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Barrel / Valve Seat Transition Question

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Barrel / Valve Seat Transition Question

Unread postAuthor: velocity3x » Tue Sep 01, 2009 10:36 am

Does anyone know how gas behaves when going through a conical transition? I'm planning a conical transition from an 1.5" valve seat to a 1.25" barrel in a distance of 3.0". Does a transition cause the gas to accelerate through the transition into the barrel or does the transition restrict flow through the valve seat? Any info would be appreciated.
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Unread postAuthor: D_Hall » Tue Sep 01, 2009 10:39 am

A tapered transition will yield a higher valve flow coefficient. How much higher is dependent upon a number of variables.
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Re: Barrel / Valve Seat Transition Question

Unread postAuthor: jeepkahn » Tue Sep 01, 2009 11:04 am

velocity3x wrote:Does anyone know how gas behaves when going through a conical transition? I'm planning a conical transition from an 1.5" valve seat to a 1.25" barrel in a distance of 3.0". Does a transition cause the gas to accelerate through the transition into the barrel or does the transition restrict flow through the valve seat? Any info would be appreciated.


Look into convergent nozzles, Velocity stacks on carburators on engines use a convergent nozzle to achieve up to and beyond 100% flowrates, by having a tapered nozzle you can bypass the 65% orifice flow rule by avoiding "vena contracta"....
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Re: Barrel / Valve Seat Transition Question

Unread postAuthor: D_Hall » Tue Sep 01, 2009 11:14 am

jeepkahn wrote:use a convergent nozzle to achieve up to and beyond 100% flowrates,

Somebody is cooking the books to make such a claim.

By definition 100% flow is 100% of the orifice at choked conditions. Baring some new laws of physics, you're just not going to beat that.
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Re: Barrel / Valve Seat Transition Question

Unread postAuthor: Technician1002 » Tue Sep 01, 2009 11:29 am

D_Hall wrote:
jeepkahn wrote:use a convergent nozzle to achieve up to and beyond 100% flowrates,

Somebody is cooking the books to make such a claim.

By definition 100% flow is 100% of the orifice at choked conditions. Baring some new laws of physics, you're just not going to beat that.


I think he may be using a baseline of a flat plate orifice with a COEF of only about 45. With the rounded edges and taper inlet, COEF of over 80 can be reached. This is the apparent flow rate of over 100% often referred to.

A flat plate orifice has a tendency to choke the flow to a smaller effective orifice than the hole size.

Image

More info is in the wikipedia article. http://en.wikipedia.org/wiki/File:Orifice.png

I have noticed this flow increase in some of my valves with highly rounded edges. The Toulie valve takes advantage of the orifice shape to achieve very high flow rates over the typical burst disk.

Image

Wikipedia article on the velocity stack http://en.wikipedia.org/wiki/Velocity_stack

Flow of greater than that possible with a flat plate orifice or square cut end of pipe is possible. Whether it's over 100% depends on the basline value.
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Unread postAuthor: jeepkahn » Tue Sep 01, 2009 11:42 am

I should have clarified that... vena contracta through a square edge orifice is about 64% of the orifice diameter, and through tapering the orifice diameter properly you can maintain a laminar flow to prevent contraction across the throat (where the taper reaches the barrel diameter) so that you are flowing more than 100% of what a square edged orifice the same diameter as the barrel would flow, NOT more than 100% of volume through a pipe/tube sans orifice...

Actually, it's about 64%+/- where you are showwing 90% in your drawing...
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Unread postAuthor: Technician1002 » Tue Sep 01, 2009 11:57 am

jeepkahn wrote:I should have clarified that... vena contracta through a square edge orifice is about 64% of the orifice diameter, and through tapering the orifice diameter properly you can maintain a laminar flow to prevent contraction across the throat (where the taper reaches the barrel diameter) so that you are flowing more than 100% of what a square edged orifice the same diameter as the barrel would flow, NOT more than 100% of volume through a pipe/tube sans orifice...

Actually, it's about 64%+/- where you are showwing 90% in your drawing...


Sorry about the drawing. I swiped it from the linked wikipedia article. I agree, the number is probably in error, but instead of quibble on the actual value, I was mostly covering the theory and effect of the rounded edges.

In the automotive application shown in the article with a throttle body and venturi after the stack, the restrictions in the path in that application may actually have an overall efficiency change in the neighborhood that they state due to the downstream obstructions.
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Unread postAuthor: jeepkahn » Wed Sep 02, 2009 3:10 pm

and when modeling a cannon using a tapered throat use this formula for calculating what you need to use for seat diameter to get a more accurate model in ggdt; ((barrel diameter surface area/.7)/pi)sqrt*2 =seat diameter

Depending on how well you've achieved a laminar flow will effect results, but on most of my toolie and tapered seat/barrel guns this formula actually gets the model within -/+3% of chrony results...

Any math whizzes can feel free to correct the formula, I know how to do it, but I don't know how to type equations... :lol:
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Unread postAuthor: dewey-1 » Wed Sep 02, 2009 4:27 pm

jeepkahn wrote:and when modeling a cannon using a tapered throat use this formula for calculating what you need to use for seat diameter to get a more accurate model in ggdt; ((barrel diameter surface area/.7)/pi)sqrt*2 =seat diameter

Depending on how well you've achieved a laminar flow will effect results, but on most of my toolie and tapered seat/barrel guns this formula actually gets the model within -/+3% of chrony results...

