## 4 Conclusions  
In the current study, the transient ignition characteristics of non-premixed methane diffusion flame under acoustic excitations has been experimentally investigated. Two high-speed cameras are synchronized to simultaneously record the color and schlieren images. The enhanced flame images show the flame structure clearly, while the turbulent flow field before and after ignition has been visualized through schlieren images. The quantitative velocity information of the flow field has been estimated using an optimized optical flow algorithm. The results indicate that acoustic excitation has great influences on the cold flow pattern, resulting in an uneven velocity distribution both in time and space. The 10 Hz perturbation has affected the cold flow field further downstream, while the cyclic variation is observed with velocity contours. For higher frequencies, the acoustic excitation effect is mainly confined in the region near the nozzle exit, which causes an uneven velocity distribution with local high-speed regions and discontinuities in the flow field. The acoustic effect also has a significant effect on the transient ignition process. The flame front has the highest velocity in all the cases, which has coupled the combustion and flame propagation velocities. For the cases with 10 Hz, 30 Hz and 50 Hz, the forming of a large vortex can be observed due to acoustic excitation. For 100 Hz excitation, the acoustic-affected region is smaller, which is mainly near the nozzle exit, and the downstream flow field is more similar to the case without acoustic excitation. The acoustic effect has caused higher velocities in the flow field, which has promoted the flame establishment in a shorter time. The ignition failure phenomena have been observed for the cases of 30 Hz and 50 Hz. The velocity at the initial ignition stage has revealed that the failure is mainly caused by the dramatic velocity change at the spark position. The large velocity gradient has induced the extinction of the initial flame kernel. The experimental results reported in the current study have shed light on the complex interactions between acoustic excitation and the fast transient ignition process. The visualization and quantitative measurement results can be served as reliable validation proof for numerical simulations. The technical means are also applicable for  
Figure 9: Velocity vectors and velocity contours 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 (the white arrow in velocity contours represents the position of ignition).  
combustors with complex turbulent conditions, which are able to supply solid and sound experimental evidence for the design and optimization of combustors. For example, in the ignition process of the combustion chamber with acoustic or mechanical perturbations, a similar low frequency acoustic perturbation in the experiment should be paid attention to as much as possible, which may lead to ignition failure.  
Conceptualization, C.J. and Q.W.; methodology, Q.W. and C.J.; software, P.Z. and Q.W.; validation, P.Z., Q.W. and T.Z.; investigation, P.Z. and D.P.; data curation, P.Z. and D.P.; writing--original draft preparation, P.Z. and Q.W.; writing--review and editing, Q.W., C.J. and T.Z.; visualization, P.Z., D.P. and Q.W.; supervision, Q.W., C.J. and T.Z.; project administration, Q.W., C.J. and T.Z.; funding acquisition, Q.W., C.J. and T.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (Grant Nos.: 51976121, 51976140 and 51306113) and the National Science and Technology Major Project of China (2017-III-0007-0033).  
The authors declare no conflict of interest.