Tuesday, May 19, 2015

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.

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Tuesday, May 12, 2015

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.

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Tuesday, April 28, 2015

Waterjetting 32d - Cutting with polymer in the water

In the recent past I have written about the use of polymers in high-pressure jets and that they can significantly improve jetting performance, with no additional changes in the power or pump and equipment used in the work. This is because of two different effects that the polymers have. Firstly they reduce turbulence in the flow from the pump to the nozzle, reducing pressure loss and increasing fluid flow, for the same pump power. As a practical consequence since the fluid flow will be greater for the same pump pressure, this will require that a larger orifice diameter be used to handle the greater, or – for the same flow rate and nozzle diameter, the pump can be operated at a lower pressure.

Figure 1. Comparison between a conventional jet and one containing the polymer additive marketed as SUPERWATER. (after Glenn Howells)

This is not immediately apparent, since the jet carrying the polymer appears smaller, but this is due to the second effect of the polymer, which is to tend to glue the water together, so that it is not dispersed as easily by the surrounding fluid – air in this case. In fact, for the same pump pressure and orifice diameter the lower jet will be operating at a higher pressure (since there is less pressure loss in the line) and there will be more water coming out of the polymer-supplied orifice at a higher velocity. But because it is not spreading into the air, it appears smaller.

This will improve the cohesion of the jet as it moves away from the jet, and as noted in an earlier post, this means that the jet will cut to a greater range from the jet, since it maintains the required critical pressure further.

The impact that the more coherent jet has on performance can be seen where the jet is used to cut into two different types of limestone, one oolitic and one crystalline.

Figure 3. Depths of cut of the polymer-containing jet (left) and plain waterjet, operating at the same pump pressure, nozzle diameter and standoff distance (Glenn Howells)

Note that at the distance where the normal waterjet has broken into droplets (as seen by the nature of the cut surface) and the jet has barely enough energy to remove the surface layer, where the jet contains polymer it retains the ability to cut.

Further, and this is more critical where cut quality is more important, the jet cut is much straighter and cleaner than the dispersed and wider normal cut. This can be seen where a different, more crystalline limestone has been cut closer to the nozzle.

Figure 4. Change in the cut shape to a narrower, deeper cut where polymer is added to the jet stream (rhs)in cutting limestone. (after Glenn Howells)

The benefit of the improved performance changes therefore with the distance of the target from the nozzle, with the more dramatic improvement being seen as the target gets further from the nozzle. Looking at the data from the original work that we did in Leeds, back in the early days of this study, this can perhaps be better realized through the use of a 3-D plot.

Figure 5. Improvement in cutting performance as a function of distance from the nozzle and jet pressure.

Note that, in these trials, the polymer improved the jet performance relatively more at lower pressures and greater standoff distance. Part of the reason for this (in hindsight close to 50-years after running the tests) is that when the jet was cutting at the lower pressures it was closer to the threshold pressure of the rock and this any drop in jet pressure had a more significant impact on cut depth than occurred at the higher pressures, where the gain was not relative to such a low benchmark.

For a number of years, until our research took us into fields where use of the polymer was precluded for several reasons, we routinely used a polymer (generally Superwater, marketed by Berkeley Chemical) rather than Polyox because it gave a relatively consistent and significant improvement in performance plus, being a liquid, it was relatively simple and inexpensive to buy a small metering unit (about the size of a small case) which would feed the polymer into the water supply line to the pump at the required concentration – typically 0.1 to 0.3%.

It tends to work better in improving abrasive jet cutting when it is used with a Direct Injection of Abrasive (DIAjet) or Abrasive Slurry Jet (ASJ) system than with conventional abrasive waterjet (AWJ) systems. The reason for this is that the AWJ system has the abrasive fed into the water stream at the mixing chamber just before the jet leaves the nozzle to strike the target. Within the mixing chamber the abrasive has to penetrate into the waterjet stream in order to acquire the jet velocity, and to distribute across the jet and give an even cut on the target.

Where the abrasive is feeding in from one side of the jet and the waterjet stream is more coherent, it becomes more difficult for the abrasive to penetrate the stream, and if the design is not adjusted accordingly, the cutting performance can be diminished, particularly relative to the gain that can be achieved where the combination is carried out effectively.

On the other hand with the ASJ systems the abrasive is mixed with the water far upstream of the nozzle and the are already thoroughly mixed together, so that the added cohesion of the jet will help to provide the acceleration that the particles need to reach close to the waterjet velocity, and achieve the improved cutting performance required.

Where an ultrahigh pressure jet does not contain abrasive, the polymer can be of benefit as a means of improving cut quality – as evidenced from this comparison in cutting a shoe sole pattern, both with and without the Superwater polymer.

