气穴是液压系统中常见的一种有害现象

气穴是液压系统中常见的一种有害现象 摘要 气穴是液压系统中常见的一种有害现象，经常发生在阀口附近。不仅破坏了流体的连续性、降低了介质的物理特性，而且引起振动和噪声。同时系统效率降低，动态特性恶化。近年来随着纯水液压技术的发展，气穴及其引起的气蚀问题变得尤为突出，直接影响到阀的性能与寿命。因此，为了设计低噪声、低能耗、高效率的液压阀，研究如何控制阀口气穴的发生和发展非常重要。 本课题针对溢流阀阀口的气穴现象，用计算流体动力学的方法对锥阀和球阀阀口气穴流场进行了数值模拟，预测了气穴发生区域，模拟得到的气体体积比分布与可视化实验得到的数字图像处理后的气穴图象非常吻合，验证了数值计算的正确性。进一步分析了锥阀、球阀和纯水液压锥阀阀口几何参数与边界条件对气穴的影响。最后，对溢流阀阀芯形状进行了改进，通过对不同结构的化实验和噪声测试，实验结果与流场分析结果一致，改进后的阀气穴强度和噪声均得到降低。 首先，本课题针对锥阀阀口喷流的特点，将质量转移方程和气体体积比方程引入RNG湍流模型，并与两层近壁模型相结合，运用商业化的CFD软k- 件FLUENT对锥阀阀口的气穴流场进行了数值模拟，预测了气穴发生后的气体体积比分布。可视化实验运用工业纤维镜与高速摄像机等组成流场可视化试验系统，多方位地观察了阀口附近的气穴现象，对其进行数字图像处理后，获得了气穴流场的分布信息，与仿真结果比较，吻合良好，表明RNG湍流模k-型能有效地描述锥阀等液压元件的阀口气穴流动。实验同时采用涡流式位移传器、激光位移器和数字应变测量仪等构成的检测系统，研究了气穴流场诱 发的阀体与阀芯振动。而且对锥阀在不同进口速度和不同出口面积的流场分别进行了模拟。 其次，在些基础上，进一步对纯水液压锥阀阀口的穴流场进行了研究，给出了不同阀口开度、不同阀芯锥角、不同进口流速和不同出口压力下的气穴流场分布。分析了进口流速、出口压力、阀口开度和阀芯锥角对纯水液压锥阀阀口气穴强度的影响。 然后，对实验观察到的球阀阀口周期性气穴现象进行了流场分析。可视化实验运用高速摄像机观察到球阀阀口气穴现象以2500Hz的频率周期性的发生。并对球阀有无气穴现象分别进行了噪声测试和频谱分析。小球及小球弹性系统在轴向的固有频率远小于2500Hz。因此推测小球有横向振动，而且小球的横向振动使气穴现象周期性发生。为了验证实验的结论，本课题对球阀阀口三维气穴流场进行了数值模拟。比较了模拟得到的对称与非对称气穴区域与实验观察到的气穴云变化，比较吻合。进一步合理地预测小球的横向振动使的气穴现象周期性发生，提示了球阀内的周期性气穴的影响，为了抑制球阀内的周期性气穴内发生机理振动、噪声的关系。 最后，针对阀口形状对气穴的影响，为了抑制阀口气穴，对溢流阀阀芯形状进行了改进，设计了几种不同的阀芯结构，分别对不同的阀芯进行了气穴流场分析和比较，来寻求优化的阀芯结构。实验采用透明的阀体，分别对改进前后不同阀芯阀口气穴现象进行了可视化实验观察和相应的噪声测试和频谱分析。比较相同条件下气穴发生过程，气穴云的变化形态及噪声频谱分析。实验结果与数值分析结果比较吻合，最后得到一种优化的阀芯结构，在相同的工况下，它的气穴区域和强度均比较小，而且噪声得到降低。 关键词:溢流阀 气穴 流场仿真 流动可视化实验 噪声测试 频谱分析 Abstract Cavitation is a common harmful phenomenon in hydraulic transmission systems, frequently occurs near the orifice of the valves. It not only damages flow continuity and reduces medium physical performance, but also induces vibration and noise. At the same time, the efficiency of a system is reduced due to cavitation, especially dynamic performances are deteriorated. In recent years, with the development of water hydraulic technology, cavitation problem and erosion due to cavitation became especially predominant, which directly effect performance and lifetime of the valves. Therefore, the research of how to control cavitation inception and development is absolutely important in order to design low noise, low energy loss and high efficiency valve. In this article, aiming at the cavitation near the orifce of the relief valves, Computational Fluid Dynamics (CFD) simulations of cavitating flow through poppet valve and ball valve were performed, cavitation region were predicted. The numerically obtained air volume fraction distribution showed a good agreement with the experimentally visualized cavitation image through digital processing, which verifies the accuracy of simulation. Furthermore, The effects of inlet velocity, outlet pressure, opening size as well as cone angle on cavitation intensity in the poppet valve, ball valve and water hydraulic poppet valve were numerically investigated. At last, the configuration of a relief valve were improved, the cavitating flow simulation. At the same time, flow visualization and noise measurement were conducted for different configurations of relief valve. The experimental results agree well with numerical results. The cavitation intensity and noise and noise level of the developed relief valve are decreased. First, applying commercial CFD software FLUENT, the cavitating flow issuing from the orifice of a poppet valve was numerically investigated using RNG κ-ε turbulence model combined with mass transfer equation, volume fraction equation and