Gap Flow Simulation Methods in High Pressure Varia

Authors: Tomasz Zawistowski Michał Kleiber

Publish Date: 2016/04/26

Volume: 24, Issue: 3, Pages: 519-542

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Abstract

High pressure variable displacement axial piston pumps are subject to complex dynamic phenomena Their analysis is difficult additionally complicated by leakage of the working fluid Analytically gap flow is calculated with the Reynolds equation which describes the pressure distribution in a thin lubricating layer The paper presents various approaches to analyze gap flow both in traditional axial piston pump and novel type of hydraulic pump designed at the Polish Gdansk Institute of Technology Because of large aspect ratio between the height of the gap and the size of pump elements the authors present the numerical simulation approach using a local model to define a lubrication gap linked to a global model of a pump from which boundary conditions were imported User defined functions implemented in Fluent and Excel were used to calculate the pressure and velocity fields and assess the fluid flow rateIt is owing to advances in computation technology that numerical simulation of complicated dynamic phenomena became possible Nevertheless not all details can be included in one model One of the reasons could be a large aspect ratio between different parts of the object under study In the case of hydraulic axial piston pumps a difference between the size of gap height and the displacement chamber diameter can reach three orders of magnitude and could render the analysis results unreliable since they would be vitiated by unacceptable errorOne of the ways to bypass a problem of large difference in scale would be a multiscale approach in which objects of considerably different scale size are modeled separately and those models are linked together with boundary conditions That is how we approached the modeling of lubricating gap flow including an energy equation in the model since viscosity of fluid significantly influences the flow rate We show how numerical simulation could enhance the design process of a novel type of high pressure axial piston pumpA hydraulic pump is a mechanical device which transforms mechanical energy into hydraulic energy It generates the flow of hydraulic fluid which is able to overcome the resistance pressure created by the load There are several types of hydraulic pumps gear piston rotary vane and screw pumps The piston pumps could be divided into axial and radial pumps Axial piston pumps could have a swash plate with variable swash angle or bent axis pumpsA piston pump transforms work of pistons that move in the reciprocating motion into pressure which is used to overcome external load through the driven fluid Axial piston pumps have an odd number of pistons that move in cylinders encased in a pump body Rotating cylinder block is driven by a shaft and through contact of a swash plate with piston slippers creates a reciprocating motion of pistons Pistons suck oil from the inlet port and subsequently squeeze fluid creating pressure which drives oil into a high pressure port Piston pumps can have variable or fixed displacementIn bent axis axial piston pumps—a shaft rotates a cylinder block with pistons while the commutation unit and pressure ports are stationary The shaft axis forms an angle with the cylinder block thus causing a reciprocating motion of pistons during the one revolution of the shaft The pistons are sealed with rings like in a typical combustion engine

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