Experiments on Shock Induced Combustion of Isolated Regions of . Hydrogen-oxygen Mixtures

The interaction of a strong plane shock wave with isolated regions of gaseous mixtures was examined through a series of shock tube experiments. Specifically, the non-uniform mixtures examined consisted of a spherical bubble of pure hydrogen or hydrogen-oxygen mixtures surrounded by either an oxygen, nitrogen or air atmosphere. Shocks in the range of Mach 1.7 to 3.7 were studied. The interaction events where recorded with high speed shadowgraphs and pressure trace recordings. No chemical reactions were observed in the interactions of shocks with strengths up to Mach 3.7 with a pure hydrogen bubble due to inadequate mixing of the reactants. In addition no reaction was observed for shocks up to Mach 3.0 interacting with a premixed stoichiometric bubble. However, shock induced combustion was observed for incident shock strengths above Mach 3.0. This demarcation between reaction and non-reaction corresponds to the classical third explosion limit for a hydrogen-oxygen mixture. This data will aid the study of the initiation and propagation of detonation waves and provide a useful set of test data for computational fluid dynamics codes involving reactive flows. INTRODUCTION The interactions of shock waves with fluid nonuniformities have long been studied. Typically these studies have looked at the interface of two fluids of different densities. Cases of both a shock moving from a light gas into the heavy gas and vice versa have been examined. Markstein [1,2], Richtmyer [3], Meshkov [4], Catherasoo & Sturtevant [5] and Brouillette & Sturtevant [6] examined the instabilities generated by the passage of a shock wave at planar interfaces between two different density gases. Abd-elFattah, et al. [7,8,9] also looked at shock waves interacting with interfaces between fluids of different densities but with an emphasis on examining the process of shock wave refraction and reflection. Haas & Sturtevant [IO] extended this work by looking at shock interactions with cylindrical and spherical interfaces. Their experimental images clearly show how the passage of a shock wave over a light gas sphere deforms the sphere into a vortex ring. The vorticity generation in these spheres due to a shock passage can easily be explained by examining the vorticity equation, The second term on the right shows that vorticity, w, will be generated whenever the density and Aerospace Research Engineer, Member AlAA ’ Professor, Member AIAA * Professor, Member AIAA This material is a declared work of the U.S. Government and is not subject to copyright protection in the United States. 1 American Institute of Aeronautics and Astronautics (c)l999 American Institute of Aeronautics & Astronautics pressure gradients are not aligned. This term is known as the baroclinic torque. The case of a shock passing over a cylindrical volume of less dense gas surrounded by a heavier gas is depicted in Figure l(a). As the shock proceeds across the test volume votticity is generated along the interface of the two gases. The maximum votticity is generated when the density and pressure gradients are at ninety degrees corresponding to the top and bottom of the cylinder represented in Figure l(b). The result is that the vorticity tends to deform the cylinder into a kidney shape shown in Figure l(c) and eventually into a pair of vortex lines. By extension it is easily seen that a spherical bubble would tend to form into a vortex ring. Marble et al. [l l] first proposed using the induced vorticity generated at a light/heavy gas interface when subjected to a steep pressure gradient as a mechanism to enhance the mixing of the fuel and oxidizer in a scramjet. The application of this mechanism to practical scramjet engines was examined by Marble et al. [12], Waitz et al. [13] and Lee et al. [14, 151. Conceptually, the mixing would occur by injecting the hydrogen fuel as a cylindrical jet into the supersonic airflow in the combustor. The combustor floor would be ramped such that a weak shock would be generated such that it interacted with the fuel jet leading to mixing of the fuel and oxidizer. For the most part the research cited above has looked at the interaction of relatively weak shock waves (I Mach 1.25) and inert gases. The thrust of the present work is to make the extension to stronger shock waves and to consider the interaction of the shocks with chemically reactive gases. Consider spherical bubbles of pure hydrogen immersed in an oxygen atmosphere. As a shock passes over the hydrogen bubble the baroclinic torque mechanism will create a pair of vortex rings and lead to mixing of the hydrogen and oxygen. The mixing coupled with the increased pressure and temperature behind the shock wave can lead to conditions were chemical reaction might take place. For the case of premixed bubbles, no additional mixing is required however the temperature and pressure behind the shock wave must be AIAA-99-0821 !