Internal Flows and Force Matrices in Axial Flow Inducers

Axial flow pump runners known as inducers are subject to complex internal flows and fluid-induced lateral and rotordynamic forces. The internal flows in inducers are three dimensional and are characterized by complicated flows. The current research investigates the boundary layer flows on the blades, hub and housing of unshrouded and shrouded axial flow inducers using flow visualization techniques. Rotordynamic and lateral force data on unshrouded inducers were also obtained under varying conditions of flow and whirl. on the internal flows showed that the blade boundary layer flow had strong radial components at off-design conditions. The flow remains attached to the blade surface of unshrouded inducers at all flow coefficients tested. The origin of the upstream swirling backflow was found to be at the discharge plane of the inducer. In addition, flow reversal was observed at the side blade tip near the leading edge in a shrouded inducer. Re-entry of the hub boundary layer flow (a downstream backflow) into the blade passage area was observed at flow coefficients below design. For unshrouded inducers the radially outward flow near the blade tip mixed with the tip clearance leakage flow to form the upstream backflow. These observations provide a better of the internal flows and the occurrence of upstream backflows in inducers. rotordynamic forces acting on an inducer due to an imposed whirl motion was also investigated. It was found that the rotordynamic force data at various whirl frequency ratios does not allow a normal quadratic fit; consequently conventional inertial, stiffness and damping coefficients cannot be obtained and a definite whirl ratio describing the instability region does not result. Rotordynamic forces were found to be significantly dependent on the flow coefficient. At flow coefficients below design, these forces are characterized by multiple zero crossings at various whirl frequencies and large peaks. Theoretical estimates of the tangential rotordynamic force on a non-whirling inducer using actuator disk theory were significantly different, in magnitude and direction, from the experimentally measured forces. effect of upstream and downstream flow distortions on the rotordynamic and lateral forces on an inducer were studied. It was found that at flow coefficients below design, large lateral forces occurred in the presence of a asymmetry. The reverse flows occurring downstream which consist of high energy fluid are the possible cause of these large forces. The imposition of uniform downstream condition reduced these forces to near zero values. Results of inlet distortion experiments show that a strong inlet shear causes a significant increase in the lateral force. However, weak inlet shear flows and the asymmetry due to a 180° upstream bend did not cause a significant lateral force. It was found that flow distortions upstream or downstream did not cause significant effect on the rotordynamic forces. Cavitation was found to have important consequences for fluid-induced rotordynamic forces. These forces destabilizing for both forward and reverse whirl. The magnitudes of the destabilizing forces were found to increase with decreasing cavitation numbers.