Analyse du champ de pression pariétale d"un diffuseur coudé de turbine Francis

Abstract

At this time, a large number of hydraulic turbomachinery installations are ageing, so that they need refurbishement to increase their efficiency and their associated output power. The items that need refurbishement are mainly the runner and the guide vanes. The draft tube and the spiral casing are generally not modified, for cost and safety reasons. An unsuitable match between the new runner and the old draft tube can lead to an unfavourable flow behavior and an efficiency drop when the hydroelectric plant works at off-design conditions, due to the rapidly changing user load conditions. Moreover, in non-optimum operating conditions, the draft tube can be the origin of unstabilities that prevent the machine exploitation. The understanding of the flow in the draft tube is therefore very important for the prediction and the control of the machine stability. The flow in the draft tube is complex, unsteady and turbulent because of its rotating nature and of the draft tube's geometry. In order to increase the understanding of the flow physics, analyses of the steady and unsteady pressure fields on the draft tube walls are lead through extensive pressure measurements in the whole draft tube. The investigation is divided in two parts. The first one concerns the analyses of the pressure measurements at the operating points near the optimum (BEP) in order to understand the recovery coefficient break-off when the flow rate increases. A particular evolution of the steady pressure field appears following the operating points around the BEP. Consequently, the flow is distributed between the 2 channels of the draft tube according to each operating point. As for the unsteady phenomena, the fluctuating field in the cone of the draft tube shows 2 components : a rotating component at the runner frequency fn and a synchronous component at 20fn resulting from the rotor-stator interaction. The influence of the spiral casing on the fluctuating pressure is pointed out by representing all phase average signals in the same absolute angular position of the runner. Unsteadiness of random nature at very low frequency, below 0.3fn, appears as well. These fluctuations propagate from the elbow to the 2 channels in the draft tube. Correlations between the steady and unsteady results are observed during the analysis. The second part concerns the analysis at the low discharge operating points. An influence of the vortex volume, from a hydrodynamic and acoustic point of view, has been found in the turbine operation for 3 values of the Thoma number σ. For these 3 pressure levels, the turning pressure field due to the vortex rotation is completely deformed by a synchronous pressure field at the same frequency. This field influences the magnitude of the fluctuations at the wall and mainly in the phase velocity of the vortex. At low σ, the big volume of the biphasic vortex interacts with the draft tube in such a way that the elbow becomes the source of strong pression oscillations that propagate to the upstream and to the downstream of the draft tube and dangerously towards the pipes of the test rig system. At mean σ, the volume of the biphasic vortex decreases and causes a regular pressure field in the draft tube and the strong oscillations disappear. At high σ, the vapor phase disappears completely and the comparison between the measurements and a monophasic numerical calculation shows good consistency in the cone of the draft tube. Thanks to this computation it is possible to explain the fluctuations shape at the cone walls.