超临界二氧化碳介质箔片轴承弹流耦合研究

发布时间：2018-09-19 14:35

【摘要】：超临界二氧化碳由于其特殊的传热以及压缩特性,非常适合作为热力循环的工质,近年来关于超临界二氧化碳闭式布雷顿循环的研究正逐渐成为热点,而由于高转速与较高的温度和压力,该循环系统中的涡轮-电机-压缩机共轴转子的支承也面临着一系列的问题。本文针对超临界二氧化碳闭式布雷顿循环的支承系统,采用理论推导与数值计算相结合的方法,对以超临界二氧化碳为润滑介质的径向箔片轴承和推力箔片轴承进行仿真研究。全文共分为三部分:第一部分,使用理论方法,从连续性方程和能量方程出发,得到两组适用于变粘度变密度工况的雷诺方程,在雷诺方程中加入随雷诺数变化的半经验常数,以此来考虑高转速大雷诺数下润滑介质对轴承特性的影响。对两个雷诺方程进行了无量纲处理,并建立合适的箔片变形模型,将润滑膜厚度与压力及箔片的变形相互耦合在一起,推导了基于有限差分法的显式松弛迭代的稳态雷诺方程求解推进格式。第二部分,使用基于MATLAB编写的程序计算空气介质的箔片径向和推力轴承的承载力与偏位角,并与文献结果进行对比,结果表明数值计算准确可靠。接着结合物性计算工具使用该求解器仿真超临界二氧化碳润滑下的两种轴承的静特性,主要包括压力、润滑膜厚度和湍流润滑系数分布、压力峰值、偏位角、承载力和摩擦力矩等,并讨论其与相同参数传统空气相比静特性的区别,对于推力轴承还确定了最佳箔片数。第三部分,将变粘度变密度的瞬态雷诺方程中的压力和润滑膜厚度在平衡位置处基于小扰动法线性展开,按动态扰动量归并后得到和静参数相关的刚度和阻尼相互耦合的偏微分方程,将该方程与箔片变形方程耦合,求解轴承的动力学特性,并分析动力学系数随轴承偏心率、间隙量、轴承数、箔片柔度、激振频率的变化规律。本研究工作中针对两种轴承不同雷诺方程所共有的结构特点,提出一种显式迭代求解雷诺方程的新思路,即将雷诺方程展开并化为标准形式,建立标准形式的通用迭代求解模块,之后通过将不同雷诺方程化为标准型并调用通用模块,可以大大简化数值编程,并增加程序可靠性。在对超临界二氧化碳物性进行计算时,通过建立物性映射表,并辅以等距分割快速查找算法,可以快速插值求取高精度的物性参数,确保计算结果的准确。在轴承的动特性计算之中,基于稳态雷诺方程,提出了一种适用于扰动频率为0工况的新型动特性计算方法,该方法与静压计算程序结合能够计算变物性的轴承特性。本文所使用的方法对于其它轴承仿真研究具有一定的借鉴作用,研究得到的一些规律和结论可用于超临界二氧化碳箔片轴承的润滑特性预测,对箔片轴承的设计具有一定的指导作用。
[Abstract]:Because of its special heat transfer and compression characteristics, supercritical carbon dioxide is very suitable as a working medium for thermal cycle. In recent years, the research on closed Brayton cycle of supercritical carbon dioxide is becoming a hot topic. Because of the high speed and high temperature and pressure, the support of the coaxial rotor of the turbo-motor-compressor is faced with a series of problems. In this paper, the radial foil bearing and thrust foil bearing with supercritical carbon dioxide as lubricating medium are simulated by combining theoretical derivation and numerical calculation for the supporting system of supercritical carbon dioxide closed Brayton cycle. The paper is divided into three parts: in the first part, two sets of Reynolds equations suitable for variable viscosity and variable density conditions are obtained by using the theoretical method and the energy equation. The semi-empirical constant with Reynolds number is added to the Reynolds equation. The influence of lubricating medium on bearing characteristics at high speed and large Reynolds number is considered. The two Reynolds equations are dimensionless and a suitable foil deformation model is established. The lubrication film thickness is coupled with the pressure and the deformation of the foil. Based on the finite difference method, an explicit relaxation iteration scheme for steady-state Reynolds equation is derived. In the second part, a program based on MATLAB is used to calculate the bearing capacity and offset angle of foil radial and thrust bearings in air media, and the results are compared with the results in literature. The results show that the numerical calculation is accurate and reliable. Then the static characteristics of two kinds of bearings under supercritical carbon dioxide lubrication are simulated by using the solver combined with physical property calculation tools, including pressure, thickness of lubricating film and distribution of turbulent lubricating coefficient, peak pressure, offset angle, and so on. The bearing capacity and friction moment are discussed, and the difference between static characteristics of thrust bearing and conventional air with the same parameters is discussed. The optimum foil number is also determined for thrust bearing. In the third part, the pressure and the thickness of the lubricating film in the transient Reynolds equation with variable viscosity and density are linearly expanded at the equilibrium position based on the small perturbation method. According to the partial differential equation of stiffness and damping coupling with static parameters, the dynamic characteristic of bearing is solved by coupling the equation with foil deformation equation, and the dynamic coefficient is analyzed with the eccentricity of bearing. Change law of clearance, bearing number, foil flexibility and exciting frequency. In this work, a new idea of solving Reynolds equation by explicit iteration is proposed, which is to expand the Reynolds equation and transform it into a standard form, according to the common structural characteristics of two different Reynolds equations of bearings. The general iterative solution module in standard form is established, and the numerical programming can be greatly simplified and the program reliability can be increased by converting the different Reynolds equations into the standard form and calling the general module. In the calculation of the physical properties of supercritical carbon dioxide, by establishing the physical property mapping table and adding the fast searching algorithm of equidistant segmentation, the high precision physical parameters can be obtained quickly, and the accuracy of the calculation results can be ensured. Based on the steady state Reynolds equation, a new method for calculating the dynamic characteristics of bearings is presented in this paper. The method is combined with the static pressure calculation program to calculate the characteristics of the bearings with variable physical properties. The method used in this paper can be used as a reference for the simulation of other bearings. The obtained laws and conclusions can be used to predict the lubrication characteristics of supercritical carbon dioxide foil bearings. It can guide the design of foil bearing.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TH133.3

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