Figure 6. Shoe sole cut comparison with and without Superwater. Note the smoother cut, with less fraying of the back side of the cut with the polymer. (after Glenn Howells)

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Saturday, April 25, 2015

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.

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Friday, April 17, 2015

Waterjetting 32b - early work with polymers

If we lower the viscosity of the fluid in a jet hitting a target, then the jet can penetrate into the fissures and flaws of that surface more easily and in this way it will see an improved cutting performance. On the other hand the thinner fluids have less resistance to the air or other fluids through which the jet has to pass on its way from the nozzle to the target, and so the amount of energy that is delivered to the target can be lower than with a less viscose jet. Some of the changes in fluid chemistry can, however, be reached using small amounts of different chemical additives that can have quite a profound effect on performance without totally changing the fluid being used.

Consider that before the jet gets to the nozzle it has already completed most of the journey from the pump to the target. And along the way, as I pointed out in one of the earlier posts I wrote, pressure losses along the line, due largely to friction, can overwhelm the power supplied by the pump. There was an occasion where a colleague at another research group was running a system where, because of the small diameter of the tubing he was using, the pressure drop from the pump was more than 75% before the water even left the nozzle. A slight increase in tubing diameter would have made the system much more powerful, with no other increase in cost.

But sometimes that option isn’t really available. I remember my first introduction to the use of long-chain polymers was after reading an article in The Sunday Times which described an investigation by the New York Fire Department. One of the problems in fighting high-rise fires is that the firemen have to haul their hoses up the long flights of stairs and 2-inch diameter hoses become quite heavy as they are moved up from level to level. In the late 1960’s (in the administration of Mayor John Lindsay) tests were carried out to see if this could be improved. Normally, when a smaller 1.5 inch diameter hose was used, the pressure loss in pumping 100 gpm through 1.5-inch diameter hose is around 25 psi per hundred feet, with 98% of that loss being due to the turbulence of the water.

Figure 1. Demonstration of the benefit of polymers before John Lindsey, the Mayor of New York City (seated far right). May 13, 1969. The two jets are of equal size and fed from pumps at the same pressure. The jet barely reaching the holding tank is the standard water, the other contains polyox.

What the demonstration showed was that more water could be sent through the smaller hose, (roughly 70%) so that it could be used, instead of the larger hose, and with better effect on putting out nearby fires.

One interesting historical comment is that the demonstration occurred in 1969, however back at the University of Leeds, I was aware of the research in 1967, and had included, as chapter 5 of my dissertation, a study of the benefits of using the polymer. Similarly Dr. Norman Franz, then at the University of Michigan, (and thereafter at UBC) had also become aware of the work, and – being smarter – had patented his findings, that were then licensed by the fore-runner (McCartney Manufacturing Company) of KMT.

The benefits were not just that the polymer reduced friction in the line from the pump, transitioning the flow from turbulent to laminar, with the consequent reduction in friction losses, but these also extended beyond the nozzle. I have always suggested that the easiest way to think of this is to compare the benefits of using the polymers to that of picking up spaghetti from a plate. Very short strands of the pasta need the eater to use a spoon, or simple fork to try and grab a small amount for a bite. On the other hand, where the pasta strands are longer, and well mixed together, when one pulls one out of the pile it is attached to another few that also rise, and before long the fork is full of a seemingly endless set of attached strands.

Given that the molecular weight of Polymerized Ethylene Oxide (Polyox) (which was first introduced by Union Carbide and is now a Dow product) can range up into the 8 million range, it is easy to make the analogy. As a result the intertwining of the long molecules facilitates passage down the feed line, but also tends to maintain the jet coherence once it leaves the nozzle.

The jet is thus not only allowed to pass through the feed line at a lower (often significantly so) pressure loss than with conventional water, but also, once the jet leaves the nozzle, the stream remains coherent to a much greater distance. This can be seen in Figure 1 at low pressure, however that result has been similarly validated at pressure of up to 50 ksi in the years since that original work was completed.

Figure 2. Effect of adding a small amount of polymer to the cutting performance of a jet at 10,000 psi.

I will spend the next couple of posts discussing the general concept of polymer additions but will close this with another anecdote. After I had carried out the first test series – which yielded the graph shown in Figure 2, I wrote to Union Carbide, who had been kind enough to send me about half-a-pound of dry product for that study. Given the favorable results we wanted to expand the study – but I received a note back from the European sales office of the company, together with an additional box of roughly three-quarters of a pound of product. The note let me know that I had now received their entire allocation of product for the continent of Europe, and that, in consequence, my research needed to be conservative from thereon out, since there was unlikely to be any more supply for some considerable time. (Which was why the research could not be pursued at any greater length until I had moved to UMR and acquired my own system – which took an additional four years).

