What happens to the flame when the velocity of the gas-air mixture through the burner ports is reduced below the flame velocity?

When the velocity of the gas-air mixture through the burner ports is reduced below the flame velocity, the correct outcome is that the flame can flash back.

Flame velocity refers to the speed at which the flame travels through a combustible gas mixture. This velocity is crucial in maintaining stable combustion. If the gas-air mixture's velocity decreases to below the flame's propagation speed, the flame can no longer maintain its forward motion. This condition can lead to a situation where the flame moves back into the burner or fuel supply line, a phenomenon known as flashback.

Flashback occurs because the flame front seeks the highest concentration of fuel and air, which may now exist within the burner or upstream of it. In practical terms, this represents a safety concern in combustion systems, as the flame is intended to remain at the burner’s end instead of moving backwards.

It's important to note that extinguishing the flame generally requires more significant disruptions in fuel supply or air mixture, while lifting away from the burner typically happens when the velocity is high enough to create a lift-off condition, and stabilization refers to the flame staying at a constant state without disruption.