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Here is a schematic cross-section of a water jet cutter nozzle design: enter image description here

As it is shown, the thin water jet grabs abrasive particles in the mixing chamber and the mix flows through the nozzle.

My question is: what makes the water flow through the nozzle despite the loss of kinetic energy due to the abrasive material and the friction inside the nozzle instead of filling up the mixing chamber and running up in the ambient pressured abrasive feeding tube?

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It's called Bernoulli's principle. It works because the fluid has momentum, so doesn't cause equal outward pressure at all points.

This principle can be used to convert a high-speed low-volume flow into a high-volume low-speed flow. Since the output flow has more volume than the input flow, additional material is sucked in.

The diameter of section 4 is a little larger than that of section 2 for this process to work.

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  • $\begingroup$ I assume that there is ambient pressure inside the mixing chamber, consequently, no pressure change occurs between the ends of the nozzle (4). Then what makes the water to flow through it despite the friction losses? $\endgroup$ – BalazsToth May 22 '18 at 13:36
  • $\begingroup$ @SolarMike As long as the water forms a free jet inside the chamber, there is no reason for the pressure to drop below the ambient. $\endgroup$ – BalazsToth May 22 '18 at 14:10
  • $\begingroup$ @SolarMike Why? Did I make any contradictions? $\endgroup$ – BalazsToth May 22 '18 at 14:16
  • $\begingroup$ @SolarMike I know the Venturi effect and it has nothing to do with the free jet. Carburetor is an excellent example, but in this case, we have a free jet with ambient pressure. $\endgroup$ – BalazsToth May 22 '18 at 14:23
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As has been mentioned in other answers it is Bernoulli's principle. Part of the pressure in the water feed line turns into momentum and eject the jet. This jet now has a high velocity and a large momentum.

When this jet in part 3 picks up the abrasive sand and accelerates it to mix and carry it, there is some loss of momentum of water in nuzzle.

They have calibrated the feed of the abrasive material such that its inertia and friction is kept well below what can cause clogging of the nuzzle or backing up.

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  • $\begingroup$ Consider a vertical straight tube with diameter d, in which water flows with a very high velocity. Now cut about a 5d long piece out the tube. Can we expect the water to flow just like before the tube was cut? No, a large part of the jet wont enter the tube after the gap because of its significant friction loss. I understand that I must be wrong, water jet cutters work well, I just want to understand how. $\endgroup$ – BalazsToth May 22 '18 at 20:23
  • $\begingroup$ I have actually used sandblasting. All the openings and junctions have been designed and sized and angled correctly to work properly. Yes if you cut a large opening you may disrupt the flow. But they size them so they don't disrupt the flow. $\endgroup$ – kamran May 22 '18 at 21:05

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