Study On The Intensification Mechanisms Of Flow-induced Noise In The Oscillating Field Of Poppet Valve

Fluid power technology, widely used in the industry thanks to the merits of high power density, high-powered and fast-response et al, becomes a crucial driving force for science and technology development. While, with the development of electric power technology and high demand for the operation comfort and environmental protection, fluid power technology, confronts with serious challenges, it has to advance towards high power transmission efficiency, high pressure, mass flow, low noise et al for its survival and development. In order to improve the power transmission efficiency, the further development of fluid power technology will aim at the closed circuit system. The poppet valve, a representative hydraulic valve which is characterized by the fast response, low cost and insensitive to pollution, is large-scalely applied in the closed circuit systems. The poppet valve, characterized by large flow force, is apt to vibrate because of the abrupt change of flow force, causing serious vibration and noise problems. Previous studies on the vibration and noise of poppet valve are based on the open circuit systems, while, some problems which are neglected in open circuit systems may become serious in closed circuit systems. So, a thorough study on the poppet should be conducted for designing the high-performance closed circuit systems. The thesis mainly focuses on the flow-induced noise in the oscillating flow field of poppet valve. Hope research of this paper can help us to understand the mechanisms of flow-induced noise generation clearly, providing theoretical basis for low noise fluid power system designing.With the means of experimental measurement, theoretical analysis and numerical simulation, the intensification mechanism of flow-induced noise in oscillating flow field of poppet valve is studied systematically and comprehensively in the present thesis. The changing laws of poppet valve vibration property, with and without cavitation, are obtained at cases with various valve seat ratio. The mechanism for the intensification of cavitation noise after poppet valve vibration is studied, proposing that the increase of bubble collapse events is responsible for the intensification of cavitation noise. In addition, a hypothesis, that the selective amplification of squeal noise with particular frequency is caused by the fluid resonant, is proposed and verified by experimental investigation. It is clear that the Helmholtz resonance, caused by fluid-acoustic interaction, is responsible for only noise signals with particular frequency are selectively amplified.The main contents of this thesis are briefly stated as follows,1. The mechanism of poppet valve vibration is evaluated by experiments. Effects of cavitation on vibration property of poppet valve are studied with experimental and theoretical analysis. Some rules on the varying tendencies of vibration property at cavitation and non-cavitation conditions are found according to the valve seat ratio R*. The reason causing discrepancies of poppet valve vibration property at cavitation and non-cavitation conditions are explained with the cavitating flow patterns.2. Observing the phenomenon that cavitation noise is intensified after poppet vibration, an idea is proposed that the intensification of cavitation noise after poppet valve vibration is mainly lead by the increase of bubble collapse events. The mechanism associated with the increase of cavitation bubble collapse events in the oscillating flow field induced by poppet vibration is analyzed by means of numerical simulation and experimental investigation, and argued that the fixed cavitation structures, located initially at the flow separation region with low local pressure, are destroyed after poppet vibration, causing the events of cavitation bubbles developing from nucleus increase, then, cavitation become more serious. Based on the mechanism of cavitation in poppet valve, an optimization on poppet valve with low cavitation noise is proposed.3. The characteristic of squeal noise spectrum is studied by means of spectral analysis. The source of squeal noise is investigated with experiments and simulations. The changing laws of squeal noise’s fundamental frequency under multi-phase flow, varying poppet lift and valve chest volume conditions are summarized with abundant experiments. An assumption that squeal noise in poppet valve is caused by the fluid-resonant is proposed and verified with the expression of the valve chamber acoustic resonant frequency. The selective amplification mechanism of squeal noise for noise with particular frequency is explained, providing some methods for squeal noise suppression.