Orifices are damaged by abrasive particles penetrating upstream of AWJ waterjet orifices. By understanding the mechanism by which the particles are carried upstream it may be possible to eliminate or at least reduce the number of particles reaching the inlet side of an orifice.
By feeding air from atmosphere to the downstream side of waterjet orifices abrasive can be prevented from reaching orifices. However this reduces the amount of air available to carry abrasive to cutting heads and complicates their design. It would be ideal if the fluid dynamic events leading to abrasive passing through orifices could be modified to eliminate the problem.
The cause of water/air reverse flow carrying particles through an orifice is probably the result of super-cavitation during the final closure of a water shut off valve. Subsequent collapse of water vapour bubbles induces reverse flow through an orifice. Speed of valve closure and compressed water volume downstream of shut off valve are factors that could influence the amount of reverse flow.
1. A transient analysis program that includes the capability to simulate valve closure with cavitation and bubble collapse. This facility is part of the transient capabilities of the Flowmaster suite of fluid flow analysis programs from www.flowmaster.com
Work in progress