| Bubbles have a great influence on the oil characteristic and the working performance of hydraulic components and system, such as reducing the oil elastic modulus, causing cavitation noise, increase the oil temperature rise, etc. is a key basic research subject of fluid power technology. Hydraulic power unit is the "heart" of hydraulic system,75%of the whole system noise volume is flow noise mainly caused by cavitation. Therefore, based on the analysis of bubbles cause harms to the operation of the hydraulic system, and the further study on bubbles flow characteristics in the hydraulic power unit and the distribution law of bubbles, bubble separation methods are proposed, has important reference value and guiding significance to reduce the noise of hydraulic power unit and improve working performance.When hydraulic system devices work under ambient conditions, it is inevitable that air will be entrained into oil, especially during the replacement of hydraulic components. As a result, the oil mixed with air can no longer be regard as the ideal state, and it does not have the basic features-incompressibility, required in the hydraulic transmission. Therefore, in the base of deeply study of the harm caused by bubbles entrained into system, discussing the specific methods discovered in the process of design, use and maintenance of hydraulic system to prevent bubbles from generating has an important guiding significance to the reliability and stability of hydraulic system.This thesis bases on the Integrated Electro-hydraulic Power Unit (IEPU) and systematical study on the flow of bubbles existing in IEPU was done by using the combined method of theoretical analysis, flow field simulation and experimental method. With the flow field simulation of bubbles with different size in oil and different oil viscosity done by using Eulerian-Eulerian Model of Fluent software, the effect factors of bubbles rise were concluded. IEPU test system platform was set up and a series of related experiments about performance test and flow observation was carried out. It was found that oil in the return pipe of the system will carry bubbles of different size into the oil reservoir and quickly disperse throughout the whole reservoir. Aiming at the existing problem of test, the model test was conducted and the bubble flow in oil was recorded by establishing the flow visualization experiment device. Finally, based on the result of bubble flow observation test and the analysis of flow field simulation calculation, three methods to achieve bubbles separation were proposed.The main content of this thesis is as follows: In chapter1, the significance and objective of this thesis were presented. The research methods and the law of the bubbles formation and bubbles motion were reviewed. The main research contents of this thesis were summarized.In chapter2, the structure and the operating principle of the IEPU were briefly introduced. The effect factors of the separation characteristics of air from oil and the rapid rise of bubbles were analyzed. With the flow field simulation of bubbles with different size in oil with different viscosity done by using Eulerian-Eulerian Model of Fluent software, the distribution of bubbles in the IEPU was obtained.In chapter3, the IEPU test system platform has been set up and a series of related experiments on IEPU performance and flow observation were carried out. Aiming at the existing problem of test, the model test has been conducted and the flow process of air bubbles was recorded by the flow visualization experiment installation. Through the contrastive analysis of results of flow visualization experiment and modeling flow field simulation, it shows that oil in the return pipe of the system will carry bubbles of different size into the reservoir and quickly disperse throughout the whole reservoir, larger bubbles can promptly float up and escape from the oil, while smaller ones are sucked into the hydraulic pump along with the oil flow. Moreover, there are bubbles remain in the hydraulic pipes.In chapter4, the research status of methods to separate air from oil was given and some kinds of relevant apparatuses were introduced. Based on the result of bubble flow observation test and the analysis of flow field simulation calculation mentioned above, three methods about changing the flow pattern of oil in the return pipe and the flow characteristics of gas-liquid two-phase flow to achieve rapid separation of bubbles were proposed and the flow field simulation was carried out by using Eulerian-Eulerian Model of software Fluent. Studies show that entrained gas bubbles can be separated from the oil rapidly by altering oil flow pattern to generate swirl flow or turbulent flow.At the end, some conclusions in this thesis are summarized and the future research proposals are suggested. |
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