Wednesday, May 15, 2013

Waterjetting 9b - the effect of standoff distance.

One of the problems with relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.

Figure 1. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target, under the jet, is not eroded, but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width, and will be subject to high lateral jet flows.

Figure 2. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.

Figure 3. Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff the central section, though still present, is being eroded.

Figure 4. Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point the cavity is starting to get noticeably deeper.

Figure 5. Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time the central section of the jet is beginning to break down into, initially short strings, that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.

Figure 6. Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

By this time the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.

Figure 7. Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice.

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size, and condition as a function of distance, and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used, if the droplets are generated properly, and used within the relatively narrow window that they exist, to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.

Figure 8. The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

There are ways of getting around this problem, but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

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