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Published April 29, 2014 | Published
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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 secondary 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. Studies 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 suction 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 understanding of the internal flows and the occurrence of upstream backflows in inducers. The 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 the 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 destabilizing peaks. Theoretical estimates of the tangential rotordynamic force on a non-whirling inducer using actuator disk theory were significantly different, both in magnitude and direction, from the experimentally measured forces. The 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 downstream asymmetry. The reverse flows occurring downstream which consist of high energy fluid are the possible cause of these large forces. The imposition of a 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 flow 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 any significant effect on the rotordynamic forces. Cavitation was found to have important consequences for fluid-induced rotordynamic forces. These forces become destabilizing for both forward and reverse whirl. The magnitudes of the destabilizing forces were found to increase with decreasing cavitation numbers.

Additional Information

Report No. E249.18. I would like to express my deepest gratitude and sincere thanks to my advisor, Professor Allan Acosta and to Professor Christopher Brennen for their guidance, encouragement and support throughout my graduate study and the dissertation process. They were available whenever I needed technical assistance and provided me with valuable comments and suggestions throughout the research. I would like to thank Professor Thomas Caughey for his technical insights throughout this work. I would also to thank the other members of my committee, Dr. Richard Murray and Dr. G. Ravichandran for their helpful suggestions. My colleagues and friends in the laboratory deserve special thanks for providing assistance during the research. In particular, I would like to thank Ron Franz, Adiel Guinzburg, Joe Sivo and Asif Khalak. I would like to express my appreciation and thanks to Cecilia Lin for her assistance with the graphics and her encouragement; and to Jackie Beard and Dana Young for help in the administrative processes. My thanks go to Marty Gould, Mike Gerfen, Louis Grimaldi and Rodney Rojas for their assistance in the fabrication of some of the components used for the research. My deepest thanks and gratitude to Lalita (my wife) for her unconditional love and encouragement which has been a source of great strength, confidence and inspiration for me. I would like to thank her for staying up and assisting me during those long hours of dissertation writing. I thank Lalita for rekindling my spirits and for providing reassurance during difficult moments. I would also like to thank my parents and sister in India for their continuous encouragement and support. Finally I would like to thank NASA George C. Marshall Space Flight Center for sponsoring the current work under grant NAGS-118.

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August 20, 2023
August 20, 2023