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

A computational fluid dynamics model of the middle chamber of a triplex positive displacement reciprocating pump is presented to assess the feasibility of a transient numerical method to investigate the performance of the pump throughout the 180 of crank rotation of the inlet stroke. The paper also investigates, by means of computational fluid dynamics, the pressure drop occurring in the pump chamber during the first part of the inlet stroke in order to gain a better understanding of the mechanisms leading to cavitation. The model includes the compressibility of the working fluid and the lift of the inlet valve as a function of the pressure field on the inlet valve surfaces. It also takes into account the valve spring preload in the overall balance of forces moving the valve. Simulation of the valve motion was achieved by providing the solver with two user-defined functions. The plunger lift–time history was defined by the crank diameter and connecting rod length. This paper will demonstrate the feasibility and reasonable accuracy of the method adopted by comparison with experimental data

A transient multiphase CFD (computational fluid dynamics) model was set up to investigate the main causes which lead to cavitation in PD (positive displacement) reciprocating pumps. Many authors agree on distinguishing two different types of cavitation affecting PD pumps: flow induced cavitation and cavitation due to expansion. The flow induced cavitation affects the zones of high fluid velocity and consequent low static pressure e.g. the valve-seat volume gap while the cavitation due to expansion can be detected in zones where the decompression effects are important e.g. in the vicinity of the plunger. This second factor is a distinctive feature of PD pumps since other devices such as centrifugal pumps are only affected by the flow induced type. Unlike what has been published in the technical literature to date, where analysis of positive displacement pumps are based exclusively on experimental or analytic methods, the work presented in this paper is based entirely on a CFD approach, it discusses the appearance and the dynamics of these two phenomena throughout an entire pumping cycle pointing out the potential of CFD techniques in studying the causes of cavitation and assessing the consequent loss of performance in positive displacement pumps.

Abstract:An advanced transient CFD model of a positive displacement reciprocating pump was created to study its behavior and performance in cavitating condition during the inlet stroke. The "full" cavitation model developed by Singhal et al. was utilized, and a sensitivity analysis test on two air mass fraction amounts (1.5 and 15 parts per million) was carried out to study the influence of the dissolved air content in water on the cavitation phenomenon. The model was equipped with user defined functions to introduce the liquid compressibility, which stabilizes the simulation, and to handle the two-way coupling between the pressure field and the inlet valve lift history. Estimation of the performance is also presented in both cases.

A CFD test case of a positive displacement reciprocating pump is presented to demonstrate the capability and benefits which numerical analysis may bring to designers in terms of information on fluid dynamic fields useful to optimize the geometry of the discussed device in all the different operating conditions; even in the worst operating conditions when cavitation appears. The paper discusses the role of design parameters such as the inlet valve shape, mass and spring preload in full cavitating conditions. The comprehensive CFD model makes use of the Singhal et al.(1) cavitation algorithm in conjunction with an Eulerian multiphase model. User defined functions add a few more functionalities to the CFD solver such as the valve dynamics model and the compressibility of water. For each of the cases, the work presented shows the capability of the CFD technique to predict quantitative results such as the volumetric efficiency loss and amount of water vapour generated when cavitation arises. Providing pump designers with this information before the design process has come to an end would give them the possibility to improve the operational life of the device as well as its efficiency. It would also result in a more economic and competitive device on the market.