Author(s): A A Fay

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Keywords: Francis turbines; Low-frequency pulsations; Torque equation; Draft tube vortex

Abstract: The power pulsations of Francis turbines are discussed which occur regularly at part load but sometimes even at full load. The main tool of the analysis is the torque equation of Francis runners which has been derived from basic mechanics neglecting only very small effects. The various potential pulsation sources are grouped according to their places of origin, namely those occurring (i) in the inlet flow path (rotor-stator interaction), (ii) at the runner (interblade vortices, rotating stall), and (iii) in the draft tube (spiraling vortex flow). Their effects are discussed assuming certain ideal symmetrical velocity distributions. The main theoretical conclusions are: (1) The shaft torque pulsations may be determined from the turbulent velocity field. (2) In case of steady quasi-axisymmetric inflow, the main pulsation source does not belong to group (i). (3) Of the pulsation sources (ii) and (iii), one is self-excited while the other is forced. (4) If the discharge is constant and the draft tube vortex rotates uniformly, then (iii) cannot affect the shaft torque. (5) If (iii) is the main source of the torque pulsations, then discharge fluctuation must appear. (6) If the discharge pulsation is too small, then either (ii) is the main source, or essential interactions of (ii) and (iii) may be expected. Thus, the torque equation is seen as a powerful tool of the analysis, and for future research attention is focused on the discharge pulsation.

DOI: https://doi.org/10.1088/1755-1315/12/1/012015

Year: 2010