Use of compact Laser Doppler Velocimetry in reduced scale model testing of hydraulic machines

Laser based flow survey methods such as Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) have long been used for studying the flow in hydraulic machines. Even if the results have crucially improved our understanding of potentially destabilizing flow patterns in the draft tube at off-design conditions and the prediction of the stable operating range, these techniques are not systematically used for reduced scale model measurements in the frame of customer acceptance tests or development projects. In a not too distant past, the price, size and complexity of the necessary equipment and procedures exceeded the acceptable range for these projects. Recent developments however made them more accessible. Furthermore, the collaboration with experienced research institutes provides the necessary know-how as well as an added academic value by providing researchers with realistic test cases. This work reports a case study for which a compact LDV probe was used to measure the velocity field in the draft tube cone of a reduced scale physical model of a Francis turbine. The axial and tangential velocity profiles were established by performing measurements at several radial positions between the turbine centreline and the cone wall. The overall objective was the validation of numerical flow simulations across the entire operating range of the turbine, from partial to full load.