One-dimensional positive displacement pump description in cavitating conditions

Student thesis: Doctoral Thesis

Abstract

The scope of the research contained in this thesis is to develop a new mathematical algorithm to model complex pump systems, capturing cavitation and pump behaviour. The approach consists of the solution of the hyperbolic wave equation, including compressibility and multiphase conditions. The second phase contains non-condensable gas and vapour formation, both important in the density and speed of sound variation. The adopted solution scheme is a finite volume method with a Monotonic Upwind Scheme for Conservational Law (MUSCL). This algorithm is second-order accurate in time and space, with a total variation diminishing (TVD) scheme to prevent spurious oscillation. In order to introduce a dissipation due to friction at the wall in a quasi-steady formulation, a source term is solved with a splitting method. To validate the code, the new simulation methodology was first applied to transient flow in a straight pipe with water hammer. The results were compared with results from pre-existing methodologies available in the literature. Thereafter, the algorithm was applied to a single chamber positive displacement diaphragm pump and then to a triplex diaphragm pump and the results compared with experimental data from an industrial test rig for both single chamber pump and multiple triplex pump network. The simulations coped with a wide range of working pump conditions and were capable of giving information on pressure pulsation, mass flow rate and volume fraction of the vapour formation inside the entire domain. The results modelled correctly the main pump behaviour especially for low cavitation formation, although cavitation was underestimated in same cases. Moreover, differences were evident out in the case of high pump rotation speed where the vapour formation also affected the discharge phase. For that condition, the algorithm was not able to perform correctly, limiting the use of the code. The algorithm may be easily extended to different positive displacement pump configurations, including a diaphragm pump where different liquids are on the driven and driving sides of the diaphragm. Such a hydraulically driven diaphragm pump requires an intermediate flow which transfers the information from the piston to the membrane. This may be embedded in the algorithm. The capability of the new algorithm to cope with different design layouts to work as a pre-design tool has been highlighted as has its ability to simulate not only the pump behaviour but also the system network response to which the pump is attached. From an industrial point of view, a reduction in terms of simulation effort with high fidelity results permits a reduction in costs for the design process and an improvement in the knowledge of a pump's operating process. Moreover, it is possible to include the practical operation characteristic, often neglected, permitting a better estimate of the NPSHR by simulation.
Date of Award14 Dec 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorMatthew Stickland (Supervisor) & William Dempster (Supervisor)

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