Waterjet Technology – Water Jet Stream Structure

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

Waterjet Technology – Water  Jet Stream Structure

In last week’s post I showed some high-speed photographs of the plain water jets that come from the small diamond and sapphire orifices and that are useful in cutting a wide variety of target materials. Before moving away from the subject of high-speed photography, this post will use results from that technique to talk about why pressure washer nozzles may not work well, and have limited range. From there it will raise the topic of adding abrasive to a waterjet stream.

Most of us, I suspect, by this point in time, have used a pressure washer to do some cleaning, typically around the house or perhaps at a car wash. The jet that comes out of the end of the nozzle is typically a fan-shaped stream that widens as the water moves away from the orifice. This flattening of the jet stream, and the resulting spreading jet is achieved by cutting a groove across the end of the nozzle to intersect either a conic or ball-ended feed channel from the back end of the nozzle.

Schematic of how a fan–generating orifice is often made.

Schematic of how a fan–generating orifice is often made.

One of the problems with this simple manufacturing process is that the very sharp edge that is produced to give a clean jet leaving the nozzle is very thin at the end. This means that with water that is not that clean (and most folk don’t filter or treat pressure washer water) the edge can wear rapidly. I have noted several designs (and we tested many) where the jet lost its performance within an hour of being installed, particularly with softer metal orifices. And in an earlier post I did show the big difference between the performance of a good fan jet and a bad.

So how do photographs help understand the difference, and explain why you should generally keep a fan jet nozzle within about 4-inches of a surface it you are trying to clean it. That does, however, depend on the cone angle that the jet diverges at, once it leaves the nozzle. We found that a 15-degree angle seemed to work best of the different combinations that we tried. If the jet remained of sufficient power, this would mean that it would clean a swath about half-an inch wide with the nozzle held 2-inches above the surface. At 4-inch standoff it will clean a swath about an inch wide, and at 6 inches, this goes up to over an inch-and-a-half. But that would require that the jet be of good quality, and evenly distributed.

Back-lit flash photograph of a fan jet, at a jet pressure of around 1,000 psi. It is less than 6 inches from the end of the orifice to the rhs of the picture.

Back-lit flash photograph of a fan jet, at a jet pressure of around 1,000 psi. It is less than 6 inches from the end of the orifice to the rhs of the picture.

In Figure 2, the lack of water on the outer edges of the stream shows that the water is not being evenly distributed over the fan. As the water volume leaves the orifice, the sheet of water begins to spread out into the wider, but thinner, sheet that forms the fan. But as it gets wider it also gets thinner, and, like a balloon, water can only be spread so thin before the sheet begins to break up. As soon as it starts to do so, the surface tension in the water causes it to pull back into roughly circular rings of droplets.

Fan jet breakup from a spreading sheet into rings (or strings) of large droplets that rapidly break down into mist.

Fan jet breakup from a spreading sheet into rings (or strings) of large droplets that rapidly break down into mist.

These droplets start out as relatively large in size, but they are moving at several hundred feet per second, and as single droplets moving through stationary air the air rapidly breaks them up into smaller droplet sizes, and then into mist, while at the same time slowing the droplets down. The smaller they get the quicker that deceleration occurs. When droplets get below 50 microns in size they become ineffective. (From a study that was done on determining the effect of rain on supersonic aircraft).

Showing the stages of the fan jet breakup from a solid sheet to mist that does little but wet the surface that it strikes.

Showing the stages of the fan jet breakup from a solid sheet to mist that does little but wet the surface that it strikes.

However, if the nozzle is held just in that short range where the droplets have formed, but have not broken down, then the jet will be more effective than it would have been at any other point along its length. This is because of something that was first discovered when scientists at the Royal Aircraft Establishment-Farnborough and at the Cavendish Lab at Cambridge University were studying what would happen if they flew a Concorde into rain, while it was still going supersonic. (They actually tried this in a heavy rain storm in Asia and found it was a seriously bad idea).

The pressures that can develop under the spherical droplet can exceed twice the water hammer pressure so that the impact pressure on the surface can exceed 20-times the driving pressure supplied by the pump. But the region effected is very small, and the effect diminishes as the surface gets wetter. And the problem, as with all waterjet streams, is that it is very hard to know where that critical half-inch range is. It varies even within the same nozzle design models due to small changes on the edge of the orifice. And as a very rough rule of thumb, a perfect droplet moving at a speed of around 1,000 ft/sec will travel 138 diameters before it is all mist. Most drops aren’t perfect and thus will travel around 30 – 50 diameters and once they turn into mist they will decelerate to having no power in less than quarter-of-an-inch. The implication of this, which we checked with field experiments, is that if you hold a pressure washer nozzle with a fan tip more than 4-6 inches from the target you are largely just wetting the surface, and spending a fair amount of money in creating turbulent air.

This story of jet breakup is a somewhat necessary introduction to two posts that I will be along before long. The first will be to discuss how we can use a different idea for nozzle designs to do a much better job, at greater standoff distances, and I will tie that in with some of the advantages of going to much higher pressure to do the cleaning job.

The other avenue that this discussion opens relates to how we mix abrasive within the mixing chamber of an abrasive nozzle design, and that will come along a little later.

