Critical Dimensions For Mixing Venturi?

Hello. Does anyone know what parts of a gas inspirator (venturi) primarily affect the fuel/air ratio? I've tried searching high and low for the answer to this and can't find it anywhere. It seems like a black art or something.
See, what I'm doing is building a reproduction of a pest scaring device which makes a loud noise to scare away birds and animals from crops, just in case anyone was curious. The goal is to have an adjustable fuel/air ratio to vary the flame speed. This changes the "tone" that is produced. Maximum flame speed occurs when the mixture is stoichiometric and tapers off on either side. Maybe someone has made their own custom burner?
Thank you, Robert

The Lightning Stalker wrote:
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Not completely. But I improved a vacuum pump (venturi principle) that Don Foreman kindly measured some years ago. I have described what I changed, so this might help you in tuning yours. <http://www.motor-manufaktur.de/werkstatt/07_vacuum_pumpe/en_index.html>
There's a link on that page to a different page with more details.
Nick

Make or buy a pulse jet. They are startlingly loud and were also used to spray insecticides around ( maybe still are). Essentially a venturi with its own power source. One of my model club members had one and you could hear it for miles. I was thinking just yesterday about how I used to chase the yearly staring invasion out of the local trees when I was a kid using only a pop-gun. They would scatter to the next forest and I would go there and do it again until they left.

daniel peterman wrote:
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And soooo easy to start! ;-))
Nick

They are easy to start. My friend did it with an ignition coil and a tire pump. I'm sure the Germans know all about this technology. I once met a man from Denmark who built very large pulsejets and used them on tethered go-karts out on a dry lake. They would run for about 4 minutes before meltdown. Then he got some exotic steel from Gremany and built some 6 footers. No meltdowns but so loud that people 3 miles away complained.

daniel peterman wrote:
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The problem is, that they have to start automatically in this application.
Nick

On 13 Dec 2006 22:36:38 -0800, with neither quill nor qualm, "The
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An aside: Your neighbors will LOVE you for that, Robert.

The Lightning Stalker wrote:
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You could always take a hint from pretty much every gas appliance out there.
Build it so it has an adjustable air supply, and replaceable jets, and experiment.
Too much time thinking, and not enough time with your hands moving, and it never gets built.
I think what I am trying to say here, is, based on personal observations, there is no CRITICAL dimension. The size of commercial burners is all over the place, and pretty much all of them have one or both inlets(fuel, air) adjustable in order to set the mixture. That is based on observations of gas BBQ's, Gas furnaces, burners for forges, and assorted gas torches.
If you start with a basic design and increase the size of the fuel orfice until it runs too rich, then replace that orfice with the last size that worked...
ALL the parts of a venturi affect the fuel air ratio. Play with the variable until it works in your application.
Cheers Trevor Jones

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You could probably put a needle valve in the orfice to vary it without having to change/experiment with various fixed sizes...
Jerry

Trevor Jones wrote:
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Yes, but which dimensions are the ones that are to be changed to alter the fuel/air mixture? Is it the orifice size, distance from the orifice to the mixing tube, taper of the converging area?
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I have tried a lot of different configurations, but none of them ever draws in more air than fuel. Right now I have a burner from a clothes dryer. It has a choke plate on it, but changing the choke doesn't change the flame any. It can go from 1/2 open to fully open and it still looks like the same fuel rich safety flame.
Thanks, Robert

The Lightning Stalker wrote:
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There are some design constrains you have to know: * Taper of the diffusor can't be bigger than say 5..10°, because you want a laminar flow that doesn't lift off of the cone's surface. This is the part that generates the suction of air.
* Orifice is up to you
* Distance orifice to mixing tube: If you make it to big, the gas coming out of the orifice (it will be a cone) hits the edge of the mixing tube and will push back (and not suck in) air. If you make the distance too small, you will throttle the air drawn in (but you can use that effect to adjust the fuel air ratio.
* Length of mixing tube (after that, you have the diffusor). Think of a conical stream of fuel coming out of the orifice. That cone has to hit the wall of the mixing tube. That is *very* important, because it works like a seal. If the "fuel-cone" doesn't touch the mixing tube, you'll have no vacuum in the mixing tube. Best would be, if that cone hits the mixing tube right where the diffusor starts. Don't know of the cone angle, but I think/guess it is about 3°. Better to make it too long than too short.
* diameter of mixing tube: Depends on the diameter of the fuel jet and the length of the mixing tube. If you have a fuel jet that is say 1mm and have a diameter of the mixing tube of 3mm you need a long mixing tube (see lenght of mixing tube and "fuel cone" above). You can't have the same or smaller diameter for the mixing tube compared to the fuel jet, it always has to be bigger.
That's my theory, I might be wrong (but not completely).
Nick

