Experimental study of unsteady hydrodynamic force matrices on whirling centrifugal pump impellers

Abstract

An experimental facility was constructed and instrumented. A study was conducted on a set of centrifugal flow pumps whose impellers were made to follow a controlled circular whirl motion. The aim was to characterize the steady and unsteady fluid forces measured on the impeller under various pump operating conditions. The postulation was that the unsteady lateral forces result from interactions between the impeller and the surrounding diffuser and or volute (via the working fluid), and that under certain flow regimes these forces can drive unstable lateral motions of the pump rotor. The lateral hydrodynamic forces were decomposed into their steady and unsteady parts, the latter being further expressed in terms of a generalized fluid stiffness matrix. A study of this matrix as a function of the whirl to pump speed ratio supported the following chief conclusions: i) the common assumption of matrix skew-symmetry is justified; ii) the magnitudes and signs of the matrix elements are such that rotor whirl can indeed be caused by the hydrodynamic forces, in pumps operated well above their first critical speed, iii) as expected, the matrix is very sensitive to the value of the flow coefficient, especially at flow rates below the design; iv) the commonly postulated quadratic variation of the matrix elements with the reduced whirl frequency, resulting in the so-called rotordynamic coefficients (stiffness, damping and inertia) is not justified for flow coefficients significantly below design; and v) surprisingly, it was discovered that the presence, number and orientation of diffuser guide vanes have little effect on the forces. Conclusions regarding the effect of impeller geometry could not be reached given the similarity of the tested designs. However, other results on phenomena such as skin friction and leakage flow are presented. Some of the findings are compared to experimental and theoretical data from other sources. Finally, the rotordynamic consequences of the results are discussed as the present data were applied by another author to the case of the Space Shuttle Main Engine's (SSME) High Pressure Oxidizer Turbopump (HPOTP).

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