## 5 Ignition Process and Mechanisms
### Ignition Process  
[PERSON] [105] divided the process of spontaneous ignition of non-premixed flame into four stages--\"no ignition\", \"random spot\" \"flash-back\" and \"lifted flame\". [PERSON] et al. also included the critical ignition stage in the division of the life cycle of turbulent diffusion hydrothermal flame [45]. After reactants were pressurized and heated in the pretreatment stage, they (fuel, oxidant and water) were injected into the reaction chamber at the same time. Due to the high flow rate of fuel and oxidant, they contacted with each other through convection and diffusion. Then through mixing and reacting, the flame front continued to develop until it caught fire successfully. Ignition was the fourth stage in the whole process. [PERSON] observed the spontaneous ignition of the ethanol-water-air system injected by laminar flow had three stages: random spots, flash-back, and stabilization (see Figure 7) [59]. The results showed that the formation and disappearance of random hot spots might occur more than once before the stable flame was finally established. The initial flame did not propagate to the nozzle to form a stable flame, mainly because the flame propagation speed was limited by temperature, fuel concentration and turbulent mixing intensity. However, the initial process of flame formation and extinction meant that there was still an exothermic oxidative reaction that increased the density and temperature gradients and induced the buoyant mixing between fuel and oxidant, which in turn continued to promote flame formation. After a series of random spots, the flame approached to the nozzle, while the flow in the area behind the nozzle gradually changed into the laminar flow, partly because of the intensification of local viscosity caused by the local temperature rise beyond the critical point. Once the flame stabilized at the burner, the area for the flame tip became more disordered. At the same time, the flame color became yellower due to the presence of soot. Compared with the airflow of 2 mL/min, the turbulence of the flow-field with a higher flow rate (7 mL/min) was stronger, so the time to reach the stabilization was shorter and the flame was brighter. The above experimental results are of great significance for the investigation of spontaneous ignition behaviors in the microgravity environment that NASA is interested in. The lack of buoyancy mixing in microgravity is not conducive to laminar spontaneous ignition, so it is necessary to generate a certain degree of turbulence by increasing injection speed or manmade fluctuations to ensure the complete mixing between fuel and air. [PERSON] proposed a thermal spontaneous ignition model to simulate the laminar diffusion flame generated by injecting fuel into a static oxidation environment [104]. With this model combining the main physical and geometric parameters in the ignition process, they found that spontaneous ignition would occur when the jet height was high enough, and that the jet temperature rose along the axial direction to the ignition point and a stable flame would form with enough energy released from the ignition core. On the one hand, this model could be used to evaluate the designated experimental  
Figure 7: Series of images showing the formation of flame kernel, upstream flame propagation and the stabilization of the diffusion flame for the 7 mL/min air co-flow case; where (**a**) reference time t = 0 s, (**b**) t = 0.033 s, (**c**) t = 0.100 s, (**d**) t = 0.133 s, and (**e**) t = 0.166 s. (adapted from [59]). parameters (such as flow velocity, fluid temperature, and reactant concentration). On the other hand, it was considered as the first step to understand the interactions among the main basic physical processes. Based on this, a more complex first-order analysis model and thermal spontaneous ignition model suitable for coaxial burners might be developed.