A Multiphase Pump Experimental Analysis

Abstract

This research project focuses on the characterization of the behaviour of a mixed-flow pump under off-design conditions and two-phase operation, in terms of performance degradation and instabilities. The recent multiphase pump models allow to process higher and higher amounts of gas in variable operating conditions. An improved understanding of the fundamental physics is therefore necessary, as, due to the concurrent physics involved and transient phenomena, a proper machine characterization cannot be limited to an overall description of the performance, but rather it needs to rely on advanced analysis tools revealing the local flow phenomena. Multiphase flow affects turbomachines operation introducing secondary flows, unsteady phenomena and other disturbances in the flow field. Besides influencing the whole system and causing losses, these disturbances introduce additional issues as vibrations and alternating stresses on the rotodynamic components, which can reduce the machine reliability. The detection of the instabilities inception, a proper definition of the safe operation boundaries and the necessary actions to recover stable operation when instabilities appear are of key importance to prevent shut-down events and failures and ensure an extended components life. The main goal of this investigation consists in the experimental description of the flow regimes in the different performance map zones, aided by flow-visualization; focus is given to the recognition of the flow mechanisms responsible for flow field modifications, which affect the machine stability, and to the relation of the performance and stability variations to the originating phenomena in the hydraulic channels. In order to accomplish these objectives, a novel multiphase pump laboratory has been specifically designed and built. An extensive literature review of the two-phase flow in turbomachines has helped to include room for the necessary investigations and carefully assess their feasibility. The laboratory facility allows a complete optical access to the pump channels and fine adjustments in the inlet configuration and the tip clearance gap. Tests are performed over a wide range of machine, flow and fluid composition parameters. As to the investigation tools employed, local measurements of the main flow parameters and a high-speed camera provide an interesting insight into the unsteady and transient flow phenomena. The numerical tool capability is evaluated under single-phase conditions, while a thorough study of the available two-phase flow simulation models and their experimental validation will be the subject of future studies. The influence of the operating parameters - pump speed, flow rates, mixture composition, suction pressure - on the machine performance and operating range has been extensively studied and the test results confirm most of the expected trends. The original contribution of the current work has been the understanding of the flow mechanisms and their connection to the outer measurements; this is very useful for a future development of a control strategy in real machines On a further stage of the research, it is desirable that the analytical description of the bubble flow and the correlations describing the switch among different flow regimes be validated experimentally for this specific application. Two-phase flow, especially at unsteady conditions, introduces additional challenges which make the use of advanced visualization techniques require a preliminary assessment, which is the object of future studies. Along with image based metrology, these ones promise to provide a qualitative description of the flow field. The Doctoral Thesis is based on five research papers which present the main outcomes of the study and are enclosed in the Appendix section. This report provides the necessary background, starting from the literature study which has led to the choice of the research approach, together with a presentation of the testing rig design and building, the experimental facility and procedure description, the test results and the characterization of the main flow mechanisms involved. It should be noted that, as the main focus is set on the flow visualization, the measurement of the pump efficiency has not been prioritized. A preliminary assessment of the numerical simulation tool is performed on single-phase operation. The Conclusion section presents a summary of the findings accomplished within the timeframe of the current work, along with the suggestion of further analyses and investigations for a deeper understanding of the flow phenomena and machine behaviour.