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How does the venturi effect work?

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How does the venturi effect work?

Unread postAuthor: kjjohn » Fri Dec 25, 2009 4:19 pm

I have heard of the venturi effect, and I know it can help alot with fuel metering. I also know that it has something to do with a pressure differential in two different sized pipes. However, how does this work, and how can it be used for fuel metering?
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Unread postAuthor: Heimo » Fri Dec 25, 2009 4:36 pm

http://en.wikipedia.org/wiki/Venturi_effect

I do not know much about it that is why I just linked the wiki page anyhow if you look up the Reil burner you will find a homemade propane burner using a venturi to suck in the correct amount of air to burn with the propane I think one can use a reil burner to inject the correct air/gas mix into a camber and use that for fuel injection then just fill the chamber with it and it would work. but this iis all guesswork and I could be way off

here is the blueprint of a Reil burner
Image

and as you can see in this picture they do a good job of mixing the correct mix and them burning it
Image

PS I made myself such a burner to do some brazing work with and it works pretty darn well

hope this is of some help :roll:
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Unread postAuthor: inonickname » Fri Dec 25, 2009 6:04 pm

A true venturi is when a choked flow increases the speed of the fluid (translation; it goes through a smaller hole). The pressure at this area will in fact be lower than the unchoked area, allowing a vacuum to be created. A similar effect can be created with a high velocity fluid set up in a similar way to a venturi.

This is used in propane torches, ciggarette lighters, carburetors etc. to great effect. As far as metering goes, it is ideal to supply a purging mix of pre-mixed fuel.

A well-made venturi can actually hold a respectable vacuum, 90% is not awfully hard with the right tooling to make one.
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Unread postAuthor: kenbo0422 » Fri Dec 25, 2009 7:17 pm

Yes, the navy uses a venturi device as a way to pump out water from a compartment, using the vacuum to suck in the water.

I'm not sure, but I would think that you would need an unrestricted flow after the venturi to keep a constant mix of fuel being entrained into the air flow. Since it works off the vacuum effect, any blockage or even pressure change after the venturi will affect the intake. The fuel would most likely need a regulator on it at 0 psi setting so that only a vacuum will draw any fuel through the system.

Somebody have the math for this?
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Unread postAuthor: inonickname » Fri Dec 25, 2009 8:06 pm

If you're metering it into a reservoir (that isn't your chamber) simply use a plastic bag or similar to allow 0 pressure filling without the issue of excess air in there.

I've brushed on some maths similar to this in aeronautics (in aerofoils) and pressure differentials between each side (keeping in mind that I was only doing math for half of a venturi) and fluid speed. I'd have to revise a bit to make any use of it here though.
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Unread postAuthor: kjjohn » Fri Dec 25, 2009 9:55 pm

How could I implement this in a fuel meter for the chamber? For example, could this take in the correct amount of oxidizer for a mixture?
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Unread postAuthor: rp181 » Fri Dec 25, 2009 11:38 pm

thats what a propane torch does. You need to make an adjustable one, and play with until it is good.
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Unread postAuthor: jimmy101 » Sat Dec 26, 2009 12:41 pm

rp181 wrote:thats what a propane torch does. You need to make an adjustable one, and play with until it is good.

That's what a lab Bunsen burner is. Trivially easy to make with say a Bernzomatic torch head, just rap a piece of thin sheet metal around the torch head's base. Slide the sheet metal up and down to block / unblock the air intake holes. On a newly lit torch you can do it with your hand. Thought the head gets too hot to touch after a minute or so.

Any venturi device changes it's behavior when the back pressure in the system changes. That's why it doesn't work very well to try to fill a chamber with "premix" made by an unlit torch. As the back pressure changes the mixture drifts. In addition, the burning flame changes the air flow through the torch significantly, so the mixture is a fair bit different with the torch lit versus unlit.

Water based aspirators can usually pull vacuum down to the vapor pressure of the water used in the aspirator. Pressure are generally in the 20mm-ish (3% vacuum, 0.03 ATM, 0.4 PSIA) range.
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