Solution and Release of Air in Hydraulic Fluids

对流体动力系统的需求不断增加，导致考虑压力介质的优化潜力。特别地，流体内气泡的存在影响压力流体的性质，并因此导致改变的系统行为。由于液压油中不可避免地存在溶解的空气，当静压下降时，空气的形成是不可避免的。这种所谓的气体空化是一种扩散过程，因此与时间有关。直到现在，还没有允许计算这种时间依赖性的模型，因此它在动态系统仿真中的集成是不可用的。因此，本项目的目的是研究液压流体中溶解空气和夹带空气的压力和时间依赖性的吸收和解吸。为了达到这个目标，首先考虑存在于液体内部的单个气泡。一个模拟模型的开发，允许通过液体的传质和通过气泡壁的相变通过使用传输方程的计算。该模拟所需的参数，描述的平衡状态，传质和随时间变化的扩散，实验研究通过使用三个测试台。其中，空气用于气相，不同的液压油形成连续相。最后将测量的参数应用到模拟中，并通过额外的测量对计算模型进行验证。在项目结束时，系统测量的流体参数存在，这些参数表征了不同油的平衡状态以及随时间变化的扩散过程。此外，还提供了一个经过验证的动态模拟模型，可以考虑不同的影响因素和机制，如静压和压力变化速度，用于计算溶解和夹带空气的时间依赖性吸收和解吸。

Increasing demands on fluid power systems lead to the consideration of the pressure medium for optimization potential. Particularly the existence of air bubbles inside the fluid affects the properties of the pressure fluid and consequently leads to a modified system behaviour. The formation of air is unavoidable when static pressure drops due to the inevitable presence of dissolved air in hydraulic fluids. This so-called gas cavitation is a diffusion process and therefore time dependent. Models allowing the calculation of this time dependency and hence its integration in dynamic system simulation are not available until now.Therefore, the purpose of this project is the investigation of the pressure and time dependent absorption and desorption of dissolved air and entrained air in hydraulic fluids. To reach this goal, first a single bubble existing inside a liquid is considered. A simulation model is developed that allows the calculation of the mass transfer through the liquid and the phase change through the bubble wall by the use of transport equations. The parameters needed for this simulation, describing the equilibrium state, mass transfer and the time dependent diffusion, are experimentally investigated by the use of three test-rigs. In these, air is used for the gaseous phase and different hydraulic oils form the continuous phase. The measured parameters are finally implemented into the simulation and the calculation model is validated with additional measurements.At the end of the project, systematically measured fluid parameters exist that characterize the equilibrium state as well as the time dependent diffusion process for different oils. In addition, a validated dynamic simulation model is available that allows the consideration of different influencing factors and mechanisms, such as the static pressure and the velocity of the pressure change, for the calculation of the time dependent absorption and desorption of dissolved and entrained air.