On the Basic Theory of Flame Ignition


On the Basic Theory of Flame Ignition
To ignite and burn the combustible mixture, in addition to self-ignition, the ignition of the mixture ignition by external energy is the most widely used in engineering. The so-called ignition refers to forced ignition. Generally, it refers to the use of hot high-temperature objects, such as electric sparks, hot object surfaces, or high-temperature burning materials in the vortex behind the flame stabilizer, to ignite two small parts of fresh combustible gas to form a local flame, and then the flame ignites the adjacent mixture. In this way, the mixture will be ignited layer by layer to ignite the whole mixture.

In principle, forced ignition (ignition) and spontaneous ignition (spontaneous combustion) are both caused by the rapid acceleration of chemical reactions, but there are the following differences in the specific process.

① The chemical reaction is accelerated only in the local area (near the ignition source) of the mixture by ignition, while the spontaneous combustion is carried out in the entire combustible mixture. For example, if a high-temperature hot object is used for ignition, the reaction is only carried out in the mixture boundary layer on the surface of the hot object, while the mixture far away from the surface of the hot object cannot be ignited due to its low temperature.

② Spontaneous combustion requires a certain external temperature To, and deflagration is caused due to the self-acceleration of the reaction, which gradually increases the temperature of the combustible mixture to the spontaneous combustion temperature. However, ignition is different. Generally, at this time, the external temperature or the temperature of the vessel wall is much lower than the temperature suitable for spontaneous combustion. Therefore, it is necessary to use a high-temperature object to contact the combustible mixture and increase the temperature of the mixture to deflagrate it. The lowest temperature on the surface of a hot object that can cause the ignition of mixed gases is called the ignition (fire) temperature. In order to ensure that the flame can spread in the cold mixed airflow, the ignition temperature is generally higher than the autoignition temperature.

③ Whether the combustible mixture can be ignited depends not only on whether the local mixture in the boundary layer of the hot object can be ignited but also on whether the flame can spread in the mixed gas flow. Therefore, the ignition process is much more complex than the spontaneous combustion process, which includes local fire and flame propagation.
Like the spontaneous combustion process, the ignition process also has ignition temperature, ignition induction period, and ignition concentration limit, but the factors that affect them are much more complex than those in the spontaneous combustion process. In addition to the chemical properties, concentration, temperature, and pressure of the combustible mixture, there are also ignition methods and the flow properties of the mixture, and the latter has a more significant impact.