Analyses of the cellular structure of detonations

Abstract

We have investigated the effects of chemical composition and dilution on the regularity of gaseous detonation cellular structure at a fixed critical tube diameter of 52 mm. Three fueloxygen mixtures were studied as function of argon dilution (0% to 80% by volume); the fuels were acetylene, hydrogen and ethane. Smoked foil records were analyzed by digital image processingto obtain a quantitative spectrum of the spatial wavelengths present in the cellular structures. With increasing argon dilution, the number of cells across the tube diameter increases from 4–10 at 0% Ar to 20–30 at 80% Ar at critical transmission conditions for acetylene-oxygen detonations, and the spectra of the cellular structures show a dramatic narrowing in spectral content. A smaller decrease in the dominant spectral wavelength and less increase in regularity with increasing argon dilution are observed for hydrogen-oxygen detonations. Ethane-oxygen detonations have a much more irregular cellular structure and the spectrum is characterized by a broad band of features with the dominant spectral wavelengths independent of argon dilution. Reaction zone lengths and sensitivity parameters have been computed using the steady-state ZND model of detonation structure. The results of these computations show that these parameters cannot account for the systematic variations in cell regularity and the differences in macroscopic behavior. A partial explanation is offered based on the response of the chemistry to large amplitude variations in shock strength, i.e., degree of overdrive. The amount of overdrive at which the reaction zone becomes endothermic is proposed as a figure of merit for instability and is shown to correlate with present observations.