非接触式机械密封中的磁流体热动力效应研究

The non-contact mechanical seal is an important shaft sealing unit installed in rotating equipment. Its sealing pressure always remains constant at invariant rotational speed and for this reason, it can not seal the medium with surge pressure as well as that with constant pressure. The idea for lubricating the end faces of non-contact mechanical seal by using the magnetofluid is proposed in this project. The alterable viscosity of magnetofluid can be utilized to generate controllable thermohydrodynamic effect of lubricant film between the end faces of mechanical seal, and accordingly the online control of sealing pressure and friction power consumption of mechanical seal can be realized by controlling the intensity of magnetic field which exerts on magnetofluid film. Based on the investigation into the influences of temperature and magnetic intensity on the viscosity and coefficient of heat conductivity of magnetofluid, the characterization method for velocity slip and temperature jump on the boundaries of magnetofluid film which is between the two end faces of sealing rings can be established. The influences of physical, geometrical and operating parameters on the thermal results of frictional heat effect of magnetofluid film will be investigated. In consideration of the coupling effect among the magnetofluid film and sealing rings, a magnetism-thermal-fluid-structure coupling analysis method will be established to investigate into the thermohydrodynamic effect in non-contact mechanical seal lubricated by magnetofluid, and then the temperature and pressure distributions of magnetofluid film and the sealability parameters, such as leakage rate, will be obtained. According to the results of the study, the mechanics of thermohydrodynamic effect of magnetofluid will be revealed. Furthermore, the results of this project will provide the theoretical base for the development of new type of non-contact mechanical seal which possesses adaptive ability aiming at varying duty.

非接触式机械密封是旋转设备防止泄漏的重要部件，其在定转速时密封压力恒定，不能适应压力波动工况。本项目提出采用磁流体作为非接触式机械密封端面润滑介质的新思想，利用磁流体粘度受磁场控制的特性产生可控的流体动压效应，实现密封压力和摩擦功耗的在线控制。研究温度和磁场强度对磁流体粘度和导热系数的影响规律，建立动环和静环端面间磁流体膜边界速度滑移和温度阶跃的表征方法；根据磁流体膜的摩擦热流密度、粘度和温度的协调关系，研究物性、几何和操作参数对磁流体膜剪切热效应的影响规律；研究磁场、静环、磁流体膜与动环的热力耦合作用特性，建立磁-热-流-固多场耦合的解析和数值分析方法；研究外磁场作用下非接触式机械密封端面间磁流体膜的热动力效应，获得磁流体膜温度、压力分布及泄漏率等密封特性参数，揭示磁流体热动力效应的形成机理并进行试验研究。研究结果可为具有变工况自适应能力的新型非接触式机械密封的研发提供理论依据。

本项目以螺旋槽机械密封为原型，通过设计润滑介质输送系统和外磁场发生系统，建立了磁流体动压润滑性能的试验装置，研究了磁流体润滑的热流体动力效应，实现了磁流体润滑膜的粘度控制，提出了磁流体动压润滑性能的控制方法，并研究了摩擦热散热方法。主要研究工作和结论如下：.（1）研究了纳米磁流体的基本物理和力学特性.建立了超薄纳米流体剪切流动的分子动力学模型，模拟发现，壁面原子的振动对纳米流体中颗粒运动的影响显著，液膜各层的切向速度在膜厚方向上呈非线性变化，且存在明显的边界速度滑移。壁面剪切速度增大，边界速度滑移率增大，系统温度对速度滑移亦有影响；.研究了均匀磁场和梯度磁场条件下纳米磁颗粒的运动情况，并提出了一种交替采用旋转均匀磁场和梯度磁场的、磁颗粒链空间姿态的控制方法，揭示了磁场对磁颗粒链的运动和变形的影响规律；.建立了磁流体的Couette模型，采用Stokes动力学模拟方法研究了磁流体膜在准静态剪切条件下磁颗粒及其簇的空间结构演化规律，发现磁流体膜的屈服应力磁颗粒结构及固体壁面密切相关，被磁化的固体壁面对磁流体动压润滑效应的影响不可忽略。.（2）提出了磁流体动压润机械密封的设计方法并制造了试验机.基于传统的非接触式机械密封，提出了一种采用磁流体润滑的非接触式机械密封结构。该结构将磁流体注入动环和静环端面之间作为密封润滑介质，并在密封环的外侧设置了一个磁场发生器。.为了测试新密封结构的性能，设计了一台机械密封试验装置，该试验装置可以控制机械密封的端面比压、密封介质压力、转轴转速以及磁场强度；.设计了用于磁流体动压机械密封的磁场发生器，并根据动环、静环、磁流体膜及磁场发生器组成的导磁结构中的磁场数值分析结果，对磁场发生器进行了优化设计。.（3）研究了磁流体动压润滑效应.研究了密封结构在外磁场作用下的磁场分布情况。磁场发生器的电流强度与密封间隙中磁流体膜的磁场强度呈正比关系，且所有磁力线均垂直于磁流体膜；.研究了磁场强度、动环转速对密封性能的影响规律。磁场强度和密封介质压力增大会导致润滑介质泵送量减小，摩擦扭矩增大，而转轴转速增大会引起润滑介质泵送量增大，摩擦扭矩减小；.根据得到的磁流体润滑机械密封中磁场发生器的电流强度与磁流体膜动压之间的关系提出了机械密封的密封性能控制方法，预期可以制造出具有自适应能力的非接触式机械密封性能。