The Influence of Swirl Brakes and a Tip Discharge Orifice on the Rotordynamic Forces Generated by Discharge-to-suction ?.. Leakage Flows in Shrouded Centrifugal Pumps

Recent experiments conducted in the Rotor Force Test Facility at the California Institute of Technology have examined the effects of a tip leakage restriction and swirl brakes on the rotordynamic forces due to leakage flows on arl impeller undergoing a prescribed circular whirl. The experiments simulate the leakage flow conditions and geometry of the Alternate Turbopump Design (ATE)) of the Space Shuttle Hi h Press ump and are critical to evaluat;n_ ÷h_ _, . .g _ ure Oxygen Turbop•_ • . ..... 6 o,*_ pump s rotordynannc Instability problems• rrevlous 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 !nduced rotordynamic forces. Also, previous experiments have shown that leakage inlet (pump discharge) swirl can increase the cross-coupled stiffness coefficient and hence increase the range of positive whirl for which the tangential force is destabilizing. In recent experimental work, the present authors demonstrated that when the swirl velocity within the leakage path is reduced by the introduction of ribs or swirl brakes, then a substantial decrease in both the destabilizing normal and tangential forces could be achieved. Motivation for the present research is that previous experiments have shown that restrictions such as wear rings or orifices at pump inlets affect th pump designs such as the St)ace tlo e leakag e forces. Recent _ Shut.._ Alternate Turbopump Design (ATD) utilize tip orifices at discharge for the purpose of establishin axial thrust balance. The ATD has experienced rotordynamic instability problems an_ one ma " charge orifices may also have an ;.... ,--. ,_• . ., y surmise that these ti dis. • ..,pu_l_ ezlec_ on _ne • P • the plane of impeller rotation T nr_,_,t _.--__ . no.rmal and tangential forces m • he ,_......... uuy ae_ermmes 11such tip leakage restrictions contribute to undesirable rotordynamic forces• Additional motivation for the present study is that the widening of the leakage path annular clearance and the installation of swirl brakes in the ATD has been proposed to solve its instability problems• The present study assesses the effect of such a design modification on the rotordynamic forces. The experimental apparatus consists of a solid or dummy impeller, a housing instrumented for pressure measurements, • a rotating dynamometer and an eccentric whirl mechanism. The solid impeller is used so that leakage flow contributions to the forces are measured, but the main throughflow contributions are not experienced. The inner surface of the housing has been modified to accommodate meridional ribs or swirl brakes within the leakage annulus. In addition, the housing has been modified to accommodate a discharge orifice that qualitatively simulates one side of the balance piston orifice of the Space S uttle ATD. Results indicate the detrimental effects of a discharge orifice and the beneficial effects of brakes. Plots of the tangential arid normal forces versus whirl ratio show a substantial increase in these forces along with destabilizing resonances at . " • when a discharge orifice is adde Wh=, r,..1 ...... ,, , some positive whirl ratios d ......... ,,,=_ _e aaaea, some of tlae detrimental effects of the orifice are reduced• For the tangential force, a plot versus whirl ratio shows a significant reduction and a destabilizing resonance appears although the overall force is not reduced, again to be eliminated. For the normal force, eliminated, a destabilizing resonance appears to be