Wednesday, February 18, 2015

Waterjetting 30b - An opportunity missed, and a question raised

In the last post I wrote about the benefits of cutting a deep slot around the edges of a tunnel, and made reference to the work done here by Dr. El-Saie back in the mid-1970’s as part of his Doctoral Dissertation.

One of the concerns that we had to address was that the waterjet had to be able to penetrate all the different rock types that it might encounter, and at the same time, since the jet would only cut a short depth on each pass we also had to find a way of cutting the slot wide enough that the nozzle assembly could enter and deepen the slot over consecutive passes around the edge. Given that the available pressures in those days were limited, for us, to 30,000 psi and this pressure was insufficient, by itself, to cut through all the rocks we might encounter, Dr. El-Saie looked at several different ways of enhancing performance. These included adding abrasive to a high pressure jet stream, inducing cavitation into the jet stream and the potential for using the break-up of the jet into droplets to enhance cutting using the impact water hammer effect.

Because of other operating conditions it was not considered practical to try and develop the droplet impact idea for this program, and the work concentrated on examining the potential differences between abrasive waterjet injection and cavitation. To simplify the comparison the same basic nozzle design was used for the tests that were then run, although the shroud fitted to create the secondary (vacuum) chamber was modified to either allow abrasive entrainment, through ports, or to create cavitation. The presence of the ports did, however, allow the strength of the vacuum generated in the chamber to be measured as the jet passed through.

Figure 1. Nozzle designs used by Dr. El-Saie. Note that the upper design has ports leading into the vacuum chamber, so that abrasive can be drawn in by the jet passage. In the lower design there are no ports, and cavitation will be induced in the chamber by the jet passage, with the bubbles then drawn into the exiting jet.

One of the advantages of cavitating the jets is that the cavitation bubble collapse will spread out over a larger area on the target surface, so that the slot generated can be quite a bit larger than the originating jet. This can be shown in two pictures of a block of dolomite exposed to the same cavitating jet, at a pressure of 6,000 psi but one with the jet traversed along the block in a minute, while in the second case the jet is moved at a slower speed, taking five minutes to cross the block, which allows the jet to exploit the cracks generated by the cavitation. A fuller description of the process is given here.

Figure 2. Cavitation damage pattern on a block of dolomite showing the initial width of the jet (red lines), and the zone of damage that is being created around the traverse path.

Figure 3. Cavitation damage on a block of dolomite at a slower traverse speed, showing the width of the damage track that can be created. The slot is about half-an-inch deep.

In the course of the test program different shroud shapes were tested, but in all cases the comparison between an abrasive-laden jet and one containing cavitation bubbles was made with shroud shapes of the same overall dimensions.

The ratio of the exit diameter (discharge) from the shroud (D2) to that of the initial jet orifice (D1) was first changed to one of four different ratios, though the diameter of the initial jet was kept at 0.04 inches (1 mm). Of the different sizes tested the greatest vacuum in the chamber was measured with the smallest of the discharge diameters was tested.

Figure 4. The effect of increasing the throat length of the shroud on the vacuum pilled in the chamber, at different pressures.

If the discharge diameter was increased to 6.35 mm then the jet pressure had to be increased to 12,500 psi to obtain the same levels of vacuum achieved otherwise at 7,500 psi.

Figure 5. Vacuum pressures measured with a larger discharge diameter from the shroud, for different lengths and jet pressures.

Impact force measurements from the jet hitting a target at varying distances from the orifice, with and without the shroud showed relatively little difference in the overall total impact force (not considering abrasive) out to a distance of 6 inches. There was thus no apparent effect due to jet disintegration from the use of the shroud over these distances.

In order to compare the performance of the abrasive-laden jet with that of a cavitating jet, a new nozzle design was developed, and small samples of granite were rotated in front of each nozzle assembly, for 20 seconds. Because the jet had to be brought up to pressure for each test, and shut down afterwards, a steel shutter plate was placed between the nozzle and the target. One of the irritants in doing the tests was that the jets kept cutting through this shutter plate.

Figure 6. Steel shutter plate cut through in 6 seconds during system start-up.

The shroud, made of stainless steel, was also wearing out within a few minutes. Unfortunately we did not recognize that this was demonstrating that abrasive waterjets were an effective method for cutting metal – that commercial development had to wait for the more perspicacious Dr. Hashish to work with Flow Research and bring the technology to the market in 1980.

Part of the reason for our lack of interest was because of a different conclusion that Dr. El-Saie drew from his work, based on the following two curves. The first comparison of different jet results occurred with a jet pressure of 7,000 psi.

Figure 7. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 7,000 psi.

Note that the three water jets do not have much significant effect on the granite at this distance and jet pressure (we had to learn some later lessons to make them more productive at this pressure). But even at this pressure the abrasive waterjet was effectively cutting the granite.

But it was the change in the relative position of these curves, as the pressure was then increased to 20,000 psi that caught our attention. (The intermediate plots are not given here).

Figure 8. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 20,000 psi.

The water feed was not useful, since the power required to accelerate that volume drew heavily from that available through the jet.

The plain jet, without a shroud will cut granite at this pressure, particularly when moved over the surface. (And we later used this system to carve the Millennium Arch_ – as well as the Missouri Stonehenge). But the performance at that time was not that impressive.

Opening the ports on the shroud, without feeding anything into the jet caused, we believed a greater jet breakup and thus some additional droplet impact effects that improved cutting performance over that of the plain, more coherent jet, in part because the jet was spread over a larger contact surface.

Closing the jets induced cavitation in the stream, and this gave the best performance of the four – including abrasive injection. Again this was, in part because of the larger area of damage that the cavitation generated on the target, over the narrower slot of the abrasive-laden and plain jets.

In comparison the abrasive waterjet did rather poorly. In retrospect this is perhaps more of a surprise – but it should be born in mind that there was little attempt at optimizing the feed condition (which later research shows has a dramatic effect on performance) or the chamber geometry. Further the slot cut was much narrower than that created by the cavitating jet.

But it certainly caused us, in that time interval, to look more at cavitation, and to totally miss the implications of the AWJ result.

Most of these illustrations come from the Doctoral Dissertation by Dr. A. A. El-Saie “Investigation of Rock Slotting by High Pressure Waterjet for Use in Tunneling”, Mining Engineering Department, Missouri University of Science and Technology, (Then University of Missouri-Rolla), 1977.

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