[TheForge] Gas forge burners

[theforge@mailman.qth.net]

In a message dated 4/9/2004 11:08:13 AM Pacific Standard Time, 
blacksmith@comcast.net  writes:
I've seen various types of mixing devices on forced air burners, but I
thought that venturi burners were supposed to have a smooth air flow (Does
laminar air flow sound right?).
It's all a balancing act, with tradeoffs made, to reach the desired end. 
Laminar flow is important in order to maintain high flow speed in the gas/air 
mixture. High flow speed is necessary in order to feed enough combustibles to the 
flame, and in order to help create a healthy pressure barrier upstream of the 
from the low pressure nozzle area (in the mixing tube portion, where you don't 
want the flame to go). A certain amount of turbulence helps to mix fuel 
molecules with the oxygen content in the entrained air. So, the best kind of 
turbulence is that which does the least to "put the brakes" on forward motion. 
Spinning the gas/air mixture gives the maximum mixing action for the minimum drag.

Maximum spin is attained by designing the whole burner with this goal in 
mind, but retrofitting is possible. It may seem that such a healthy spin would 
consume a lot of power, but this isn't necessarily the case. No doubt you have 
seen skaters start into their spin with a fairly weak twirling motion, only to 
increase their speed enormously by retracting their arms. A naturally aspirated 
burner which employs an accelerator (small diameter tube) instead of a hole 
for an orifice can increase flow speed, even on old fashioned burners. It works 
best in conjunction with a bell reducer or some other form of funnel at the 
entrance of the mixer tube on smaller burners. Properly built burners in larger 
diameters will create their own spin without any assistance (this would also 
be true of the smaller sizes if perfect accelerator tube diameters were easily 
available, but that is a whole different conversation).

On any such burner a nozzle is necessary in order to create a low pressure 
area, which keeps the flame from being blown right off the burner's end. As 
already discussed, this low pressure area also constitutes an effective barrier 
against back-flash. This same trick can be performed by the right size and shape 
of ceramic portal, which acts to replace the nozzle's functions.
Mikey


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