A Numerical Approach for the Evaluation of the Effects of Air Release and Vapour Cavitation on Effective Flow Rate of Axial Piston Machines

Abstract

This work illustrates a numerical methodology for the description of effective flow rate of axial piston pumps and motors. The presented mathematical model is similar to classical lumped parameter approaches that are commonly used to simulate hydraulic units; however, this work uniquely utilises a mathematical formulation for compressible flows based on an original description of fluid density. Assuming the behaviour of typical mineral oil, the model can evaluate fluid density for all possible values of pressure while considering the occurrence of gas cavitation (due to the release of air normally dissolved into liquid) below saturation pressure, and of vapour cavitation (due to liquid change of phase) below vapour pressure. The developed simulation model allows a description of several characteristics of the machine (i.e. instantaneous cylinder pressure and density, delivery and inlet flow rates, etc.) in its whole field of operation taking into account conditions of insufficient flow due to cavitation at the low pressure port. Tests were carried out on a swash plate type axial piston pump for open circuit applications to verify potentials of the developed numerical model. Experiments were conducted to test the pump under typical operating conditions as well as situations critical from the point of view of cavitation (high shaft speed, low values of inlet pressure), thus permitting the comparison between the prediction given by the developed model and experimental results over a wide range of data. Results highlight how fluid density changes can be used to characterize effective flow rate but also to justify, in particular operating conditions, the utilization of the approach for compressible flows. Results show that the developed model uniquely allows the calculation of effective flow rate through the pump at fair and extreme conditions, thus permitting the ability to predict limitations of the machine. Furthermore, the realistic prediction of pressures throughout the machine in these conditions leads the accurate predictions of pressure forces and of flow through the lubricating gaps that may be critical in other models.