Transition from Fast Deflagration to Detonation Under the Influence of Wall Obstacles

Abstract

In the present study we investigate the transition of a fast deflagration (planar shock followed by reaction front propagating at about half the C-J detonation velocity) induced by perturbing it with transverse pressure waves generated by a series of small obstacles placed periodically along the channel wall. To obtain the fast deflagration an initial cellular detonation wave is caused to fail by damping the transverse waves associated with the cellular structure by acoustic absorbing walls. The experiments were carried out with three different mixtures: 2C2&2 + 5O2 + 75%Ar, CsHg + 5O2 and CH4 + 2C>2; and four different obstacle configurations were used to control the perturbation transverse waves frequency. The initial pressure varies between 25 Torr and 160 Ton. The transition process is studied using Schlieren framing and streak photography. The results indicate that for a given mixture with a given obstacle geometry, there is a minimum pressure below which transition is not observed. Above this pressure, transition is observed and the distance it takes for transition to occur is dependent on the initial pressure. The results also indicate that there is an optimal obstacle spacing (or, equivalently, transverse wave perturbation frequency) of the order of the channel height at which transition is most efficient, i.e., transition can be induced for less sensitive mixtures. The results demonstrate the essential role of transverse pressure perturbation on the formation of the detonation. The present results also show that transition can be induced much easier for mixtures having irregular cells (C3Hg + 5O2 and CH4 + 2O2) than for mixtures having regular cells (2C2H2 + 5O2 + 75%Ar). Copyright © 1992 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Graduate Student, Department of Mechanical Engineering. ' Professor, Department of Mechanical Engineering. •r Professor, permanent address: Institute of Pure and Applied Mechanics, USSR Academy of Science, Novosibirsk. 270 TRANSITION FROM DEFLAGRATION TO DETONATION 271