Waterjetting 33b - More on flow inlet conditions

The structure of the jet flow from an orifice makes a tremendous difference to the ability that the jet then has in terms both of its range and its cutting ability. And one of the major factors that control the structure of the jet lies in the flow conditions just upstream of the orifice itself. From time to time, over the decades, we would go out and buy every nozzle that was available for a certain purpose, and run tests between them, trying to find which would, under otherwise similar conditions, provide the best performance. Rarely did the most expensive nozzle give the best result. And the performance of even the best nozzle was also controlled by the flow channel upstream of that nozzle. This, in turn, controlled the condition of the water entering the nozzle. To illustrate the point let me use the example of some tests we made with fan-jet nozzles. In this particular case to objective was to clean large surfaces, but the generalized conclusion also holds true of nozzles of different shapes and jets up to even the highest of pressures used (and we have gone up to 10 million psi).

Simple cleaning nozzles, of the sort that are used in most pressure washers, have historically produced fan-jets that spread in one plane away from the orifice. There are a large variety of these on the market, of varying flow rate and geometry, and it was an initial challenge to find a simple way of relatively ranking the jet quality. Our initial answer, for the first cut, was to take blocks of polystyrene foam and traverse these at a fixed speed under the jet at different pressures and distances from the nozzle. This foam is very easily cut by a jet. So the tests were carried out at 1,000 psi and 2,000 psi, which is the range of pressures of the electrically powered pressure washers found in most hardware stores these days. The difference between two nozzles that were nominally supposed to achieve the same performance was striking:

Figure 1. Comparison of performance between a “better” fan nozzle (top) and a “poor” one (lower sample) when cutting polystyrene packing foam at low pressures.

As you may note at 1,000 psi the poor design was barely able to remove the surface of the polystyrene, rather than cutting deeply into it, as was the case with most of the nozzles tested, and as exemplified in the top cuts.

My point today however, is not the inherent faults in the design of the nozzle shape itself, but rather to highlight the problems that the particular design had, as a result of the way that water was fed into the orifice.

For in this case, unlike many of the conventional nozzles, where the flow is directed directly at the orifice down a channel aligned with the orifice, the nozzle were small discs arrayed along a spray bar, of the type that is used for car and truck washing rigs where a single channel feeds a number of sprays.

The flow in this case is primarily along the distribution manifold, and, as such, perpendicular to the axis of the resulting jets. When the water, therefore, exits from the individual nozzles it retains a component of this lateral velocity, and this tears the jet apart relatively close to the nozzle. The results are evident in the cut made in the lower half of Figure 1.

It is surprisingly easy to remedy this. A short tube inserted behind the nozzle orifice, and protruding up into the manifold channel allows the water some chance to collimate in the direction of flow before it accelerates through the nozzle orifice, and the result, relative to the original cut is quite significantly better.

Not that short lengths of tube are completely effective, but they are a start. One of the more effective means of getting a water jet to move as a cylinder in short jets (such as those seen at Disneyworld and at Detroit Airport is to run the water from the supply pump through a small stabilizing chamber and then pass it into a collimating tube full of drinking straws (or their technical equivalent) which sit just behind the nozzle. Providing the geometries are properly selected you can get the very smooth cylinders of water that are a feature of the jumping streams.

A similar structure lies upstream of the the nozzle at the Gateway Geyser across the river from the Gateway Arch in St. Louis. The fountain shoots a jet of water to the same height (630 ft) as the Gateway Arch on the other side of the river, and to quote Wikipedia:

the Gateway Geyser was designed and constructed by St. Louis–based Hydro Dramatics. It was completed in 1995 at a cost of $4 million. Three 800-horsepower (600 kW) pumps power the fountain, discharging 8,000 U.S. gallons of water per minute (50 L/s) at a speed of 250 feet (76 m) per second. The fountain has an axial thrust of 103,000 pounds-force (460 kN); water is jetted out of the 6-foot (1.8 m)-tall aerated nozzle at a pressure of 550 pounds per square inch (3.8 MPa).

These are more complex flow straighteners than the simpler ones that are used in low pressure cleaning systems, and with considerable effect in controlling the jet flows from the monitors of hydraulic mining equipment. By channeling the water into a multitude (perhaps 200) small diameter channels and then recombining the water at the nozzle the resulting flow is laminar.

