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How to calculate a venturi?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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How to calculate a venturi?

Unread postAuthor: inonickname » Thu Apr 23, 2009 8:27 am

I require to make a custom venturi (think of the nozzle on a blowtorch, lighter, bunsen burner..) to make a stoichometric (as close to perfect as possible) mix of air/butane. (for fueling a mini)

Now I have several basic options:

1) Trial and error
2) Mathematical formula
3) Adjustable via screw/collar.

Unless I can get a reliable formula for this I'll probably just use an adjustable collar which would also allow for changing the mix without a regulator in cold conditions.

So if anyone knows, how do I calculate it? Do I use the flow of gas relevant to the circumference of tube and mixture or what.. Unless somebody knows this magical formula adjustable sounds good.
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Unread postAuthor: psycix » Thu Apr 23, 2009 1:00 pm

1) Always works, just make sure your stuff is as adjustable as possible.
2) Will become very hard
3) Basically the same as 1
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Re: How to calculate a venturi?

Unread postAuthor: jimmy101 » Thu Apr 23, 2009 1:14 pm

I would go with the adjutable collar, just like a good Bunsen burner.

The problem with trying to calculate the correct vent area is that the fuel delivery rate, especially for butane, is going to be vary depending on the temperature and how long you actually open the fuel valve (as the butane in the resevoir cools the delivery pressure will drop).

With the adjustable collar you can just open the butane source and ignite the fuel flow. Now tweak the collar until you get a flame that is blue with no yellow or orange.

Remember that the efficience of mixing will depend a lot on any back pressure in the system. You can't use a venturi type setup to correctly fuel into a closed chamber.

To displace the air in the chamber with the premixed fuel you'll need to use at least several chamber volumes of premix. Personally, I would figure it takes 10x the chamber volume to completely replace the air with the premix.
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Re: How to calculate a venturi?

Unread postAuthor: Technician1002 » Thu Apr 23, 2009 2:20 pm

jimmy101 wrote:I would go with the adjutable collar, just like a good Bunsen burner.

The problem with trying to calculate the correct vent area is that the fuel delivery rate, especially for butane, is going to be vary depending on the temperature and how long you actually open the fuel valve (as the butane in the resevoir cools the delivery pressure will drop).

Remember that the efficience of mixing will depend a lot on any back pressure in the system.


A venturi generates a low pressure area. Don't use pressurized fuel. Regulate to 0 PSI or slightly below for fuel cut off at no air flow. Using a plastic bag as a bladder is one way to supply fuel. Canister pressure isn't used at the venturi.

Venturi delivery can be used at elevated pressure. Sand blasting equipment uses them as well as pressure washers.
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Unread postAuthor: inonickname » Fri Apr 24, 2009 8:20 am

I've decided that I'll just go with a reservoir that will give the correct fuel charge for the chamber, and vent the chamber like the tippy c3..

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Air will be allowed back in through a check valve.

Btw, the reservoir will refill automatically through a needle valve so not to overcharge but not have to touch anything either.
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Re: How to calculate a venturi?

Unread postAuthor: jimmy101 » Fri Apr 24, 2009 10:52 am

Technician1002 wrote:A venturi generates a low pressure area. Don't use pressurized fuel. Regulate to 0 PSI or slightly below for fuel cut off at no air flow. Using a plastic bag as a bladder is one way to supply fuel. Canister pressure isn't used at the venturi.

Venturi delivery can be used at elevated pressure. Sand blasting equipment uses them as well as pressure washers.

How would a venturi work with the gas supply regulated to 0 PSI(g)?

I assume he wants to essentially use an unlit torch head (Bunsen burner) to create a proper air+fuel mix. Regulating the fuel source to 0 PSIG gives no motive force to make the system operate.

The bag as a bladder is a good idea. I believe it was discussed on the old forum. The annoying part is how big the bag has to be. As I posted before, it probably needs to be at least several times larger than the chamber, personally I would go with say 10X the chamber volume. That would be for a single fueling cycle of the chamber.
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Re: How to calculate a venturi?

Unread postAuthor: Technician1002 » Fri Apr 24, 2009 11:09 am

jimmy101 wrote:
Technician1002 wrote:A venturi generates a low pressure area. Don't use pressurized fuel. Regulate to 0 PSI or slightly below for fuel cut off at no air flow. Using a plastic bag as a bladder is one way to supply fuel. Canister pressure isn't used at the venturi.

Venturi delivery can be used at elevated pressure. Sand blasting equipment uses them as well as pressure washers.

How would a venturi work with the gas supply regulated to 0 PSI(g)?

I assume he wants to essentially use an unlit torch head (Bunsen burner) to create a proper air+fuel mix. Regulating the fuel source to 0 PSIG gives no motive force to make the system operate.

The bag as a bladder is a good idea. I believe it was discussed on the old forum. The annoying part is how big the bag has to be. As I posted before, it probably needs to be at least several times larger than the chamber, personally I would go with say 10X the chamber volume. That would be for a single fueling cycle of the chamber.


The venturi would work because the air is provided at pressure by a pump and draws the fuel, not the pressurized fuel drawing the air like a torch, which is often too rich of a mix.

If you use a pump for the air, you can draw the fuel in a carburetor, which is what I think the goal is. The bladder containing fuel would need to be a minimum of 4.02% of the chamber volume.

inonickname wrote:I've decided that I'll just go with a reservoir that will give the correct fuel charge for the chamber, and vent the chamber like the tippy c3..

Air will be allowed back in through a check valve.

Btw, the reservoir will refill automatically through a needle valve so not to overcharge but not have to touch anything either.


Sweet design. :D Build it, shoot a vid of it, and enter it in the competition. It's worthy of serious competition. Well done :!:

:?: On second thought, a carb would provide a better mix without a fan. The air may be turbulent enough after the fill stroke to provide a good mix without a fan with the pump. I'd like to see it work.
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Last edited by Technician1002 on Fri Apr 24, 2009 11:58 am, edited 3 times in total.
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Re: How to calculate a venturi?

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Re: How to calculate a venturi?

Unread postAuthor: jimmy101 » Fri Apr 24, 2009 3:35 pm

Technician1002 wrote:The venturi would work because the air is provided at pressure by a pump and draws the fuel, not the pressurized fuel drawing the air like a torch, which is often too rich of a mix.

If you use a pump for the air, you can draw the fuel in a carburetor, which is what I think the goal is. The bladder containing fuel would need to be a minimum of 4.02% of the chamber volume.

So, you're just swapping the pressurized fuel for pressurized air. That probably would work a bit better. Especially since bunsen burners draw a different fuel + air mix when lit as opposed to unlit.
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