Counter-rotating mixed-flow pumps design and analysis

Abstract The application of the counter-rocating (CR) impeller design on mixed-flow pump was investigated in this chesis with the aim to improve the system power-density. Well known in other turbomachinery applications, the counter-rotating design principle counts as powerful design feature in the development of high energy turbomachineries. The natural adaptability of a independent CR dou-ble shafts driving system is combined to a mixed-flow hydraulic character, which exploits the prerotating velocity on a downstream radial impeller, to intensify the machine energy concentration. A serics of different CR mixed flow pumps were developed and both numerically and expcrimentally tested. Comparison of the CFD predictions with the available experimental data, in terms of hydraulic efficiency and head coefficient showed an acceptable agreement. Rising design head and rotational speed characterize the impeller Prototypes' series, which were hosted in the same volute having the same nominal flow-capacity. Speed ratio and head rise distribution between the front and rear rotors, (FR) and (RR) respectively, were considcred as decisive design parameters. Suction recirculation and cavitalion inception, beside the conventional performance curve, were included to the simulation post process. Reference conventional geometries designed for the same flow capacity, speed and casing were also both numerically and experimentally investigated under same operating conditions, in order to assess the advantages of CR layout in terms of head coefficient. Three different layouts were tested in a closed loop test circuit, where performance measurement and cavication investigations were carried out. The measurement results showed larger head coefficients and an enhanced power-density, deceriorated by a narrow efficiency curve, which reached comparable maximum value. In general unfavourable cavitation behaviour was observed especially at rear roter inlet, where high relative velocity caused a sharp drop of static pressure. On the other hand, the additional degree of freedom of the system represents a key feature of this innovative layout, which was varied to benefit from different flow condicions at off-design operations. Application of such a high-energy density system is multiple in the industrial field, however the double-shaft driving devices represcnt still a limit in terms of manufacturability and costs. Driving system for this kind of pump layout is also proposed together with design guidelines about the hydraulics. This thesis states a first step in the development of CR mixed-flow pumps and counts as one of the few published investigations with experimental validation in this ressarch subject.