Numerical Investigation of the Transition Process from the Optimal Operating Point to the Maximum Load of the Francis Turbine

The flow in the draft tube of water turbines at the point of maximum load operation is associated with many instabilities. In addition to the loss of efficiency caused by the vortex breakdown, the resulting low frequency pressure fluctuations can cause the resonance phenomenon and reduce the life of the turbine. In the current study, the process of forming the maximum load vortex core (torch) in the transition from the optimal operating point to the maximum load has been studied and the transient regimes formed during this transition process have been discussed and studied. It is clear that the core of the torch vortex is the result of the bubble vortex breaking phenomenon and its main factor is the formation of a stagnation point and the creation of a recirculation flow in the axis of the draft tube. This recirculation flow is created due to the loss of axial momentum in the conical region of the draft tube. The results of the vortex core show that in the stages of growth and formation of the burner vortex core, the core diameter of the optimal charge base grows first. Then, waves will be created on the surface of the vortex core structure, which with the passage of time and the strength of the rotation in the flow field, will break the integrated structure of the core and will be divided into several parts. Then the diameter and length of the vortex core structure increases and affects many parts of the draft tube cross-section, the length of this structure will progress to the draft tube diffuser. Therefore, it is clear that the main place of effect of the torch is the end of the conical area and the elbow inlet of the draft tube. By examining the numerical results, it was determined that with the passage of 1.5 to 2 seconds after the start of the transition process, there are significant changes in the rotational, axial and radial velocity components to will exist.