Detection of cavitation in situ operation of kinetic pumps: Effect of cavitation on the characteristic discrete frequency component

Cavitation within a pump causes structural vibration with noise, among other by-products. Experiments have shown that there is a discrete frequency or broadband peak within the audible noise spectra, which is in strong correlation with the development of the cavitation process in the pump. Furthermore, the peak of the discrete frequency, or broadband peak, coincides with the net positive suction head (NPSH) critical value, which corresponds to a 3% drop in the total delivery head. Therefore, the discrete frequency tone can be used to detect the incipience of cavitation and its development as well as to determine the NPSH required or critical value with in situ operation of a pump. In this study, we wanted to clarify the mechanism of noise generation, which is responsible for the discrete frequency component and on which cavitation has an important effect. For this purpose, three different measurement methods were used: the first is based on measurement of the sound pressure level (SPL) in the surrounding air, the second is based on measurement of the underwater acoustics and the third is based on measurement of the structural vibration. Experiments have shown that the characteristic discrete frequency tone, which is in close correlation with the cavitation process, is a result of structural vibrations (modes) or resonances caused by implosion of bubbles and bombardment of the inner surfaces of the pump.     

Noise as an indicator of cavitation in a centrifugal pump

One of the sources of instability in a centrifugal pump is cavitation within the pump. Cavitation of a centrifugal pump is the result of insufficient net positive suction head (NPSH) and can occur within the entire range of operating conditions. Cavitation may cause three different and undesirable effects: (1) a drop in head-capacity and efficiency curves, (2) damage to the impeller by pitting and erosion, and (3) structure vibration and resulting noise. Therefore, the cavitation process must be prevented by all means. To prevent the onset of cavitation we have to detect the beginning of the cavitation process in the pump. To detect the beginning of the cavitation process, the emitted noise can be used, among other possibilities. Specifically, a noise spectra structure can be used to detect the beginning of cavitation and its development. Experiments have shown that there is a discrete frequency tone, at 147 Hz, which is strongly dependent on the cavitation process and its development. Therefore, noise spectra can also be used to determine the NPSH required or the critical value, representing the upper limit of the permissible pump operation without cavitation.     

空化流动诱导离心泵低频振动的实验研究

空化流动严重影响泵安全稳定运行,为充分认识泵内空化发展程度及其诱导的低频振动特性,设计了一台离心式模型泵作为研究对象,同时采用高速摄影及振动加速度测试手段,实现了叶轮内部空化流动的可视化及对应的泵体低频振动信号提取,分析了空化泡形态随空化发展的演化规律,对比了空化发生前后泵体低频振动频谱特性,探讨了泵体总振级水平、离散频率下振动加速度幅值随空化发展程度的变化,提出振动临界汽蚀余量可作为空化程度的另一判据,叶频时的振动加速度幅值变化亦可作为泵内空化程度的表征。     

Experimental research into time–frequency characteristics of cavitation noise using wavelet scalogram

The cavitation has become the main cause of the damage to the hydraulic machine. Cavitation detection is very important to guarantee the safe running of the hydraulic machine. The sound, especially the audible sound, based methods are becoming attractive due to their simplicity and logicality in the application. However, the cavitation noise is easy to be contaminated by the background noise. In order to understand the characteristics of the cavitation noise deeply, using the wavelet scalogram analysis, this paper presents an experimental study to investigate the time–frequency characteristics of the cavitation noise of various cavitation states and the relation between the cavitation noise and the cavitation process. In addition, the method of parameters optimization for the wavelet is used to improve the transform performance of the wavelet scalogram. The results show that: the cavitation noise is composed of the components with wide band frequency and obvious impulse feature; but the cavitation noise of different cavitation stages has different time–frequency characteristics and compositions; in addition, the cavitation noise can be distinguished from the background noise because they have totally different frequency characteristic. This study validates that the cavitation noise can be used to detect the cavitation state and monitor the cavitation process.     

