A Study of High-Speed Pulsed Water Jets

对活塞冲击产生的速度为200~500m/S的脉冲水射流进行了数值分析和实验研究。采用锥形喷嘴和直喷嘴，出口直径分别为5 mm和10 mm。在实验中，利用Xe-闪光灯和脉冲红宝石激光器测量了射流的喷头速度和冲击压力，并拍摄了照片。在数值分析中，采用有限差分方法对喷管内的流动进行了分析。结果表明：(1)两步Lax-Wendroff法结合FCT控制的人工扩散法在非定常两相流分析中是有效的。(2)短过渡后，喷管内的流动可视为准定常流动。当活塞减速导致流速降低时，在喷嘴出口处形成蒸汽空腔。特别是在直喷嘴出口，形成了收缩流动和强烈空化的分离。蒸汽腔也扩散到水柱中。在假设喷嘴内流动稳定的前提下，可以估算出喷嘴的最大射流速度。(4)直喷嘴的喷头速度随射流的移动而增大，而锥形喷嘴的喷头速度随射流的移动而减小。由于锥形喷嘴的射流速度是冲量增大的，而直形喷嘴的射流速度增长缓慢。(5)在空气中形成水射流时，由于射流速度的变化，会出现严重的碰撞和射流轴线周围的水盘。(6)在一定距离处的射流冲击压力不仅取决于射流的速度，而且由于喷嘴的形状而影响射流的流动结构。

Pulsed water jets in the range of speed =200-500 m/s produced by piston impact are examined by numerical analysis and experiments. Taper nozzles and straight nozzles are used with outlet diameter 5 and 10 mm. In experiments the jet-head velocity and the impact pressure of the jets are measured and photographs are taken using Xe-flash and pulse ruby laser. In the numerical analysis finite difference method is employed for the flow analysis in the nozzles. The following conclusion can be made.(1) 2 step Lax Wendroff method with FCT controlled artificial diffusion proves to be effective in the transient two phase flow analysis.(2) After short transition the flow in the nozzles can be regarded as quasisteady. Vapor cavities are formed at the nozzle outlets during flow velocity decrease due to the piston deceleration. Especially in the straight nozzle outlets the contract flow and the separation with strong cavitation are formed. The vapor cavities also spread into the water column. The density and the velocity of flow change discontinuously at the interface of the two phase.(3) On the assumption that the flow in the nozzles are steady maximum jet speed can be estimated. This estimation is enough applicable to the jets of the straight nozzles.(4) The jet-head velocity of the straight nozzles increase as the jets travel but that of the taper nozzles decrease. Because the jet speed of the taper nozzles increase impulsively but that of the straight nozzles increase slowly.(5) When the water jets are formed in the air severe collision and water disks around the jet axis are observed because of the change of the jet speed.(6) The jet impact pressure at some standoff distance depend on not only the jet speed but also flow structure of the jets due to the nozzle shape.