Cavitating Flow And Noise In Hydraulic Throttling Valves

With the development of hydraulic technology toward high-speed, high-pressure and large power directions, the internal problems of which like noise and vibration problems etc. have been seriously exposed. As the requirements of environmental comfortableness and steady flow performance increases for the meantime, the noise and vibration problem has been addressed with much attention. The hydraulic valves, which play important roles on hydraulic control is considered as one of the major sources of hydraulic noise and vibration. The flow noise is considered much more serious and important compared with structure noise, which can be divided into three types with the noise mechanism, cavitation noise (monopole), flow fluctuation noise (dipole) and general turbulent noise (quadrupole). Cavitation occurs in a wide kind of flow machineries, brings not only noise problem but also erosion, bad flow performance problems etc., researches on which are much abundant and comprehensive within long history. Since the phenomena are much complicated, investigations are not fully satisfied. Researches on hydraulic valve cavitation are always restricted by its complex and narrow flow structure and high speed flow velocity, the detailed cavitating flow state cannot be easily observed, most of the investigations still relay on theoretical hypothesis and experimental summarization. As for the flow fluctuation phenomena, the occurrence of which always accompanies with the cavitating flow in hydraulic valves, often be ignored or confused with cavitation noise somehow. Though the flow fluctuation phenomena are widely cognized and investigated in many flow components, literatures in hydraulic valves are much few.In this thesis, the flow noise induced by cavitating flow and (self-sustained) fluctuation flow are investigated profoundly and comprehensively with theoretical analysis, flow simulation and visualization technology and noise & vibration spectra analysis. Using the high-speed camera technology, the travelling cavitation bubbles and their variation law with the pressure parameters are visually obtained, which provide validations to the simplified theoretical model for throttling valves cavitation noise. The theoretical and experimental data of the peak frequency of bubble implosion noise are obtained and compared, the reason why the theoretical result is always larger than the experimental result is explained by the hydraulic oil's high viscosity. Moreover, the fixed large cavitation structure can be also observed in U-notch besides the travelling cavitation bubbles. The formation and instable behavior, including its influence to the travelling bubble noise are investigated, the research of which presents more detailed and aboard view to the cavitating flow and noise in hydraulic valves. As for V-notch, intense shear flow plays the predominant role compared the swirling flow in U-notch, no distinct fixed cavitation structure exists. The cavitation noise shows familiar properties with that of U-notch bubble noise. However, the shear flow arouses intense flow fluctuations, with the flow dynamic feedback form the downstream impingement, serious self-sustained flow fluctuation occurs, correspondingly, the noise spectra shows remarkable frequency peak in relatively lower frequency range compared with cavitation noise, which explains the reason of more powerful and whistling noise in throttling valves with V-notch.The outline of this thesis is as follows,In chapter 1, the significance and purpose of this thesis are proposed. The reviews on cavitating flow noise and self-sustained flow fluctuation noise are introduced. The literatures of the researches on the two types of noise in hydraulic valves are summarized. At the end, main research contents, research methods and experimental apparatus and system of this thesis are presented.In chapter 2, with the acoustic and under-water sound radiation theory, the individual bubble noise model is introduced, which illustrated the relationship of the bubble volume change with the noise radiation. The theoretical value of the bubble implosion noise is figured out with the observed bubble size, which is always larger than the experimental results, the reason is explained by the considerable viscosity effects of the hydraulic oil. Besides, the travelling cavitation bubbles and their variation law with the pressure parameters are visually obtained, which provide good validations to the simplified theoretical model for throttling valves cavitation noise.In chapter 3, while observing of travelling bubbles, the fixed large cavitation structure was also discovered in U-notch. CFD simulation is employed to analyze the flow state in U-notch, which illustrates that the spiral shape of fixed cavitation structure is induced by the notch wall guided swirling flow. The cavitation length is normalized of different flow structural characteristics with the critical inception cavitation number since that the inception cavitation number is the only function of flow structure. At the end, the mathematical derivation is conducted to describe the cavitation structure instable behavior. The characteristic dynamic frequency is then analytical obtained. Moreover, the influence of the unstable behavior to the bubble flow noise spectra is also studied.In chapter 4, with the comparison of the noise power and its spectra distribution between U-notch and V-notch, the reason of more powerful whistling noise comes from V-notch is explained that, in V-notch, the flow noise not only comes from cavitation noise, but also from intense shear flow self-sustained fluctuation. The self-sustained flow fluctuation in V-notch is studied with abundant experiments, the results shows good common regularities of this phenomenon. Besides, the influence from the accompanied cavitation phenomenon is also studied.In chapter 5, the research work of this thesis is summarized with further prospect.