How to maintain the stability of flame combustion?
The stability of a flame refers to the ability of the flame to maintain a certain position and volume under specified combustion conditions, without flashback or continuous fire.
The fundamental reason for backfire is that the flame propagation speed is greater than the airflow ejection speed, leading to the disruption of the dynamic balance between the flame propagation speed and the airflow ejection speed. Therefore, in order to prevent flashback, the velocity of combustible mixture gas flowing out of the burner must be greater than a certain critical velocity, which is related to factors such as gas composition, preheating temperature, nozzle diameter, and airflow properties. For example, for gas with a high flame propagation speed (such as coke oven gas), the spraying speed should not be less than 12m/s. When air or gas is preheated, its spraying speed should also be increased.
In addition to ensuring that the ejection speed of the airflow is not less than the critical tempering speed, attention should also be paid to ensuring a uniform distribution of velocity on the outlet section to avoid external disturbance of the airflow. For burners with high combustion capacity, cooling the nozzle head is also an important measure to prevent backfire. When the nozzle diameter is small, air cooling can be used, and when it is large, water cooling can be used.
In terms of flame interruption (flame detachment and extinguishing), the flame is relatively stable when burning with flame. This is because under diffusion combustion conditions, the gas fuel and air near the nozzle outlet can form various concentrations of combustible gas mixtures during the mixing process, including gases with high flame propagation speed, which is conducive to forming a stable ignition heat source. On the contrary, during flameless combustion, combustible gases that have already been mixed in a chemical equivalent ratio, or even slightly lean gases (with an air excess coefficient greater than 1), flow out of the burner. This gas, due to being diluted by the atmosphere, has a significant decrease in flame propagation speed, which can easily cause the flame to detach and extinguish.
——Author：EBICO