Visualilisation hydrodynamique de l'écoulement dans une maquette de turbomachine axiale

Abstract

To enhance the knowledge of the penomena characterising flow in an axial turbomachine, visual inspection in a hydrodynamic tunnel was performed by the ONERA, drawing the existing analogy between aerodynamic and hydraulic flow conditions. The visual scrutiny of walls and space is obviously easier and finer in water than in air, especially in non-stationary state. The scale-model used is vertical and includes a mobile runner fitted with 20 cylindrical vanes (circumference 1 = 50 mm) placed in a cylindrical channel (radius of casing R = 146 mm). As the flow velocity V1 and the speed of rotation N of the runner are controlled and regulated independently, all operating conditions can be simulated using this assembly. In addition to the main testing layout (isolated mobile runner), a few variations have also been tested (mobile runner with a rectifier, mobile runner with a rotating keel). The assembly has been designed to the used for the following scanning methods : - emissions of a coloring agent of density 1 produced from fixed or mobile elements (carcen, hub, vanes, etc.) - air bubles in suspension in the water. They are observed through lit sections placed logitudinally (diametrically cut or half-way up the vanes) or transversally (cut at the edge of leakage of mobile vanes or half-way along the chord of the rectifier blades). The power and efficiency of the stage are a function of the volume flow and the percentage by comparison with optimum power due to ventilation. The transition zone between motor and receiver modes of the stage is approximately (QV)* = 0.5. Maximum power intake is a major share of optimum power, especially when the variation of (Q V)* is associated with an increase in downstream pressure. The hypotheses adopted and the analytical findings are in agreement with experience and yield an acceptable accurate forecast of the performance of a turbine stage under partial load. The conclusion from the various observations made is that terminal stage flow is highly sensitive to the load variations necessitated by its environment. A separated flow rotating en bloc with the mobile blading occurs at the downstream section of the blade when the volume flow is reduced to 25 % below its rated value. The stall then increases and forces the flow to the central part and the periphery up to the point at which a second toric vortex is formed upstream and at the tip of the blade. This eddy rotates at virtually the same speed as the mobile blading. At zero flow, the two eddies rotating in opposite directions invade the channel and produce called losses due to ventilation, in which the centrigual effect is dominant.