Waterjetting 14e - Cavitation and Comminution

In the last post on this subject I discussed how, by adjusting the back pressure in the relatively stationary fluid surrounding a high-speed jet of water, it is possible to intensify cavitation damage. The simple way to find the optimal value for the back pressure for a given jet pressure and size was, we found, to listen to the sound of the cavitation collapse, and by adjusting the back pressure tweek both the sound and range of the cavitation cloud surrounding the jet.

The damage that the cavitation would induce on samples of rock is a function of the time that the cloud plays on the surface. By slowly moving the sample under the nozzle, in a confined cell, different levels of damage could be achieved, based on the speed at which the sample moved.

Figure 1. Traversing specimen cell, with the front cover removed to show the sample in the holder.

When the sample was moved under the nozzle at 2-inches a minute, the cavitation cloud attacked the surface relatively uniformly, with only localized increases in damage. In Figure 2 the red lines mark the width of the cavitation cloud on impact, it then spreads and collapses over the surface to give the wider erosion path.

Figure 2. Traverse over the surface of a dolomite sample at 2-inches a minute. The red lines define the width of the jet. Note the additional depth of removal under the sample ID (15) where the ink chemical had slightly weakened the rock making it more susceptible to erosion.

As the speed of the sample movement is slowed, however, the cavitation attack starts to find weakness planes in the rock and preferentially begins to erode these. As these channels are formed so the jet will flow into them to escape from the following flow of water in the consequent jet flow. As the cloud moves into these narrower spaces, so the pressure increases, inducing more of the bubbles to collapse and thus intensifying the erosion attack along that weakness plane (Figure 3).

Figure 3. Traverse of a cavitating jet over dolomite at a speed of 0.5-inches per minute. Note how the jet is now eating into zones of weakness which are beginning to define pieces of rock that are then liberated as cracks grow all around them.

As the traverse speed is further reduced to 0.4-inches per minute the erosion pattern which is developing in Figure 2 becomes consistent under the full width of the cavitation cloud, and the intersection of developing cracks means that the rock is now being removed in larger pieces and the erosion rate suddenly increases significantly.

Figure 4. Effects of moving the cavitating jet over the rock at 0.4-inches per minute. The cavitation is now developing cracks in the rock that join and break out larger pieces of rock, to a depth of around 0.5 inches over the cloud width.

This ability to focus the jet attack on weaknesses in the rock structure can be useful if, for example, the rock under attack is a mineral ore. Because the ore is defined with weakness planes around the individual constituent grains of the minerals and host rock, at a slow traverse speed the cavitation cloud will preferentially attack those boundaries, in the process liberating the individual grains, and separating the rock into its constituent materials. This liberation can be achieved as the rock is being mined (we have demonstrated this in the lab) so that the valuable mineral can be separated from the waste rock at the mining machine. This means that the waste can be left, in a larger size range than is conventionally left after separation, at the mining site, and does not have to be transported to the surface and ground to powder in order to separate out the valuable minerals. The energy savings that this achieves can be potentially as high as 75% of the total energy currently used at the mine.

Where the mining breaks out the rock without achieving complete liberation a secondary process can be used where the particles of material are fed into a secondary tube, where the particles pass through a second cavitation cloud. The attack of the very small bubbles on the mineral particles is such that the fragments of ore are rapidly broken (comminuted) into much smaller sizes in a process which, because there are so many events occurring sequentially , can appear almost instantaneous.

Tests at Missouri University of Science and Technology, for example, have shown that 0.5-inch sized pieces of coal can be reduced to 5-micron size in a single step. There is a video attached to this post of one of these tests. Figure 5 shows the equipment, with the coal in the inner metal tube, while the surrounding space is filled with water under slight pressure. 

Figure 5. Equipment to comminute coal to 5-microns (The size of the feed coal can be seen in the plastic box on the right).

Water to the cell does not have to be at any great pressure. The test has been successfully run with the water fed from a pressure washer obtained from the local hardware store for less than $100.

Figure 5a. Early in the test the water flowing out of the inner tube is filled with fine particles of coal as the cavitation breaks the pieces down to the required size.

B) A short while later and the outer tube begins to fill with the fine material.

One of the advantages of coal at 5-microns is that it can be mixed with water in about a 50% slurry and fed into a diesel engine, which will then run. GE has tested a locomotive and shown that it is possible to run the engine on the mixture, should conventional diesel no longer be economically available.

The process also works when a harder rock, such as dolomite (a host for galena and other minerals) is placed in the inner tube. The cloud color in this case is white.

Figure 6. Using cavitation to crush dolomite. The original particle sizes are in the box on the left, the cloud of particles is at around 5-microns.

Waterjetting 14d - Traversing Cavitation

Within the normal range of everyday fluid flow cavitation is something to be guarded against. Because it occurs when water is put under conditions where, through geometric or working conditions, it is either pulled in tension or shear to create the small cavities of sensibly vacuum that lead to cavitation damage, there are many ways it can be formed. In an earlier post I mentioned the cases where moving water past a blunt-ended surface at high speed can cause the bubbles to form. In an alternate form, this cavitation can be generated when solid bodies are dragged through relatively stationary water at high speed.

