Hydrodynamics of High Specific Power Pumps for Off-Design Operating Conditions

Part load operation of high energy centrifugal pumps is associated with increased vibration levels which may adversely affect the machines operational safety. The main sources of vibrations in centrifugal pumps are of hydrodynamic nature. The interaction between rotating and stationary flow fields yields spatially distributed pressure fluctuation patterns, which excites mechanical vibrations of the rotating and stationary pump components. As virtually all modern, high-energy machines are operated using variable speed drives, an operation, where the excitation by rotor-stator interaction pressure fluctuations matches one of the natural frequencies of the impeller or the pumps stationary components, appears to be unavoidable during the impeller life span. This may yield, provided these vibrations are not sufficiently damped, structural damage to the centrifugal pump components. Moreover, at off-design operation of centrifugal pumps, further phenomena are superposed to the Rotor-Stator interaction effects. Local flow separations affect the flow structure in the hydraulic components, modifying existing and creating supplementary hydraulic forces as excitations for deformations and vibrations at additional frequencies. In the present work, experimental investigations of part-load hydrodynamic phenomena as sources of mechanical excitations in a conventional, high-energy centrifugal pump stage are presented. These investigations include unsteady pressure fluctuation measurements in the rotating and stationary elements of the investigated pump stage, performed at different rotational speeds and operating points. The unsteady pressure measurements were accompanied by measurements of impeller deformations using strain gauges embedded in the impeller shroud wall. The acquired data have been completed by measurements of shaft and bearing housing vibrations. The measurements at part-load operation unveiled the existence of stationary and rotating instabilities in the diffuser of the pump stage, referred as stall. Stationary stall, which expresses itself as a non-rotating high pressure zone in the Rotor-Stator interface, yields additional pressure fluctuations at the rotational frequency and its harmonics. At specific relative flow rates, this high pressure pattern begins to rotate around the impeller circumference. This slowly rotating high pressure zone has a dramatic impact on the mechanical behavior of the pump stage. On the one hand, it forms the highest contribution to the impeller shroud strain, generating deformations several times higher than the ones generated by rotor-stator interaction pressure fluctuations. The perturbations in the circumferential pressure distribution yield further a radial net force, slowly rotating around the impeller circumference. This effect has been identified using the shaft vibration measurements, where it can be shown, that the shaft centerline displacement directly follows the radial force direction. This can negatively affect the rotor system dynamic stability but can also be used to detect rotating stall during the operation of the pump, as external pressure fluctuation measurements not always allow the detection of rotating stall. The impeller blade passage through the stall zone strongly affects the flow in the impeller side rooms. The entrainment of fluid with low circumferential velocity reduces strongly the rotation of the flow in the side rooms and thus affects the steady pressure distribution and the axial net force. The blade passage through the detachment zone adds periodic variations of the axial force acting on the impeller, which can be identified in the axial shaft vibration signature, allowing a detection of stationary stall by a thorough analysis of the axial shaft vibrations.

Multiscale Computational Methodology Applied to Hydroacoustic Resonance in Cavitating Pipe Flow

Nicolas Ruchonnet

The present work is a contribution to the physical analysis and numerical simulation of the pressure surges in hydraulic machinery and connected conduit systems. Localized hydrodynamic instabilities including cavitation are prone to interact with the entire conduit through the propagation of acoustic plane waves. At resonance, the superimposition of the acoustic waves leads to the formation of large amplitudes standing waves along the entire conduit. The resulting fluctuation of velocity and pressure in the source region may have a significant role on the hydrodynamic instability. A computational methodology based on two fields is proposed to simulate this interaction in the time domain: a 1D hydroacoustic model (HA model) is selected to analyze the entire acoustic field including the source region, a 3D incompressible hydrodynamic model (HD model) is used to describe the flow in the source region. The acoustic perturbation due to the instability is precisely evaluated with the HD model and injected in the HA model through discrete sources. To describe the complete interaction between the fields, two methods are proposed: the acoustic feedback is either fully modeled (two way coupled simulation) or accounted for using interaction parameters (one way concurrent simulation). In the first method, the boundary conditions of the HD model are adjusted dynamically using the solution field of the HA model and all components of the sources are evaluated in the HD model. In the second method, the acoustics and hydrodynamics components of the sources are considered as independent. The components due to the hydrodynamic field are evaluated in the HD model with steady boundary conditions and injected in HA model through discrete sources. The components of the sources due to the acoustic fluctuations are accounted for with specific parameters of the HA model; those interaction parameters, i.e. cavity compliance and mass flow gain factor, are evaluated with the help of the HD model. A reference case study has been setup; video analysis and dynamic pressure measurement have been performed to validate the simulations. The case study consists in a straight pipe connecting two constant pressure tanks. A bluff body is placed at 3/4 of the pipe length, the resulting flow instability is characterized by the alternate shedding of vortices at a frequency proportional to the flow velocity. The hydroacoustic resonator is the pipe itself. Measurements have been performed for resonant and non-resonant conditions in cavitating and cavitation free flow regime. In cavitation free flow regime, the hydrodynamic source is identified as a pure momentum source associated with the drag force on the bluff body. The flow conditions leading to resonance can be evaluated with the two way coupled simulation. At resonance, the distribution, frequency and amplitude of pressure fluctuation predicted in the simulation is in good agreement with the measurement. For the selected case study, the acoustic feedback is very weak and has no significant effect on the momentum source. In cavitating flow regime, two hydrodynamic sources have been identified, the momentum source and the mass source. The momentum source is associated with the drag force on the bluff body, the mass source is associated with the volume fluctuation of the fl vapor phase in the wake of the bluff body. The strength of the mass source is orders of magnitude larger and the momentum source can therefore be neglected. One way concurrent simulations using dynamic update of the interaction parameters have been performed. Fair agreement is obtained between the simulations and the measurements. The amplification of the fluctuation observed experimentally below incipient cavitation is reproduced. At intermediate cavitation index, the pressure fluctuation amplitude and the spectral energy distribution is in fair agreement with the experiment. The modifications of the pipe eigenmodes and eigenfrequencies due to cavitation is satisfactorily reproduced with the cavity compliance.