two layer zonal model. The air volume fraction distribution was predicted. The finished experiments are conducted to catch cavitation images around the valve seat of the poppet valve from the perpendicular directions, using a pair of industrial fiberscopes and high-speed video cameras integrated visualization system. The results therefore indicate that RNG turbulence model could achieve a reasonable prediction for the cavitating flow within the poppet valve as well as other hydraulic components. Besides, the vibrations of the valve body and poppet induced by the cavitating flow are detected using vortex displacement transducer, laser displacement meter and digital strain device. Moreover, the cavitatig flow in poppet valve were simulated for different inlet velocity and different outlet area, respectively. Secondly, based on the above study, Furthermore, the cavitating flow through a water hydraulic popper valve were performed. The flow field distribution were obtained with different inlet velocity, different outlet pressure, different opening size as well as different cone angle. The effects of inlet velocity, outlet pressure, opening size as well as cone angle on cavitation intensity in the water hydraulic poppet valve were numerically investigated. Afterwards, the periodical cavitation inception near the orifice of a ball valve was numerically analysed. The visualized cavitation occurs near orifice periodically in experiment and its frequency is about 2500 Hz using high speed video camera. Furthermore, the noise spectrums of the ball valve in cavitation and non-cavitation cass are measured. The natural frequency of the ball in the axial direction is far from 2500Hz. Therefore, It is expected that the ball vibrates in the lateral direction and the periodic cavitation occurs due to the lateral vibration of the ball. In order to verify above results, the three-dimensional cavitating flow issuing from the orifice of a ball valve was numerically investigated, The cavitation region was predicted for axisymmetric and non-axisymmetric case when the ball moves to one side. The predicted cavitation region for axisymmetric and non-axisymmetric case was compared with experimentally visualized cavitation image. The predicted and observed cavitation region agrees well. Therefore, it was expected reasonably that the ball vibrates in the lateral direction and cavitation occur frequently. The relationship between periodical cavitation inception and vibration was disclosed. At last, according to the effect of orifice configuration on the cavitation, the several poppet configurations of a relief valve were designed in order to suppress cavitation, the cavitating flow simulation for different configuration were performed and compared in ordet to find a optimal conriguration. The experiment adopted transparent acrylic resin valve body. Cavitation flow visualization experiment were experimentally conducted and compared. Furthermore, cavitation noises were measured and noise spectrums were analyzed. The experimental results agree well with numerical results. A optimal poppet configuration has been obtained. Its cavitation region and cavitation intensity are reduced under the same condition, and it causes the decrease of cavitation noise level. Keywords: relief valve, cavitation, cavitating flow simulation,flow visualization, experiment noise measurement, spectrum analysis