Analyse expérimentale de l'écoulement dans la zone axiale des canaux adducteurs des turbines-Hélices

The paper presents sorne results of the measurements carried out, in water, in the axial region of a propeller turbine admission duct (Hy), without runner, and, in air, in an annular section cylindrical duct fed by a radial distributor (1MB) (see Fig. 2). The measurements have been performed with different wicket-gate openings a and the attention has been drawn on the small openings where different phenomena have been observed (see Fig. 1) : a variation of the radial distribution of the tangential velocity near the hub, associated with the arising of a quite forced vortex ; the decrease and sometimes the inversion of the axial velocity; the presence of radial flows directed towards the hub; the drop of the total pressure, due to the viscous dissipation, These phenomena mainly develop in the axial region of the two ducts, rapidly extend their influence and finally settle on a practically constant area (see Fig. II). The distribution of axial and tangential velocity components change almost similarly in the two ducts according to the decrease of the wicket-gate opening, as it can be observed in figures 3 and 4. These figures show the distribution of axial and tangential velocity components divided by the average axial velocity, as a function of the ratio between the radius of the measurement points and the external one in the closest sections of the two ducts (1MB 2 and HyG). The similarity of the flow evolution in the two ducts is even more underlined by the comparison (see Fig. 5-8) of the velocity distributions for comparable values of the m parameter, called swirl rate, defined by the equation (1) and usually utilized in technical literature for the evaluation of the flow vortex intensity. In the diagram of figure 9 the radius limiting the central core, which is characterized by the decrease of axial velocity (ri) and the radius of the core characterized by the quite forced vortex distribution of tangential velocities (rf) have been associated to mparameter values, calculated by means of the geometric data of the duct according to the equation (2). The mand m values have been compared in the diagram of figure 10 versus a. Some experimental tests carried out on the Hy model with runner showed that, at small openings, a central core similar to the one previously described in the case without runner is developed (see Fig. 12). In axial turbines this phenomenon causes efficiency drop and cavitating vortexes which, as it happens without the runner (see Fig. 14), detach near the hub and pass through the runner (see Fig. 15) in a helix spiral form, thus causing vibrations and noise at low operation loads. The operation areas concerned by this phenomenon are indicated in figure 13 on the hill diagrams of a propeller and a Kaplan turbine with medium specific speed.