Do Bellmouths count towards the length of the runner? 1

[jminahan]

I've been reading books, talking to masters and I'm still confused. I wanted to get some opinions from the local propeller heads. Especially Pat.

http://www.profblairandassociates.com/RET_Articles.html

Good info for you guys if you haven't read up. Read the bellmouth article.
The Bell mouth article is probably the only really worthwhile article imho because the others are mostly ads for his $13k software. The articles are brutal to read, but there is brilliance there.

My question is...Does the bellmouth count towards the length of the runner? How much?

I've been told by very reputable sources:
no.
Yes.
and Some.

Any thoughts?

Jason.

 

[GregLocock]

Without reading the article (I'm on slow diallup today) , from an acoustics point of view

(b) some.

Otherwise trumpets wouldn't sound like trumpets.

It's actually a pretty complex field, I think the best practical approach is to try it and see.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jminahan]

If I may play devils advocate:
We're sucking through the trumpet. Not blowing. No sound to sound like a trumpet.

That also brings me to my next question:
Should I put a set of bellmouths on the bottom of my carburetor going into my plenum? (assume chevy v8)
The modern carbs are building them into the top of the carb, no doubt. How about the bottom?

 

[IceStationZebra]

I am a newbie to the acoustics field, but I have read books/papers that say a tapered expansion has an acoustical effect. This effect diminishes as the angle gets closer to perpendicular. There was no specific angle vs. effect given probably because it has too many variables such as flow rate, entrance diameter, etc. A bellmouth should act like a complicated taper.

So yes, I agree with Greg.

Your other question "Should I put a set of bellmouths on the bottom of my carburetor going into my plenum?"
The extreme version of this would be a tunnel ram or individual runner manifold. The simple version would be a 4-into-1 tapered carb spacer.

http://www.freepatentsonline.com/6595175-0-display.jpg

ISZ

 

[patprimmer]

The trumpet analogy is correct in my opinion, and Greg is far more experienced in this than I am.

My understanding is the bell mouth is there to ease the transition of still air surrounding the intake into the column of air travelling down the manifold and port.

It allows the air to accelerate into the tube and minimises turbulence at the entrance to the tube and has nothing directly to do with tuned length.

The tuned length is to do with sound waves bouncing back as they exit the runner. The more defined the end of the runner the stronger this bounce back, but even with a poorly defined end point there is still some bounce back of sound wavea.

The bell mouth makes the end of the tube less defined, but still works fairly well. The gains in steady state airflow from the bell mouth outweigh the sharper definition of the endpoint for reflected wave or pulse tuning.

The bigger and more gradual the flare of the bell mouth, the more it assists steady state and the more it diminishes the pulse peak.

My experience tells me bigger bell mouth designs have a wider but less intense peak band. This kind of fits with less powerful and less defined pulse and better steady state flow.

I don't know, but I expect that the reflected wave is propagated where it emerges from the high speed column and hits slower or stationary air, so point to measure from is where the air really starts to accelerate into the runner.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jminahan]

Interesting Pat; however, as I understand the Blair article (It really is worth the read, only a couple pages), the bell mouth or trumpet works to *increase* the strength of the reflected pulse and ram tuning, not just steady state.
He also gives a very detailed description of the "best" shape for a bell.

I've recently been told by a very respected engine guy that:
If the bell mouths are entering a plenum you can count the whole length of the bell.
If the bell is entering free atmosphere you don't count it at all.
WTH?

btw, I'm designing a single plane 4 barrel manifold, very long runner for a v8.

Jason

Minahan Modifieds

 

[SomptingGuy]

An interesting article. Strange though that Blair mentions Fluent many times, but does not name the 1D code anywhere. I wonder if it was his own or someone else's?

To sum up the article: The aim of a bell mouth is to minimise the flow separation at the duct entry. Flow separation leads to a reduction in the available flow area, the ratio of available flow area to actual area is the Cd. The amount of separation depends on the shape (primarily radius) of the corner and the speed the air is going around it. Hence it depends on geometry and pressure ratio. CFD can be used to make predictions of Cd vs pressure ratio and geometry.


The effective runner length issue is equally interesting but not touched upon by the article.

If you take an open pipe and calculate a natural frequency based on its length and the speed of sound, you'll get a value slightly higher than what's measured. The pipe behaves as if it is slightly longer than its geometric length (caused by complex interactions between the sound propagation in the duct and the sound radiation at the exit). This has obvious implications for runner length tuning - a simple (unflanged) pipe behaves as if it were about 0.3d longer than its geometric length (d is the pipe diameter). Stick a flange on the exit and the additional length goes up to 0.4d. This additional length is commonly called the "acoustic end correction".

As expected, the end correction is highly dependent on the form of the duct end. For a bell mouth, it's common to define the geometric length as the length of the straight part of the duct and then look up an end correction based on the duct and bell mouth radii. The sum of these is the effective length of the duct. For example, if the bell mouth radius is 2x the duct radius, the acoustic end correction would be about 1.0d. So the effective length of the duct would be its length without the bell mouth, plus its diameter. If the radii were equal, the end correction would be about 0.6d.

---

The above values are taken from an excellent paper on the subject:

"Wave reflections from duct terminations", A.Selamet & Z.L.Ji (Ohio State University), R.A.Kach (Ford Motor Company). J. Acoust. Soc. Am. 109 (4), April 2001.

The paper shows end corrections (and reflection coefficients) for many different duct terminations.

---

So.. the answer to the original question is:

Yes, but then so do all forms of termination.
How much depends on the radius, but the data is available.

- Steve

 

[evelrod]

Well, here goes the old guy again---

I have done quite a few tests on 40 and 45 DCOE Weber carbs on a 1594cc Lotus Twincam engine. On the dyno, changing the air horn length does show peak power and VE changes but not at all like changes in the exhaust system. Adding, changing taper, etc. of a megaphone shows some big effects. On paper it works out to about (that's real iffy) half length on a slow taper. On the bell mouths of the Webers, I could not tell any difference in the smaller "Weber type" and the larger Mikuni/Solex type bell mouths. Length, yes. Shape, no. I'm sure there is most likely a difference, just not enough for my very unsophisticated test equipment.

Rod

 

[jminahan]

Wow steve! Thanx,
The manifold I'm designing is going to be a compromise of several things anyway, but I feel more comfortable when I know what I'm compromising and what I'm not.

jm