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Power loss in over under vs inline

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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Power loss in over under vs inline

Unread postAuthor: jor2daje » Sun Apr 17, 2011 8:45 pm

Hey guys Im starting work on a fairly large combustion with 3" chamber and 1.5" gb barrel, I think I have most of the design figured out but cannot decide whether to go over under or inline, I like the compactness of the over under but Im worried i will be limiting power if I do because the bend will be made from 2x 1.5" elbows not 2x 3" elbows. I seem to remember a thread about this but couldn't find it with the search so any input would be greatly appreciated.
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Unread postAuthor: SpudBlaster15 » Sun Apr 17, 2011 9:01 pm

The energy losses provided by the bend will be relatively minor, and likely won't be noticeable with an atmospheric pressure combustion launcher.

The over-under configuration has the advantage of allowing you to use a much longer barrel and chamber per given launcher length, which will affect performance far more drastically than minor flow restrictions.
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Unread postAuthor: mark.f » Sun Apr 17, 2011 10:35 pm

Another thing to note is that the elbows themselves act as more "chamber volume", so there is no dead space between the chamber and projectile. Only factor here (as noted) is the very minor flow detriment, which is more than made up for by compactness and the things SpudBlaster mentioned.
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Unread postAuthor: Technician1002 » Sun Apr 17, 2011 11:54 pm

At transonic flows, 90 degree elbows do restrict the flow considerably. If you are launching spuds or other normal projectiles at 600 FPS or slower, there should not be a problem.

On the other hand if you are trying to pancake empty soda cans at 800 FPS or above, the flow restriction becomes significant.
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Unread postAuthor: SpudBlaster15 » Mon Apr 18, 2011 12:31 am

Technician1002 wrote:On the other hand if you are trying to pancake empty soda cans at 800 FPS or above, the flow restriction becomes significant.


Not with the very low gas pressures generated by the combustion of hydrocarbon fuels at 1ATM, methinks. Especially since an elbow is a streamlined transition with no sharp edges in the flow.

If we were dealing with reasonably high pressures (KPSI range), yes, there would be a significant effect as gas velocities approached the upper transonic range. For a combustion spudgun, well, you'd probably see greater velocity deviations if your fuel meter pressure was off by a couple percent.
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Unread postAuthor: Technician1002 » Mon Apr 18, 2011 1:16 am

Maybe this graphic will show in an elbow the area with high flow is restricted. After the restriction of the elbow the velocity is lower. the closer you aproach SOS, the worse this effect chokes flow.
Image
Look for areas of dark blue and violet. The velocity is very low in those areas. They are about the same as blocked as there is no real flow in those areas.
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Unread postAuthor: jimmy101 » Mon Apr 18, 2011 5:25 pm

I seriously doubt you will get anywhere near the SOS in the chamber of a combustion gun. Figure the gas temp is up over 2000C so the SOS is considerably more than 1100FPS. Figure the SOS at that kind of temperature is more like 3000 FPS. So, unless you are building a 1X combustion gun that you expect to have a multi-mach muzzle velocity the SOS in the chamber is pretty irrelevant.

Edit: I would expect the more relevant issue would be the heat lost to the pipe as the gases negotiate the bend.
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