Effects of area discontinuity at nozzle inlet on the characteristics of high speed self-excited oscillation pulsed waterjets

High speed self-excited oscillation pulsed waterjet (SEOPW) offers many advantages over continuous jets or external-excited pulsed jets and has a large number of practical and industrial applications. In order to take better advantage of SEOPWs, effects of area discontinuity at nozzle inlet were analyzed based on the previous related research and then experimentally investigated with respect to the axial pressure oscillations. A jet-driven Helmholtz oscillator, which is capable of producing effective SEOPWs, was employed in the experiment. It was found that area discontinuity has a capacity of enhancing the peak, which largely depends on the inlet pressure and standoff distance. The enhancement decreases with increasing inlet pressure and only happens within small standoff distances. Compared with the continuous case, area enlargement enhances the peak by 25%, 21%, and 16%, corresponding to inlet pressures of 10, 15, and 20 MPa, respectively; while area contraction turns to be a better one by improving the peak by 8% at inlet pressure of 25 MPa. However, at inlet pressure of 30 MPa, both enlargement and contraction decrease the peak. Moreover, area discontinuity cannot influence the optimum standoff distance where the maximum peak appears. For the pressure oscillation amplitude, even both area discontinuities have the ability of increasing the amplitude regardless of inlet pressure and standoff distance, contraction enhances the amplitude much more than enlargement does at all the testing inlet pressures. In addition, area discontinuity has nearly no influence on the oscillating frequency and causes perturbations that may only affect the intensity of self-excitation by amplitude.