flow straighteners - adpub.com straighteners 2005-07.pdfstraighteners by david summers, ph.d. a...
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40 • July 2005 • CleanerTimes
onsider paddling a canoe down a smooth, narrow canal.
The water runs fast, but once steady in the middle of the stream,
you are carried forward without a great deal of effort to steer.
Now consider trying to steer where there are whirlpools along
the edge of the stream that grab the bow and pull it over, so that
even working as hard as you can, you can’t steer straight.
Water particles have the same situation as they move down
pipe that is smooth, neatly aligned, and with a smooth transi-
tion into and through the noz-
zle, as opposed to a system that
has been slapped together just
to get a jet out of the end of a
lance. In the former case there
is no turbulence generated
(which causes the water to
spin), and the stream that
comes out of the nozzle is
solid and carries the energy out
a long way from the nozzle. In
the second case the water
enters the nozzle spinning,
and so when it comes out it is
still spinning and the jet fans
out and the distance over
which the jet is effective is
much shorter.
Now you might think that no one would put a system
together where the jet is made deliberately bad, and that is true,
but without thinking and taking a few precautions many of us
C
HIGH PRESSURE
Flow
Straightenersby David Summers, Ph.D.
A variety of flow straightener designs.
Type A Type C
Type DType B
CleanerTimes • July 2005 • 41
can create a bad flow situation lead-
ing into a nozzle. Here are a few
thoughts so that when you go to put
a system together, it will give you
the better of the two alternatives I
have described.
The first step is to make sure,
where you can, that the section of
high pressure tubing leading into the
nozzle is straight for, if possible,
one hundred tubing diameters back
from the nozzle. In other words if you
have a lance with a 0.5-in. internal
diameter, it is best if there is a straight
feed to the nozzle that is at least 50
in. long. This gives the water a
chance to ”straighten out” after any
bends, and to stabilize so that the
direction of all the water particles
is along the axis of the tube.
Of course, this might not be pos-
sible. There is an alternative that
can be just about as effective, and that
is to put a flow straightening device
into the line. What this does is to
reduce the diameter of the effec-
tive tube by dividing the channel
into a series of smaller segments.
The flow straightener does not
have to be that long to be effec-
tive. (Most of them range from an
inch for small pipes to perhaps a
foot for the very large ones.) By
dividing the channel into smaller
segments, the flow straightener
makes it easier to develop laminar
flow in the water as it moves to
the end of the tube. (The technical
rationale is that by reducing the
Reynolds Number, a number that is
controlled by the diameter of the flow
channel, below 2000, the flow tran-
sitions from turbulent to laminar.)
It turns out, that the flow straight-
eners do not have to be in the straight
section of the pipe. The then U.S.
Bureau of Mines proved this in
some work done for Dr. Savanick in
the 1970s by TRW1. The study first
theoretically, and then experimen-
tally showed that by matching the
curve of the bend in a pipe from ver-
tical to horizontal with a specially
designed flow straightener that
curved around the bend with the
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water, a jet could be produced that
would throw the energy as far as
though the nozzle were attached
to a straight lead-in section of the
right length to normally give opti-
mized nozzle flow.
Now you might think that at this
point you are done. Unfortunately, it
is at this point that most folk make the
next mistake. When the nozzle fits
over the outlet of the feed tube it
should be a smooth fit, with both
being at the same diameter, accu-
rately aligned and in firm contact. In
many nozzle assemblies there is not
a smooth transition from the tube
diameter to the nozzle inlet. Either one
or the other is of a different size.
This means that there is a step in
the edge flow along the jet, and this
induces turbulence. Since it occurs
as the water is entering the nozzle,
then the water spins as it goes
through the nozzle, and keeps that
spin as it comes out. This means
that the jet has a sideways throw that
will disrupt the jet much closer to the
nozzle. This gets worse in cases
(and I have seen a number) where
the nozzle does not contact the end
of the tube. Now there is a much
larger step between the end of the
tube and the nozzle, and so, a much
greater turbulence can be created.
(You can think of it as a large
whirlpool around the flow stream
just as you go into a set of rapids.)
At this point there are only two
more things to worry about. The first
is easier to do something about than
the second. If the nozzle assembly has
more than two orifices, then you
need to try and ensure that the flow
is evenly and smoothly divided to
each orifice, and that there is at least
a small conic lead-in to each of the
orifices. While this may be a little
difficult to do, Doug Wright2 has
shown that this stops damage in the
throat section of the nozzle and can
significantly improve jet quality.
The other thing that helps is to
polish out any burrs or manufac-
turing steps on the inner nozzle
surface. This is often harder to do
(which is why we have students do
it) but if the surface can be made
smooth enough that you can see
your face in it then it is about the
level that you need, around 6 micro-
inches. Normally this can’t be done
with a carbide nozzle, and trying will
not really gain you very much, but
it works in softer metals.
All of this may sound as though it
is a lot of effort for not that much gain,
but you might want to consider that,
by doing just the steps that I have
outlined above, Bruce Selberg and
Clark Barker3 increased the effec-
tive throw of a waterjet through a
nozzle from somewhere around 125
orifice diameters (5 in.) to 2000 orifice
diameters (40 in.) for a 0.04-in. diam-
eter nozzle. Admittedly, for that
work they had to have the nozzles
especially made to get the high level
of polish on the inner surfaces, which
were made of an electro-formed
nickel. But that aside, the rest of the
steps that they carried out to improve
jet throw are relatively common-
sense applications of simple fluid
mechanics to make sure that the
water has not reason to move in other
than a smooth, steady, straight direc-
tion once it gets out of the nozzle.
David Summers is the Curators’
Professor of Mining Engineering at
the University of Missouri-Rolla, and
Directs the High Pressure Waterjet
Laboratory at UMR.
References
1. Lohn P.D. and Brent D.A.
“Inlet Nozzle Performance
Study” TRW Defense and
Space Systems group for USDI
Bureau of Mines, Final Report
Contract JO255024, July 1977.
2. Wright D. , Wolgamott J., and
Zink G. “Waterjet Nozzle
Material Types”, Proc. 2003
WJTA American Waterjet
Conference, Houston TX,
paper 4-B, August 2003.
3. Barker C.R., & Selberg B.P.
“Water Jet Nozzle Performance
Tests”, paper A1, 4th Int
Symposium on Jet Cutting
Technology, Canterbury, UK,
April 1978. CT
For information circle 38242 • July 2005 • CleanerTimes