It was not until years later, as spreadsheets became ubiquitous, and data analysis facilitated from the hand-calculations that we were reduced to before the days of computers, that I was finally able to fully comprehend the data that we had, but I will leave that discussion until the next post.

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Wednesday, April 8, 2015

Waterjetting 32a - jet fluids

From time to time I am asked why we use water in almost every application of high-pressure waterjet use, rather than using any one of a number of other choices for the fluid. There are a couple of major reasons for this. The first is that most of the uses of the tool will require that we use a fairly large quantity of fluid, over time, and water is the cheapest and most readily available of the alternatives that can be used in such volumes. The other is that, in many circumstances, it works as well, if not better than those alternatives. As an additional point it can often be cleaned up relatively cheaply and simply before being disposed of after use.

But the conclusion isn’t an absolutely true one, and along the way there have been a number of different investigations to find alternatives that can provide a better answer than just water alone. Some of these choices suggest applications where a different fluid might work better, for example if the target is a food product that contains a lot of sugar then it might be better to cut with a form of vegetable oil that does not dissolve the sugar on contact. In other cases there is an advantage to adding a chemical to the water to change its properties.

The changes in performance that decide whether the change in cutting fluid is worth the effort are (as with so many other choices) going to be based on the particular job that has to be carried out. In some cases, as with cutting candy for example, the benefits of using an oil stream may make the process practical in a food that would otherwise not be feasible to be cut with a high-pressure jet, for aesthetic reasons if no other.

Figure 1. Effect of changing fluid properties on cut depth at an impact velocity of 330 ft/sec. (after Rochester and Brunton) on a nickel target.

Several different fluids were used to generate the curves shown in Figure 1, ranging up to mercury in density, and including four different oils as a way of investigating viscosity.

One of the down sides to the use of oil as a cutting fluid comes from the increased viscosity of the fluid, with some of the oils that were initially used as the tool went into commercial use for cutting confectionary items.

The increased viscosity of the oil raised the pressure required to drive the fluid from the pump through the delivery lines to the cutting nozzle arrays. This in turn lowered the cutting pressure at the nozzle, and made the differential between the operating pressure at the pump during the drive stroke and during the reversal significantly greater. This greater fluctuation in pressure produced a greater fatigue on the drive train, and in turn led to a more rapid failure of components within the system. While the initial answer was thought to be in using a different cutting oil, the final, easier, solution was to move the pump closer to the cutting zone, reducing the pressure losses and bringing the fatigue back to acceptable levels.

Changing fluid properties can therefore have a significant impact on system operation, even though they may have little impact on the actual cutting performance, although in many cases there are significant changes as the fluid properties are changed.

The chemical impact is one that is perhaps addressed more commonly in cleaning applications, where it is often suggested that chemical agents be added to the water as a way of weakening the bond between dirt and the underlying surface. While this is theoretically possible, it should be remembered that the jet, even at the relatively low pressure of most pressure washers and car wash units, will be travelling at speeds of hundreds of feet per second. The residence time of the chemical on the surface can thus be measured in milliseconds and is rarely long enough for much change to occur.

In these circumstances it is usually more fruitful (and less demanding of chemical) to spray the chemical cleaning solution onto the surface first, and then allow a short period of time for the interaction to fully occur before applying the pressure wash. In these conditions (as we saw when we watched as our house was pressure washed the other week) even though the nozzle was held too far from the surface for there to be much pressure on the wall by the time the jet reached it, the chemical cleaner had broken the bonds of the algae and dirt, and the house walls were rapidly cleaned with relatively little additional effort. (This is contrast with the time that I had cleaned it without chemical, where it was necessary to keep the nozzle within six inches of the wall for the jet, unassisted, to remove all the surface contamination).

In the case of a house cleaning this highlights an additional benefit from using the chemical, since the lower pressure jet impact on the building means that the jet will not be strong enough to erode any of the timber surfaces around the house that were starting to weather with the passage of time.

There are, however, some chemicals that are used in mining and civil construction that will interact relatively quickly, when introduced into the cracks within rocks, particularly if they can be pushed to the tips of the cracks where they can rapidly reduce the strength of the bonds across the tip of the crack, making it easier for the crack to grow at lower fluid pressures. The fluid characteristic that controls this effectiveness is often related to its Zeta Potential. However much discussion of that topic would rapidly take us a considerable distance from the current subject, and so I would recommend that any of those interested might want to follow along a line that might start with the work at Brookhaven.

This series will return with a look at some of the mechanical changes that can be made to a waterjet, to improve cutting of everything from shoes, to ships and similar subjects.

