Cavitation and Fluid Dynamics of Cryogenic Turbopumps for Liquid Rocket Fuel Feed System

This work synthetically reviews the open literature, both scientific and technical. currently available on cavitation and its impact on the steady and unsteady fluid dynamics of cavitating turbomachines, with particular attention to high power density cryogenic turbopumps for liquid rocket fuel feed systems. The first part of this review is concerned with the general aspects of cavitation phenomena that are most important in turbomachinery applications. Consideration is given to the nucleation process, the mechanics of inception, the morphological types of cavitation, the classical theory of cavitation scaling and its limitations (cavitation scaling effects), and the relations of cavitation with bubble dynamics. Cavitation scaling effects are examined with special reference to the case of developed cavitation because of its dominant impact on the performance degradation and final breakdown of hydraulic machines. Thus, only the occurrence and prediction of thermodynamic effects in travelling bubble cavitation and in fully developed attached cavitation are considered in some detail. The influence of the thermodynamic state and properties on the extrapolation of test results obtained in different liquids and the currently available correlation methods are outlined. Finally an introduction is given to the mathematical analysis of cavity flows and supercavitatioo because of its important applications in the study of steady, quasi-steady and unsteady performance of cavitating turbomachines. In the second part of the review the most significant aspects of non-cavitating turbomachinery flow modeling and analysis are outlined, with specific emphasis on the fully wetted incompressible flow in the inducer. The various formulations of the fluid dynamic problem in terms of the Navier-Stokes, Reynolds averaged, Euler, stream function, vorticity and potential equations are presented and their general mathematical characteristics are summarized. The numerical techniques .(finite difference, finite element and spectral methods) most frequently used for the solt1tion of the above equations are briefly outlined and problems related to stability. speed, complexity, and flexibility are indicated. A review is given of the current computational methods for inviscid and viscous flow analysis in turbomachines. The comparative merits and limitations of individual methods are pointed out, with special reference to their applicability to specific flow situations, state of development and computational requirements. Finally, an introduction is presented of the fully wetted flow in axial inducers, with a brief survey of the relevant theoretical analyses and experimental results. Special attention is given to the identification of the various fundamental phenomena involved and to the illustration of their combined effects in determining the overall inducer flow field. The third part of the review deals with the specifics of cavitation in hydraulic pumping systems. The mechanisms and behavior of steady cavitation in hydraulic machinery is briefly discussed and reference is made to cavitation models based on free streamline theory of cavitating cascades and on bubbly flow representations. Finally, the main aspects of the unsteady dynamic performance of cavitating turbopumps is examined and the relevant fluid dynamic parameter~ are introduced. Special attention is paid to auto-oscillations (surge) of hydraulic systems, which represent the most technologically significant form of cavitation induced instability. The available theoretical methods for the evaluation of the dynamic properties and for the linearized dynamic stability analysis of cavitating turbopumps and pumping systems are outlined and compared to the relevant experimental data in the literature.