Fundamental Aspects in Waterjet Cleaning
Louis and Schikorr* wrote an article back in 1982 to address some fundamental issues in waterjet cleaning. The article still has a great reference value today. Here a review of the article.
The article started with listing out 15 typical waterjet cleaning applications. It also sorted out the types of applications by the types of substrates and layers to be removed. Characteristics of substrates were described in details. Two noted characteristics of metal substrates are “Wöhler behaviour”(fretting) and corrosion. Layers were classified into organic (naturally grown and artificial) and inorganic types. The characteristics of each type of layers were also described in details. The mechanisms (mechanical, specific, and chemosorption) of adhesion were also discussed.
The article also talked about two types of free jets: round and flat. Both types are fully turbulent and they disintegrate into small fluid particles shortly after they leave the nozzles. Flat jets have a higher tendency of disintegration than the round jets.
A continuous jet flowing perpendicular to the target surface produces a normal force as well as a shearing force that is parallel to the surface. In the case that the jet is hitting a kerf, the flow will change direction and a large portion of the kinetic energy will be converted into pressure, which could open up a crack. In reality the jet becomes a stream of small fluid particles which could produce pulses of impact pressures multiple times of the steady-state pressure. A high temperature in the jet is helpful in removing layers consisting of oil and grease.
Regarding the layer removal process in waterjet cleaning, the authors suggested that crack formation is necessary for removing a hard and/or brittle layer and dynamic loading (impact) is preferable. For ductile types of layers, a high impact speed is preferable because a ductile material will become more brittle with higher impact speeds. For viscous and viscoelastic layers (bitumen, chlorine-caoutchouc, oil-grease), a quasi-static load is sufficient and a higher temperature of the jet is helpful.
The last session of the article addressed the topic of process optimization, assuming metals as the substrate material. Metals can be damaged by the jet after an incubation period because of the “Wöhler behaviour”. Achieving the highest productivity without causing damage to the substrate can be achieved by optimizing these three variables: (1) operating at a pressure which can cause significant mass-loss for the layer to be removed but below the incubation pressure of the substrate; (2) loading time should be below the incubation period of the substrate; (3) a proper operating distance between the nozzle and the target should be either just above or just below the peak erosion zone.
* Louis, H. & Schikorr, W. (1982) Fundamental aspects in cleaning with high-speed water jets, Proceedings of the 6th International Symposium on Jet Cutting Technology, Guildford, England, April 6-8, 1982, pp. 217-228.
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