ArtMetal

Nelson,

The common wisdom is that the flare should have a 1:12 taper ratio. There are a number of authorities who cite this as an absolute, as though nothing else will work. They are, as is so often the case with absolutes, quite wrong. I'll expand on that a bit:

The purpose of a flare on a burner end is to slow the velocity of the air/gas mixture so that it does not "blow off" the end of the burner and burn in the forge chamber as a free mixture. The objective is to have a controlled flame at the tip of the burner, without having the flame either blow out or burn back up inside the burner tube. To maintain the flame at the right point, the ari/gas mixture must be traveling at just the right velocity; too fast and it blows the flame off the end of the burner, too slow and the flame front burns back up the tube, causing backfires. The optimum point balances the air/gas mix velocity with the flame-front velocity (speed of combustion, or burn rate). A flare at the end of the burner tube does this, but other methods can work as well.

While the taper is stated to be 1:12, a plain cylinder will often do every bit as well as a precisely machined flare, and sometimes better. Why is this? When a larger piece of pipe is slipped over the end of the burner tube, the junction between the two creates a "step" at the point where the pipes overlap. This step creates an eddy current in the flow of the air/gas mixture, slowing its velocity markedly. That eddy allows the flame to remain at the burner end, instead of being blown off. In the case of the highly machined flare, the smooth surface is actually less effective at slowing the velocity than the slightly rougher surface inside a piece of common pipe. The rougher pipe acts to increase turbulent flow, (the Reynolds factor), slowing the velocity.

In the venturi, a low Reynolds factoris important, in order to maintain the velocity necessary to create a pressure differential and draw in the gas. In the burner tube, however, some turbulence is desireable, as it gives the propane and air a better chance to mix thoroughly, in the hopes of achieving a perfect stoichiometric mixture that yields the highest Btu output and most nearly neutral flame characteristics. So, polish the redlukcer venturi, but leave the burner tube rough inside. Some people have even used a spiral spring inside the burner tube to increase turbulence, or shoved some rolled wire mesh down the tube. The necessity for such measures can be reduced by having a burner tube of sufficient length. For a 3/4" pipe venturi burner, the minimum length for the burner tube should be nine inches (9"). Any less and the fuel and air don't get thoroughly mixed. On the other hand, if the tube is way too long, the velocity can be slowed so much by turbulence that backfires are inevitable. 9-1/2" to 11" seems to be about right.

I hope this helps clear up some of the confusion.

Rich