Nozzle design for pulsed water jets an analytical, numerical and experimental study

University dissertation from Luleå : Luleå tekniska universitet

Abstract: A study is made of the early time behaviour of pulsed water jets generated by the action of a shock wave moving through a waterfilled nozzle. In particular the influence of nozzle geometry on the exit velocity is examined. A quasi-one-dimensional small perturbation analysis revealed the importance of the geometry near the lip on the jet front velocity. This analysis also suggested the use of divergent-convergent nozzle shapes for attaining high jet front velocities. A numerical and experimental study was done for nozzles with an area contraction ratio of 1/11. The numerical results revealed a more complex dependence of the exit velocity on the nozzle area, but still did not exclude the use of divergent-convergent nozzle shapes. In the experiments the jet stagnation pressure near the lip of the nozzles was measured with a strain gauge technique. These measurements were supplemented by high-speed photographs of the jet profile. Jet velocities over 200 m/s were obtained. The comparison with the numerical results was made difficult due to proven distensibility effects in the nozzles and due to different spreading of the jets. Nevertheless, qualitative agreement was found taken the above mentioned observations into account. The major result of this study was, however, the discovery that divergent-convergent nozzle shapes have been proven to be a potential tool in the optimum design of nozzles for pulsed high-speed water jets. In the construction of nozzles awareness should be made of the possibility that major effects of distensibility may distort the characteristic performance of a nozzle, as regards e.g. maximum jet speed.

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