Analyse des erreurs dues à l'écoulement dans la mesure de débit par la méthode acoustique: approches expérimentale et numérique
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.