A review of active control of combustion instabilities

: This book has several purposes, including a broad historical summary of combustion instabilities in propulsion systems; a concise compilation of the main mechanisms for instabilities in liquid and solid fueled rockets, ramjets, thrust augmentors and gas turbines; development of a theoretical framework for investigating unsteady motions in combustion systems; and accessible surveys of the basic material required to understand the subject. Emphasis is placed throughout the book on observed behavior. For well-understood reasons, the best, and in many respects most useful quantitative data, have been obtained for solid propellant rockets. Hence that type of device occupies a special position in the subject. The book comprises nine chapters and eight annexes. Material in the annexes is not essential for reading and broadly understanding the main part of the book, but is likely interesting for those choosing to do research on the subject. The nine chapters divide into three parts.

Unsteady Motions in Combustion Chambers for Propulsion Systems

In the past 20 years, periodic vortex shedding as a source of acoustic energy inside solid propellant rocket motors has been continuously studied, in connection with several motors that exhibited oscillatory behaviors although they were predicted stable by means of conventional linear stability methods. On that subject, correlations through Strouhal numbers are commonly used. In this article, the meanings of various Strouhal number definitions are discussed. The basic mechanisms for sound production in ducts or chambers are then discussed through simple tools, such as the linear hydrodynamic stability analysis, Flandro's method and the acoustic balance technique, which provide good insights into the physics of the phenomenon. Necessary conditions for vortex-shedding-driven motors are then arrived at and illustrated by actual firing results. Finally, the full numerical approaches are shown to provide unprecedented insight into the mechanisms behind the vortexshedding phenomenon and are believed to open the way to quantitative predictions of frequencies and oscillatory levels.

Vortex-Shedding Phenomena in Solid Rocket Motors

The objective of these studies is to experimentally characterize the mean and fluctuating flow field that develops along the length of a simulated cylindrical port rocket chamber. Flow simulation was accomplished by injecting ambient temperature nitrogen uniformly along the walls of 10.2-cm (4-in.) diam, porous-tube chambers connected to a choked sonic nozzle. Experiments were conducted with chamber L/D ratios of 9.5 and 14.3, at injection Mach numbers and Reynolds numbers typical of rocket motor values. Maximum Reynolds numbers based on injection and centerline velocities were, respectively, 1.8 x 10 and 1.6 x 10. Mean and fluctuating speed and turbulent shear stresses were measured in the principle coordinate directions using three-element hot-wire anemometers. The data show that noticeable velocity fluctuations in the head-end region generally decrease in intensity, relative to centerline speed, over the first five port diameters. At this point, regular velocity oscillations appear near the wall, just prior to the transition to turbulent flow. The oscillation frequency characteristics suggest the occurrence of vortical disturbances which exhibit pairing as they move away from the wall. The downstream turbulence development is characterized by a slow spreading toward the centerline: peak values of turbulence intensity and shear stress occur a few tenths of a port radius from the wall and remain relatively constant. Mean velocity profiles prior to transition show fair agreement with those derived for a rotational inviscid flow injected normal to the surface. A slow transition from these profiles occurs downstream in the turbulent region. Two surprising features of the flow were the occurrence of both buoyant flow influences and flow spinning in forward regions of the chamber.

S. W. Young

Papers

Vortex Shedding as a Source of Acoustic Energy in Segmented Solid Rockets

Vortex shedding studies in a simulated coaxial dump combustor