| The external gear pump is broadly used in the hydrostatic transmission systemsdue to its structure simplicity and operating robustness. Along with the increasingindustrial and environmental requirements to the hydraulic machines, the gearpumps are moving towards the direction of low noise emission, low flow fluctuationand high pressure and speed. Generally due to the limited back pressure and the lowsuction pressure, cavitation is prone to happen in gear pump, negatively affecting itsoperating characteristics. Regarding the limitations of gas cavitation model forhydraulic oils and fluid dynamic model for gear pumps, research is conducted aroundwith two main lines in this thesis, with one indicating the gas evolution process ingear pump and the other indicating the cavitation effects on the pump operatingcharacteristics. By combining the approaches of theoretical modeling, CFD simulationand experimental analysis, a cavitation evolution model in the multi-connectedvolumes of gear pump based on the lumped parameter method is constructed and thegas evaluating process is revealed. This work provides a powerful tool for furtherenhancing the gear pump design methods and investigating the influence of cavitationon other physical domains existing in gear pumps, thus implying important theoreticaland practical value.On the basis of lumped parameter approach, a dynamic gas-oil cavitation modelfor the closed volume is proposed, accounting for the dynamic features of air releaseand absorption. Then the constitutive relations between oil properties (such asdensity and fluid bulk modulus) and gas content are derived. The proposed model isvalidated by experimental tests and is used to explain the “hysteresis” phenomenonexhibiting in fluid density during the compression/expansion cycle. Results showdensity is mainly affected by gas mass fraction and bulk modulus is mainly affected bygas volumetric fraction; air release velocity is bigger than air absorption velocity inhydraulic oils.The dynamic cavitation model is extended to the condition of open volume, inwhich not only air release and absorption but also the external mass exchangebetween the adjacent volumes and the volume change are taken into account. For this reason, the gas evolution equation is derived for open volume. A lumped parametermodel for the throttling segment in pump inlet is formulated using this equation. Thelumped parameter model is then applied in the study of the two-phase critical floweffects and the influence of throat diameter, upstream pressure and air releasecoefficient on the critical pressure and limited pump speed of oil supply is obtained.Results show the fluid velocity is equal to the sound speed in the throat region underthe critical condition. Reducing the air release velocity has an important effect forimproving the pump operation.In terms of the multi-volume flow field in the external gear pump, a cavitationevolution model for gear pump is developed to explore the dynamic process of airrelease, transport, distribution and absorption. From this model, the internal pressureand gas content in tooth space volume of gear pump is analyzed. The correlationbetween gas evolution and the pressure built-up, volumetric efficiency and the flowpulsation is demonstrated. It shows an increase of internal leakage and pressurepulsation and a decrease of volumetric efficiency in the cavitating gear pump.In the end, the influence of cavitation on the thermodynamic behaviors of theopen-looped gear pump circuit is studied. The heat rejection rate of the components inthe loop is calculated. Considering the gas evolution in tooth space volume of gearpump, the additional heat induced by cavitation can be determined. By using thethermal-hydraulic equations of the hydraulic system, a thermal-hydraulic model forthe pump circuit is built and the change of oil temperature is analyzed. It is found thatthe heat induced by cavitation comes from the work done for compressing thereleased air, resulting in a faster increase of oil temperature in the pump circuit. |
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