喷嘴出口结构参数对自振射流振荡特性的影响
Effects of nozzle outlet geometry on the oscillating characteristics of self-excited waterjet
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摘要: 自振射流的振荡特性决定其自激振荡及冲蚀效果,受喷嘴出口结构影响较大,但其影响规律尚不十分清晰。首先从自振射流产生机理出发,分析射流频率组成及特性影响因素,探究出口结构与振荡特性的内在联系。基于信号检测手段,拾取不同出口结构下自振射流压力及噪声信号,实时调整射流作业围压以连续改变自激频率,同时对上述信号进行时频分析以实现射流自激频率及谐振腔声学固有频率的解耦。试验结果表明,扩口角度及扩口长度等喷嘴出口结构参数对自激频率具有显著影响,进而改变自振射流振荡特性。喷嘴扩口角度决定了射流自激振荡的产生与否,恰当的扩口角度能够促进射流剪切层涡环脱落,进而引起初始压力激励,而扩口角度为0°时无法产生有效自激振荡;当扩口角度由10°增加到40°时,自激频率先减小后增加;扩口角度亦会促进自振初生,一定范围内随扩口角度减小,其自振初生空化数呈增高趋势。扩口长度会影响自激频率及自振初生空化数,随扩口长度增加,自激频率减小而自振初生空化数增大。研究揭示了喷嘴出口结构参数对自振射流振荡特性的影响规律,并初步探究了其作用机理,同时定性讨论了喷嘴出口结构参数对射流自振初生空化数的影响,为深入认识自振射流振荡特性提供了支撑。Abstract: The oscillating characteristics of self-excited waterjet determine the jet's resonating and erosion effects, which are greatly affected by nozzle outlet geometry, but the influence law is unclear. In this study, firstly, the influence factor of the jet's oscillating characteristics is analyzed based on the self-excited waterjet's mechanism, and the internal relationship between nozzle outlet geometry and oscillating characteristics is studied theoretically. The ambient pressure is adjusted to make the self-excited frequency changing continuous, and the self-excited waterjet's pressure and noise signal are captured based on the signal detection method. Meanwhile, time-frequency analysis is carried out to decouple the self-excited frequency from the acoustic natural frequency. Experimental results indicate that the nozzle outlet geometry, including the expansion angle and length, could significantly affect on the self-excited frequency and change the oscillating characteristics. The nozzle expansion angle determines whether the self-excited oscillation occurs or not. An appropriate expansion angle can promote the vortex ring shedding, and then cause the initial pressure excitation. However, an effective self-excited oscillation cannot occur when the expansion angle is 0°. When the expansion angle increases from 10° to 40°, the self-excitation frequency decreases first and then increases. The expansion angle also can promote oscillation initiation, and the incipient cavitation number increases with the decrease of the expansion angle in a specific range. The expansion length will affect the self-excited frequency and the cavitation number of initial oscillation. In the present working condition, the self-excited frequency decreases, and the incipient cavitation number increases with the expansion length. The effects of nozzle outlet geometry on the oscillating characteristics of self-excited waterjet are uncovered, and the mechanism is preliminarily explored. In the meantime, the influence of nozzle outlet geometry on the incipient cavitation number is qualitatively discussed, which supports a further understanding of the oscillating characteristics of self-excited waterjet.