Flow measurements around guide vanes of Francis turbine: A PIV approach

A guide vane cascade is developed for the study of flow in the distributor of a low specific speed Francis turbine. Velocity and pressure measurements are done with Reynold's number 1.15 E+07, at 80% of BEP flow as in a reference prototype turbine. This work illustrates the development of test setup and focuses on investigation of PIV methods applied for the velocity measurements. Techniques developed for 'in-situ' calibration of PIV setup and methods applied for image processing are discussed in details. Approach to estimate total uncertainty in PIV measurements and minimum no of image pairs required for statistical convergence of velocity field is presented. Reference measurements are done along the plane of chord, from guide vane wall to its mid-span. Flow velocity exceeding 35 m/s, at the runner inlet of Francis turbine, is reported for the first time from such experimental studies. Flow phenomenon inside Francis turbine distributor are characterized and comparison are done with the cases for prototype turbines. The cascade setup is found to reproduce the flow conditions inside a Francis turbine distributor, except the rotor-stator-interaction. PIV methods are generalized for the cases of similar measurements and the results will be applicable to validate numerical studies.