基于空化辐射噪声的检测方法实验研究

空化空蚀严重影响水利设备的正常工作和使用寿命,为此本文研究了基于水听器信号的水泵空化检测方法,从空化噪声辐射基本特性出发,分析并选取强度、脉冲以及频谱结构等特征参数组成最优分类特征向量,使其既有较强的稳定性,也有较好的灵敏度。以此特征向量训练获得的支持向量机(SVM)分类器进行水泵空化状态识别准确率平均能有99.6%。检测其他结构水泵的空化,识别准确率也在96.5%以上。在较高环境噪声单一能量特征无法识别的情况下,依然有较好的识别准确率。表明该方法对不同结构的水泵及较高环境噪声具有一定的鲁棒性,有较好的应用价值。     

基于时频分析的离心泵空化状态表征研究

空化检测对于保障离心泵运行的安全性和可靠性具有重要意义,已有研究侧重于信号采集和特征提取,对于空化诱发的振动噪声形成机理研究不够深入。为了实现离心泵空化状态的准确表征和有效识别,本文建立了基于信号调制理论的流体机械振动噪声信号模型,将流体激振信号和调制信号视为空化表征的有效信息成分,在此基础上提出了一种基于频带能量和峭度的主导频带时频分析方法,并结合卷积神经网络实现空化状态智能识别.最后,仿真信号和实际数据的分析结果验证了流体机械信号模型的合理性,也证明了所提出的主导频带时频分析方法的有效性。     

超声清洗装置空化噪声谱分析法空化噪声级测量

空化强度是用以衡量液体介质中空化活动的剧烈程度,同时空化效应在超声清洗中起关键作用,因此,测量超声清洗槽中的空化强度便可了解其中空化活动的情况.当发生空化时,液体介质中会产生成分复杂空化噪声,对空化噪声谱进行分析和计算得到空化噪声级,据此可判断空化强度.实验测得结果表明:超声清洗装置内稳态空化分布广泛、均匀,瞬态空化分...     

Acoustic characterization of cavitation intensity: A review

Cavitation intensity is used to describe the activity of cavitation, and several methods are developed to identify the intensity of cavitation. This work aimed to provide an overview and discussion of the several existing characterization methods for cavitation intensity, three acoustic approaches for charactering cavitation were discussed in detail. It was showed that cavitation noise spectrum is too complex and there are some differences and disputes on the characterization of cavitation intensity by cavitation noise. In this review, we recommended a total cavitation noise intensity estimated via the integration of real cavitation noise spectrum over full frequency domain instead of artificially adding inaccurate filtering processing.     

Detecting incipient cavitation by assessing low frequency acceleration and analysis of CMC

Cavitation is a common phenomenon in a fluid circuit especially wherever local pressure is lower than fluid saturated pressure. The cavitation negatively affects a fluid system and structure in different ways: i.e. erosion, flow rate reduction, noise and vibration. In order to diminish cavitation, adding some nanomaterials seem to be applicable in different ways. This research aims at assessing the effects of CMC (Carboxy methyl cellulose) additives on incipient cavitation by analyzing the resultant change in low frequency acceleration. Furthermore, this study attempts to examine the accuracy of low frequency acceleration for detecting incipient cavitation.     

Quantifying the distortion products generated by amplitude-modulated noise

When sinusoidal amplitude modulation (SAM) is applied to noise or tone carriers, the stimuli can generate audible distortion products in the region of the modulation frequency. As a result, when bandpass-filtered SAM noise is used to investigate temporal processing, a band of unmodulated noise is typically positioned at the modulation frequency to mask any distortion products. This study was designed to investigate the distortion products for bandpass noise carriers, and so reduce ambiguity about the form of this distortion and its role in perception. The distortion consists of two distortion-noise bands and a distortion tone at the modulation frequency. In the first two experiments, the level and phase of the distortion tone are measured using two different experimental paradigms. In the third experiment, modulation-frequency difference limens are measured for filtered SAM noise and it is shown that performance deteriorates markedly when the distortion tone is canceled. In a fourth experiment, masked threshold is measured at low frequencies for bands of high-frequency, unmodulated noise with the same levels and spectra as the SAM noises in the earlier experiments. The results confirm that unmodulated noise also produces quadratic distortion which may explain some aspects of earlier reports on remote masking.     