Rochester M. and Brunton J. The Influence of the Physical Properties of the Fluid on the Erosion of Solids, CVED/C-MAT/TR10, University of Cambridge, UK 1973. (This formed part of Mike’s doctoral dissertation).

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Wednesday, April 1, 2015

Waterjetting 31d - thickening a waterjet to improve downstream pressure

There is a trick that one can learn while a teenager, which comes with the introduction “I so strong that I can blow a brick over!” Upon finding a suitable victim to impress, the brick is placed over a deflated balloon, which is then inflated, raising the brick which then, if suitably placed, topples onto its side – proving the strength of your lungs.

The critical part of the activity is to have the air that you blow be confined within the balloon, and equally exert pressure over the surface of the brick, so that a low pressure translates into a much more significant and powerful force. It is the confinement of the pressure that allows the build-up that moves the brick.

In most cases the use of high-pressure water as a cutting tool does not see much confinement of the water over the cutting process, with the water flowing into the cut, removing some material, and then flowing on out. Yet the water still has considerable energy as it leaves, and this means that the process is usually quite inefficient. How then can the contained energy in the jet be used in a secondary way to improve the removal efficiency of the process?

One answer to the question comes with the use of long-chain polymeric additives. These have recently seen a fair amount of publicity because of their use as the “slick water” components of the “slick-water-fracking” tools that have helped improve the production of oil and natural gas from long horizontal wells drilled into the hydrocarbon deposits in places such as the Bakken fields of North Dakota and the Barnett and Eagle Ford Shales in Texas. The long horizontal wells are separated into short intervals, within which the pressure within the well is raised until the surrounding rock cracks (fracks) with s series of cracks that extend out into the surrounding rock. The crack makes it easier for the hydrocarbons in the rock to escape and reach the well, improving the recovery to the point that the well can be economic to operate. The reason that the “slick water” is used is that the crack would normally close back up after the borehole pressure was lowered back down. In order to stop this happening the fluid in the well during the frack is made up with long-chain polymers and also contains grains of a sand or similar proppant. When the crack is formed the fluid in the well flows into the crack, carrying the sand with it, and this then holds the crack open when the pressure falls.

The polymer thickens the water which makes up most of the fracking fluid, so that it can carry more of the sand, and at the same time, the polymer reduces the friction of the water against the rock, so that it is easier for the water and sand to penetrate deeper into the cracks. Once the proppant particles catch against the sides of the crack they become held in place, while the fluid moves on and eventually returns back out of the well.

Back in the days when I was carrying out the research for my doctorate, I had used the long-chain polymer Polymerized ethylene oxide (Polyox) to reduce the friction in the delivery line from the pump to the nozzle. The increased cohesion of the jet (which I will cover in posts that follow this) meant that it would cut to a greater distance from the nozzle, with less decline in cutting power. However the increased cohesion of the jet had an additional benefit, which was noted by Chapman Young, as part of his development of tools for removing loose rock from around tunnels.

In a typical tunnel excavation, the miners drill a pattern of holes in the face of the tunnel, and then partially fill these with explosive, which is then set off in a controlled pattern of blasting. The central core of rock on the face is broken out by the explosive force, but some portion of the rock at the edge of the blast is only loosened from the solid, and still hangs in place. One of the more dangerous mining jobs (which I have done) is to take a long pry bar and insert this behind the loose pieces of rock around the opening, hoping to be able to wedge these loose, so that they no longer pose a risk to miners who then pass underneath.

Seeking to automate this process Dr Young and his colleagues tried using high-pressure waterjets to blast these lumps free from the wall. Subsequently investigators at Colorado School of Mines have shown that the jets give an improved cleaning of the wall, over other methods – but as normally applied they do not have the confined power to be able to get behind the block with sufficient confined force to be able to pry larger blocks free.

And this is where the balloon analogy comes in, because Dr Young realized that if he could increase the viscocity of the water in the stream sufficiently so that, for a short instance, it would be confined behind the block and could acquire some of the pressure from the following impacting jet, then enough pressure over a large enough area would provide the force needed to dislodge the block. He tried it, and it worked.

It is not, however, a simple process to carry out, since the jet path must be carefully aimed to ensure that there is enough confinement of the water behind the target block for pressure to be built up, and this requires that the jet contain a relatively high concentration of polymer. That in itself brings another problem, which can be anticipated by the “slick water” nickname. Where the water gets onto the floor the friction reduction properties mean that it makes it quite difficult to walk on the wetted rock. Now while that, in turn, opens up a new avenue for business (the chemical is sometimes referred to as Banana water in riot control) it makes it unpopular with those that have to work with it in the confines of a mining tunnel and so the technology has not caught on. But it does provide an introduction to the topic of different cutting fluids, which will be the next topic of discussion.

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