How is the tangential velocity at the outlet of the runner at part load? The flow is assumed attached, i.e., no separation occurs. Explain with the help of velocity vectors
It is assumed that the flow still follows the runner blade at part load, i.e., Vrel still has the angle β at the outlet. Since Q=2πr2b2Vrel sin(β) is lower than at best efficiency, then Vrel is lower since β is constant. Therefore, Vrel cos(β)<ωr2 , and Vabs cos(α)≠0 and Vabs cos(α)>0. Therefore, the fluid has more swirl at part load in the same direction as the runner.
What is a vortex breakdown and when does it occur?
Vortex breakdown is the creation of a recirculation zone in the central region. It occurs at part load.
Why is there a small tangential velocity at the outlet of the runner at the best efficiency point?
At the best efficiency point, there is a small swirl. It is beneficial to the performance of the draft tube, especially for low heads machine where the kinetic energy represents a significant part of the head.
Mention and describe the diffferent steps in the design of a Francis turbine runner using conformal mapping
Determine flow passage & blade limits
Design streamlines in meridional plan
Determine inlet & outlet blade angles
Assume an energy distribution along the blade
Design streamlines in the radial plane with conformal mapping
Blade thickness
What does the energy distribution represent?
The energy Distribution defines the variations of circulation corresponding to transferable energy along the blade between leading and trailing edge. It should be a smooth function
Explain the consequences of the energy distribution on the blade loading and the losses
It is desirable to utilize most of the energy at the beginning of the blade. Here, the Highest pressure occurs which corresponds to higher blade loading and therefore blade thickness
-> exponential function shows that more energy needs to be used at the beginning.
Also, at the end, the runner outlet should have a higher area (with thinner blades) so that the flow can go on more smoothly
Losses: depend on the velocity, an therefore most of the velocity should be taken away at the beginning of the blade
Why is the use of a scale-up formula necessary to obtain prototype efficiency from model efficiency?
There is no dynamic similarity (Re_M = Re_P)
Are the losses more important on the model or the prototype?pro
Prototype
Where are the relative losses more important?
On the model, because Re is lower
Last changeda year ago
