Unsteady numerical simulation of an air-operated piston pump for lubricating greases using dynamic meshes

This paper presents the numerical simulation of an air-operated piston pump for lubricating greases. The model, based on the dynamic mesh technique, describes the unsteady displacement of the piston using a layering algorithm. The pump driven velocity has been modified using a parametric user defined function (UDF) that covers the whole range of operating points for the pump, as a function of the discharged outlet pressure. The leakage flow across the feeder plate gap has been analyzed in detail, monitoring its evolution in both forward and backward strokes as a function of the pressure in the adjacent chambers. The comparison between the numerical results and the predictions from the theoretical formulations in the literature for non-newtonian fluids in annular ducts has confirmed the accuracy of the simulation and the correct selection of the discretization for small gaps and inner passages. In addition, the analysis of the flow delivered overtime has provided the numerical distribution and mean values expected from the experimental performance curves supplied by the manufacturer. Finally, the flow conditions in the aspirating region of the pump have been studied in terms of dynamic viscosity and grease fluidity. It has been observed that the fluidity is restricted to those zones presenting large areas of velocity gradients, like the feeder gap and the vicinity of the aspirating inlet, especially close to the solid walls of pump and drum. For the intermediate positions of both advancement and returning stages, when the piston velocity is maximum, the obtained maps reveal that the induced suction reach significant portions of grease within the drum, similar in distance to the pump diameter, and confirms the order of magnitude estimated with an inspectional analysis of the governing equations of grease motion in the aspirating zone. (C) 2011 Elsevier Ltd. All rights reserved.