Analyse des erreurs dues à l'écoulement dans la mesure de débit par la méthode acoustique

The precise flow-rate measurement is a basic problem in the management and the exploitation of a hydraulic power-plant. The efficiency characterisation of an hydraulic machinery or of a water distribution net is linked to the accurate knowledge of the flow-rate. As well as the usual measurement methods, acoustic discharge measurement has become of great importance during the last ten years, specially due to the last development in the field of electronic and signal processing. The acoustic flow measurement uncertainties show that geometry, integration method, influence of the flow field and the interaction between the acoustic transducer and the pipe have an important influence on the measurement accuracy. In this work, the influence of the flow field on the accuracy is of great importance. To estimate the uncertainties of the acoustic discharge measurement for disturbed flow fields, laboratory tests are carried out at the hydraulic machinery laboratory LMH in order to qualify the measurement accuracy depending on the upstream distortion induced by different typical pipe configurations. Furthermore, numerical flow computations with software packages are performed to simulate the turbulent flow profiles at the location of the acoustic flow measurement. These simulations are performed for laboratory and also industrial flow configurations. Comparative discharge measurements for typical disturbed flow fields are performed on a special test rig of LMH between a reference electromagnetic flow meter and an 8 paths in two crossed planes acoustic flow-meter. The accuracy analysis, based upon all laboratory measurements shows that globally the measurement quality is excellent even in the case of a heavily disturbed flow field in the meter section. A specific analysis has to be performed at the flow-rate measurement installation in order to take into account the pipe configuration influence (for example an elbow or a gate), the developing length between the meter section and the disturbance and finally the use of crossed planes. Such analysis allows the reduction of the acoustic measurement uncertainties. The protrusion effect can also introduce an important measurement error in the acoustic measurement. Laboratory measurements in the high speed cavitation tunnel of LMH show clearly the influence of the protrusion effect. In such small dimensions in comparison with the industrial pipes, these effects are amplified and they introduce important errors. The second part of this work is based on the simulation of the velocity profiles at the location of the metering section by numerical flow computations. Comparisons between numerical results and Laser Doppler flow measurements show that the simulation and the flow physics results are in agreement with each other. Based on these convincing results, numerical flow computations are performed for an industrial power-plant in order to obtain an approximation of the flow profile in the metering section. Based on these results, an optimisation of the acoustic path installation is proposed in order to limit the uncertainties due to the flow-field in the pipe. This study is applied to the laboratory configurations and allows one to find the optimum measured orientations of the acoustic paths. Finally, this study also applies to an industrial installation.

Dynamics of isolated confined air bubbles in liquid flows through circular microchannels: an experimental and numerical study

Navid Borhani, Sepideh Khodaparast, Mirco Magnini, John Richard Thome

Experimental and numerical studies are performed to characterise the dynamics of isolated confined air bubbles in laminar fully developed liquid flows within channels of diameters d = 0.5 mm and d = 1 mm. Water and glycerol are used as the continuous liquid phase, and therefore, a large range of flow capillary numbers 10(-4) < Ca < 10(-1) and Reynolds numbers 10(-3) < Re < 10(3) are covered. An extensive investigation is performed on the effect of bubble size and flow capillary number on different flow parameters, such as the shape and velocity of bubbles, thickness of the liquid film formed between the bubbles and the channel wall, and the development lengths in front and at the back of the bubbles. The micro-particle shadow velocimetry technique (mu PSV) is employed in the experimental measurements allowing simultaneous quantification of important flow parameters using a single sequence of high-speed greyscale images recorded at each test condition. Bubble volume and flow rate of the continuous liquid phase are precisely determined in the post-processing stage using the mu PSV images. These parameters are then used as initial and boundary conditions to set up CFD simulations reproducing the corresponding two-phase flow. Simulations based on the volume of fluid technique with the aim of capturing the interface dynamics are performed with both ANSYS Fluent v. 14.5, here augmented by implementing self-defined functions to improve the accuracy of the surface tension force estimation, and ESI OpenFOAM v. 2.1.1. The present approach not only results in valuable findings on the underlying physics involved in the problem of interest but also allows us to directly compare and validate results that are currently obtained by the experimental and computational methods. It is believed that similar methodology can be employed to rigorously investigate more complex two-phase flow regimes in micro-geometries.

