Inner Flow-field Analysis Of A Variable Displacement External Gear Pump

Because of the advantages of high efficiency, low fuel consumption, the variable displacement gear pump has become the main trend of the engine oil pump. However, due to the existence of the non-full meshing state of the variable displacement gear pump, it is different from the traditional fixed displacement gear pump. The problem of cavitation and trapped oil in the variable displacement pump is more prominent, which will reduce the effect of lubrication. In order to improve the overall performance of the variable displacement gear pump and provide reference for the further optimization design a three dimentional CFD model of variable displacement gear pump is established. And cavitation and flow characteristics of the variable displacement gear pump are analyzed. The optimal unloading groove is designed, and the influence of the design parameters of the gears on the transport properties of the fluid is studied. The major research work includes:Firstly, the quantitative analysis method is used to analyze the inner flow-field of the model of variable displacement gear pump because of the characteristics of variable displacement gear pump at different rotational speed corresponding to different gear meshing width. The tooth width formula of gear meshing of the variable displacement gear pump is obtained. The established model and the formula are verified by tested data. The cavitation characteristics of variable displacement gear pump are analyzed based on the verified model. The cavitation of variable displacement gear pump is mainly occurred in the oil chamber region. And the influence of different factors on the cavitation characteristics of the pump is also studied. Then, three kinds of main gap leakage of variable displacement gear pump are analyzed. The influence of different gap on the flow-field of the pump is studied.Secondly, the trapped oil phenomenon of the variable displacement gear pump is studied, and the unloading groove is optimized. By analyzing the process of the variable displacement gear pump the volume of trapped oil region is solved, which provide theoretical basis for the analysis of the trapped oil of variable displacement gear pump. The pressure fluctuation of the trapped oil region is analyzed after simulation of the CFD model. The influence of different factors on the pressure fluctuation of the trapped oil region is also studied. Based on the operating mechanism of the variable displacement gear pump the structure of wedged unloading groove is designed, and the optimal unloading groove is selected by the orthogonal experimental method. By comparing the simulation results with adding new unloading groove, the effectiveness of the unloading groove is verified. The results show that, under the condition of same pump flux, the optimized unloading groove can reduce the pressure fluctuations in the suction and discharge chambers by 9.9% and 8.3%.Thirdly, by studying gear parameters of the number of teeth, modulus, pressure angle, tooth width, overlap coefficient, modification coefficient and addendum coefficient, the influence of the gear parameters on the fluid transport characteristics of the pump is obtained. And according to the conclusion the new gears are designed. The variable displacement gear pump with new gears is simulated. The results show that the new pump model can improve the output volume by 3.3% and reduce the flow fluctuation coefficient by 2.1%.The aim of this thesis is to establish a fluid simulation and optimization procedure of variable displacement gear pump. The proposed approach and drawn conclusions can be directly applied to the design of other variable displacement gear pump. And they can also provide certain technical support for the theory research and design of variable displacement gear pump.