| The role of the combustion zone microstructure of the diffusion flame in the pressure-coupled response of composite rocket propellants was investigated. The leading edge portion of the diffusion flame, referred to as the leading edge flame (LEF), is thought to play an important part in the response of the surface of a pyrolyzing propellant to imposed pressure fluctuations.; The T-burner was used to create a standing pressure wave in the presence of a combustion environment, thus allowing for observation of the interaction of the propellant combustion and the acoustic field. This interaction is characterized by the pressure-coupled response function, and as measured in the T-burner, provides insight into how a propellant will behave in a rocket motor, where stability is governed by the balance between the gains and losses of acoustic energy in the combustion chamber.; In order to quantify the behavior of the LEF in an oscillatory environment, a bimodal ammonium perchlorate-hydrocarbon propellant was fired in the T-burner over a characteristic pressure range. It was proposed that the LEF's on diffusion flamelets associated with fine oxidizer particles behave such that under certain conditions of small oxidizer particle size and/or low pressure, the classical diffusion flame structure "breaks down," yielding a transition to a more remote and as yet undetermined flame location. Such unstable conditions, manifested as a fluctuation of the LEF's between an "attached" state (in the classical structure) and a "detached" state, are presumed to contribute strongly to overall dynamic response.; The bimodal oxidizer size distribution consisted of nominal 400 {dollar}mu{dollar}m coarse ammonium perchlorate (AP) and fine AP in the range of 10-100 {dollar}mu{dollar}m. Crucial to the study was the use of very narrow size distributions within the modes, especially the fine component. It was the wide difference in size between the two modes, facilitating stable flamelets on the coarse AP particles and unstable flamelets on the fine, that was expected to isolate LEF behavior and give a unique, recognizable dependence of oscillatory combustion response on mean pressure and particle size of the fine mode.; The T-burner results indicate a definite trend in the response function with pressure at frequencies of 500 and 800 Hz. Pronounced peaks in the pressure-coupled response at pressures of 250-300 psia are indicative of singular LEF behavior, brought about by unstable flamelets on the fine AP particles in this pressure range. The higher combustion response is attributed to a high population of these flamelets fluctuating within the overall flame complex, between the aforementioned attached and detached flame configurations. |
