A premixed burning flame occurs when:

A premixed burning flame occurs when fuel vapors are mixed with air and there is no ignition source present. This scenario describes the phase where the fuel and air are in an ideal ratio for combustion but have not yet been ignited. In a premixed flame, the reaction can only begin once an ignition source is introduced.

Understanding this concept is crucial, as it highlights the importance of the fuel-air mixture in combustion processes. Without an ignition source, the mixture remains unreacted, even though it is ready for combustion. When the right conditions are met, the introduction of an ignition source can lead to a rapid transformation into a burning flame.

In contrast, an ignition source alone does not create a premixed flame if the fuel and air are not properly mixed. Similarly, the state of the fuel, whether gas or liquid, becomes relevant only after the ignition source has acted on the fuel-air mixture. Thus, the precise combination of fuel vapors and air, without the initial ignition source, characterizes a premixed burning flame.