Airflow in ducts with different shaped openings

[Sciguyjim]

Here are the airflow efficiencies I found in a library book for smooth duct openings with different shapes. (Ignore all the dots, I had to add them to get the solid lines to position correctly.)
I thought you guys making your own cold air intakes and ductwork would find this useful.

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Plain opening with sharp edge, efficiency = 72%

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Flanged opening, efficiency = 82%

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Funnel-like tapered opening, efficiency = 90%- 98%
Angle of taper 20° to 65°

Curved "Bellmouth" shaped opening, efficiency = 98%

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Can type opening, efficiency = 82%

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Can with funnel-like tapered bottom, efficiency = 90% to 96%
Angle of taper 20° to 65°

Can with curved bellmouth shaped bottom, efficiency = 97%

The sharp edged plain opening is the least efficient because most of the air is sucked in from right next to the duct. As it makes the sharp turn into the duct, its inertia makes it overshoot a little bit and the flow constricts to as little as 70% of the duct diameter a short distance inside the end of the duct.

 

[impala_sc]

thanks for the info.

HINT: to draw using ASCII it helps to use the CODE option.

without code:
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with code:

code:

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[digitaldrifter91]

cool man..i have often ponderd about building a custom intake with the advantages of cold air and short ram. this will really help. do u build custom intakes?? u have one on ur car?? tell me about it...

 

[Sciguyjim]

quote:

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Originally posted by Jay:

quote:

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The sharp edged plain opening is the least efficient because most of the air is sucked in from right next to the duct. As it makes the sharp turn into the duct, its inertia makes it overshoot a little bit and the flow constricts to as little as 70% of the duct diameter a short distance inside the end of the duct.

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I'm assuming these efficiency figures are with a stationary duct with a vacuum on one end. But which would be more efficient if the duct were a ram duct? Would the flanged opening possibly create more turbulence and be less efficient than the plain opening?

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Jay,

The book did not specify how they did the tests, but I have seen examples in books showing bellmouth shaped intakes in various positions on cars, from vertical intakes on dragsters, to small, ram effect intakes mounted behind fog light openings on street cars. It doesn't matter where they are mounted, or if the flange or bellmouth is mounted to a backing panel, or if they are on a ram intake or not. The presence of a properly curved and aerodynamic shaped intake just works better than the lack of one. Where turbulence from the flange seemed to be a consideration they seemed to either be mounted to a backing panel or the edges of the bellmouth were curved back on themselves to form an aerodynamic trailing edge.

Digitaldrifter91,

Well, yes and no. No, I don't build custom intakes, but yes, I do have one on my car. I wanted to make a cold air intake and ran some flexible aluminum ducting from the snorkle on my airbox into the space inside my fender. I then made further improvements by using a bakeable hardening hobby putty to fashion a bellmouth shaped opening on the original edge of the ducting. It was very easy to do. Because of tight spaces, it was harder to get the duct positioned inside the fender afterwards, but it was doable.

Before I installed the finished duct though, I went even further and smoothed out the inside of the duct to make it more aerodynamic too. I realize some of you may think this is going too far, but it was easy, and clever too I think. I simply took a big bottle of Elmer's glue and poured it inside the flexible duct, then rotated it so that the entire inside surface was coated with glue. I drained the excess and let it dry while trying to keep the coating as even as possible. This first coat is the trickiest. I then gave it 2 more coats. Now the inside of the ducting has only slight ripples instead of the original deeper ones. Now my intake duct is as aerodynamic as I can possibly make it and I only used cheap, easy to shape materials. I like making things simple but effective (and saving as much money as possible.)

 

[OneQuartLow]

Sciguyjim, I've done that with paint & resins before, when smoothing interior parts. For the last 10% oil-base floor paint seems indestructible and fills/sticks well. Always wanted to try it with plasti-dip.

Waterproof Elmers?

David

 

[Jay]

quote:

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The sharp edged plain opening is the least efficient because most of the air is sucked in from right next to the duct. As it makes the sharp turn into the duct, its inertia makes it overshoot a little bit and the flow constricts to as little as 70% of the duct diameter a short distance inside the end of the duct.

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I'm assuming these efficiency figures are with a stationary duct with a vacuum on one end. But which would be more efficient if the duct were a ram duct? Would the flanged opening possibly create more turbulence and be less efficient than the plain opening?

 

[Sciguyjim]

OneQuartLow,

No, just ordinary Elmer's. The intake is high enough inside the fender that even in extreme rain, no water should find its way in there. Besides, as Elmer's ages, it gets less water soluble. Even now with it new, if water got in it would evaporate in the airflow before the glue had a chance to soak up enough and get soft. Even if it did break up somehow, any pieces that came off would only get as far as the air filter.

 

[digitaldrifter91]

Sciguyjim u are a clever person with that glue!! haha...i never would of thought of it. i am a weird person too and would try to do something in a way that it would be cheap for me...im going to keep ur idea handy haha. do u have a pic of ur work..if u cant sent it through here send it to my email if u got one [email protected]

 

[Sciguyjim]

No, sorry. Never thought to take a picture before I installed the duct.