Any math whizzes can feel free to correct the formula, I know how to do it, but I don't know how to type equations... :lol:


Could you give some actual calculations to determine the correct formula?

The .7 reference, what is that?
Is it 70% or is it an approximated value?

The reason I ask is because that 1/2 of the square root of 2 is .7071068.

A little clarification would help.
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Unread postAuthor: jeepkahn » Wed Sep 02, 2009 4:53 pm

1" bore with no taper will flow at roughly 65% efficiency due to vena contracta....

a 1" bore barrel that is tapered from a larger diameter will flow the same as a 1.19" bore with no taper....

It's basically a formula to give you the diameter of a 30% larger surface area to basically bypass the 65% limit imposed by ggdt...

i.e. 1" barrel with taper= (((.785"/.7)/3.14).597614)2=1.195228"diameter flowing at 65%

Am I making any sense???
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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 5:08 pm

dewey-1 wrote:
jeepkahn wrote:and when modeling a cannon using a tapered throat use this formula for calculating what you need to use for seat diameter to get a more accurate model in ggdt; ((barrel diameter surface area/.7)/pi)sqrt*2 =seat diameter

Depending on how well you've achieved a laminar flow will effect results, but on most of my toolie and tapered seat/barrel guns this formula actually gets the model within -/+3% of chrony results...

Any math whizzes can feel free to correct the formula, I know how to do it, but I don't know how to type equations... :lol:


Could you give some actual calculations to determine the correct formula?

The .7 reference, what is that?
Is it 70% or is it an approximated value?

The reason I ask is because that 1/2 of the square root of 2 is .7071068.

A little clarification would help.


The .7 is typically the effective flow. The physical hole is larger through the plate. The pinched area to the flow behaves like a hole .7 that area.

For example a proper stack can change the flow from 70% to 100%.

If the flat plate is assumed to be 100% then the stack can improve the flow to more than 100% of normal flow.
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Unread postAuthor: jeepkahn » Wed Sep 02, 2009 5:13 pm

Technician1002 wrote:
dewey-1 wrote:
jeepkahn wrote:and when modeling a cannon using a tapered throat use this formula for calculating what you need to use for seat diameter to get a more accurate model in ggdt; ((barrel diameter surface area/.7)/pi)sqrt*2 =seat diameter

Depending on how well you've achieved a laminar flow will effect results, but on most of my toolie and tapered seat/barrel guns this formula actually gets the model within -/+3% of chrony results...

Any math whizzes can feel free to correct the formula, I know how to do it, but I don't know how to type equations... :lol:


Could you give some actual calculations to determine the correct formula?

The .7 reference, what is that?
Is it 70% or is it an approximated value?

The reason I ask is because that 1/2 of the square root of 2 is .7071068.

A little clarification would help.


The .7 is typically the effective flow. The physical hole is larger through the plate. The pinched area to the flow behaves like a hole .7 that area.

For example a proper stack can change the flow from 70% to 100%.

If the flat plate is assumed to be 100% then the stack can improve the flow to more than 100% of normal flow.


Right, the formula I gave is so that you can figure out what # to put into the seat size in ggdt to get a proper representation of flow via velocity stack/taper...

And an fyi, the higher the pressure, the less differance it makes...
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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 5:38 pm

For my calculations I use the actual valve seat, but use higher COEF figures in GGDT. This is how I started to find that one of my cannons valves exceeds the flow of the burst disk in GGDT. The results were very confusing at first as I thought NOTHING could out perform a burst disk, so I kept looking for other failures in my measurements.

In the end, I found the rounded end of the tank, the rounded polished ports, and open flow design does indeed out flow a typical burst disk due to the shape of the porting into the barrel. :shock: The toolie is another that will out flow a burst disk with a flat plate orifice.
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Unread postAuthor: dewey-1 » Wed Sep 02, 2009 8:21 pm

Jeep & Tech;
Now I understand what you were referring to.

I made my own explanation to clear up my own confusion.

Does this explanation kind of sum it up correctly? Ea is Effective area.

(pi*r^2)/.7= 1.1219974 sq in effective surface area Ea with stack

or (pi*r^2) times 1.4285714 larger area which is 1.1219974 sq in or Ea

Approximately 42.86% more surface area


(pi*r^2) = .7853982 sq in or d = 1.00 in versus

Ea = (pi*Er^2) = 1.1219974 sq in or Er=.5976143 or Ed = 1.1952286
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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 8:29 pm

You got it. Good job. :D

EDIT;

The transition issue is the main reason I am building a 3 inch QDV. The valve will be oversize and then transition into smaller barrels, so it will be used primarily with 1.5 to 2.5 inch barrels. To round out the variety a 3 inch and 4 inch will also be built for big heavy stuff for massive recoil.

I expect to exceed burst disk performance on a golf ball barrel. I will have near at or over SOS. The attempt is for exceeding SOS with a marshmallow.

A 200 PSI shot with the 3 inch valve transitioned to a 1.25 inch barrel may do the trick. Bondo will be used to make the venturi transition.

I am in a serious competition in a marshmallow tossing contest. They have already gone supersonic.
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