Caractéristiques des poussées axiales

Characteristics of axial thrust

¹ Ecole Polythechnique Fédérale de Lausanne
² Hydroat S.p.A. Milan
³ Neyrpic Grenoble

Abstract

The first chapter deals with the theoretical calculation of the axial thrust, taking into account the various components due to external and internal forces on the runner. Internal forces are induced by both pressure and momentum effects. As a result, non dimensional parameters corresponding to the internal and external components are calculated and plotted versus specific speed for the optimum operating range. Chapter II covers the influence of the operating range on axial thrust. A complete example is given of the relationship between static pressures, discharge and axial thrust for a turbine rotating at average specific speed, as well as comments on the trends of other specific speeds. Main conclusion is that high specific speeds are more sensitive to flow variations than low specific speeds. In the third chapter, special attention is paid to similarity and transposition problems. First, a distinction must be made between turbines of high specific speeds, for which the main part of axial thrust is of internal nature and, therefore, offers a good similarity between model and prototype, and low specific turbines for which the main part of the thrust is of external nature. In the latter case, there is a problem regarding similarity, due to differences in Reynolds numbers in the wearing rings which leads to high differences between pressure losses on model and prototype. Recommendation is given concerning the homogeneity of wearing ring diameters and the similarity of static pressures between runner and head cover which must be obtained without homogeneity of wearing ring clearances. Chapter four studies the fluctuations of axial thrust in partial load range. A distinction is made between the effect of pulse flow which relates to all the components of axial thrust and the effect of local pressure fluctuation which only relates to certain components. Example of frequency analyses show the influence of different pressure frequencies on the dynamic behaviour of axial thrust. The paper concludes on both scientific and practical viewpoints. From a scientific point of view, the calculation of axial thrust at optimum load may be considered as safe, but the same calculation in partial load range remains inaccurate due to three dimensional and non-stationary effects. In practice, it can be observed that any progress in axial thrust prognosis may lead to significant money savings and to better safety of the thrust bearing.