Measurements of Hydrodynamic Forces on a Two-Dimensional Impeller and a Modified Centrifugal Pump

Abstract

Previously force measurements have been made on a centrifugal pump impeller. The test section inlet of these tests was modified by increasing the volume of the annular region surrounding the shroud. Force measurements were subsequently made on this modified configuration. In addition, a two dimensional model of the impeller was tested. The references provide a description of the facility prior to the current modifications. Briefly, the dynamometer, composed of two parallel plates connected by four strain gaged posts, is mounted between the impeller and the drive shaft. It measures the six components of a generalized hydrodynamic force vector {F} acting on the impeller. The impeller is made to whirl in an orbit eccentric to the volute center, in addition to the normal shaft rotation. Since the eccentric motion is in the lateral plane, perpendicular to the impeller centerline, only the two components of the force vector {F} in this lateral plane will be discussed. Referring to Fig. 1, these forces, in the stationary volute frame of reference, can be represented by [equation included in scanned report's introduction, p. 4](1). The lateral forces, represented by F_x and F_y, can be considered as the sum of two forces: a fixed force, represented by F_(ox) and F_(oy), which the impeller would experience if located at the volute center, and an unsteady force due to the eccentric motion of the impeller, represented by a force matrix [A]. The gravitational and buoyancy forces on the rotor are subtracted out. Dimensionless quantities are used throughout (see Nomenclature for definitions). The data presented in this paper are derived from measurements with the impeller whirling in the circular orbit. When there is no whirl the matrix [A] and vector {F_o} can still be extracted from measurements taken at four fixed eccentric positions, 90 degrees apart, except that these results are limited to the zero whirl value. In general, the force matrix [A] is a strong function of the whirl speed ratio Ω/ω.