Flame Velocity and Burner Size

Flame velocity only really matters with 'quiet' burners, be they gas or Shellite stoves. Normally the whole flame sits nicely on the burner in a pattern of blue mini-flames from the holes. However, with some stove/cartridge combinations one can see a rath

Flame velocity only really matters with 'quiet' burners, be they gas or Shellite stoves. Normally the whole flame sits nicely on the burner in a pattern of blue mini-flames from the holes. However, with some stove/cartridge combinations one can see a rather frightening sight when the stove is turned right up: the flame starts to 'lift off' the face of the burner. If this goes too far the flame could actually 'blow out', and then you would an explosive air/gas mix filling whatever space is around your stove. Clearly, this would be very dangerous. What is going on here? We must delve deep into physical chemistry to understand this.

When fuel burns the energy from the combustion heats up the gas molecules and helps them dissociate. That is, the complex molcule breaks into singel atoms. This has to happen before the fuel can burn. Also, the atoms of oxygen in an oxygen molecule bonded to each other and they won't bond to something else while they are bonded to each other. Similar disassociation has to happen with the fuel molecules to separate the hydrogen and carbon atoms from each other. This disassociation takes time and energy: it does not happen at an infinite speed.

The end result of all this is that the flame front in an air/gas mixture travels at a finite speed. This can be called the 'maximum flame velocity'. (Slightly different terms may be used in some texts.) In a gas explosion, the high pressure created by the rise in temperature as some gas burns causes the flame front to travel *extremely* fast. In an oxy/acetylene flame, the flame velocity is also very fast. In both air/butane and air/propane mixes the maximum flame velocity is moderately low, around 44 cm/sec, while an air/isobutane mix has a slightly lower flame velocity of about 39.8 cm/sec (these are figures I have found on the web).

The gas coming out of the burner jet is travelling very fast. It drags air in from the air inlets (the large holes in the burner column beside the jet) as it shoots up into the burner head, and this air mixes with the fuel gas as it goes. Then the fuel/air mix comes out the small holes in the burner head. In fact, without this high velocity you would not get the air being dragged in through the air inlets, and the fuel could actually leak out of the air inlets. It is gas momentum which sucks air in the holes near the jet and out the holes in the burner head.

The holes on the face of the burner are deliberately kept very small to prevent the flame from getting inside the burner head - this principle was used by Davey in his early Miner's Lamp. Once outside the burner the fuel/air mix spreads outwards and slows down, and provided there is enough local heat energy to start breaking down the molecules, the mix burns. But it won't burn without an initial source of energy.

But if the fuel/air mix is travelling very fast at the exit holes in the burner face, the gas speed may be greater than the maximum flame velocity. The gas may fly away from the burner face faster than the flame can reach back towards the burner face. The gas will burn some distance away from the burner surface where the gas has spread out some more and the velocity has dropped below the maximum flame velocity. When this happens you have 'flame lift off'. Needless to say, things are getting a bit risky here as there is no guarantee of where the flame will be. In short, too high a pressure at the burner face can cause flame lift-off.

There is also a problem with a very low gas velocity or a very low flow. This leads to a very small flame. A very small flame can have a problem heating up the incoming gas enough, so the combustion can die. Also, the very low gas velocity means the flame can be blown around by a draft, and blown away. Once again we have a dangerous situation, but at a minimum flow rate.

If you look at a range of stoves you will realise that they sort of fall into two categories: those with little burners and those with big burners. This applies to both gas and Shellite stoves. The ones with big burners spread the gas out more and this leads overall to a lower gas velocity. Thus they can handle a bigger over-all gas flow, and can pump out more heat. However, they can be harder to turn right down as the larger area requires more flow to get above the minimum flow rate. On the other hand, the little burners are obviously more risky at high flow rates but they do generally allow you to turn the burner down to a lower setting. Designing a burner head to get fast boiling but gentle simmering is not that easy!