The aim of this paper is to present the results obtained with a 3-D numerical method allowing the prediction of the cavitation behavior of a centrifugal pump and to compare this prediction to model tests. The influence of the diffuser on the pump performances, for a cavitating flow, is taken into account by performing coupled computations. The proposed method, which allows the performance drop prediction, consists of assuming the cavity interface as a free surface boundary of the computation domain and in computing the single phase flow. The unknown shape of the interface is determined using an iterative procedure matching the cavity surface to a constant pressure boundary (pv). The originality of the method is that the adaptation process is done apart from the flow calculation, allowing to use any available code.

The objective is to define a prediction and transposition model for cavitation erosion. Experiments were conducted to determine the energy spectrum associated with a leading edge cavitation. Two fundamental parameters have been measured on a symmetrical hydrofoil for a wide range of flow conditions: the volume of every transient vapor cavity and its respective rate of production. The generation process of transient vapor cavities is ruled by a Strouhal-like law related to the cavity size. The analysis of the vapor volume data demonstrated that vapor vortices can be assimilated to spherical cavities. Results are valid for both the steady and unsteady cavitation behaviors, this latter being peculiar besides due to the existence of distinct volumes produced at specific shedding rates. The fluid energy spectrum is formulated and related to the flow parameters. Comparison with the material deformation energy spectrum shows a remarkable proportionality relationship defined upon the collapse efficiency coefficient. The erosive power term, formerly suggested as the ground component of the prediction model, is derived taking into account the damaging threshold energy of the material. An erosive efficiency coefficient is introduced on this basis that allows to quantify the erosive potential of a cavitation situation for a given material. A formula for localization of erosion is proposed that completes the prediction model. Finally, a procedure is described for geometrical scale and flow velocity transpositions.

The behavior of the flow in a vertical semi-axial mixed flow pump has been analyzed by numerical flow simulations of the entire stage, and the results have been compared to test data. As the flow is expected to be unsteady at part load in such a pump, the steady-state simulations were complemented with unsteady flow simulations of the entire machine at one part load operating point. Pressure measurements at different locations in the casing of the pump provided valuable data for the validation of the calculated pressure head. This paper shows that the pump characteristic can be quite accurately predicted from full load to part load by modern numerical tools. Simulations of the unsteady flow, which use much more computer resources, are also feasible in an industrial environment and yield detailed information about the flow patterns and pressure fluctuations in the pump.

In centrifugal pumps, the interaction between the rotating impeller and the stationary diffuser generates specific pressure fluctuation patterns. When the pump is operated at off design conditions, these pressure fluctuations increase. The resulting rise of mechanical vibration levels may negatively affect the operational performance and the life span of mechanical components. This paper presents detailed pressure fluctuation measurements performed in a high speed centrifugal pump stage at full scale at various operating conditions. The impeller and stationary part (diffuser, exit chamber) of the pump stage have been equipped with piezo-resistive miniature pressure sensors. The measured data in the impeller have been acquired using a newly developed onboard data acquisition system, designed for rotational speeds up to 6000 rpm. The measurements have been performed synchronously in the rotating and stationary domains. The analysis of pressure fluctuations at the impeller blade trailing edge, which had significantly larger amplitudes as the pressure fluctuations in the stationary domain, allowed the detection and exploration of stalled channels in the vaned diffuser. This stall may be stationary or rotating with different rotational speeds and number of stalled channels, depending on the relative flow rate and the rotational speed of the pump. The stall yields pressure fluctuations at frequencies which are multiples of the rotational speed of the impeller and generates additional sources of mechanical excitation. [6] performed pressure fluctuation measurements in the impeller and different diffusers at varying radial gaps between the impeller and the diffuser. He observed sidebands in the impeller pressure fluctuation spectra generated by a modulation between the vane passing frequency and the rotation frequency due to an uneven circumferential pressure distribution. Guo and Maruta [7] performed pressure fluctuation measurements in a centrifugal pump impeller and they found at high flow rates sidebands in the pressure fluctuation spectra. These sidebands were also a result of a modulation of the vane passing frequency with an uneven circumferential pressure distribution. Eisele et al. [8] used LDV and PTV techniques for a detailed flow analysis in a centrifugal pump diffuser at different operating points of the pump. At part-load flow rate they observed recirculation from the diffuser back into the impeller. Sano et al. [9] used CFD for the numerical simulation of the flow in a diffuser connected to an impeller using a moving grid method. The calculations were made "quasi" 2D, the mesh in the ground view was only one element high. Resulting from the simulation, diffuser rotating stall rotating with 10% of the rotational speed has been found. The simulated flow pattern was in accordance with the measurements of Hergt and Benner [5], Sinha et al. [10] and Wang and Tsukamoto [11]. While in the case studied by Sinha the gap between diffuser and impeller vanes was relatively large (20% of impeller radius), in ...

In a centrifugal pump the interaction between the rotating impeller pressure field and the stationary diffuser pressure field yields pressure fluctuations as the result of a modulation process. These fluctuations may induce hydroacoustic pressure fluctuations in the exit chamber of the pump and could cause unacceptable vibrations. This paper presents a methodology for the prediction of hydroacoustic pressure fluctuations resulting from rotor-stator interaction in a multistage centrifugal pump. The method consists in the one-way coupling of incompressible CFD and hydroacoustic simulations. In a first step the rotorstator pressure fluctuations are calculated using a commercial 3D-RANS CFD-code (CFX 10) for different flow rates. The acoustic simulations are performed in two consecutive steps. Initially a free oscillation analysis using white noise pressure fluctuations is performed, which provides hydroacoustic eigen frequencies and mode shapes of the outlet casing. In a second step the spatially distributed pressure fluctuations from the CFD simulation are used to perform a forced oscillation analysis. This approach allows the prediction of possible standing waves in the hydraulic collection elements in the last stage of multistage pumps. NOMENCLATURE