EPFL2010

The effect of pressure gradient on the collapse of cavitation bubbles in normal and reduced gravity

Marc Tinguely

When water flows through hydraulic turbomachines, the local pressure can become low enough to vaporize the water and create vapor cavities. This phenomenon is called cavitation. When the cavities collapse, shock waves and liquid jets traveling through the inclusions can erode nearby solid surfaces. The collapse of cavitation bubbles has been extensively investigated in the case of a single bubble in a liquid at rest. However, in the case of hydraulic machines, the bubbles collapse in a flowing liquid subject to strong pressure gradients. The objective of this thesis is thus to investigate the effect of the pressure gradient on the collapse of a spherical cavitation bubble. We have performed a preliminary investigation of bubble dynamics in a flowing liquid with a pressure gradient. To this end, we have placed a Naca0009 hydrofoil in the test section of EPFL High Speed Cavitation Tunnel and used a high energy pulsed laser focusing technique to generate a single vapor bubble close to the hydrofoil’s leading edge. We have observed a significant influence of the pressure gradient on the bubble dynamics. Particularly, if the collapse phase occurs near the minimum pressure point, the microjet is no more directed towards the solid surface but towards the lower pressure zone in the stream wise direction. We have also observed a peculiar feature of a cluster of bubble dynamics, which behaves almost similar to a single bubble, exhibiting a microjet during its collapse. These unprecedented re- sults are of major importance for a better understanding of the cavitation erosion mechanism in hydraulic systems. The qualitative results obtained in the cavitation tunnel led to the investigation of the effect of a constant pressure gradient on the collapse of the bubble. An experimental setup is built to observe the dynamics of the bubble in water, subject to the gravity induced hydrostatic pressure gradient, and to measure the pressure fluctuation due to the shock waves. The bubbles are generated with a high energy pulsed laser and recorded with a high speed camera. The experimental setup is taken onboard parabolic flights. The parabolic manoeuvres allow the gravity level to be varied in the plane, which modulates the intensity of the pressure gradient in the liquid. The high speed movies taken during the flights reveal that vapor jets appear with the rebound bubble. An empirical law for the prediction of the volume of the jet is deducted from the experimental results. The volume of the jet, normalized with the volume of the rebound bubble, is found to be proportional to a non dimensional parameter ζ0=|∇p|Rmax/Δp, where ∇p is the pressure gradient, Rmax is the maximal bubble radius and Δp is the driving pressure. This dependance is enforced by a theoretical development based on the concept of the Kelvin impulse. Moreover, we identify a threshold for the apparition of the vapor jet: ζ > 4 · 10 − 4 . A new approach for the study of the bubble collapse is proposed: we look at how the energy in the initial cavitation bubble is partitioned between the collapse channels, namely the rebound, the shock wave, the jet, and the luminescence. The microgravity phases of the parabolic flights prevent the apparition of the jet. The collapse of the bubble, in this case, is perfectly spherically symmetric. Moreover, the energy dissipated through luminescence is negligible. Therefore, the study reduces to the energy partition between rebound and shock wave. The measurements uncover a systematic pressure dependence of the energy partition between rebound and shock. We demonstrate that these observations agree with a physical model relying on a first-order approximation of the liquid compressibility and an adiabatic treatment of the non-condensable gas inside the bubble. Using this model, we find that the energy partition between rebound and shock is dictated by a single non-dimensional parameter ξ=Δp(γ^6)/(pg0^1γ)/(ρc^2)^(1−1/γ), where γ is the adiabatic index of the non-condensable gas, pg0 is the pressure of the non-condensable gas at the maximal bubble radius, ρ is the liquid density, and c is the speed of sound in the liquid.

EPFL2013

Simulation tool linking hydroelectric production sites and electrical networks

Philippe Allenbach, François Avellan, Christophe Nicolet, Jean-Jacques Simond

Since many years the EPFL Laboratory of Electrical Machines develops SIMSEN, a numerical software package for the simulation of electrical power systems and adjustable speed drives, in transient and steady-state conditions, having an arbitrary topology. SIMSEN is based on a collection of modules, each corresponding to one system component (machines, converters, transformers, control devices, etc.). For more information, one refers to: http//simsen.epfl.ch. This contribution presents the extension of SIMSEN to the hydraulic components of a hydroelectric power plant including pump-turbine, valve, penstock, surge tank, gallery, reservoir etc. The basic idea is to define for each hydraulic component an equivalent electric component which can be introduced in the existing electric version of SIMSEN. Doing this, it becomes possible to use the modularity of the electric version to define the complete topology of the hydroelectric power plant and the connected electrical network. This extension makes possible a numerical simulation taking precisely into account the interactions between the hydraulic and the electric parts of the system during transients. It is therefore useful for transient and stability analyses as well as for the design optimization.