(For those interested in more reading there have been a series of Conferences on Rain Erosion, and then “Erosion by Solid and Liquid Impact” which were held under the aegis of John Field at Cambridge for many years. See, for e.g.. Field, J.E., Lesser, M.B. and Davies, P.N.H., “Theoretical and Experimental Studies of Two-Dimensional Liquid Impact,” paper 2, 5th International Conference on Erosion by Liquid and Solid Impact, Cambridge, UK, September, 1979, pp. 2-1 to 2-8. The founding conference was held under the imprimatur of the Royal Society, which devoted a volume to the Proceedings. Phil. Trans. Royal Society, London, Vol. 260A.)

 

Waterjet Technology – An intro to water jet structure

 

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

Waterjet Technology – An intro to water jet structure

Once a waterjet starts to move out of the nozzle with any significant speed, as the pump pressure begins to build, it becomes more and more difficult to look at the stream of water and get any realistic idea of its structure. Mainly what is seen is the very fine mist that surrounds the main body of the jet, and while some idea of the structure can be obtained by making cuts through material, it can be quite expensive to actually see within that structure. Part of the problem is that though the mist is very fine, it is also moving at speeds in the range of a couple of thousand feet per second. The human eyeball isn’t quite that fast. But we can use a very high-speed flash (in this case it was on for two millionths of a second) which has the effect of “freezing” the motion.

Figure 1. 40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit.

Figure 1. 40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit.

However this mist still hides the solid internal structure of the jet and does not change much in relative structure, even when the internal jet conditions can be quite different. Fundamentally the internal structure was described by Yanaida at the 1974 BHR Group Waterjet Conference, and his description has been validated by many studies since.

Figure 2. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

Figure 2. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

This structure holds for jets across a wide range of pressure and flow volumes, but it is difficult to determine the exact transition points of that structure conventionally. And this can lead to very unfortunate results. I have twice seen people back a nozzle away and then move their hand in front of the jet to show that even high-pressure jets (these were being used to cut paper products and had no abrasive in them at the time) could be “safe.” If both cases the individuals were very lucky to escape injury (water can penetrate the pores of the skin and lacerate the internal parts without any surficial signs of injury, and, as I showed last time, if the nozzle is too close it will slice through flesh and bone). I thought to take today’s post to show, though the use of photographs, why that was such a stupid action.

The photos that follow were taken in Baxter Springs, KS, which has been recognized as the Birthplace of Waterjet Cutting.

Baxter Springs, Kansas. Birthplace of Waterjet Cutting.

Baxter Springs, Kansas. Birthplace of Waterjet Cutting.

In the early 1970’s we used, what was then a McCartney Manufacturing waterjet intensifier, (today KMT Waterjet Systems) to shoot jets of varying pressure, and nozzle diameter along a path, so that we could see how coherent the jets were. As I mentioned above, the problem with looking directly at the jet is that the internal structure is hidden by the surrounding mist. To overcome that part of the problem we shone the light along a ground glass screen (to diffuse it) that was placed behind the jet, so that we could see the outline of the internal structure.

Figure 4. Arrangement for taking photographs of a high-speed jet.

Figure 4. Arrangement for taking photographs of a high-speed jet.

This more of the downstream mist from the photograph, and a much better idea of the internal structure of the jet, and where the solid section ended could be measured.

Figure 5. Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches.

Figure 5. Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches.

The benefit of the technique is perhaps more evident when nozzles at different pressures and diameters and different chemistry are compared. First consider the change with an increase in jet diameter. From the front-lit view there is little difference in the jets. From the backlit, it is clear that the smaller diameter jet only reaches 3-inches across the screen, while the larger jet barely reaches the end of the range.

Figure 6. The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches.

Figure 6. The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches.

One of the parts of the study we were carrying out in 1974 was to examine the effect that adding different long-chain polymers had on jet structure. The ones that we were looking at include some that are now used in the oil and natural gas industry to make the “slick water” that is used in the fracking industry to improve production from shale reservoirs. But it also has an advantage in “binding” the jet together. And so, in the study, Dr. Jack Zakin and I tested a wide range of different polymers to see which would be give the best jet.

There were a number of different things we were looking for. In cutting paper, soft tissue and water sensitive material for example, the polymer can bind the water sufficiently well as to further lower wetting to the point where it doesn’t have an effect. It also can improve jet cutting under water – but I’ll cover those in a few post on polymer effects that will come to later in the series.

The effect of a polymer (in this case an AP273) is shown in two tests where the only change was to add the polymer to the water for the lower one.

Figure 7. Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi, the range is 6 inches, and the lower jet has had the polymer AP273 added to the water.

Figure 7. Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi, the range is 6 inches, and the lower jet has had the polymer AP273 added to the water.

The narrower stream in the lower frame is the effect that we were looking for. Putting change in diameter and the better polymers together gave, as an example, the following:

Figure 8. The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion.

Figure 8. The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion.

It might be noted that the jet in the bottom frame has as much relative concentration (and power) at the end of the range as the top jet had at the beginning of the range.

Now it all depends on what you want the jet to do, as to which condition you wish to achieve. Inside abrasive mixing chambers the object is much different than it is when the object is to cut a foot or more of foam with high quality edges. And there have been some interesting developments with different polymers over the years, but I’ll save those stories for another day.

But bear in mind that those individuals who could slide their fingers under the jet in the top frame of figure 8 would have had them all cut off if the jet had been running instead under the conditions of the bottom two frames, and in all three cases, to the naked eye the jets looked the same.