Sunday, March 1, 2015

Waterjetting 30d - Applying cavitation damage

Much of the work that we carry out with high pressure waterjets requires that they cut with precision and, in consequence much of the focus has been on controlling the stream of the jet to obtain the tightly constrained cutting action that produces this result.

Yet two of the larger growth sectors of the industry, the sub-divisions that have now been given the titles “hydro-demolition” and “hydro-excavation” don’t have that focus. Rather they seek to remove critical volumes of material, generally to some specific depth, but with less concern over the edges of the hole that is being created (provided water doesn’t penetrate the edge materials).

Depth of cut control is a little more of a challenge using an abrasive waterjet system since I have seen AWJ cuts that penetrated through feet of reinforced concrete and have mentioned the problem that new owners of systems sometimes run into when they run the nozzle for too long in a fixed position over a target and discover that the jet has not only cut the material, but also penetrated through the bottom of the holding tank, and put a hole into the underlying concrete floor.

Precisely controlling depth then becomes a matter of controlling the length of time the jet cuts on a surface, and to get to a fixed depth that will also depend on the amount of abrasive in the water, the jet pressure and the distance from the nozzle to the surface. It can also, to a degree, be controlled by the pressure of the surrounding fluid, although that is an interaction with the driving pressure that can become a little more complex.

In the last post I mentioned that when cavitation is formed around the outside of a jet cutting down through water which is itself pressurized (perhaps only because the jet is under a significant depth as water, such as for example a diver cutting apart an oil platform in the North Sea) then the damage from the cavitation bubble collapse occurs most intensely over a short distance from the nozzle. That distance changes with the cavitation number (simplistically the ratio of the pressure in the water around the jet to the pressure driving the jet itself), the volume flow and in a secondary relationship to the surrounding fluid pressure as well as other factors.

The latter impact of chamber pressure on the cutting range of the jet can be demonstrated with a Lichtarowicz cell, which allows one to see the jet as it cuts through surrounding fluid to the jet, and where, by adjusting the chamber fluid pressure the jet and cavitation cloud length can be extended to and beyond the sample, or reduced so that the jet barely reaches the target.

Figure 1. Backlit picture showing the cavitation bubbles forming and hitting the target.

The problem with generating this type of cavitation cloud as a means of drilling forward is that the bubbles are on the outside of the jet, and so as the jet hits and flows across the surface it protects the surface from the bubbles which flow on the outside of the lateral action.

The bubbles need to be confined against the target surface, and this is easier to do where the bubbles are formed in the center of the jet. The ways of doing this were discussed in an earlier post but can be summarized as being either by creating a turbulent swirl in the jet, or by placing a flat-ended probe into the jet stream.

Figure 2. Methods of creating cavitation bubbles in the center of a jet. (After Johnson et al)

Of these two methods, that using the central probe is more effective over greater distances, since the jet remains relatively coherent, while the swirling jet tends to broaden and lose energy after much shorter distances.

Tests of the central probe device showed that it could very quickly drill a hole more than 18 inches deep – at which point, unfortunately, the probe within the nozzle was itself destroyed by the cavitation action.

These tests were, however, carried out with nozzles with orifice diameters on the order of 0.04 inches, with the probe diameter being roughly half of that. Such designs are difficult to make and then align – ensuring that the probe is centered within the orifice throat, as shown.

In contrast with abrasive waterjet damage, the damage from an individual event is not as critically affected by the particle size nor by the main jet velocity. The collapsing pressure jet from a cavity collapse is at around 1 million psi – as Dr. Al Ellis theorized and we were able to confirm at Missouri S&T. This occurs with relatively little control by the surrounding fluid, or originating jet (which instead is more influential in controlling the intensity of cavitation generation and the location of the collapse).

This means that it is quite possible to use larger jet streams and still achieve quite destructive effects. In Johnson’s early paper on the topic he was using a jet pressure of 1,600 psi and able to drill through blocks of granite. The best advance rate that he could achieve at that time was around 3.5 inches/hour – which is not a practical value for commercial operations.

And unfortunately, for a while, this led us to be distracted into seeking higher and higher operating pressures to drive the jet, forgetting that this did not really change the bubble collapse pressure. It was only later, when we followed Dr. Lichtarowicz’ advice that we started adjusting the back pressure in the system and then we began to achieve useful material removal rates (on the order of cubic inches per minute).

However we did not carry out tests at larger flow rates, where we know, from the evidence at the Tarbela High Dam that much greater volumes of material may be removed, even at relatively low operating pressures.

At the Boulder Dam in the United States cavitation generated a cavity some 100 ft long and roughly 25 ft wide cutting into the rock wall to a depth of 40 ft. along the spillway during the course of a season, as reported by Warnock.

As a result of these tests it is clear that there is a considerable development potential for the practical use of cavitation – at significantly higher production rates than achieved to date, and over the wide spectrum of minerals (since the high destructive pressures exceed those necessary to disintegrate all natural materials).

It will be interesting to see when interest in the topic regenerates.

Johnson, Kohl, Thiruvengadam and Conn “Tunneling, Fracturing, Drilling and Mining with High-Speed Waterjets Utilizing Cavitation Damage.” First ISJCT
Benjamin T.B. and Ellis A.T. “The Collapse of Cavitation Bubbles and the Pressures Thereby Produced against Solid Boundaries,’ Proc. Royal Society (London), A262, pp.221-240.
Wanock J.E. “Experiences in the Bureau of Reclamation,” Cavitation in Hydraulic Structures – a Symposium, ASCE vol 71, no 7, p 1053. (Sept. 1945)

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