Where the water flow is much lower, such as when being used in an ultra-high pressure system, the flow can be stabilized by allowing a long straight run-up of the pipe leading into the nozzle. (Typically the rule of thumb was that the length should be around 125 pipe diameters, however work at the U.S. Bureau of Mines showed that the length of straight section did not need to be this long – a length of around 4-inches proved effective.)

Figure 2. The improvement in jet performance with a straight inlet section (after Kovsec et al*)

A similar improvement can also be seen when the flow conditions are correct when working with higher pressure jets.

As a general rule, however, such care is not taken in the construction and lead-in to the nozzles, and the jet will begin to taper and reduce in effective diameter from the time that it leaves the nozzle.

I’ll talk more about that, next time.

*Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10.

Waterjetting 33a - Waterjet structure and its effect

When a waterjet first comes out of an orifice the flow (providing the upstream conditions are properly aligned) will form a cylindrical stream, with the jet pressure across that stream relatively constant. Within about an inch, depending on the flow conditions, ripples start to appear on that smooth cylinder (Rayleigh waves) and these grow and gradually disrupt the jet as it travels further from the nozzle.

Looking at the jet under ordinary light, this makes the jet appear to grow larger, and potentially more powerful as it moves away from the nozzle. However, when the jet is back-lit, or when a pressure profile is taken of the jet at different distances from the nozzle, a different picture emerges.

Figure 1. Pressure profiles across a 6,000 psi jet at 6-inch intervals from the nozzle

What this shows is that the initial even pressure distribution across the stream gradually transforms into a curve very similar to that described as “Gaussian” in mathematical literature. These pressure profiles are generally carried out only at lower jet pressures, because of the way that we have to protect the pressure transducer from the direct jet impact.

Figure 2. Instrumentation for measuring pressure profile.

At higher jet pressures the erosion from the jet on the protective steel cap very rapidly wears the entry hole into the pressure transducer channel, and makes the readings less reliable as the channel shape begins to change. For this reason we have relied on either physical damage to the target, or photographs of the jet, to see what the jet structure had transitioned into, with backlit photographs giving the better set of information.

This damage, interestingly, comes more from droplet impact caused by the breakup of the outside of the jet, which can be easier shown through a front-lit photo.

Figure 3. Microsecond exposure of an ultra-high pressure jet, the orifice is at the 10-inch mark on the rule.

At the nozzle the jet emerges as a smooth cylinder, but small ripples develop on the edge of the jet as it flows. There has been a considerable amount of study of this, and the development of the waves relates to the surface tension in the liquid, and the relative densities of the two fluids (in the case the water in the jet, and the air into which it is injected). At very low pressures (such as water from a tap) surface tension effects dominate and the jet stream is pulled into droplets over a relatively short distance.

Figure 4. Breakup of a low pressure jet into droplets. (Taken from an MIT lecture on fluid jets)

Incidentally, for my male readers, this is why you should stand within 6-inches of the wall of a urinal. (Read to the bottom of the post).

As the jet velocity increases small surface waves develop on the outside of the jet. They look similar to these stationary capillary waves, except that they grow in magnitude as they move away from the nozzle, and were first discussed by Rayleigh, for whom they are named.

Figure 5. Wave generation on the surface of a jet (the grid has a 1-mm spacing for scale). (From an MIT lecture on fluid jets ).

At the jet velocity grows higher – as can be seen by looking closely at the jet in Figure 3, these waves grow large enough to be pulled from the surface of the jet, and, being slowed by the surrounding air, appear to be pulled backwards as the jet flows.

The wave deceleration and break up into a fine mist reduces the size of the central core of the jet, and also the drag reduces the velocity of the outer layers of the jet, giving the pressure profile that is shown in Figure 1.

When cutting with a plain jet (i.e. without polymers or abrasive) a slight deceleration over the edge of the jet is helpful in eroding and removing the material in the target. When a jet impacts close to the nozzle, and the pressure across the target is relatively uniform (as it is 15-cm or 6-inches from the nozzle in Figure 1) then there is no pressure difference between the water that penetrates into the cracks on the target within that central zone. Because of this, while the material may compress, there isn’t enough difference in the forces on the material to cause it to be removed, and the central stub of material will therefore remain in place.