The most common example of this is with propellers and underwater craft, where the relative flow paths over a moving, submerged body can cause bubbles to form and collapse. When the bubbles collapse, even though individually tiny, they can, because of their number, create a fair amount of noise. That noise, generated behind the spinning blades of a driving propeller, was one way in which submarines could be detected and located during the Second World War.

Figure 1. Cavitation forming as water flows around a probe.

Notice that, in Figure 1, the bubbles form and collapse over the length of the probe, which was, in this case, held stationary while water flowed over the surface within a tube. The relative motion is the same as though the probe, a potential submarine shape, was moving at speed through stationary water.

There have been a number of different flow chambers built at different research centers, each in their own way trying to build a device that would allow study of the ways in which cavitation damages surfaces, and to evaluate different materials for cavitation resistance. I have mentioned the ASTM test methods earlier.

Figure 2. German cavitation test apparatus

In one such design, (Figure 2) German investigators built a flow channel where the flow channel was narrowed, and then expanded to induce cavitation in the downstream flow. By then placing a test specimen at the point of maximum bubble collapse, a test could then evaluate the different material responses.

The concept of creating shear, as well as tension in the water around a flowing jet can similarly be imagined where the design above is modified so that the jet that issues into the downstream flow is pressurized to higher velocities. This was the basis for the cavitation cell developed by Andrej Lichtarowicz at the University of Nottingham. This can be applied in a number of ways, in the one below, for example, Canadian investigators had developed a portable version of the concept.

Figure 3. Early method for inducing cavitation around a submerged jet.

However, it was the Nottingham cell that provided the basis for a move forward in the technology as a number of us, around the world, collaborated with Dr Lichtarowicz in trying the new concept. Early on we noticed that if one listens to the noise made by such a jet, it is possible to hear a change in the pitch of the sound as the relative pressure in the surrounding water changes, relative to that of the driving jet. This relationship is defined by the definition of a value known as the Cavitation Number of the condition.

In this equation Pd is the pressure in the downstream fluid, Pv is the vapor pressure of the fluid, and Pu is the driving pressure behind the jet.

For higher pressure cavitation flows the vapor pressure is sufficiently small that the equation can be simplified to the ratio of the downstream pressure (say 50 psi) divided by the jet pressure (say 10,000 psi) which would give a close approximation to the cavitation number ( 0.005).

Dr. Lichtarowcz simplified the design of a cell in which a submerged jet could be directed at a target, with the back pressure in the cell adjusted to control both the intensity of the resulting cavitation, and also its position of maximum damage.

Figure 5. Early design of a Lichtarowcz Cavitation Cell

This design was of interest to us, since it allowed rock samples to be used and evaluated, and we built and tested several different models based on this design. The two windows allowed the jet and specimen to be lit and viewed during a test.

Figure 6. View of a cavitating jet, with the cavitation cloud of bubbles collapsing at the surface of the specimen on the right.

Dr. Hood, in Australia, has shown, with high-speed photographs, how changing the back pressure in the chamber changes the effective damage range of the jets.

Figure 8. Back-lit photographs of jet and cavitation cloud collapse as the ambient chamber pressure is increased.

The above pictures show why, in underwater applications, the range of a high-pressure waterjet becomes increasingly restricted as the pressure increases. This is of great importance where, for example, high-pressure jets are being sent to the bottom of an oilwell to clean the filter screens. The range of the jet is controlled, in part, by the jet diameter, as well as the pressure, but can also be expedient to add different chemicals to the flow in order to enhance the range, and I will write on that in a later piece.

However, it was through the control of the range, and the intensity of the cavitation that we discovered, in applying the cavitating stream to rock, that the damage was now occurring at a fast enough rate that the small samples, and longer test times of the conventional test were no longer viable. The small samples were being consumed in a very short time, and so the design was modified, so that a target block of rock could be moved under the jet, at a rate of around an inch a minute, while maintaining the cell pressure to intensify the damage.

Figure 8. Section of a 2-inch deep hole drilled (at 6,000 psi jet pressure) into a block of dolomite. The damage is caused by cavitation since, at that jet pressure, the fluid would otherwise not damage that particular rock. Note that in this test the sample was not moved relative to the nozzle, and the jet was impacting at the top of the rock, which is to the right end of the hole drilled.

Figure 9. Traversing specimen test cell – schematic view from the top showing the starting position of the jet. The jet is positioned on the end plates until the test starts, and the sample is moved under the jet until the second end plate is reached, when the test is concluded.

The sample had to be moved at a controlled speed since, as the jet cut down into the rock, so the target surface moved away from the jet, and the depth of focus for maximum cavitation damage is relatively narrow – depending on the test condition.

It is in this balance between the effective damage range of the jet, and the intensity of that damage, that is yet to defined in a way that will focus the intensity of damage to its greatest potential. However there are other ways of using that potential, and I will describe those next time.

