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RE: Aquatic Plants Digest V5 #55

> -----Original Message-----
> From: Aquatic-Plants-Owner at actwin_com [mailto:Aquatic-Plants-
> Date: Fri, 26 Apr 2002 09:13:56 -0700
> From: Alan Kaufmann <Alan4ever at pacbell_net>
> Subject: Re: Spraybars
> I noticed a mention of spraybars in the last APD.  I have found the
> making of spraybars to be a very interesting endeavor.  They
> illustrate the complexity of hydrodynamics...or maybe I am just easily
> entertained. :)

Unfortunately me too. And it seems to get easier as I get older.  :)

> I have made all my spraybars out of black, semi-rigid 1/2 inch
> irrigation riser pipe.  The type available in most home improvement
> places.  There are fittings to attach the riser pipe to standard PVC
> pipe for the input, but I never found any to block the far end of the
> pipe as this is a non-standard application.  So I use a piece of a hot
> glue stick.  I just soften it a bit with a torch, jam it in the
> opening, and cut off the excess.  Works fine.

Good idea, but I am surprised that there are not some sort of end caps
available.  I am sure that there is some sort of 1/2" cap somewhere.
Maybe when I get some free time . . .

As I thought about this I recall my sprinkler system has a riser (I
think it may be too rigid for this) but there are screw on caps for it.
Perhaps one of those would fit.

> My first spray bar I made with all the same size holes along its
> length.  I was surprised to find that the water came strongest out the
> furthest hole, not the first holes.  I guess the water rushes down the
> pipe past all the holes and when it hits the end it starts to exit the
> spraybar with the greatest force there.  This is not what I would have
> guessed.

I think this has something to do with the cross sectional flow profile.
There is a boundary layer along the wall of the pipe that has a 0 flow
velocity.  If my memory is correct, I think it is formed by friction
with the wall of the pipe and is a layer of relatively high turbulence.
As you move away from the wall and out of the boundary layer, the
velocity increases.  It is at its maximum at the center of the pipe.
Once you pass the center the flow velocity drops until you get to the
boundary layer on the opposite wall where it again goes to or near 0.
It has been a long time but I know this applies to compressible flow and
I think it also applies to incompressible flow but I seem to recall that
the velocity difference between the wall and the center may not be as
great.  Or again I could be wrong and it is substantially greater.  Ten
years is a long time for a guy who has trouble remembering breakfast
these days. ;)  Again, I'll call my old roommate or my old partner (also
an ME) to find out if anyone else really wants to know.

The boundary layer is larger at low flow velocities and small at high
flow velocity.  Once the water gets to the "plug" at the end of your
pipe the pressure builds up substantially.  The highest pressure
differential across all of the holes would be at that end, hence the
highest flow rate through that orifice.
> I tried making spraybars with the holes varying in size from larger to
> smaller along the length to even out the flow.  This worked O.K., but
> the best spraybar that I came up with incorporates internal baffles.
> I tried several different approaches before hitting on an easy and
> effective way of doing this.
> First I clamp the riser pipe in a vise and mark where I want the
> holes.  Then, using the point of a razor knife, I poke a tiny starter
> cut on the end side of the hole.  I then place a 1/4" hole punch (the
> hollow type) against the cut at an angle and whack it with a hammer a
> few times.  The result should be a 1/4" hole in the pipe with the
> "hanging chad" inside the pipe acting like a little scoop to catch the
> water as it rushes by.

I suspect that your "scoop" gets out past the boundary layer and into
the flow stream, exposing the hole there to water with some pressure.
> This design produces a very even output along the length of the pipe
> with almost no back pressure.  I hope someone finds this helpful.
> Alan

I sure did.  I knew the theory (I think - it has been a while and as I
said I am an EE/controls type, not an ME/Physics guy) but your idea with
the semi-rigid pipe jogged my memory about flow and was a great
inspiration.  I have got all sorts of ideas now.  If only I had the time
to try a few of them.  Maybe this summer . . .

Thanks Alan.

Charles Kuehnl