## 1 Introduction  
The ignition process of fuel has been one of the most important research topics in the field of fundamental combustion. For example, blowout and re-ignition of internal combustion engines, gas turbines and aeroengines and fire development caused by fuel leakage are closely related to the ignition stage [1; 2; 3]. The establishment of the initial flame kernel and the following flame development in the ignition process can directly affect the combustion stability, combustion efficiency and emission characteristics. The ignition process is jointly affected by a variety of factors, such as the equivalent ratio at the location of the flame kernel, flow velocity and acoustic pulsation, etc. Therefore, the study of ignition characteristics is essential to deeply understand the mechanism of combustion stability and is also of great significance to the combustion chamber design. In previous research work, the ignition characteristics have been intensively studied in constant volume bombs and turbulent jet flow, as well as confined combustors. The involved factors include flame kernel formation, the minimum ignition energy (MIE), ignition position, equivalent ratio and fuel compositions, etc. [PERSON] and [PERSON] [4] theoretically described the development process and the internal mechanism of flame from flame kernel to flame ball and then to stably propagating flame in a static flow field. The results demonstrated the influence of flame stretching and radiant heat loss on the ignition process. [PERSON] et al. [5] investigated the minimum energy necessary to ignite a laminar premixedmethane-air mixture through an experimental study. Ref. [6] studied the ignition problem in a wide Lewis number (Le) range and found that when Le is less than a certain value, the flame ball smaller than the theoretical minimum diameter can be successfully ignited. [PERSON] et al. [7] found that the ignition fuel ratio has considerable influence on combustion efficiency, combustion product and flame stability. In actual combustion devices, the jet diffusion combustion mode is mostly adopted [8]. The combustion process of a diffusion flame is accompanied by a complex flow phenomenon and a heat and mass transfer process of chemical reaction. Existing research results show that the dynamic characteristics of a jet diffusion flame are closely related to the flow field distribution [9]. [PERSON] et al. [10] experimentally investigated the laser spark ignition of a jet diffusion flame. It is reported that the success of ignition depends on whether or not the spark-initiated reacting gas could undergo a transition from hot plasma to a propagating flame. [PERSON] et al. [11; 12] presented experiments on ignition in turbulent non-premixed methane jets in air. The spark position, energy, duration, electrode diameter/gap, jet velocity and air premixing of the fuel stream are examined to study their effects on the ignition probability defined as a successful flame establishment. [PERSON] [13] has reviewed the initiation of turbulent non-premixed flames through auto ignition and spark ignition. [PERSON] et al. [14] experimentally investigated the ignition characteristics of hydrogen-enriched methane diffusion impinging flames, while the results indicate that the flow field velocity increases with an increasing hydrogen percentage. [PERSON] et al. [15] investigated the effect of adding hydrogen to methane on the thermal characteristics and ignition delay in a methane-air, oxygen-enriched and oxy-fuel MILD combustion mode, which concluded that the use of hydrogen as an additive to methane is an effective strategy. The influences of a head geometry structure on the ignition process are investigated using experimental and numerical methods [16]. The results have shown that the ignition performance can be improved with the shortening of the sleeve length. [PERSON] et al. [17] investigated the ignition process of a sonic ethylene jet penetrating to heated supersonic crossflows.