Part-load flow instabilities in a radial pump measured with TR-PIV NICO KRAUSE, ELEMER PAP, DOMINIQUE THEVENIN Laboratory of Fluid Dynamics and Technical Flows University of Magdeburg ''Otto von Guericke'' Universitaetsplatz 2 D-39106 Magdeburg GERMANY Abstract: - Pumps are used in a variety of important applications. Often, the pipe characteristic is unsatisfactory known and the selected pump has more power than needed. It will thus be throttled or the flow is throttled during operation. In such cases the pump operates under part-load flow and the inflow condition differs from the dimensioning point. In this paper results for an impeller blade designed using a single arc of a circle are presented. The pump characteristic curve shows energy transmission and losses. For the investigation of the flow in the impeller Time-resolved Particle Image Velocimetry (TR-PIV) is used. Using TR-PIV it is possible to visualise the vortices in the stall regime. The results have been obtained at different operating points. It is possible to observe different stages of the developing rotating stall. The first vortices are observed at a throttled flow rate of 50% of the design point. They exist only in two channels of the impeller. The first rotations of the stall cell through the impeller are observed at flow rates smaller than 40% of the design point. These velocity fields are further post-processed to analyze the possible development of the rotating stall cell. Key-Words: rotating stall, radial pump, turbomachinery, blade design, TR-PIV, pump characteristic, part-load flow 1 Introduction In many applications pumps are not used at the design point. Wurm [1] indicates that in head installations pumps operate at under 25% of the design flow-rate over 50% of the time. In such cases the pump operates under part-load flow and the inflow condition differs from the dimensioning point. For turbomachines operating with gases the problem of rotating stall has been known for a long time. The first description was given by Emmons et al. [2]. In turbomachines there exists a well-known velocity triangle with the components c (absolute velocity), u (peripheral velocity) and w (relative velocity) (Eq. 1, Fig. 1a). c u w = + (1) If the flow rate is reduced, the meridian component of the relative velocity w m also decreases (Fig. 1b). For this reduced flow rate the angle of attack rises and the stagnation point is displaced to the pressure side of the blade. When the angle of attack exceeds a certain threshold, the flow on the suction side will detach and stall can appear. The static pressure inside the stall region is smaller than in the surrounding flow; vortices can therefore appear with the same rotating direction as the impeller. In the outlet of the blade passage a second vortex can be formed with an inverse rotational direction. These vortices can grow until the complete blade passage is blocked. Then, the medium has to pass through the following channel. This leads once more to a displacement of the stagnation point in the following blade passage, and a new stall cell will be formed in this channel. On the other hand, the flow in the first blade passage will reattach and the flow conditions in this channel will improve. It is furthermore possible that several stall cells exist simultaneously in the impeller. a) b) Fig.1a-b Velocity triangles at design point (a) and for throttled flow (b). With this mechanism the stall cell(s) can turn around through the impeller. Because the angular velocity of the stall cell is lower than the impeller angular velocity, it turns inside the impeller against the rotational direction. Proceedings of the 4th WSEAS International Conference on Fluid Mechanics and Aerodynamics, Elounda, Greece, August 21-23, 2006 (pp319-324)