Springer Verlag2015

Application of micro particle shadow velocimetry mu PSV to two-phase flows in microchannels

Navid Borhani, Sepideh Khodaparast, John Richard Thome

Micro particle shadow velocimetry (mu PSV) is performed in the present study for simultaneous velocity measurement and interface tracking in both liquid-liquid and gas-liquid two-phase flows through circular microchannels of 5001 mu m diameter. The back-lit illumination using a non-coherent LED light source, combined with full refractive index matching of the liquid phases, the tube wall material and the channel exterior medium, allowed velocimetry to be done both within and around the liquid droplets, and even close to the interfaces and boundaries. Moreover, post-processing methods are proposed and implemented in order to resolve motion of isolated gas bubbles and immiscible liquid droplets in laminar flows quantitatively. In particular, simultaneous measurements of local instantaneous phase velocities and flow rates, liquid film dynamics and its thickness, shape and volume of the dispersed phase, and development length in front and at the back of the bubbles are obtained using one single sequence of gray scale shadowgraphy images. Such results are valuable for validation of corresponding numerical simulation codes. It is believed that this approach significantly reduces the size and cost of the experimental setup while increasing the accuracy and reliability of the measurements. (C) 2014 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2014

Part load flow in radial centrifugal pumps

Olivier Braun

Centrifugal pumps are required to sustain a stable operation of the system they support under all operating conditions. Minor modifications of the surfaces defining the pump's water passage can influence the tendency to unstable system operation significantly. The action of such modifications on the flow are yet not fully understood, leading to costly trial and error approaches in the solution of instability problems. The part-load flow in centrifugal pumps is inherently time-dependent due to the interaction of the rotating impeller with the stationary diffuser (Rotor-Stator Interaction, RSI). Furthermore, adverse pressure gradients in the pump diffuser may cause flow separation, potentially inducing symmetry-breaking non-uniformities, either spatially stationary or rotating and either steady or intermittent. Rotating stall, characterized by the presence of distinct cells of flow separation on the circumference, rotating at a fraction of the impeller revolution rate, has been observed in thermal and hydraulic turbomachines. Due to its complexity, the part-load flow in radial centrifugal pumps is a major challenge for numerical flow simulation methods. The present study investigates the part-load flow in radial centrifugal pumps and pump-turbines by experimental and numerical methods, the latter using a finite volume discretization of the Reynolds-averaged Navier-Stokes (RANS) equation. Physical phenomena of part load flow are evidenced based on three case studies, and the ability of numerical simulation methods to reproduce part-load flow in radial centrifugal pumps qualitatively and quantitatively is assessed. A numerical study of the flow in a high specific speed radial pump-turbine using steady approaches and the hypothesis of angular periodicity between neighboring blade channels evidences the relation of sudden flow topology changes with an increase of viscous losses, impacting on the energy-discharge characteristic, and thus increasing the risk of unstable operation. When the flow rate drops below a critical threshold, the straight through-flow with flow separation zones attached to the guide vanes changes to an asymmetrical flow. Energy is drawn off the mean flow and dissipated in a large vortex-like structure. Besides flow separation in some diffuser channels, time-dependent numerical simulations of the flow in a double suction pump evidence a flow rate imbalance between both impeller sides interacting with asymmetric flow separation in the diffuser. Viscous losses increase substantially as this imbalance occurs, the resulting segment of positive slope in the energy-discharge characteristic is found for a flow rate sensibly different from measurements. Different modes of rotating stall are identified by transient pressure measurements in a low-specific-speed pump-turbine, showing 3 to 5 zones of separated flow, rotating at 0.016 to 0.028 times impeller rotation rate, depending on discharge. For operating conditions where stall with 4 cells is most pronounced, velocity is measured by Laser-Doppler methods at locations of interest. The velocity field is reconstructed with respect to the passage of stall cells by definition of a stall phase obtained from simultaneous transient pressure measurements. Time-dependent numerical simulation predicting rotating stall with 4 cells shows velocity fields that are in reasonable agreement with the measured velocity fields, but occurring at a sensibly higher flow rate than found from experiments. In consideration of the quantitative shortcomings of the numerical simulation, a novel modelling approach is proposed: Replacing the costly 3-dimensional simulation of the major part of the impeller channels by a 1-dimensional model allows a significant economy in computational resources, allowing an improved modeling for the remainder of the domain at constant computational cost. The model is validated with the challenging cases of rotating stall and impeller side flow rate imbalance. The satisfying coherence of the results with the simulation including the entire impeller channels qualifies this approach for numerous turbomachinery applications. It could also provide improved, time-dependent boundary conditions for draft tube vortex rope simulations at reasonable computational cost. Parameter studies modifying deliberately some quantities of mean flow and turbulence at the modeled boundary surfaces can be implemented in the framework of the method.