Abstract: The tester of the draft tube of Francis turbine is specially designed and constructed for internal laser flow measurement model Francis turbine testing, where the testing planes of the draft tube is entirely made of transparent organic material. Instantaneous velocity is tested by PIV (Particle image veloci-metry) on different guide vane opening in different positions, including the draft tube cone, the elbow and the inlet and outlet of draft tube diffusing section. By time averaged data process, the Internal-flow velocity, vorticity and distribution of stream-lines are obtained. It is noted that it has round flow on radial planes, and back flow on meridian planes under different guide vane opening on the test conditions. There are secondary flow on the draft elbow and draft tube diffusing section. And more-over it is unsymmetrical on the left and right planes of draft tube diffusing section. These experiment results not only help to understand the flow feature of draft tube, but also verify numerical simulation results of the flow of draft tube.

Key words: Draft tube, Flow, Guide vane opening, PIV