Bell mouth outlet pipe?

[rmitchell62]

http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Contains a CFD analysis of Bell openings that applies to this thread.

[retired2]

http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Contains a CFD analysis of Bell openings that applies to this thread.

Interesting article, even though it made my head hurt trying to understand it.  What was interesting was the writer claims a mass flow rate improvement of 16% for a simple radius bell-mouth vs. a straight pipe.  I looked back at the data I published when testing my separator build, and it shows a 13% improvement.  Given the lack of sophistication in my testing procedures that is pretty close.

The article goes on to say that a bell-mouth with an elliptical radius would yield another 4% improvement.  Unfortunately, I doubt there is a source anywhere for off the shelf elliptical radiused bell-mouths.  So, that 4% will probably elude us wood butchers forever.

Never the less, the article provides more verification that all top hat separators should be built with a commercially available simple radius bell mouth on the outlet pipe - and even at the pick-up ports where possible.

Regards,
Retired2

[rmitchell62]

@retired2

One other thing I believe was in this article was a description of the optimal ratio of dimensions (but I've been reading alot).  If the pipe diameter is x then bell rim diameter is 2X and optimal length of pipe is also X.  So if I plan an 8" wide pipe outlet (inlet to fan/vacuum) then the pipe should be 8" long.  I haven't found any research on the distance below the bell, to the baffle.  I know yours is about 3" or so but that seemed really low to me.  I'm going to try 8" above the bell and >1x below.  We will see. 


Maybe we should bite the bullet and do a computational analysis of the design.  Any current University Engineering students lurking?

.
Rob

[retired2]

@retired2

One other thing I believe was in this article was a description of the optimal ratio of dimensions (but I've been reading alot).  If the pipe diameter is x then bell rim diameter is 2X and optimal length of pipe is also X.  So if I plan an 8" wide pipe outlet (inlet to fan/vacuum) then the pipe should be 8" long.  I haven't found any research on the distance below the bell, to the baffle.  I know yours is about 3" or so but that seemed really low to me.  I'm going to try 8" above the bell and >1x below.  We will see. 


Maybe we should bite the bullet and do a computational analysis of the design.  Any current University Engineering students lurking?

.
Rob

It makes sense that there is an optimal neck length for a bell mouth, but it never occurred to me at the time, so mine is arbitrary in length.  Also, my air straightener probably extends into the optimal neck area, so that may also have some unknown effect on performance.

With regard to bell mouth position relative to the baffle,  my air flow was highest when the bell mouth was closest to the baffle.  That always puzzled me because that is not what I would have expectecd.  I did not leave it there simply because Phil's testing suggests the position to be 1/2 the pipe diameter.

It's been 30 years since I've done any serious engineering, and that was fluid flow, so any analysis beyond elementary basics is over my head!  The real question is whether it is worth it?  I don't doubt that a few more cfm's could be squeezed out of the typical separator, but I suspect most DIY dust collection systems have other "low hanging fruit" that would produce bigger gains.

[Geezer]

Youtube DIY link on how to make a flared port tube:

http://www.youtube.com/watch?v=fzdnlBkG2mI

Not sure about HSE practices (electrics and H2O!) but the technique looks fairly simple. Might give this a go on my 150mm (6in) connector pipe between separator and impeller housing.

Any suggestions on how to square off a large diameter pipe? I received a rough hand sawn length of pipe which needs squaring first. I'm even struggling to draw a straight line around the pipe so I can cut and file to the line.

Thanks.

[dabullseye]

if u need to draw a line around a pipe , get a long  lenght of paper with a straight edge and wrap it  @ the pipe at least twice and keep the st edge straight is you go around the pipe then just mark the line and cut