How a bell mouth affects flow

[mwstewart]

Hi there, I hope you can help. I am working on an inlet manifold for a 4 cylinder turbocharged engine.

Without a flowbench I cannot anser this so I hope someone on here can, what is the optimum position for a bellmouth in relation to the wall of the plenum. EG Where the inlet runner enters the plenum, how far into the plenum should the bell mouth protrude?

Thanks in advance.

 

[AboveRedline]

For overall best power, I think the runner entry should be flush with the plenum wall. But because of packaging this is not always possible, without compromising runner length. Therefore the designer combines the desired runner length, the desired plenum volume, and the exterior constraints, and the amount of protrusion becomes the dependant variable.

 

[mwstewart]

Thankyou for your help, it's much appreciated.

Mark

 

[jamesforsyth]

Would it be desirable to follow the air flow pattern and see in what way it enters each bell mouth? Depending on the shape of the entry port to the plenum, I'd hazard a guess at the furthest bell mouth (let's call it number 4 of 4, at the far end of the plenum) receiving the highest volume of air, due to it travelling horizontally along the plenum and hitting the far end, deflecting it downwards. This would result in bell mouth 4 being the first to be fed. Then I'd imagine that mouth by mouth back towards the entrance port (i.e. going back from 4-1) that less and less air would be received.

So, if this is a correct assumption, would it be fair to say that profiling the entry to each bell mouth according to this distribution of air to tune the amount drawn into each runner would even out this distribution? By this I mean that a large "lip" on bell mouth number 1, a smaller lip on 2, and even smaller on 3, and perhaps no lip on 4, would cause each bell mouth to "attract" the air flow in varying degrees which might go some way to cancelling out the unequal flow of air into the bell mouths that I described at the top of this reply.

Does anyone understand and/or agree with any of this?

James.

 

[Tmoose]

The Hydraulic Institute (now ANSI) standards have a whole section on how to design and fix wet wells which can have many pumps drawing from a common sump. One of their requirements is good relatively un-disturbed inlet flow because pumps HATE inlet vortices and stuff. I never checked the Reynolds numbers, but it sure looked like "their" problems are similar to "our" problems.

 

[jamesforsyth]

This is all making me think more and more that a one-into-four construction for the charge carrier-inlet leaders might be a better approach, space permitting. This would do away with the plenum altogether, and perhaps reduce the chance of uneven amounts of charge being delivered to each chamber. A similar setup is used on turbocharged motorbikes, which have a tiny amount of distance to get all four exhaust pipes into one large pipe to bolt to the turbo. All four are brought together into one to meet the turbo, so if this is reversed and the one pipe (charge-carrying pipe from turbo) is mated up with the four leaders in such a way that an even mass flow rate is delivered to each leader, it would surely do away with the problems you're facing with the current plenum/bell mouth/leader idea?

I know space isn't at a premium, so maybe this, along with the cost aspect, is a stupid idea anyway! I'm sure you could slap a sleek bonnet bulge on if you wanted to try it and needed an inch or few extra above.

James.