Functions of Nozzle in Abrasive Jet Machining (AJM)

In general, nozzle is an isentropic steady flow device that produces a high velocity jet by converting static pressure of fluid to kinetic energy (while a diffuser slows down fluid velocity and increases static pressure). It also delivers fluid at a particular angle. In abrasive jet machining also, a nozzle is used to produce high velocity jet (100 – 300m/s) and deliver it at a particular angle to strike the work surface. When such high velocity jet strikes the work surface, it removes material by impact erosion, sometime assisted by brittle fracture.  Basic functions of nozzle in abrasive jet machining are discussed below.

Nozzle inner diameter needs to be about 20 times larger than the average media size (abrasive grain size).

Three functions of nozzle in AJM

Delivering high velocity jet—At inlet of the nozzle, pressure of gas-abrasive mixture remains high (15 – 20bar) but velocity remains low. When this mixture passes through the nozzle, static enthalpy (pressure) of the mixture is transformed to kinetic energy and thus high velocity jet is obtained.

Controlling jet diameter—Diameter of abrasive jet depends on inner profile of the nozzle as well as exit diameter. For a constant carrier gas pressure and mixing ratio, larger diameter reduces jet velocity but increases jet cross-sectional area and vice versa.

Regulating impingement angle—Apart from delivering a high velocity jet, AJM nozzle also controls impingement angle. It is the angle between work surface and jet axis. Theoretically its value can be 0º – 90º; however, practically it is kept between 60º – 90º. It influences material removal rate (MRR) and accuracy of machining. High MRR can be obtained at an impingement angle of 70º – 80º. For making deeper but narrower slots, an angle of 90º is preferred while for cutting wide area, impingement angle can be reduced. Therefore on the basis of work surface inclination, AJM nozzle can be oriented accordingly for harnessing best performance.

References

  • Book: An Introduction to Energy Conversion: Turbomachinery by V. Kadambi and M. Prasad (New Age International Publishers).
  • Paper: M. W. Chastagner and A. J. Shih, Abrasive Jet Machining for Edge Generation, Transactions of NAMRI/SME, Volume 35, 2007.