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