## 3 Results and Discussion
### Analysis of Ignition Failure Caused by Acoustic Perturbation  
In this experiment, when the igniter starts to work, continuous electric sparks with a frequency of 250 Hz would be generated; the energy of a single pulse spark is about 200 mJ. For the cases without acoustic excitation, multiple attempts show that this energy exceeded the minimum ignition energy of the methane jet flame at the test position. Through the visualization of the color and the schlieren images, it can be found that the first electric spark generated by the igniter can successfully ignite the fuel. However, for the cases of 30 Hz and 50 Hz, the ignition failure was observed at the initial several electric sparks. The flame can be ignited later with a certain spark generation. For each acoustic excitation case, more than twenty attempts were conducted, which excludes the accidental occurrence. As shown in Figure 8, the ignition failure phenomena are visualized by high-speed schlieren images, while the ignition success processes are also presented for comparison. The time interval between images was 0.5 ms, while the onset time was recognized at the moment when a single spark was generated. As can be seen from the figure, in the unignited cases (Figure 8a,c), only a small hot gas kernel was generated, as shown by the red arrow in the white dotted box. The hot gas mass disappeared quickly, lasting less than 2 ms. As can be seen from the figure of successful ignition cases (Figure 8b,d), the electric spark heats the surrounding gas, forms the initial flame core and then develops into flame successfully.  
In order to further study the reasons for the failure of single spark ignition caused by acoustic excitation, the velocity contour and vector of a flow field at 0 ms under the above conditions were calculated by the optical flow algorithm, as shown in Figure 9. In Figure 9, the white arrow represents the position of the ignition probe, where the spark is generated. By observing the schlieren images in Figure 8 and the velocity vectors in Figure 9, it can be found that, in the cases of ignition failure, the wave peak excited by acoustic excitation just propagates to the ignition probe position. The velocity at this position increases significantly, which reaches around 2 m/s. The velocity gradient at the boundary increases sharply, resulting in the failure of an electric spark to ignite the fuel/air mixture. In the case of successful ignition, the acoustic wave peak has crossed the ignition probe to reach the downstream. Thus, the velocity of the flow field at the ignition probe does not change drastically, which has enabled the successful ignition. It should be pointed out that the ignition failure is not observed for the cases of 10 Hz and 100 Hz. It can be seen from Figure 5b that the 10 Hz excitation has influenced the large vortex structure, while  
Figure 7: Flame image, schlieren image and velocity contours during ignition under different conditions: (**a**) no acoustic excitation; (**b**) 10 Hz excitation; (**c**) 30 Hz excitation; (**d**) 50 Hz excitation; (**e**) 100 Hz excitation.  
there is no local velocity peak region near the spark position. For the case of 100 Hz, as shown in Figure 5e, the local velocity peak appears near the nozzle exit, which is confined in a small area. Therefore, the main reason for the failure of single spark ignition caused by acoustic excitation is that the propagation of an acoustic wave leads to the drastic change of velocity at the ignition position in the flow field.  
Figure 8: Schlieren images for: (**a**) ignition failure of 30 Hz; (**b**) ignition success of 30 Hz; (**c**) ignition failure of 50 Hz; (**d**) ignition success of 50 Hz.