An interesting BMH effect

**dmorse** · 1st September 2019, 03:13 AM

Actually two observations. Now maybe these are obvious to everyone but I found them interesting. First, adding a 125mm orifice plate with an eased edge to a 150mm duct end results in less resistance and thus more flow. Second is that the airflow into the BMH generates lift that tries to pull it off the pipe.

I made some restrictor plates the other day for airflow testing in a 150mm duct. I chamfered and rounded the edges to try to reduce noise.

Orifices.jpg Orifice125 Drawing.JPG

I did a quick preliminary test to make sure that my sizes gave a reasonable spacing to the data points. I left the DC running while changing orifices and all of the plates except the 125mm would be firmly sucked into place. The 125mm however needed to be pushed into place. I decided to investigate with some 100mm parts that were convenient.

When I later plotted the data I saw that the readings were nearly identical for the open pipe and the 125mm restriction.

OrificeTest.jpg
I didn't do a traverse because I was only interested in checking the set of sizes so it could have been an anomaly in the flow profile if it had only been the velocity reading. But I actually have velocity measurements from both a hot wire anemometer and a pitot-static tube. I also have pressure readings from the static port on the pitot-static tube and a piezometer ring on the duct. I also recorded motor current. All five measurements show a slight increase in flow for the 125mm orifice compared to the open pipe.

Calling them nearly equal and assuming a 0.8 loss coefficient for the bare pipe and abrupt expansion loss factor for the expansion from the orifice says that the loss coefficient for my "eased restrictor plate" is about 0.37, or a bit better than a flanged end. Assuming of course that my arithmetic is correct!

**QC Inspector** · 1st September 2019, 11:58 AM

I think because the air moving over the radius would be moving faster than the backside air which wouldn't be moving much if at all you would have the lift you demonstrate. Much the same as an aircraft wing with the airflow over the top having to travel farther than the bottom creating low pressure. Does it do the same with a 150mm/6" BMH?

Pete

**dmorse** · 1st September 2019, 07:41 PM

I agree that the high velocity air moving smoothly over the radius generates the lift. I do wonder though how much the radius contributes to this effect. Maybe I'll make a version of that fitting with a flat flange for the front and see how much the increased turbulence affects it.

I'm sure the effect would be the same with a 150mm BMH, after all, it was a 125mm quasi-BMH on a 150mm pipe that first made me aware of it. I would have used that for the video if I had a 150mm vertical pipe easily available. The visual effect just isn't as good with a horizontal pipe.

**LanceC** · 2nd September 2019, 03:06 PM

There was a thread just recently where the "popping off hood" issue was discussed. See Bell mouth hoods

Kind regards,
Lance

**dmorse** · 2nd September 2019, 09:06 PM

I remember seeing that thread, I totally forgot about the "popping off" part. Maybe I've just become desensitized to all of the "popping off" posts.

As predicted, I had to make a flanged version and try that.
Flange.jpg
I didn't make a video because most of the difference between tho two is felt rather than seen. Subjectively, the flange face has a lot less push and is much more unstable when close (1 to 10mm or so) to the pipe. There's a slight weight difference, 58g for the bell face and 62g for the flange face. Dimensionally they're identical except for the edge treatment.

Dave

**LanceC** · 2nd September 2019, 09:40 PM

Originally Posted by dmorse

I remember seeing that thread, I totally forgot about the "popping off" part. Maybe I've just become desensitized to all of the "popping off" posts.

You know Dave, when I read over the post before pressing “send” that phrase did give me pause, then upon consideration thought, “it will be fine, this is a discussion amongst adults, no-one else will read it that way”.

Kind regards,
Lance

**dmorse** · 2nd September 2019, 10:03 PM

Originally Posted by LanceC

... “it will be fine, this is a discussion amongst adults, no-one else will read it that way”.

Oh, THAT way. No, I was thinking more about posting the first thing that pops into one's head without giving it further thought. I guess
I should have rethought my post before hitting "send".

Dave

**John G** · 4th October 2019, 07:13 PM

Originally Posted by dmorse

I made some restrictor plates the other day for airflow testing in a 150mm duct. I chamfered and rounded the edges to try to reduce noise

Quick q: how are you making the BMH? They look 3D printed?

**dmorse** · 4th October 2019, 08:26 PM

Originally Posted by John G

Quick q: how are you making the BMH? They look 3D printed?

You are correct, 3D printing is one of my current obsessions.

**woodPixel** · 4th October 2019, 09:52 PM

BMH's are the very best!

BobL makes them on a lathe, heat gun and a mold. Easy peasy.

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