Laser Velocimeter Measurements in the Leakage Annulus of a Whirling Shrouded Centrifugal Pump

  • K. Caughey
  • Published 2004

Abstract

Previous experiments conducted in the Rotor Force Test Facility at the California Institute of Technology have thoroughly examined the effect of leakage flows on the rotordynamic forces on a centrifugal pump impeller undergoing a prescribed circular whirl. These leakage flows have been shown to contribute substantially to the total fluid induced forces acting on a pump. However, to date nothing is known of the flow field in the leakage annulus of shrouded centrifugal pumps. No attempt has been made to qualitatively or quantitatively examine the velocity field in the leakage annulus. Hence the test objective of this experiment is to aquire fluid velocity data for a geometry representative of the leakage annulus of a shrouded centrifugal pump while the rotor is whirling using laser velocimetry. Tests are performed over a range of whirl ratios and a flowrate typical of Space Shuttle Turbopump designs. In addition to a qualitative study of the flow field, the velocity data can be used to anchor flow models. NOMENCLATURE d Location across leakage gap measured normal from impeller shroud H Clearance between impeller shroud and housing Q Volumetric leakage flow rate E Eccentricity of impeller's circular whirl orbit w Main shaft radian frequency R Whirl radian frequency INTRODUCTION Previous experimental and analytical results have shown that discharge-to-suction leakage flows in the annulus of a shrouded centrifugal pump contribute substantially to the fluid induced rotordynamic forces (Adkins, 1988). Experiments conducted in the Rotor Force Test Facility (RFTF) at Caltech on an impeller undergoing a prescribed whirl have indicated that the leakage flow contribution to the normal and tangential forces can be as much as 70% and 30% of the total, respectively (Jery, 1986). Recent experiments at Caltech have examined the rotordynamic consequences of leakage flows and have shown that the rotordynamic forces are functions not only of the whirl ratio but also of the leakage flow rate and the impeller shroud to pump housing clearance. The forces were found to be inversely proportional to the clearance and a region of forward subsynchronous whirl was found for which the average tangential force was destabilizing. This region decreased with flow coefficient (Guinzburg, 1992a). In recent experimental work, the present Caltech authors demonstrated that when the swirl velocity within the leakage path is reduced by the introduction of ribs or swirl brakes on the housing, then a substantial decrease in both the destabilizing normal and tangential forces could be achieved (Sivo, 1993).The motivation for the present research is that no previous experiments have examined the flow field within the leakage annulus. No measurements have been made of the destabilizing swirl or tangential velocity or demonstrated the complex nature of the flow. Analytical models of such flows use bulk average velocities to examine the rotordynamic forces. Such bulk flow models may be seriously flawed when it comes to estimating the meridional and tangential velocities. It is hoped that this present research will serve as preliminary insight into the complex nature of leakage flows.

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Cite this paper

@inproceedings{Caughey2004LaserVM, title={Laser Velocimeter Measurements in the Leakage Annulus of a Whirling Shrouded Centrifugal Pump}, author={K. Caughey}, year={2004} }