Fluid Mechanical Challenges in High-Speed Liquid Turbomachines

Abstract: Pumps are one of the most common components of any hydraulic system. Their reliability and efficiency are of ubiquitous and paramount importance. Despite many indications of unfamiliar phenomena, academic interest in the fluid mechanics of these devices was scant indeed for a large part of the late 20th century and the design tools used in industry remained confined to steady flow hydraulic analyses and a few empirical vibration criteria. It was not until extreme versions of unsteady flow difficulties arose in the development of the high speed turbopumps in liquid-propelled rocket engines that serious attention began to be paid to the flow instabilities and fluid-structure interaction problems in pumps. Methodologies had to be developed to investigate these unsteady flows and practical design tools had to be identified to predict and ameliorate their consequences. This lecture will review some of these key issues and the new fluid mechanics that was developed in response to those challenges.

Though the development of liquid-propelled rocket engine pumps was a primary trigger for this research, it is now recognized that the phenomena and methodologies are common to many liquid turbomachines. For simplicity, however, the present paper will focus on the rocket engine application. Two key milestones are worth noting. The first was the identification in the 1960s of the Pogo instability that plagued many of the early launch vehicles and caused the destruction of some. This led eventually to an understanding of the dynamic characteristics of the pumps and how to use this knowledge to limit the instability of the fuel and oxidizer feed systems. In this talk I review some of the resulting investigations of the dynamics of cavitating pumps. The second milestone occurred during the development of the high speed pumps in the Space Shuttle Main Engine when it became apparent that fluid-induced rotordynamic forces substantially affecting the critical speeds of the high speed pumps and thereby limiting their operational range. Herein I briefly review some of the newer understandings of these fluid-induced rotordynamic effects.

In more recent times, knowledge of these unsteady flow phenomena is used in a wide range of pump applications. But new and hybrid variations of these instabilities continue to be uncovered and require attention.