A Study on the Operation of Gerotor Type Units Considering Fluid Structure and Mechanical Interaction Effects
thesisposted on 03.01.2019 by Matteo Pellegri
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Gerotor units are widely used in low-pressure (up to 30 bar)
fluid power applications, injection and lubrication systems, due to their compact package and low cost. Their performance in terms of volumetric efficiency, flow pulsations, internal pressure peaks or localized cavitation depends on many parameters, such as the rotors profiles and the manufacturing tolerances. In this work, a multi-domain simulation approach for the numerical analysis of the performance of Gerotor units is proposed. The model can be used for analysis and virtual prototyping of units considering the actual geometry of the rotors, their geometrical tolerances and the properties of the working fluid and materials.
The approach is based on the coupling of different models: a numerical geometric model evaluating the instantaneous volumes and flow areas inside the unit; a lumped parameter fluid dynamic model describing the displacing process of the tooth space volumes; and a mechanical model evaluating the internal micro-motions of the rotors axes according to their tolerances. In this way, the model determines the actual loading of the rotors, considering also the actual location of the points of contact. Advanced 2D CFD models for the analysis of lubricated interfaces have been developed to study the axial and the radial gaps. This approach allows an accurate prediction of the power losses and radial micro-motion of the rotors taking into account fluidstructure interaction effects. A line contact model is used to describe the contact interface using an elastohydrodynamic approach and non-Newtonian fluid behavior for the prediction of the friction between the rotors.
Specic experiments were performed by connecting the pump outlet to a variable loading orifice and by measuring the delivered flow, pressure ripple and required torque under dfferent operating conditions of speed and outlet pressure. These experiments allowed a detailed model validation through the comparisons with simulation data in terms of signicant steady-state as well as transient pressure, flow and torque features. After the model validation, gaps compensation solutions to minimize the volumetric losses are developed and studied showing the potential improvements on the performances of a prototype unit by reaching up to 55% volumetric effciency at high pressure (pmax > 100bar).
The approach used in the current work along with its proven accuracy through experiments can be considered a valuable tool when studying the impact of real-life technological clearances and gears geometry on the fluid-dynamic performance of the pump.