To fill the gap in the literature in terms of numerical studies of positive displacement (PD) pumps in a cavitating condition, a comprehensive and transient computational fluid dynamics (CFD) model of a PD pump, simulating the cavitation arising during the suction stroke, was created. The 'full' cavitation model was utilized to study its capability on PD pump cavitation. A set of three plunger speeds were simulated. Using the highest plunger speed, an assessment was made of the effect of 1.5, 3, 4.5 and 15 parts per million (ppm) of air mass fraction on pump performance and cavitation. An experimental test rig, replicating the CFD model, was designed and built in order to validate the numerical model and find its weaknesses. CFD modeled, in a consistent way, the fluid dynamics phenomena related to cavitation (the chamber pressure approaching the vapor pressure, the vaporization/condensation and the pressure spike occurrence at the end of the suction stroke marking the end of cavitation). On the other hand the CFD pressure trends calculated appeared stretched along the time axis with respect to the experimental data, and this highlighted issues in the multiphase and cavitation models: the vaporization/condensation rate calculated by CFD did not follow the real dynamics correctly because the non-condensable gas expansion was overestimated. This was seen when comparing the CFD/experimental results where the simulated pressure drop gradient at the beginning of the suction stroke and the pressure peaks as the valve closed exhibited a delay in their occurrence. The simulation results were sensitive to the dissolved air mass fraction as the delay depended on the amount of air dissolved in the water. Although the influence of the air mass fraction was considered consistent, the 3 ppm CFD case was the closest to the experimental results, whereas the analyst expected the 15 ppm case to be more accurate.
ARTICLE HISTORY
The design of safety relief valves depends on knowledge of the expected force-lift and flow-lift characteristics at the desired operating conditions of the valve. During valve opening the flow conditions change from seal-leakage type flows to combinations of sub-sonic and supersonic flows It is these highly compressible flow conditions that control the force and flow lift characteristics. This paper reports the use of computational fluid dynamics techniques to investigate the valve characteristics for a conventional spring operated 1/4” safety relief valve designed for gases operating between 10 and 30 bar. The force and flow magnitudes are highly dependent on the lift and geometry of the valve and these characteristics are explained with the aid of the detailed information available from the CFD analysis. Experimental determination of the force and flow lift conditions has also been carried out and a comparison indicates good correspondence between the predictions and the experiment. However, attention requires to be paid to specific aspects of the geometry modeling including corner radii and edge chamfers to ensure satisfactory prediction.
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