A flame lifts off and goes out

The balance between flame speed and gas speed.

A flame lifts off and goes out when the gas rushing out of a burner starts moving faster than the flame itself can travel back into that gas, so that the point of steady burning is first pushed away from the burner tip and then, if the mismatch grows large enough, blown out entirely.

A front that walks into the oncoming gas

A flame is not a fixed object sitting still in space, it is a moving front, constantly propagating into whatever unburned fuel and air sit just ahead of it at a speed set by the chemistry and the mixture, called the flame speed. A flame stays anchored at a burner's tip only because that propagation speed roughly matches the speed of the gas flowing out to meet it, the front pushing into the oncoming gas at almost exactly the rate the gas pushes outward against it, like two people leaning on each other with matched force.

Increase the gas flow rate and the oncoming gas starts moving faster than the flame can propagate back into it. The front is carried downstream, lifting away from the tip to a position further out where, for a while, it can still find a balance. Push the gas speed higher and even that fails, since a lifted flame draws in more surrounding air at its base and loses heat and stability faster than it can sustain itself, until it blows away and goes out. A lifting flame rarely does so silently, either. The extra air mixing suddenly into its base gives a rougher, louder, rasping note than the anchored flame made, an audible warning before it blows out completely.

Turn the flow down instead and the opposite mismatch appears. Gas moving more slowly than the flame can propagate lets the front advance back toward the burner, potentially travelling inside the tube where gas and air first mixed, a condition called flashback that can be far more dangerous than lift-off because it moves combustion somewhere it was never designed to be.

Walking up a down escalator

Walking briskly up a downward-moving escalator, a person makes progress as long as their walking speed beats the escalator's, each step carrying them a little further up than the escalator carries them back. Match the two speeds and they stay almost in place, walking hard. Let the escalator win, because it speeds up or the walker tires, and they are carried down and off the bottom however hard they keep walking.

The flame is the walker and the outflowing gas is the escalator. As long as the flame can walk into the gas at least as fast as the gas moves away, it holds its ground, and once the gas outpaces it the flame is carried downstream, lifting off and, if the mismatch is severe enough, losing its footing and going out. On a slower escalator the same walker drifts upward past where they started, just as a flame facing gas that moves too slowly advances back toward the burner.

A flame slower than a stroll

The walker in this picture is surprisingly slow. A flame in a well-mixed blend of natural gas and air propagates at around 40 centimetres per second, roughly a third of ordinary walking pace, and propane in air is only slightly quicker. Gas pushed hard through a burner's throat can easily outrun that, which is why lift-off and blow-out are everyday practical limits on how hard a burner can be run.

Every burner therefore has a maximum gas flow above which the flame cannot keep pace, fixed by the chemistry of the mixture, and a burner turned up hard all at once is far more likely to blow itself out than the same burner opened gradually, which gives the flame a chance to stay anchored while the gas speed climbs. A burner's usable turndown range, the ratio between its highest and lowest safe firing rates, is bounded at the top by blow-out and at the bottom by flashback, whatever fuel happens to be available.

Helping the flame hold on

This balance describes flames anchored freely in open air. A flame held very close to a solid surface, or stabilised with a pilot light or a small recessed shelf built into the burner, can hold its position at gas speeds that would otherwise blow it away, because the surface or pilot does part of the anchoring work the flame's own propagation can no longer manage. That is why a well-designed burner can be pushed harder, safely, than the bare flame-speed argument would suggest. A crosswind works the other way, stripping heat from the flame front and narrowing the margin between an anchored flame and one that lifts or blows out, which is why outdoor burners and camping stoves are shielded from wind, a fix that keeps their full gas rate available.