Unsteady Numerical Simulations of the Flow Related to the Unstable Energy-Discharge Characteristic of a Medium Specific Speed Double Suction Pump
Regions of positive slope in the pressure-discharge characteristics can result in a reduction or even lack of damping of system instabilities. They are therefore one of the major concerns in design and operation of centrifugal pumps as they are limiting the admissible operating range. The considered preliminary hydraulic design of an industrial double suction pump of medium specific speed n = 0.410 (0.205 per impeller side) shows a marked saddle in the energydischarge characteristic associated to a sudden drop of efficiency versus discharge at part load. Unsteady RANS type flow simulations are performed using hexahedral meshes with 2.5 million nodes to model the inlet casing, the shrouded double sided impeller with 2 x 7 blades, the diffuser with 12 blades and the volute. In unsteady solution monitoring, low frequency (below blade passing frequencies) phenomena are noticed. Therefore, simulation times of up to 10 impeller revolutions at each flow rate are requested to achieve statistically steady behavior of the flow judged on global performance numbers and circumferential flow rate distribution. The numerical simulations emphasize a drop in the characteristic, though at a lower flow rate than found on the test rig. It is shown to be associated to a one-sided separation in the diffuser, further leading to an unbalanced flow rate distribution of about 10% of flow rate between both sides of the impeller. There is a region of hysteresis, where both configurations with balanced and unbalanced flow rate distributions can be obtained for the same global flow rate. The asymmetric flow distribution leads to asymmetric velocity profiles at the impeller-diffuser interface which results in a strongly vortical flow in the diffuser channels, where an important amount of energy is dissipated in regions of increased viscous and turbulent shear.
Investigations of time dependent flow phenomena in a turbine and a pump-turbine of Francis type
The present work deals with time dependent phenomena, which develop in turbines and pump turbines of Francis type. These phenomena may cause serious damages to hydropower plant. The overall objective of this study is to bring a better understanding of two kinds of time dependent phenomena such as rotor-stator interaction (RSI) and precessing vortex rope (PVR) by numerical simulations of fluid flows and measurements. Two case-studies appropriate to the investigated unsteady phenomena are selected. The analysis of RSI is performed on the results of a pump-turbine scaled model. A Francis turbine model is selected to investigate PVR in the draft tube. For the flow numerical simulations, CFD package, ANSYS-CFX, is used, which is a finite-volume based flow solver. For the both case studies, the fluid is assumed to be incompressible, and hence the effects of the hydro-acoustic waves are neglected. Prior to performing the numerical simulations, the effects of grid resolutions, time increments, configurations of the computational domains and turbulence model are studied, and thereby, the optimum parameter sets for each of the test cases are found. With respect to rotor-stator interaction, several operating points from part load to full load conditions are investigated. At full load operating point corresponding to a strong rotor-stator interaction, the pressure fluctuations resulting from numerical simulations and measurements are analyzed. The computed results of static pressure fluctuations are validated with the measurements on the casing wall of the spiral casing, the bottom wall of a stay-vane and guide-vane channels for the RSI case and on the wall of the draft tube cone for the PVR case. The predicted fluctuations in the static pressure fit well with the measured counterparts in terms of both magnitude and frequency for RSI and PVR cases, except for several points such as on the wall in the spiral casing. These results confirm that with the exception of vibration condition, cavitation or fluid-structure interaction, the incompressible flow hypothesis is valid. For the RSI simulations, the diametrical mode of the pressure fluctuation is clearly identified by decomposing the results obtained from the numerical simulations. The hydro dynamical reason why two peaks in the pressure fluctuation appear during one passage of the blade pitch is also described. Moreover, a non-dimensional number based on discharge, the period of runner revolution and the time during which, flow passes through the vaneless gap between the guide vanes and the runner blades is proposed. Accordingly, it is found that the magnitudes of the pressure fluctuations well correlate with the proposed function for the three operating conditions investigated from the part load through the full load conditions.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.