Rehabilitation of ageing power plant is a growing market with specific needs: we have to deal with non up-to-date elements in terms of hydraulic behavior. Refurbishment projects are often focused on the guide vane and runner modification. But for low-head refurbishment projects, the draft-tube is a non negligible part of the overall performance. In some cases, pathological draft-tubes have been encountered with a significant efficiency drop near the best efficiency point. Modification of the draft-tube in order to attenuate, and even eliminate, this effect can thus have a real impact on improving the performance of the power plant, and therefore become economically justified (that heavily depends on the addition or removal of material from the draft-tube walls). Laser Doppler Velocimetry (LDV) measurements near draft-tube outlet have been achieved for both original and modified draft-tube. It offers 2D velocity profiles and permits to calculate discharge repartition in each bay. Pressure recovery factor measured separately for each bay is used to quantify losses.

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