Identification approach of acoustic cavitation via frequency spectrum of sound pressure wave signals in numerical simulation

In a sono-reactor, complex ultrasound pressure wave signal can be detected, containing multiple information related to acoustic cavitation. In this present study, acoustic cavitation in a cylinder is investigated numerically. Via Fast Fourier Transfer (FFT), the sound pressure signals from sonotrode emitting surface are separated into harmonics, sub/ultra-harmonics and cavitation white noise: (1) the appearance of harmonics proved the non-linear propagation of ultrasound, (2) at the vibratory amplitude from 5∼20μm, only harmonics exists in the frequency spectra, corresponding to expansion and compression of non-condensable gas (NCG), (3) at the vibratory amplitude range of 30∼50μm, the occurrence of sub/ultra-harmonics demonstrated gaseous cavitation occurred, and (4) at the vibratory amplitude higher than 55μm, cavitation white noise arose, pointing out the initiation of vaporous cavitation. Based on the combination of frequency spectra and cavitation zones distribution, the acoustic cavitation state in water liquid is determined.     

The effect of temperature and viscoelasticity on cavitation dynamics during ultrasonic ablation

Inertial cavitation has been shown to enhance heating rates during high intensity focused ultrasound treatments. Cavitation dynamics will be affected by heating and by the changes in mechanical properties of tissue resultant from thermal denaturation; however, the nature of the change is not known and forms the focus of the current study. A Keller-Miksis equation is used to find the variation in inertial cavitation threshold with temperature in water and, when coupled with a Kelvin-Voigt viscoelastic model, in biological tissue. Simulated thermal ablation treatments in liver and muscle are used to explore the changes in cavitation dynamics, and the resultant frequency spectra of secondary acoustic emissions, due to tissue denaturation. Results indicate that viscosity is the key parameter controlling cavitation dynamics in biological tissues. The increase in viscosity during denaturation is predicted to increase inertial cavitation thresholds, leading to a substantial decrease in the higher harmonic content of the emitted pressure signal across a wide range of bubble radii. Experimental validation of these observations could offer improved methods to monitor therapeutic ultrasound treatments.     

Copropagating and counterpropagating pumps in second-order- pumped discrete fiber Raman amplifiers

The gains and noise figures of discrete second-order-pumped fiber Raman amplifiers utilizing copropagating and counterpropagating pump configurations were experimentally obtained, and the gain results were compared with computer simulations. It was found that the additional gain that is due to second-order Raman pumping is larger for the copropagating pumps than for the counterpropagating pumps, in agreement with simulations. In contrast to distributed second-order-pumped fiber Raman amplifiers, a slight increase in noise figure, by as much as ~1 dB was observed relative to the single-pump scheme. However, the advantages of second-order pumping in discrete amplifiers include greater flexibility in design of the gain distribution along the fiber and the ability to spectrally distribute the pump powers to avoid undesired nonlinear effects.     

Investigation of cavitation noise using Eulerian-Lagrangian multiscale modeling

We have employed the large eddy simulation (LES) approach to investigate the cavitation noise characteristics of an unsteady cavitating flow around a NACA66 (National Advisory Committee for Aeronautics) hydrofoil by employing an Eulerian-Lagrangian based multiscale cavitation model. A volume of fluid (VOF) method simulates the large cavity, whereas a Lagrangian discrete bubble model (DBM) tracks the small bubbles. Meanwhile, noise is determined using the Ffowcs Williams-Hawkings equation (FW-H). Eulerian-Lagrangian analysis has shown that, in comparison to VOF, it is more effective in revealing microscopic characteristics of unsteady cavitating flows, including microscale bubbles, that are unresolvable around the cloud cavity, and their impact on the flow field. It is also evident that its evolution of cavitation features on the hydrofoil is more consistent with the experimental observations. The frequency of the maximum sound pressure level corresponds to the frequency of the main cavity shedding for the noise characteristics. Using the Eulerian-Lagrangian method to predict the noise signal, results show that the cavitation noise, generated by discrete bubbles due to their collapse, is mainly composed of high-frequency signals. In addition, the frequency of cavitation noise induced by discrete microbubbles is around 10 kHz. A typical characteristic of cavitation noise, including two intense pulses during the collapsing of the cloud cavity, is described, as well as the mechanisms that underlie these phenomena. The findings of this work provide for a fundamental understanding of cavitation and serve as a valuable reference for the design and intensification of hydrodynamic cavitation reactors.     

