Suction dynamics of axial piston pumps

Abstract: Computational fluid dynamics (CFD), extensive experimentalstudies and a one-dimensional model based on two pressurereduction coefficients are used in this thesis to analyse thesuction dynamics of an bent axis axial piston pump. Specialattention is given to cavitation since a great deal of problemsare related to this very important phenomenon.In previous experimental investigations, a simpleone-dimensional mode derived from the Navier-Stokes equationshas proven relatively accurate in predicting when thecavitation starts in different types of positive displacementpumps. The one-dimensional model, which uses two pump-specificpressure-reduction coefficients to describe theviscosity-dependent and density-dependent pressure drop insidethe pump, was extended and thoroughly analysed in thiswork.An extensive experimental analysis of a bent-axis axialpiston pump showed that the one-dimensional model coulddescribe the behaviour of the pump over a fairly wide range ofoperating conditions. Exceptions are at low viscosities incombination with high rotation speeds where the pump deviatesfrom the behaviour predicted by the one-dimensional model.CFD simulations in both two and three dimensions were madeon the same geometry as the axial piston pump used in theexperiments. Due to the rotation of the cylinder block and dueto the piston movement, sliding and deforming grids were usedin the three-dimensional CFD analyse. The two-dimensional CFDsimulations suggested two critical regions inside theinvestigated pump where cavities could start to form, insidethe cylinder during the opening and closing phase onto and fromthe valve plate suction port. The results of thetwo-dimensional CFD simulations must however be treated withsome caution, since the agreement with the experimental resultswas poor. The three-dimensional CFD simulations, which agreesbetter with the experiments, suggested a third critical regioninside the pump where cavities could start to from, the areabehind the moving front wall of the cylinder port. A vortexwith low pressure was generated behind the moving front wall ofthe cylinder during the entire suction phase. Thethree-dimensional CFD simulations offered an explanation forthe pump behaviour in the criticalrange of operatingconditions. When the pump operates under non-criticalconditions the lowest pressure is found inside the cylinderduring the closing phase, but when the pump operates in thecritical range of operating conditions, the lowest pressure isfound in the vortex area behind the moving front wall of thecylinder. Since the one-dimensional model assumes that thecavitation always starts in the same position, such a change inthe cavitation point could be a reasonable explanation to thedeviation.In general, it is demonstrated that CFD is a useful tool inpump design, even though the results from such analyses must betreated with some caution.Keywords:Fluid power, Cavitation, Axial piston pumps,Suction dynamics, Computational fluid dynamics (CFD)