Nick Mueller wrote:
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When you say diffusor, which part of the unit are you referring to? Would this be the end where the fuel/air mixture is flowing out?
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I noticed that all the dryer and stove orifices have a gradual taper inside. The Reil burner on the other hand is just a hole drilled into the side of a piece of tubing. Do you think this is preventing more air from being mixed in?
Thank you for your help.
Robert

Nick Mueller wrote:
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Actually, this version will be manually operated.
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When you say diffusor, which part of the unit are you referring to? Would this be the end where the fuel/air mixture is flowing out?
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I noticed that all the dryer and stove orifices have a gradual taper inside. The Reil burner on the other hand is just a hole drilled into the side of a piece of tubing. Do you think this is preventing more air from being mixed in?
Thank you for your help.
Robert

The Lightning Stalker wrote:
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Yes, the "exhaust" side.
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Sorry, I have no explanation for that.
Nick

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Venturi mixers are more efficient than orifice mixers because the pumping losses are smaller. They work by the jet pump principle. Energy from the high velocity jet causes air to be drawn into the throat of the venture. Pumping efficiency can be as high as 90%. for a jet drawing air through an orifice efficiency is rarely greater than 60%. Which to use depends on how much pressure you need on the down stream side of the pump, and how much energy is available in the supply pipe. The mixing is a side effect of the pumping action.

The Lightning Stalker wrote:
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Yes! All of the above.
They are variables. None is critical, all can be worked with to change the flame. Also consider fuel pressure.
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Larger tube diameter, with a larger area for the air to draw into, and a smaller jet at higher pressure, with a better venturi, to give better airflow, are all variables to play with.
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Then you have too large a orifice. What pressure are you using? You do have a variable pressure regulator, yes?
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I have seen burners made from 1/2" copper pipe, on up to burners made of 2" pipe fittings that all worked fine. Play with the variables. Too rich, then less fuel or more air required, too lean, less air or more fuel.
Cheers Trevor Jones

Trevor Jones wrote:
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The pressure is about 1/2 psi, the same as what commercial propane cannons use. Mongo and t-rex burners can inspire excess air at almost 0 psi according to Ron Reil's page.
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Obviously.

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Along with everything else, keep in mind that not only is fuel rate a factor but fuel velocity is also. Increasing the jet size is not the same as increasing the supply pressure or opening the valve further. Your velocity may need to be increased by using a smaller jet and more pressure.
Don Young

On 13 Dec 2006 22:36:38 -0800, "The Lightning Stalker"
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I have made a couple of burners. When experimenting with burners (e.g., Reil burner), it seemed to me that it was easiest to make an aspirator that ran lean (excess air) and then choke the air intake with a butterfly, slide plate or whatever. The orifice size (and gas pressure) just controls rate of fuel consumption and hence BTU content of the flame. Once "tuned", ratio doesn't seem to change much with varying fuelgas pressure and consequent heat output.
You'll find several very easy-to-make designs and concepts at http://www.abana.org/ronreil/design1.shtml
These use a lot of gas, produce a lot of heat and "roar" while in operation. They certainly could be scaled down, but a smaller continuous burner (e.g., Bernz-O-Matic torch) doesn't produce much sound.
I'd guess that maximum sound production would require intermittent operation and hence spark ignition, perhaps in a resonant combustion tube. This suggests that the burn be self-extinguishing, perhaps by means of a pressure-activated fuel valve -- again perhaps in a resonant combustion cavity. This might produce periodic pressure pulses. My inclination (guessing here) would be to place the ignitor distant from the fuel-air inlet. A flame front would then propagate backwards relative to fuel-air velocity, thus creating a rearward-moving pressure wave and consequent pressure impulse in the cavity. With a long enough tube you might even get detonation near the back, if pressure (and temperature) builds to the point where a volume of fuel-air mix all ignites at once. This would make one hell of a racket! It might also self-extinguish (blow itself out) without requiring a valve.
Keep us posted, please!