This investigation is concerned with the improvement of the nozzle design for water and abrasive water jet machining. The mechanism of formation and characteristics of pure water and abrasive water jets are investigated in order to determine quasi-optimal process conditions.; To improve the pure water jet machining, a pulsed water jet nozzle, which employs the principle of the Helmholtz type resonator, is investigated experimentally and numerically. The experiments show the advantages of this nozzle over the commercial nozzle in cutting and cleaning. A numerical solution of the differential equations of continuity, momentum conservation, turbulent kinetic energy and dissipation for two dimensional axi-symmetric flow by employing the FIDAP package is developed and used for the numerical prediction of pulsed turbulent flow inside the nozzle. The determination of the optimal nozzle parameters aided by numerical simulation is carried out and the best ratios of the parameters are: h (cavity length)/{dollar}rm dsb1{dollar} (diameter of upstream nozzle) = 3.0 and {dollar}rm dsb2{dollar} (diameter of downstream nozzle)/{dollar}rm dsb1{dollar} (diameter of upstream nozzle) = 1.3. The results of simulation agree well with the experiments. The numerical prediction of the velocity at the exit of the pulsed nozzle is validated by the velocity measurement by a laser transit anemometer. The obtained velocity changes periodically and ranges from 190 m/s to 230 m/s. A numerical analysis enables us to evaluate nozzle design and the effectiveness of the numerical prediction is validated experimentally. The numerical solutions and experimental results present the improvement on the pure water cutting and cleaning and provide a technological basis for the improvement of pulsed water jet machining and technology.; To increase the efficiency of abrasive water jet machining, an improved abrasive water jet nozzle is developed and experimentally investigated. The performance of the abrasive water jet is improved by the modification of the abrasive particles path prior to the collision with the water jet. This modification is obtained by control of the angle ({dollar}alpha{dollar}) between the topshaped surface of the focusing tube and the water flow direction and change of distance (H) between the water nozzle and focusing tube. The improvement of water-particles mixing increases the rate of material removal and simplifies the alignment procedure. It is found that the optimal parameters for the nozzle design are: {dollar}alpha{dollar} = 45{dollar}spcirc{dollar} and H = 1.587 mm. The experimental results and analysis show the potential of this modified nozzle for applications in abrasive water jet machining.
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