The Effect of Inlet Swirl on the Rotordynamic Shroud Forces in a Centrifugal Pump

Abstract

The role played by fluid forces in determining the rotordynamic stability of a centrifugal pump is gaining increasing attention . The present research investigates the contributions to the rotordynamic forces from the discharge-to-suction leakage flows between the front shroud of the rotating impeller and the stationary pump casing. In particular, the dependency of the rotordynamic characteristics of leakage flows on the swirl at the inlet to the leakage path was examined . An inlet guide vane was designed for the experiment so that swirl could be introduced at the leakage flow inlet. The data demonstrates substantial rotordynamic effects and a destabilizing tangential force for small positive whirl ratios; this force decreased with increasing flow rate. The effect of swirl on the rotordynamic forces was found to be destabilizing.

Measurements of the Flow in Backshroud/CasingClearance of Precessing Centrifugal Impeller

“…6. It is noteworthy that the effect of the leakage flow on the fluid force moment on the precessing back-shroud is the same as that of the inlet swirl on the fluid force on the whirling shroud (Childs [1989] and Guinzburg et al [1993]).…”

“…Moreover, rotordynamic fluid forces on an impeller shroud with a small shroud/casing clearance are destabilizing (Ohashi et al [1988] and Adkin et al I1988]). For the shroud forces of a whirling centrifugal impeller, numerical (Childs [1989] and Baskharone et al [1994]) and experimental (Guinzburg et al [1993]) investiga-tions have been carried out to see the effect of leakage flow in the shroud/casing clearance and inlet swirl at the entrance of the clearance. It was found that the fluid forces on the shroud become more rotordynamically destabilizing with increasing the inlet swirl at the impeller outer radius.…”

Fluid-induced rotordynamic forces and instabilities

“…Though some early data on the rotordynamic forces in a centrifugal pump were obtained by Shoji and Ohashi [8], the Caltech program added greatly to the data base. Data were obtained for many different types of impellers (centrifugal and axial, shrouded and unshrouded, with and without cavitation), for many different types of volutes and for parametric variations of the forces with pump operating point, pump cavitation and various clearances and other factors [6,7,[9][10][11][12][13][14][15][16][17][18][19]. Typical data for the dimensionless normal and tangential forces, F n and F t ; as a function of the frequency ratio, o=O; are presented in Figure 3 for a conventional centrifugal pump impeller operating in a single vaneless volute.…”

“…Guinzburg et al [13,14,29], Uy and Brennen [16,17] and Hsu and Brennen [18] also conducted experimental investigations of the effect on the rotordynamic forces of the inlet swirl to the leakage flow path. This was accomplished by installing guide vanes at the leakage inlet to introduce pre-rotation in the direction of shaft rotation.…”

“…Figure 9 shows a typical installation designed for a swirl angle of 68 other vanes with angles of 18 and 28 were also fabricated. With this installation and assuming that the flow is parallel with the vanes, the swirl ratio, G (defined as the circumferential fluid velocity divided by the radial fluid velocity), depends on the flow coefficient and turning angle, a; according to G=j ¼ H=B tan a where B ¼ 0:318 cm is the width of the channel containing the vanes [13]. Some of the early data and analysis assumed, not unreasonably, that the flow angle would be equal to the vane angle; later this was found to be a serious source of error and the correct effect of inlet swirl was finally established by Hsu and Brennen [18] by direct measurement of the fluid swirl velocity and flow angle at inlet.…”