Flow characteristics of high speed water jet and effective nozzle shape for machining

采用实验方法和数值分析方法对两种高速水射流进行了研究，找出了有效的喷嘴形状。一种是水下水射流，恒定输送压力为50-200MPa，喷嘴直径为0.155 mm；另一种是脉冲水射流，从锥形喷嘴向大气发射，速度为500m/s，喷嘴直径为2mm。主要研究结果总结如下：(1) CIP有限差分法对于流场中含有不连续界面的气液两相流的数值分析是有效的。(2)采用高速浸没水射流进行加工时，冲孔、钻孔或切削时，材料与喷嘴的距离应尽可能短，因为射流的动量对去除材料起着重要的作用。另一方面，脱离距离取决于抛光或喷丸的输送压力，因为空化在表面处理中起主要作用。在这两种情况下，喷嘴盖的出口长度应短，直径应小。(3)利用彩色施利伦法和高速摄像机对活塞冲击产生的冲激水射流进行了复杂激波射流的碰撞和破碎观察。计算结果与定性观测结果基本一致。用CIP差值法可以检测喷嘴形状、水柱在喷嘴内的初始位置和分离距离对固体材料冲击压力的影响。

Two types of high speed water jets are examined by experimental method and numerical analysis to find effective nozzle shape. One is submerged water jet with constant delivery pressure of 50-200MPa, and nozzle diameter of 0.155 mm, the other is impulse water jet issuing from a taper nozzle into the atmosphere with velocity of 500m/s and nozzle diameter of 2mm.The main results are summarized as follows.(1) The finite difference method of CIP is effective for the numerical analysis of gas-liquid two phase flows which include discontinuous interface in the flow field.(2) In case of machining with high speed submerged water jet, the stand off distance of material from a nozzle should be taken as possible as short for piercing, drilling or cutting, because the momentum of jet plays an important role in removing material. On the other hand, the stand off distance depends on the delivery pressure for polishing or peening, because cavitation plays a main part the surface treatment. In both cases, outlet length of nozzle cap should be taken short and the diameter small.(3) On the impulse water jet which is produced by piston impact, collision and break up of the jet with complicated shock waves are observed by colored Schlielen method and high speed video camera. The computational results show the same characteristics of the jet flow as observed one qualitatively. The effect of the nozzle shape, the initial position of water column in the nozzle and stand off distance on the impact pressure on a solid material can be examined by CIP difference method.