Mechanism of Instabilities of Exothermic Hypersonic Blunt-Body Flows

Abstract

An experimental and analytical investigation of the regular, periodic instability observed in exothermic hypersonic blunt-body flows has been conducted. A ballistic range instrumented with submicrosecond-spark dual-schlieren photographic equipment and ionization probes has been used to identify the distinguishing flow characteristics about spheres fired into lean acetylene-oxygen and stoichiometric hydrogen-air mixtures. The variation of the instability wavelength with initial gas state, body size, and body velocity has been recorded. An instability mechanism is proposed and is shown to be consistent with the experimental results and with the significant gas-dynamic wave-interaction processes The instability originates in the induction zone which separates the bow shock and the exothermic reaction front in the nose region of the flow field. It arises from the effect on the chemical induction time of entropy waves which are generated by the interaction of compression waves with the bow shock. The compression waves are in turn generated as a result of the time-varying reaction rate. The induction times derived from the measured instability wavelengths compare favorably with shock-tube data.