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

采用实验和数值分析相结合的方法，对两种类型的高速水射流进行了实验研究，找出了有效的喷嘴形状。一种是恒定输出压力为50-200 Mpa、喷嘴直径为0.155 mm的浸没水射流，另一种是锥形喷嘴向大气中喷射的脉冲水射流，其速度为500m/S，喷嘴直径为2 mm。主要结果如下：(1)有限差分方法对于含有不连续界面的气液两相流动的数值分析是有效的。(2)在高速浸入水射流加工时，为了穿孔、钻孔或切割，应尽量缩短材料与喷嘴的距离。因为射流的动量对物质的去除起着重要的作用。另一方面，由于气蚀在表面处理中起主要作用，因此抛光或喷丸的输送压力决定了抛光或喷丸的支撑距离。在活塞冲击产生的脉冲水射流上，用彩色Schlielen法和高速摄像机观测到了复杂激波下射流的碰撞和解体过程。计算结果与定性观测的结果一致。用CIP差分法研究了喷嘴形状、喷嘴内水柱的初始位置、喷嘴离地距离等因素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.