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 Turbulence and baffles in a small bench 
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Joined: Wed Aug 25, 2010 7:54 am
Posts: 402
Location: Riverina Australia
I'm just finishing off a small orifice bench for Kart use, 2 motors, total flow 193 cfm @ 28"
I designed it quite small due to space limitations and it can be lifted and carried by one person.
In the small design I tossed around a lot of thought and ideas with baffling above and below the orifice. The layout allowed a fairly straightforward baffle set up between the orifice and transfer port from the motors but above the orifice was a different story.

The 2.6" orifice is directly below the test port approx 5 1/2" so to put a baffle halfway up worried me, I thought a baffle would restrict the flow or create turbulence from being too close to the orifice so I left the area open, hoped for the best, glued, screwed and finished the main cabinet.

In the picture I hope you can see what I'm talking about. The pic was taken before the floor, front and side were fitted. You can see the lower baffling and the straight shot from orifice to test port.

Jim


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Fri Dec 02, 2016 9:41 am
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Joined: Wed Aug 25, 2010 7:54 am
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Location: Riverina Australia
When I first fired the bench, I ran it in intake mode and did a leak check. No problems, close to zero @ 28", the motor speed was fairly low at this point, everything sounded good so I left the test port open and took the motors up to max speed.

The noise was INSANE! Howling and whistling and the air was so unstable it vibrated everything close by. HMMMMM. Problem. Massive turbulence. Next I brought down the motor speed and had a quick check of calibration with a PTS 100 @ 28" plate. The reading was way off. I didn't record it but think 40 or 50 cfm. It's at this point I know I need a top side baffle. Due to the chamber being totally glued, screwed and painted, I spent some time changing the calibration in the DM to see what effects it would have. With the CD set at a really crazy level, I could get the 100 plate to read 100 cfm every time.

Next I tried a 150 plate. Computer shows 115 cfm! I didn't bother with the 200 plate.....

So I shelved the bench for a few days, spoke to Bruce a few times, read through the forum and tried to think of any possible way to fit a top side baffle with my only access possibility through my 2 1/2" test port.

I was explaining the problem to a fabricator friend who I thought could offer another idea, he said - break the bench you have to go inside. Later that day he rang back, don't wreck the bench, he had something for me to try.

He came in with a block of hard foam. I said no straight away, it would surely break up and go through the motors. I tried to tear it, but it was really tough. HMMMMM, time for a test. ( Away from the bench! ) 3 shop vacs hooked together won't tear this stuff. Strong as hell and coarse like sea sponge too so maybe it could work.
Turns out it's baffling foam from a fuel cell that got removed when a local racer switched to alcohol I later found out.

In short, I cut a cube, squeezed it in so it sits on top of the orifice and 1 1/2" below the test port. Success! No more turbulent noise, no vibration and calibration is now no problem.

Jim


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Fri Dec 02, 2016 10:37 am
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Location: Riverina Australia
So now its calibrated really nice and repeats the same every time. At the moment I"m laminating and finishing the control panel, I'll post a picture when its done.
The foam does restrict flow, that's the only drawback of using it. The motors each flow 112 cfm @ 44" so using the rise of 16" and taking some peak off for grouping loss, the motors combined potential at 28" should be 200-210 cfm. With the foam baffle the limit is 193 cfm @ 28".

An interesting side point for the thinkers here:

With the baffle out and a 200 plate on top, you can wind the motor speed up and the depression goes up. (obviously) The thing is, when you go up in depression, the cfm reading goes up, peaks at about 80 then goes down as the depression goes up. At approx half to 3/4 motor speed the cfm reading has zeroed out and will not change, BUT if you REVERSE PT2+ and PT- at this point ( the pickups above and below the orifice ) the cfm readings come back up and at full motor speed register 50 cfm!
It will only do this with a cal plate on top.

Jim

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Fri Dec 02, 2016 11:09 am
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Joined: Wed Aug 25, 2010 7:54 am
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Location: Riverina Australia
Screenshots from calibration, all at the same CD, all the same internal orifice. Gotta love the PTS DM!
100 plate @ 28
150 plate @ 28
200 plate @16
300 plate @ 10


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Fri Dec 02, 2016 11:14 am
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Joined: Mon Feb 23, 2015 6:10 am
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Location: Northern NSW, on the wrong side of the Great Divide.
Interesting. I wonder if the bench was taller, and not as wide, would you be able to use a baffle.

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Fri Dec 02, 2016 11:48 pm
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Joined: Wed Aug 25, 2010 7:54 am
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Location: Riverina Australia
Yes. the height is the main restriction

Jim

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Sat Dec 03, 2016 10:03 am
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Joined: Fri Jan 08, 2010 3:35 pm
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Interesting . . . glad you made some progress!

I can see some MDF cutting in my future trying out some of mine and Rick's thoughts on a small bench design. Seems like there is a push for something like this.

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Sat Dec 03, 2016 11:11 am
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Joined: Thu Mar 04, 2010 9:01 pm
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I'm familiar with fuel cell foam, I'm a little surprised it works so well sitting directly on top of the orifice. Anyways, the foam is quite durable however it doesn't last forever eventually it will break down. It's cheap enough that it would probably be wise just to treat it as a consumable and swap in a fresh piece once every few years.


Sat Dec 03, 2016 12:47 pm
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Joined: Wed Mar 03, 2010 3:40 pm
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Location: Melbourne, Australia
I suspect that foam may accumulate dust fairly readily, and that may tend to gradually increase the pressure drop over time. But an occasional dunking in a bucket of hot soapy water should fix it, if the foam piece is readily removable.

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Sat Dec 03, 2016 4:55 pm
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86rocco wrote:
I'm familiar with fuel cell foam, I'm a little surprised it works so well sitting directly on top of the orifice.


I can relate! The plane I crewed spent much time in the fuel cell hanger and I removed and stuffed foam in many tanks over 23yrs!

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Tue Dec 06, 2016 12:46 am
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