Pelton jet visualization
The quality of a jet of a Pelton turbine has major impact on the overall efficiency of the turbine. Jet disturbances ensue primarily from upstream flow conditions in the distributor, the injector and the nozzle. The resulting secondary flows and non-uniformities in the velocity distributions of the exiting jet are inherently connected with energy losses. More important, however, is the suboptimal energy conversion of the disturbed jet in the runner buckets.
Downstream of the nozzle exit internal flow disturbances immediately build up surface deformation of the jet. Parts of these deformations are steady while others vary with time. In the worst case the jet may be deformed in a way that some of the water does not even touch the Pelton buckets, but bypasses the runner.
For example, typical steady deformation of the jets surface arises from the secondary flow induced by an upstream bend or supporting structures within the injector. Vortex stretching creates longitudinal structures or streaks on the jet surface as do local mechanical defects of the nozzle surface, which leave “scratches” on the jet’s surface.
Flow visualization proves to be a good tool for diagnosis of efficiency problems for prototypes.
Installation of equipment for prototype visualization is tricky since the best positioning of camera and lighting can not be found on the basis of trial and error but must be based on experience. Furthermore, the mechanical forces of possible water impingement on the camera and lights call for a rigid mounting system. Housings of camera and lights have to be waterproof and measures must be taken to avoid condensation building up on the lenses. The high Reynolds and Weber numbers lead to a further complication since they are responsible for the formation of small droplets and fog in the housings of a prototype. Water velocities range between 50 and 200 m/s and motion blur of the images must be avoided.
In order to achieve acceptable image quality under these adverse circumstances special equipment is needed. etaeval is well prepared for such installations.