However, in the zone where the pressure differential has developed, on the sides of the jet, there will be a large difference in the pressures in the fluid within the cracks on the target as one moves away from the jet center. This will cause significant material removal. I wrote about this in an earlier post and used the same illustration of what we called a butterfly – which is the erosion pattern that a 10,000 psi jet makes on an aluminum target, where the pressure profile of the jet is still relatively even, close to the nozzle.

Figure 6. Erosion pattern around the point of impact of a waterjet on an aluminum target. The erosion occurs on the outside of the impacting jet, while the central core, being under an even jet pressure, is not removed.

Having digressed a little from the initial topic to explain what happens with the water on the outside of the jet as it is peeled away from the central core, I will return to the topic in the next post, because it is the tapering in of the cut, as the central higher-pressure cutting jet moves away from the nozzle, that is the subject of this short sequence, and so we will return to that topic, next time.

Waterjetting 32c - more tests with polymers

In the last post on this topic I pointed out that one of early drivers to the use of long-chain polymers in water came from the reduction in friction that it provided to fluid flow through long pipes. In many instances this has been the driving force for the selling of the product, and in industries such as oil well drilling and fracking the reduction in friction down long relatively small diameter drilling pipe has been a significant selling argument.

The cohesion of the jet, once it leaves the nozzle, is a secondary consideration in overall economics, yet in some applications, such as the cleaning of down-hole completion screens, the ability of a polymer-laden waterjet to penetrate through the pressurized fluid in an oil well to reach and clean the screen has been the main reason that the market developed.

Figure 1. Improved jet power underwater when polymer is added (after Zublin)

One of the first steps to be addressed was the practical considerations as to how we got the polymer into fluid that made up the jet stream. The original polymer that I used was polyethylene oxide (Polyox) which was marketed in the form of small prills of chemical. The problem that we were then faced with is that, when these are just dumped into a container full of water, that the outer edge of each prill soaked up some water, became gel-like and adhesive, and stuck to the next particle, in a way that made a large collective lump that was very difficult to dissolve into the surrounding water flow. Even when the particles were fed in slowly into a fluid mixer the particles initially tended to concentrate in one layer of liquid, which only slowly dispersed into the main body of the fluid. That concentrated polymer has a number of interesting properties.

Figure 2. Lifting a thick concentration of polymer from a bucket by hand.

For example it can be thick enough that one can grab it with one's fingers and lift it that way out of the bucket, as the picture above shows, or it can cause a unique problem in a mixing tank.

The polymer can wind up around the mixing paddle shaft and work its way up the shaft until it hits the retaining screw at the top. It then piles up at this point until it reaches a critical mass, when a tendril can be thrown out of the tank, through the centrifugal force exerted through rotation of the paddle shaft. The tendril falling outside the tank falls to the floor, which is lower than the fluid in the tank, and thus the concentrated layer of polymer is drawn up the inside of the tank, over the side and down the outside of the tank since it is still attached to the escaping tendril. The result clearly showed that liquid could flow uphill, when pulled by the cohesion inherent in the high concentration polymer.

This, in turn, gives either a disadvantage (if you are using this in a factory) or an advantage to the use of the polymer. The reason comes from the fluid nickname – Slippery Water.” The addition of the polymer, while reducing friction in the pipe, also reduces it between a person’s shoe and the floor, and thus it becomes a hazard in the workplace, since it increases the risk of slipping. It has the impressive title Anti Traction Mobility Denial System . We used to call it Banana water, but that seems to have faded from use.

The need to reach the very low concentrations of polymer that are all that is necessary to enhance jet cutting required a better way of mixing, The recommended answer was to briefly suspend the particles in a suspension of isopropyl alcohol (swirling it in a cup worked well) and then dumping it into the tank in a way that ensured that the individual prills were distributed away from one another. And while this worked, it was somewhat cumbersome and worked well only when mixing up individual batches of water – useful in a laboratory but not so much in a factory that must operate steadily for a full shift.

A number of different chemical liquid additives, most particularly polyacrylamides and derivatives of guar gum, have been tested, with the original work (carried out with the help of Dr Jack Zakin) being carried out in special section of the Baxter Springs plant where we could photograph jets at one-millionth of a second in order to study their structure. To do that we set the system up so that the jet was back-lit, so that we could determine how solid the core jet was, and used a high-speed strobe to illuminate the jet for the short-time needed to freeze the jet motion, leaving the camera shutter open for that time. This meant that the room was totally dark, and since the tests were carried out in the middle of summer, it made for an interesting couple of weeks.

