A Study of High-Speed Pulsed Water Jets

本文通过数值分析和实验研究了活塞冲击产生的速度为200-500 m/s的脉冲水射流。采用锥形喷嘴和直形喷嘴，出口直径分别为5 mm和10 mm。实验中测量了射流头速度和射流冲击压力，并用Xe-flash和脉冲红宝石激光器拍摄了照片。在数值分析中，采用有限差分法对喷管内的流动进行了分析。可以得出以下结论。(1)采用FCT控制人工扩散的两步Lax Wendroff方法对瞬态两相流进行了有效的分析。(2)经过短暂的转捩后，喷管内的流动可视为准定常流动。由于活塞减速，流速降低时，在喷嘴出口处形成蒸汽空穴。特别是在直喷嘴出口处，形成了收缩流和分离流，并伴有强烈的空化现象。蒸汽空穴也扩散到水柱中。在两相界面处，流体的密度和速度呈不连续变化。(3)假设喷嘴内的流动是稳定的，可以估算出最大射流速度。这一估算方法对直喷管的射流是足够适用的。(4)直喷嘴的射流头速度随射流行程的增加而增加，而锥形喷嘴的射流头速度随射流行程的增加而减小。由于锥形喷嘴的射流速度呈脉冲式增长，而直喷嘴的射流速度增长缓慢。(5)当水射流在空气中形成时，由于射流速度的变化，在射流轴线周围观察到剧烈的碰撞和水盘。(6)喷距一定时的射流冲击压力不仅取决于射流速度，而且由于喷嘴形状的原因，还与射流的流动结构有关。

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.