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This can't be right (chamber fan flow velocity)

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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This can't be right (chamber fan flow velocity)

Unread postAuthor: D_Hall » Sat May 03, 2008 4:30 pm

So I'm looking at cooling fans (ie, chamber fans). I want to know what kind of air velocities are being generated by them.

Looking over at McMaster, I see 80 mm (3.15" models).
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There's a bit of housing there so I'll say that the fan itself is 3" in diameter. The hub? Call it 1.5". That means that the streamtube has a cross sectional area of... pi/4*(3^2-1.5^2) = 5.3 in^2

There are three fans listed in McMaster of that size. Flows are 21, 23, and 36 cfm. I'll squint a bit and use 25 cfm as a "standard."

25 cfm = 0.417 cf/s = 720 in^3/s.

720 in^3/s through 5.3 in^2 = 136 fps? WTF?




edit: D'oh! 136 INCHES/s = 11.3 fps.

Nevermind folks.
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Unread postAuthor: LikimysCrotchus5 » Sat May 03, 2008 4:31 pm

Wait, so is there still something wrong?
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Unread postAuthor: dongfang » Mon May 05, 2008 10:08 am

Hi,

That's why NASA switched to Metric :D :D :D

Regards
Soren
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Unread postAuthor: POLAND_SPUD » Mon May 05, 2008 10:25 am

omg... I am glad I don't have to use imperial units
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Unread postAuthor: Antonio » Mon May 05, 2008 11:17 am

dongfang wrote:Hi,

That's why NASA switched to Metric :D :D :D

Regards
Soren


yeah I was ganna say that as well.... haha lol
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Unread postAuthor: D_Hall » Mon May 05, 2008 3:43 pm

POLAND_SPUD wrote:omg... I am glad I don't have to use imperial units

LOL... The "doh" moment would have been just as likely in SI units... I would have been looking at a velocity in cm/s wondering WTF before properly converting to m/s. ;)
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Unread postAuthor: jimmy101 » Mon May 05, 2008 5:41 pm

dongfang wrote:Hi,

That's why NASA switched to Metric :D :D :D

Regards
Soren

Not really. NASA switched to metric for the same reason the US military is mostly metric. It avoids the errors assocaited with working with partners that use a different system.

Imperial units can be a PITA, but engineers havn't had any problems with the units. We'll have to wait and see how many errors the switch causes. An engineer with 30 years experience is much more likely to make an error after the switch than he was using the imperial system. For example, he probably has a much better intuitive feel for how many BTUs a reefer should be than how many kilojoules it should be.

Look as D_Halls original post ...
720 in^3/s through 5.3 in^2 = 136 fps? WTF?

He had the wrong answer and he intuitively knew something was wrong, he just couldn't find it for a while. If he had been doing the calculation in a system of units he was unfamiliar with he may well have accepted the answer as correct because he wouldn't have any intuitive feeling for what the answer should be.

Don't underestimate the importance of working in a system of measure that the person is comfortable with.
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