Boundary-layer combustion in propulsion

Abstract

A review and discussion of the general status of propulsion-oriented, subsonic, boundarylayer combustion research is presented. Because propulsion applications usually imply high rates of surface-mass addition and high Reynolds Numbers, the paper is confined primarily to turbulent boundary layer combustion considerations. Within this realm, diffusion-limited combustion has received the greatest attention and is the only case for which a relatively complete theoretical description is available. This theory is summarized in some detail, and its capabilities and limitations are demonstrated through comparisons with data for the simple Plexiglas-oxygen propellant combination. The majority of the paper is concerned with three other relatively unexplored, but important, aspects of the boundary-layer-combustion problem that warrant further consideration from the standpoint of propulsion applications: (1) radiative heat transfer; (2) the role of chemical kinetics; and (3) transient phenomena (particularly low-frequency combustion instability). In an endeavor to better define the associated problems and suggest possible directions for future investigations, some tentative hypotheses and preliminary theoretical concepts are presented. Although highly oversimplified, a gray-body radiation treatment has proven useful in applications and here serves to illustrate the complex interaction between flow, kinetics, and radiation parameters. Also, a simple analysis indicates that the kinetics of heterogeneous reactions are likely to be important only in the low-pressure regime where gas-phase kinetics play a role. Finally, a brief theoretical study suggests that the profile of energy release in the boundary layer, as affected by heterogeneous reactions, etc., may be of great importance in transient phenomena.