Gate driven adiabatic quantum pumping in graphene

We propose a new type of quantum pump made out of graphene, adiabatically driven by oscillating voltages applied to two back gates. From a practical point of view, graphene-based quantum pumps present advantages as compared to normal pumps, like enhanced robustness against thermal effects and a wider adiabatic range in driving frequency. From a fundamental point of view, apart from conventional pumping through propagating modes, graphene pumps can tap into evanescent modes, which penetrate deeply into the device as a consequence of chirality. At the Dirac point the evanescent modes dominate pumping and give rise to a universal response under weak driving for short and wide pumps, even though the charge per unit cycle is not quantized.     

Surface functionalization by nanosecond-laser texturing for controlling hydrodynamic cavitation dynamics

The interaction between liquid flow and solid boundary can result in cavitation formation when the local pressure drops below vaporization threshold. The cavitation dynamics does not depend only on basic geometry, but also on surface roughness, chemistry and wettability. From application point of view, controlling cavitation in fluid flows by surface functionalization is of great importance to avoid the unwanted effects of hydrodynamic cavitation (erosion, noise and vibrations). However, it could be also used for intensification of various physical and chemical processes. In this work, the surfaces of 10-mm stainless steel cylinders are laser textured in order to demonstrate how hydrodynamic cavitation behavior can be controlled by surface modification. The surface properties are modified by using a nanosecond (10–28 ns) fiber laser (wavelength of 1060 nm). In such a way, surfaces with different topographies and wettability were produced and tested in a cavitation tunnel at different cavitation numbers (1.0–2.6). Cavitation characteristics behind functionalized cylindrical surfaces were monitored simultaneously by high-speed visualization (20,000 fps) and high frequency pressure transducers. The results clearly show that cavitation characteristics differ significantly between different micro-structured surfaces. On some surfaces incipient cavitation is delayed and cavitation extent decreased in comparison with the reference – a highly polished cylinder. It is also shown that the increased surface wettability (i.e., hydrophilicity) delays the incipient cavitation.     

Photoacoustic cavitation and heat transfer effects in the laser-induced temperature jump in water

The finite element method is employed to analyze photoacoustic cavitation and heat transfer occurring when modest temperature jumps (T-jumps) are induced by a laser in D₂O solution, which may contain a small concentration of a protein or peptide sample. Cavitation can be initiated through a photoacoustic mechanism at intensities well below optical breakdown thresholds. Cavitation probability is related to test medium properties, initial temperature, T-jump magnitude and test region geometry. Parameters affecting thermal conduction losses are also examined because such losses limit the useful duration of the T-jump induced in protein folding experiments. From this study, guidelines are offered for reducing the occurrence of cavitation and extending the useful duration of the T-jump. Received: 2 April 2001 / Revised version: 17 October 2001 / Published online: 29 November 2001     

低A声级贯流风机叶片周向角不等距优化设计

叶轮产生离散噪声的原因是叶片间周向夹角均匀布置,因此降低叶轮离散噪声的有效手段就是破坏叶片周向夹角的均匀布置,通过合理布置各叶片夹角降低叶轮离散噪声。本文提出了采用叶片周向角不等距布置降低叶轮总离散噪声A声级的一种优化设计模型,模型充分考虑了A计权函数与波动力对脉动流场响应参数Sc乘积有关的综合衰减曲线的高低频段衰减较大的特性和人可听域频率范围在20-2000Hz之间的特点,通过算例验证采用该模型可以降低总离散噪声A声级。     

Modulation control and spectral shaping of optical fiber supercontinuum generation in the picosecond regime

Numerical simulations are used to study how fiber supercontinuum generation seeded by picosecond pulses can be actively controlled through the use of input pulse modulation. By carrying out multiple simulations in the presence of noise, we show how tailored supercontinuum spectra with increased bandwidth and improved stability can be generated using an input envelope modulation of appropriate frequency and depth. The results are discussed in terms of the nonlinear propagation dynamics and pump depletion.