\), and the main stage has an axial swirler with 20 blades of 30\({}^{\circ}\). The swirl numbers of the pilot and main stage can be estimated to be 0.68 and 0.5, respectively [34]. The methane-and-air flow entering the burner is controlled by mass flow meters. After being mixed in two separate premix tanks, two premixed gases are passed into the pilot stage and the main stage, respectively. The nozzle outlet is constrained by a 90 mm \(\times\) 90 mm \(\times\) 90 mm quartz glass square cylinder. The CH\({}^{*}\) chemiluminescence image of the swirling flame is recorded using a high-speed camera (Photron SA-Z, Tokyo, Japan) and an image intensifier ([PERSON], Groningen, The Netherlands) with a visible light lens (Nikon 50 mm f/1.4 G, Tokyo, Japan) and a narrow-bandpass filter (Semrock, 439 \(\pm\) 77 nm, Rochester, NY, USA).  
Pre-experiments were conducted to find the experimental conditions that are close to the LBO limit with typical LBO phenomena, such as flame oscillations and local extinguishment and reignition. It was found that the flame oscillation phenomenon that mainly occurred in the total equivalent ratio \(\phi\) was about 0.68. Then nine experimental conditions listed in Table 1 were used in the present research, where \(f\) means the total airflow rate, and \(\phi_{p}\) and \(\phi_{m}\) are the fuel-air ratio of premixed gas in the pilot stage and the main stage, respectively. \(\phi\) is the total fuel-air ratio of gas, and \(R_{s}=\phi_{p}/\phi_{m}\) is the stratification ratio. A stratification ratio is a dimensionless number that has a significant effect on the structure of the stratified swirling flame combustion flow [35; 37].  
After preliminary processing, the resolution of flame images obtained by the experiments is \(1024\times 1024\). Due to the limited computer capacity, the captured CH\({}^{*}\) flame images were taken to be discretized by 1 frame every 20 frames to reduce the data size, which means that there are 2 ms between each frame. For each case in Table 1, there are about 1450 images for the following process.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Case**} & \multicolumn{2}{c}{**Total Air**} & \multicolumn{2}{c}{**Fuel-Air**} & \multicolumn{2}{c}{**Fuel–Air**} & \multirow{2}{*}{**Stratification**} & **Total** \\  & **Flow Rate \(f\)** & **Ratio in the** & **Ratio in the** & **Ratio in the** & **Ratio \(R_{s}\)** & **Fuel–Air** \\  & **(g/s)** & **Pilot Stage** & **Main Stage** & **Ratio \(\phi\)** \\  & & \(\phi_{p}\) & \(\phi_{m}\) & & **Ratio \(\phi\)** \\ \hline
1 & 2.5 & 0.96 & 0.64 & 1.495 & 0.677 \\
2 & 2.5 & 0.93 & 0.64 & 1.443 & 0.674 \\
3 & 2.5 & 0.98 & 0.64 & 1.526 & 0.680 \\
4 & 3.0 & 0.96 & 0.65 & 1.477 & 0.685 \\
5 & 3.0 & 0.96 & 0.63 & 1.517 & 0.669 \\
6 & 3.0 & 0.94 & 0.64 & 1.464 & 0.675 \\
7 & 3.5 & 0.96 & 0.65 & 1.473 & 0.686 \\
8 & 3.5 & 0.96 & 0.63 & 1.521 & 0.668 \\
9 & 3.5 & 0.96 & 0.64 & 1.496 & 0.677 \\ \hline \end{tabular}
\end{table}
Table 1: Experimental conditions.  
Figure 1: Scheme of BASIS swirler and CH\({}^{*}\) chemiluminescence experiment system [34].