[TheForge] Re: Why gas burners are shaped the way they are.

[info@reciprodyne.com]

Dear list,

The reason that burners are shaped the way they are is all about
velocity(ies).

When a jet of pressurized fuel gas is released from an orifice, air from the
atmosphere is entrained by the column of gas, and carried along with the
gas. The turbulence of the interacting fuel gas and air will at some point
cause a combustible mixture to form. Direct this mixture into a straight
tube, and you have a burner of sorts.

In a combustible mixture of gas and air, a flame will propagate (move
through the mixture)  at X ft/sec velocity. In order for the burner to work
properly, the velocity of the of the combustible gas mixture as it moves
through the burner must be higher than the rate of flame propagation, or the
flame will burn back to the mixing point, and overheat the burner. If the
velocity is too high, the flame will jump off the end of the mixing tube,
and the burner will be unstable.

In order for the burner to have certain desirable properties, such as a
stable flame, the velocity of the gas mixture can be controlled by changing
the diameter of the mixing tube at different points.

A simple burner can be made by directing a jet of gas into a straight tube,
but in order for this to work at all, a small orifice, and high gas pressure
are required to generate sufficient velocity to keep the flame away from the
orifice. Usually, this velocity is so high that the gas and air do not have
time to mix, so the flame generally burns mostly outside the tube (where
secondary air is available) , and very slight changes in gas pressure or
back pressure (in the furnace) will make the flame pop back or jump off.
Also, the high pressure turbulence causes the straight tube burner to be
very noisy. Straight tube burners generally are loud and temperamental.

Now if we neck down the inlet of our mixing tube, (hourglass shape) the
velocity of the column of gas and entrained air accelerates as it goes
through the neck, providing the needed velocity to prevent burnback, and
with much less gas inlet pressure. Many industrial atmospheric gas burners
will run well at 1/4 p.s.i. gas inlet pressure.

After the gas and air mixture rushes through the neck at high velocity, it
is already partly mixed, and needs only a little further distance to mix for
efficient combustion, then a way to slow back down before it gets to the end
of the mixer. 

Fancy burners often have a longish diverging tapered mixer. What happens
here is: as the gas and air mixture (which is now combustible) travels along
the diverging section, its velocity drops with the increasing diameter. In a
properly tuned burner, the velocity of the combined air/fuel mixture drops
steadily until it exactly matches the natural propagation rate of the flame
just inside the end of the mixer, or nozzle. Ta-da, a stable burner that
needs very little fuel pressure to work properly.

If you can't make a long tapered divergent section, a sharply diverging exit
nozzle, (trumpet shape)  or a turbulence inducing device (like a washer
inside the end of the tube), can help slow down and stabilize the flame at
the end of the mixer.

So, in summary: a low pressure, low velocity jet of gas leaves the gas
orifice, entrains primary combustion air, and is directed into a converging
nozzle. As the mixture passes through the converging section, its velocity
is increased above the flame propagation velocity for that mixture. The
gasses then mix and decelerate as they pass through a diverging nozzle, and
reaches equilibrium with the flame propagation at the end of the mixer, or
nozzle, if any.

I hope this is helpful.

Tom Troszak