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Abstract

Flow in a elbow draft tube is a turbulent complex unsteady flow caused by its rotating nature and the draft tube's geometry. The understanding of the main phenomena that could appear near the BEP and at off-design conditions is important to master the instability of the machine. The results shown in this work are a summary of a very large experimental database. The aim of this investigation work is to analyze the steady and unsteady pressure fields at the wall of the draft tube through extensive pressure measurements. A particular evolution of the steady pressure field appeared according to the operating points around the BEP chosen to study the draft tube recovery break-off. Consequently, the flow is distributed between the 2 channels of the draft tube according to the operating point. The fluctuating pressure field at the cone can be divided mainly in 2 components: first a rotating component at the runner's frequency (f*) and second a syncronous component at 20f* resultant of the interaction rotor-stator frequency. 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. The results at the low discharge operating point without vapour phase show the main pressure fluctuations at the cavitation free vortex frequency as well as the advection of these fluctuations in the elbow and the channel walls.

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