低温流体非稳态汽蚀形成机理可视化研究

Cavitation can occur in any hydraulic machinery. Cavitation in cryogenic fluids generates substantial thermal effects and strong variations in fluid properties, which takes a deep insight into the complex thermodynamic process. Understandign and quantifying these thermal effects are important for the design of liquid rocket turbomachinery systems that pump liquid hydrogen and liquid oxygen. The present project investigates the thermal effects on the unsteady cavitation of cryogenic fluids. The temperature and pressure distributions in the cavity are quantitatively measured when liquid nitrogen flows over blunt bodies with different headform shapes. The results are used to investigate the close relationship between the cavitation intensity and the inlet conditions as well as the thermodynamic effects of cryogenic fluids. High-speed camera is used to observe and record the unsteady development of the cavitation configuration from sheet one to cloud one. After that, the cavitation dynamics and the way of the rear re-entrant flow to force the bubble cloud to depart from the attached wall are illustrated. Considering the so-called "full cavitation model" is based on the steady flow condition, the preject develops a new cavitation model,in which the bubble radius is the function of local pressure. And, the model is derived from combining the Gibbs-Duhem equation and the Young-Laplace equation. Using the developed cavitation model, we dedicate to biuld a computational fluid dynamic (CFD) model for modeling the cavitating flow of cryogenic fluids. The CFD model is based on the homogeneous equilibrim flow model and gas mass fraction transport method, as well as the LES turbulent closure. Then, the simulations are performed on the quasi-steady and unsteady cavitating flow of liquid nitrogen flow over the different blunts. The achievements from the project researches will lay the solid foundations for the optimization of the more complex cavitating mechanisms of cryogenic fluids in rocket propellant system.

汽蚀现象在流体机械中广泛存在。由于热效应不可忽略,低温流体汽蚀展现了更复杂的热力学过程。理解热效应影响对设计液氢/液氧为燃料的火箭推进系统汽蚀部件关系重大。本项目研究低温流体非稳态汽蚀过程的热效应特性。测量不同形状钝体中液氮汽蚀的温度和压力分布，研究边界条件（汽蚀数）及热效应对汽蚀强度的动态影响。通过高速摄像机观察汽蚀形态从片状汽蚀到云状汽蚀的非稳态发展过程，分析其发展规律和尾部再入流引起的汽泡云与壁面剥离机理。考虑到所谓的"完全汽蚀模型"基于稳定流动条件得到，本项目结合热力学Gibbs-Duhem方程和Young-Laplace方程，推导得到气泡半径为压力函数的动态汽蚀模型。并用该汽蚀模型，基于均相流理论及传输方程法，结合LES湍流模型，构建低温流体非稳态汽蚀的数值模型，分析低温流体在钝体中的非稳态汽蚀特性。本项目研究有望为诱导轮等火箭系统中更复杂低温流体汽蚀机械的优化设计打下基础。

汽蚀指液体流动过程中，局部压力小于饱和压力而汽化的现象，汽蚀现象在阀门、诱导轮及文氏管等低温流体机械中广泛存在。相比于室温流体，低温流体汽蚀过程由于热效应而展现了更复杂的物理机理。. 本项目研究低温流体非稳态汽蚀过程的热效应特性。测量不同形状钝体中液氮汽蚀的温度和压力分布，研究边界条件及热效应对汽蚀强度的动态影响。通过高速摄像机观察汽蚀形态非稳态发展过程，分析其发展规律和脱落机理。考虑压力零阶效应推导得到动态汽蚀模型，基于均相流理论及传输方程法，结合LES 湍流模型，分析低温流体在钝体中的非稳态汽蚀特性。. 经过研究，我们成功发展得到了考虑压力零阶效应的动态汽蚀模型(DCM)，基于LES建模了水中水翼和不同几何结构的类稳态汽蚀特性。计算的压力以及汽蚀形态等结果，比基于FCM汽蚀模型得到的结果与实验结果更加吻合。进一步，我们建模了NACA66水翼的非稳态汽蚀特性，获得了与实验更一致的汽蚀区气相体积含量、压力振幅及频率等结果，再现了汽蚀区尾部引起汽蚀脱落的反射流流动过程，揭示了尾部压力振动两次达到峰值及其对汽蚀脱落的影响机理。. 在建模液氢动态汽蚀特性时，我们首次发现了液氢汽蚀尾部“下蛋式”小汽蚀云高频脱落模式，这种模式不同于液氮的整体汽蚀脱落模式。除了分析液氢汽蚀脱落频率、压力振幅等特性，通过研究液氢可压缩性、热效应、反射流以及涡流形态等相互作用，系统揭示了这种下蛋式汽蚀脱落机理。. 另外，我们数值分析了液氮等低温流体在三维水翼等的汽蚀特性，研究不同攻角时的汽蚀动态特性，揭示其热效应及涡流流态与汽蚀脱落的内在机理。结论是热效应虽然能抑制汽蚀强度，但促进了汽蚀脱落频率，在相同流动条件，液氮汽蚀脱落频率一般大于200Hz，远大于水的脱落频率。对液氢，小汽蚀云脱落模式甚少达到了~1300Hz。另外一个影响是，热效应影响下的低温流体汽蚀云很难覆盖流向水翼表面一半面积，而水则往往可覆盖整个水翼表面，变成超汽蚀。. 在实验研究方面，我们研制了一套可视文氏管液氮汽蚀实验系统，以及一套三维水翼可视液氮汽蚀实验系统，基于高速摄像机捕获了汽蚀动态发展过程，测得的汽蚀区动态压力、温度及流量等宝贵数据已经用于验证数值计算结果。. 研究成果为进一步研究减小汽蚀影响的低温流体机械优化设计打下了基础。