Waterjetting 14b - More on Cavitation

In the last post on this topic I introduced the concept of cavitation, and the way in which individual bubbles collapse, leading to erosion of adjacent surfaces. Under many conditions these bubbles do not immediately cause much problem. This is, perhaps illustrated by the standard test by which the American Society for Testing and Materials (ASTM) recommends evaluating cavitation erosion resistance. (And a confession, I was on G02, the ASTM Committee that covered this topic, for several years.)

Simply put a small test specimen is shaped, with a threaded upper end, that fits into a specially shaped horn. This horn is itself attached to an ultrasonic transducer, so that the tip of the horn will vibrate at 20,000 cycles/sec, with the face of the specimen rising and falling over a distance of 50 microns on each cycle.

Figure 1. Schematic of the ASTM test apparatus

The test specimen is positioned so that the lower face is set just under water, in a specially designed and sized container, which is kept at a constant temperature during the test, and the test is ready to be run.

Figure 2. Size and location of the Test Specimen for the ASTM test.

The specimen is weighed before the test, and at pre-determined intervals (perhaps hourly) as the test continues. After test runs that may last 40-hours the test is discontinued and the mass loss plotted as a function of time. It usually appears in the following form:

Figure 3. Typical result from the ASTM test, the slope of the steady state rate is used to provide the comparative resistance of the material to cavitation attack. Note that, for metals, it make take several hours to generate the plot.

There are some materials which cannot be easily threaded to make up the dimensions of the test specimen, and I was on the Committee when we worked to develop a modification to this test, where the sample was placed underneath a dummy tip, so that the cavitation cloud developed was driven down onto the specimen. Because the specimen did not move, we called this the Stationary Specimen Test, though it is now more commonly called the Indirect Cavitation test.

Figure 4. Geometry to be used with a stationary specimen

Since much of my work dealt with rock, this was the test we initially used, and we had to modify the test a little since fragments of rock would break into the gap and add undesired damage. To overcome this a small central hole was drilled through the center of the specimen and a very low water flow was used to keep the two surfaces clean, and at constant temperature. The test still took several hours, though it was a little faster than the original.

Figure 5. Specimens of aluminum, granite and dolomite after testing.

Figure 6. Comparison of results from the ASTM conventional test, and that using the Stationary Specimen.

Most of the time engineers are interested in designing structures that avoid cavitation, and metals provide enough resistance that small amounts on occasion can usually be coped with. However, in the civil engineering world the scales are often different, and while the individual bubble sizes can be tiny large flows can generate a lot of them in a very short time.

Consider the case of the Tarbela High Dam, where they have just had to open the spillways to release the rising waters in the lake behind the dam.

Figure 7. The Tarbela High Dam in Pakistan spilling water (Pakistan Today)

Back when the Dam was first constructed, however, the concern was more with the water tunnels that carry water from the top of the dam, down through the turbines, used to generate electrical power. The design was a single tunnel fed by three channels from within the construction. However, when the dam was first tested it was decided to close the side gates, and only open the central channel.

Figure 8. Upper – model showing the shape of the channel, and the cavitation cloud developing downstream.
Lower, the location of the damage to the lining and rock in the tunnel.

This change in the shape of the flow channel meant that the water in the side channels (not flowing) was pulled upon by the flow in the main channel. This induced cavitation at the interface, with bubbles being dragged into the main flow volume. These then collapsed, under the pressure of the flow in that channel. The damage from the combined bubble collapse during the less than one–day test was such that, as the lower part of Figure 8 shows, a cavity was chewed some 5 m deep and 13 m in diameter into the floor of the channel.

Figure 9. Damage to the floor of the channel

The ends of the secondary channel support columns, where they joined the main channel were also eaten away by the cavitation. The extent of the damage in Figure 9 can be assessed by the size of the men shown at the tip of the arrow.

This is not just a rare event in a distant country consider the following:

Late spring, 1983. Heavy snowmelt and steady rains create the worst flooding in nearly a century in the Colorado River basin. Lake Powell, a 185-mile-long reservoir on the Utah-Arizona border, is the hardest hit. Both spillways at the reservoir's 710-foot-high Glen Canyon Dam must be opened for the first time to prevent the reservoir from breaching its top. . . they find a crater 32 feet deep and 180 feet long at its elbow, and the holes Burgi discovered in June are now cavities 10 feet deep and 20 feet long. . . . . fill holes with 3,000 cubic yards of concrete.

Figure 10. The damage to the spillways at Glen Canyon (Popular Science )

With this much more rapid rate of erosion demonstrated as happening in nature, it was clear that there should be some way of speeding not only the testing of materials for erosion resistance, so that the test would take minutes instead of hours, but also (appealing to those of us who are interested in excavation) there should be a way of intensifying and focusing the damage so that we could remove material in a controlled way.

There were two different approaches taken originally, the first suggested by folk at what was then Hydronautics in Maryland, and the other by Dr. Lichtarowcz at Nottingham University in the UK. I will explain the two approaches, and how they can be applied, in the next post.