La Houille Blanche
Number 7-8, Novembre 1982
|Page(s)||607 - 613|
|Published online||01 November 2009|
Recherche expérimentale et numérique sur le comportement d'une roue réversible au point de tracé
Experimental and numerical invegtion of the behaviour of a reversible pump-turbine runner at and off design conditions
Professeur Istituto di Macchine Université de Padoue (Italie)
2 Professeur adjoint Istituto di Macchine Université de Padoue (Italie)
3 Chercheur Istituto di Macchine Université de Padoue (Italie)
Object of this paper is the comparison of actual flow configurations in a turbomachine with flow configurations estimated by numerical procedures. Blade-to-blade solutions of the differential equation of motion are generally referred to an isolated runner, and in fixing the boundary conditions at the impeller periphery the influence of the casing or of the stationary system is completely disregarded. While this may be acceptable for centripetal turbine runners, it gives unsatisfactory results in pumps and compressors, where flow pattern at the runner outlet changes considerably with the discharge rate because of the influence of the volute and/or the diffuser. The present investigation has been conducted on a model reversible pump-turbine stage fitted with a fully bladed diffuser-conveyor. The runner tip clearance has been traversed in pump operation in order to examine the distorsion of the flow induced by the stationary system. The averaged values of flow angle, velocity and total pressure better agreed with theoretical estimates only at the matching point of runner and diffuser, which occurred at a discharge different from the b.e.p. rate. At lower or higher rates the averaged value of the meridional component of the absolute velocity differed considerably from actual through-flow. And also, the runner input head obtained from the averaged values of the tangential component of the velocity , which on account of the impeller losses was expected to be always greater than the total head rise through the runner, appeared to be smaller than the latter at flows below the design rate and greater at higher flows. The discrepancies have been ascribed to the interaction of runner and stationary vanes, which distorted the flowlines in the clearance annulus. It is also shown that fewer interference problems are present in turbine operation. The stationary vanes were then removed from the diffuser channel so as to obtain an unobstructed annulus where free flow conditions would set up comparable to those existing outside an isolated runner. Flow measurements showed now better consistency with theoretical estimates. It could also be appreciated that the ratio of slip velocity to runner tangential velocity did not change appreciably with the rate of flow. Axial distribution of velocity and pressure in the clearance annulus was also considerably affected by the presence of the diffuser vanes. Known numerical solutions of the equation of motion may yield satisfactory results provided no external action is exerted on the runner flow other than the pressure forces exchanged by the runner itself. For pump runner with volute or vaned diffuser casing, the boundary conditions at the impeller exit should be defined by an equation expressing the distorting effects of the stationary system on the flow.
© Société Hydrotechnique de France, 1982
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