混流式喷水推进泵空化流动及其诱导振动特性的试验研究

Experimental study on the cavitation flow and the induced vibration characteristics of a mixed-flow water-jet pump

为了研究喷水推进泵内非定常空化流动及其诱导的振动特征,搭建了高速摄像与数字信号同步测量平台。对不同空化程度下的泵内空化流动、压力脉动信号、振动信号进行了采集和分析。引入经验模态分解及希尔伯特黄变换(Hilbert-Huang transform,HHT)对振动信号进行了处理和分析。试验表明:喷水推进泵叶顶区出现了叶顶间隙空化和叶顶泄露涡空化等空化现象;随着空化程度的加剧,叶顶区的空化结构先由丝状逐渐连接成薄片状,继而发展为云状;快速旋转的空化云会呈现出极不稳定状态,尾部发生脱落形成垂直空化涡(perpendicular cavitating vortices,PCVs);小尺度的垂直空化涡消散在流道中部;随着空化程度的进一步加剧,垂直空化涡尺度增大并向相邻叶片的压力面运动,造成喷水推进泵性能的下降。空化程度加剧对应的流动结构特征会导致压力脉动产生相应的变化,从而诱导振动的增加。经验模态分解和HHT结果显示片状空化及大尺度云空泡溃灭时会诱导高频区的宽频振动增大;空化云尾缘的低频脱落会产生垂直空化涡,诱导低频的压力脉动和振动的快速增加。希尔伯特边际谱上高频幅值,叶频和轴频幅值的变化特征可在一定程度上用来反映泵内的流动状态。

In order to study the unsteady cavitation flow and the induced vibration characteristics in a water-jet pump,a high-speed camera and digital signal synchronization measurement platform was built.The cavitation flow,pressure pulsation,and vibration signals in the pump at different cavitation levels were acquired and analyzed.Empirical modal decomposition and Hilbert-Huang transform(HHT)were introduced to process and analyze the vibration signals.The results show that the cavitation at the top gap and the leaky cavitation at the top of the lobe appear in the water-jet pump.With the decreasing of inlet pressure of pump,the cavitation flow structure in the tip area of the blades develops from filamentary to sheet and then to cloudy.The rapidly rotating cavitation cloud will show an extremely unsteady characteristics,and the tailing of cavitation cloud will fall off to form perpendicular cavitation vortexes(PCVs).The small-scale PCVs dissipate in the middle of the flow channel,and with the further intensification of the cavitation,the scale of PCVs increases,which then move to the pressure surface of the adjacent blade,resulting in the degradation of the water-jet pump performance.The change of flow structure characteristics corresponding to the aggravation of cavitation level leads to a corresponding change in pressure fluctuation and induces an increase in vibration.The results of EMD and HHT show that the development of sheet cavitation and the collapse of large scale of cloudy cavitation vortexes induce the increasing of broadband vibration in the high frequency region;the PCVs induce rapid increasing of pressure fluctuation and vibration at low frequencies.The changing trends of the amplitudes at high frequency,rotating frequency and shaft frequency in the Hilbert marginal spectrum can be used to reflect the flow characteristics in the waterjet pump.