Figure 3. Improved cohesion of a 30,000 psi jet when polymer is added (lower picture) the jet range shown in the picture is about 8 inches.

We also ran a pressure transducer across the different jets, at different standoff distances, so that, for the most promising additives, we could measure the differences in impact pressure and jet cohesion as the transducer moved away from the nozzle. The results were reported in the Proceedings of the 3rd ISJCT with the different chemicals tested ranked according to their ability to improve jet cohesion and reduce jet spread.

One of the problems with some of the additives is that they are temperature sensitive, and the jet was coming from the nozzle at temperatures between 95 and 115 deg Fahrenheit (it was a hot summer and the water reservoir was not chilled). This was not recognized at the time, and it did have some impact on the performance of some of the chemicals, which also showed a tendency to rapidly age once mixed, due to the storage conditions. Nevertheless the results showed that while Polyox was the best compound, there were liquid alternatives that also were effective, and the technology has since switched to liquid additives of which I will have more to say next time.

Zublin, C.W., "Water Jet Cleaning Speeds - Theoretical Determinations," 2nd U.S. Water Jet Conference, Rolla, MO, May, 1983, pp. 159 - 166.
Zakin, J.L., Summers, D.A., The Effect of Visco-Elastic Additives on Jet Structure," paper A4, 3rd International Symposium on Jet Cutting Technology, Chicago, IL, May, 1976, pp. A4-47 - A4-66.

Waterjetting 25a - choosing jet parameters for range.

The range over which a waterjet is able to cut material can widely quite significantly, depending on a wide range of factors, including abrasive content. An earlier post described the way in which students in a waterjet class were shown some of the difficulties in assessing risks arising from the use of a waterjet, and the range over which it was dangerous. Simplistically the students first cut along a plywood panel to see how far from the nozzle the jet would remove wood.

Figure 1. By slightly tilting the 4-ft wide panel and then having students move the jet past the board along the left-hand edge, a measure of the range of the jet could be obtained.

However, after the students had decided that, for the 10,000 psi 0.03-inch diameter jet, the cutting range was about ¾ of the way across the width (i.e. 3 ft) they were then tasked to pass the jet, as fast as they could, over a piece of pork that was at least a foot further away from where they estimated the distance that the jet stopped cutting.

Figure 2. A piece of pork after being “sliced” by a 10,000 psi waterjet.

The pork was typically cut to a depth of over an inch, grooving into the bone at a distance that the student had previously decided was “safe.” It was pointed out to the class that the pork was a good simulator for human flesh.

The point of the demonstration was fairly obvious, but it does highlight that the distance at which a jet stops cutting one material because of insufficient energy, may still be quite a distance closer than that critical distance for other softer materials.

In one of the earlier scientific papers on waterjet cutting Leach and Walker plotted the drop in jet pressure from two different nozzle shapes, against the distance from the nozzle.

Figure 3. Decline in jet pressure with distance from the nozzle (Leach and Walker)

With poorer nozzle designs and in cutting many harder materials the critical distance at which the jet pressure falls below half the original pressure, and thus in many materials stops cutting, is at around 125 nozzle diameters. For a 0.03-inch diameter jet, cutting a distance of 36 inches takes the range to 1,200 diameters. And while that range is partly because we have significantly improved the fluid flow into the nozzle it relates, as noted, also to the strength of the material being cut.

One reason to mention this is that I have seen, both in photos and real life, people foolish enough to hold their hands in front of a 40,000 psi waterjet, as an illustration of the safety of the tool at even a short range. (Typically they were using jets of around 0.006 inches diameter with the hand about a foot from the nozzle). A slight increase in nozzle diameter, undetected by the operator, or a change in fluid content (such as by adding a long-chain polymer (such as Superwater) could extend the range of the jet several-fold, so that the unsuspecting operator might lose several fingers before realizing the change in conditions.

An earlier post described the work of Clark Barker and Bruce Selberg, who demonstrated that an increase in polish of the inner surfaces and a smooth transition path into the orifice could extend the cutting range of a jet in harder materials from 125 diameters to over 2,000.

