## 3 Results and Discussion
### Autoignition Process  
Flame growth following autoignition is demonstrated in Figure 7a-c. As mentioned earlier, spray combustion creates a turbulent environment in the combustion chamber. The flame is not continuous during the turbulent combustion process; instead, it is a process comprising extinction and re-ignition [12]. In the state of steady combustion, there is an autoignition process of the flame. In Figure 7, the change of the flame surface shape is similar to that of a forced ignition; however, the degree of wrinkle of the flame surface is obviously different. The ignition time, nearly 10 ms, i.e., much less than that of a forced ignition, indicated that the combustion was more intense. A Comparison of Figures 2 and 7b shows that the initial condition of the interface between fuel and air may affect ignition time. The distorted flame front implies that the turbulence intensity is higher than before. If we focus our attention on the degree of the wrinkled structure, we will, surprisingly, find a similar trend of contraction and expansion, which requires further study. Though the flames trembled violently, the phenomenon of stratified flame combustion was still investigated. The existence of this phenomenon may play an important role in stabilizing combustion. The same method is applied for autoignition processing. We still focus  
Figure 5: The displacement vector diagram of wrinkled structure and flame surface. The arrow represents the normal line of the flame surface. The flame propagation direction is from right to left, and the distance coordinates correspond to the field of view.  
Figure 6: The displacement velocity of flame in forced ignition process.  
attention on information about the velocity, which is estimated in Figure 8. The velocity gradually decreased, whereas three autoignition processes expressed different displacement velocities of the flame surface in Figure 7a-c. Analysis of the images suggests that this may be due to different heat release rates [21]. The temperature of the combustion chamber periphery gradually increased, resulting in intense burning in combustion chamber, thus showing different velocities.  
In this experiment, the frequency of the laser is 400 Hz; this is sufficient to detect flame growth following ignition, especially in terms of the trajectory of the wrinkled structure. However, it is not enough to get more information about combustion, so a laser with a high repetition rate which has been adapted to complex environments is desirable. It can be found that the connection between the displacement velocity of the wrinkled structure and the flame shape is associated with the fact that the distorted flame shape may correspond to a greater combustion rate. In other words, the extent to which the wrinkled structure promotes combustion concerning the combustion rate is large.  
Figure 8: The displacement velocity of flame in autoignition process.  
Figure 7: Selected sequences of OH-PLIF images about flame evolution following autoignition (**a–c**). Images in time-sequence from left to right only, with 2.5 ms between consecutive frames, field of view = 110 mm \(\times\) 110 mm. The oxygen concentration is 33%. The excess air coefficient \(\alpha\) = 1.652. The oxygen consumption is 103 L/min, and the total gas flow is 676 L/min.