In the initial instant that a flat ended jet hits a flat surface at a shallow angle, two jet flows are formed. A small micro-jet is formed along that surface that moves at a much higher speed in the direction of the arriving jet. A slower jet moves in the opposite direction.
Figure 1. The acceleration of a small segment of an arriving jet on a flat surface (after Mazurkiewicz et al).
By replacing the flat lower surface with a second jet co-axially aligned in the plane of the jets, a similar effect can be achieved, with the second jet moving considerably faster that the arriving jets.
Figure 2. The impact of two jets inclined toward one another at a shallow angle.
For the tests that I described last time, the two jets were inclined toward one another at a relatively small angle (in the range from 1 – 10 degrees) which was partly controlled by the geometry of the cutting head in which they moved.
The improved velocity of the secondary jet can be shown by one of the photographs taken of the impact of two small jets, operated at 60 psi, and intersecting at an angle of ten degrees, using a high speed camera to capture the result.
Figure 3. Intersection of two jets viewed from the top. The jets had broken into droplets at the point of impact, and the shock waves generated by the high speed of the secondary jet formed on impact can be seen around the impact point.
With better quality jets (made from electro-formed nickel built up on a flame-polished mandrel) it was possible to get the jets to intersect while still coherent, and the resulting jets, formed from jets at 10,000 psi, were able to cut thick lenses of pyrite in the field. This was not possible using the 10,000 psi jets alone, without the use of the augmented jets produced by their convergence.
There is a second benefit that can occur where these convergent jets are used in working with harder materials (than coal). It can be illustrated by a photograph of two separate pieces of Berea sandstone into which two different sets of converging jets had been fired.
Figure 4. Two blocks of Berea sandstone each of which had split after having had a pair of convergent jets fired into the top of the block.
The pair of jets converged at the point where the belled-out shape of the cavity transitions to a narrower tapering hole. The larger upper volume is created by the back-flow of the slower moving jet as it cuts back towards the entry hole, reaming out the original passage.
Apart from the evidence of the smaller accelerated jet (through the shape of the cavity) the other interesting point (which was confirmed in a number of tests) is that the restriction of the outflow of water from the cavity, because of the narrowing of the cutting jet paths with depth, and the augmentation of pressure at the impact point, produced enough internal pressure in the blocks to cause them to rupture.
This augmentation was used in Rolla in a number of different applications over the years, although, because of the expense of building the high precision nozzles, these were not used extensively in later work. Rather the jets were formed from two separate flows to nozzles on the end of two short lengths of hose. These jets could then be adjusted to change the intersection angle of the jets, which was also adjusted through raising and lowering the head, so that the intersection point fell below the surface of the target. This meant that the jets had to penetrate a little into the rock by themselves, before they intersected and generated the higher pressure small penetrating jet and concomitant increase in local pressure of that jet.
Figure 5. Different approaches to the use of converging jets on a rock surface. That on the left is the MS&T version, that on the right was carried out at the University of New South Wales**.
The MS&T approach was based on the work we had carried out in the field, where the jets were to converge on the surface of the target, so that the jet would be able to penetrate through rock materials that it would not normally be able to cut. In the trials in the mine an intersection angle of 2 degrees was found to be best.
The Australian approach followed on Frank Roxborough’s ideas of trying to generate larger chips when cutting into rock, in order to lower the energy required for material removal.
The Australian team however, found it more useful to converge the jets, closer to the nozzle which was less tightly manufactured, and focused the streams within the rock body. In this way the stresses set up within the rock were found to invariably produce large single chips of rock roughly conical in shape with an angle similar to that of the impacting jets. Interestingly it was reported that there was little evidence of jet cutting action in these tests where a jet at a pressure of 40,000 psi was cutting into a 30,000 psi uniaxial compressive strength basalt – something normally impractical even at those jet pressures. The results also were reported to show that the specific energy required for this technique was one to three orders of magnitude less than for conventional cutting of slots by jet action. Subsequent traversing tests on the rock were preliminarily reported to substantiate the results from the static testing.
* Mazurkiewicz, M., Barker, C.R., Summers, D.A., "Adaptation of Jet Accumulation Techniques for Enhanced Rock Cutting," in Erosion: Prevention and Useful Application, ASTM STP 664, W.F. Adler, ed, ASTM, 1979, pp. 473 - 492.
** Lin, B., Hagan, P.C., Roxborough, F.F., "Massive Breakage of Rock by High Pressure Water jets," 10th International Symposium on Jet Cutting Technology, Amsterdam, Holland ,October, 1990, pp. 399 - 412.
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