Achieving a smooth flow path to the orifice is critical to superior performance, though – as I have mentioned before – it was at one time surprising to me how many contractors did not even have the nozzle insert mating with the end of the supply pipe. Rather, with the nozzle insert held in a holder, they just turned the latter until it was tight, not always achieving contact between the back of the nozzle insert and the pipe. In addition there have been many cases I have seen where the nozzle insert inlet diameter differs from that of the internal diameter of the connecting pipe Again this will interfere with performance away from the nozzle.

Assuming, however, that one has stabilized the flow into the nozzle, and that it is of the right shape, how can one increase the jet throw distance further? The obvious, and wrong, answer is to up the pressure that is driving the jet.

Why is this the wrong answer? Well, if one considers what happens when a jet shoots out into the air, as one can see in a high-speed flash photograph:

Figure 4. Flash photograph (exposure at about one-millionth of a second) of a high speed waterjet showing the structure.

As the jet travels through the air, so the relatively stationary air around the jet strips off, and decelerates, the jet in layers starting from the outside. These show up as backward pointing stringers flowing out from the main jet stream. As the outer layers are peeled off (as with stripping the layers from an onion) so the remaining diameter gets less until, as in the picture above, there is no jet left.

Consider that with a higher driving pressure that there is a greater differential between the air speed and that of the jet, and obviously the stripping action will occur more rapidly, reducing the overall range of the jet.

Now consider if, instead of putting that additional power into pressure/jet velocity one were, instead to put it into additional flow. Then there are more layers of the jet to strip away, and the differential is not as great. As a result, when one compares the performance of two jets one gets:

Figure 5. Comparing the performance of two jets.

Notice in this case that relatively close to the nozzle the two jets, cut to roughly the same depth, and in this range the higher pressure, smaller jet has advantages in that the thrust it applies to the holding tool is less, and the total amount of water used is also less (roughly 4.3 gpm rather than 7.3 gpm). However if one is cutting at a greater standoff distance between the wall and the target, then at about 4 ft from the nozzle (1000 diameters of the larger, 1500 diameters of the smaller) the lower pressured, higher flow rate jet becomes more effective.

This relative change in nozzle effectiveness with pressure and diameter was also reported from results at lower pressure when developing nozles for cutting coal in Germany.

Figure 6. Comparing the pressure profiles of jets at two different diameters and pressures, as a function of distance from the nozzle.(Benedum et al)

Note that here, again, at about 8 m from the nozzles, both jets are producing about the same impact pressure, while closer to the nozzle the smaller (blue line) jet has a better profile (at 0.78 inch diameter, and 1,300 psi) than the larger (black line) jet (at 1-inch diameter, and 1,000 psi). But at greater distances the lower pressure, larger diameter jet becomes more effective.

There is, in short, significant benefit to determining, before one starts, what the objective is and over what range the jet is expected to cut, since both will help decide what set of jet operating conditions will give the better result.

References Leach, S.J., and Walker, G.L., "Some Aspects of Rock Cutting by High Speed Water Jets," Phil. Trans. Royal Society, London, Vol. 260A, pp. 295 - 308.
Barker, C.R. and Selberg, B.P., "Water Jet Nozzle Performance Tests", paper A1, 4th International Symposium on Jet Cutting Technology, Canterbury, UK, April, 1978.
Benedum, W., Harzer, H., and Maurer, H., "The Development and Performance of two Hydromechanical Large Scale workings in the West German Coal Mining Industry," paper J2, Proc. 2nd Int. Symp. Jet Cutting Tech., BHRA.

Waterjetting 9b - the effect of standoff distance.

One of the problems with relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.


Figure 1. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target, under the jet, is not eroded, but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width, and will be subject to high lateral jet flows.


Figure 2. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.


Figure 3. Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff the central section, though still present, is being eroded.


Figure 4. Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point the cavity is starting to get noticeably deeper.


Figure 5. Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time the central section of the jet is beginning to break down into, initially short strings, that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.


Figure 6. Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

By this time the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.


Figure 7. Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size, and condition as a function of distance, and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used, if the droplets are generated properly, and used within the relatively narrow window that they exist, to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.


Figure 8. The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

There are ways of getting around this problem, but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

Waterjetting 7a - An intro to 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.

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.)

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 were taken down at Baxter Springs, KS in the early 1970’s and involved the use of what was then a MacCartney Manufacturing Co intensifier, 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 3. 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 4. 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 5. 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 6. 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 7. 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 7 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.