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
The prediction of the performance characteristic in centrifugal pumps at flow rates other than the design flow is of high relevance but relatively uncertain. In the present study, an evaluation of the existing shut-off head prediction methods and an assessment of both, the empirical approach and the numerical simulation are performed. For this purpose, after a review of existing methodologies, the empirical predictions are compared against available statistical data for single stage volute pumps. The CFD study, performed for 3 different single stage volute pumps with specific speeds between nq = 12 and nq = 54 is quantitatively compared against available model test data, which were obtained using precision manufactured hydraulic components. Several empirical methods from open literature have been reviewed for the shut-off head prediction. An overview of different prediction methods has been presented by Dyson [1], these and an empirical correlation proposed by Guelich have been compared against available data. The numerical study at shut-off condition requires a full-pump geometry, which also includes long suction and pressure pipes. Unlike several analyses present in the literature (Dyson & Texeira [2], Benigni et al. [3], Liu et al. [4]), which consider the internal leakage flow in the outlet boundary, the shut-off condition is obtained by imposing walls at inlet and outlet boundaries, but leakage flow through the gap between casing and impeller which is essential for the recirculation of the flow is numerically modeled. Because of the unsteady nature of the pressure in the pump volute, a transient simulation is performed and the predicted shut-off head is the average of the whole range of results for a minimum of 10 impeller revolutions.
Deformations, mechanical stresses and vibrations in centrifugal pumps are the result of pressure fluctuations, which are acting as excitation forces. When a pump operates at its optimum, the pressure pulsations are at minimum, but for a pump operating in part-load, pressure pulsations increase and subsequent vibration and deformation levels increase. In a recent experimental research, the pressure pulsations and the resulting structural stresses in the last stage impeller of a multistage pump have experimentally investigated for different operating conditions [1]. The experimental investigations have been complemented by transient numerical simulations using a commercial CFD code and structural analysis using the pressure pulsations resulting from the CFD code as boundary conditions. In the present study, a validation of these CFD and FEM simulations is presented. The analysis has been performed in three steps. In the first step, the transient CFD results for different load cases are analyzed and compared with the experimental results in order to evaluate the CFD simulations. In the second step the time domain pressure pulsation data are post-treated and decomposed into a series of rotating pressure waves. These pressure waves are then applied as boundary conditions to an FEM model and one full impeller revolution is simulated as steady calculations for 72 angular positions. The pressure pulsations in the best efficiency point are regularly distributed in space and time and dominated by rotor-stator-interaction. For part-load operation, the pressure distribution becomes more and more unsteady. The CFD results for part load exhibit stationary stall in the diffuser for a flow rate relative to best efficiency point of q* = 0.9 and unsteady stall behavior for a q* = 0.8. While the numerical CFD results agree well with experimental data for q* = 1 and q* = 0.9, at lower part load (q* = 0.8) the CFD didn’t reproduce the experimentally observed flow behavior, especially the rotating stall. The FEM results at design conditions show relatively low tangential stresses at the impeller outlet, which agree well with the measured deformations and stresses.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.