Detonation Structure and Dynamics

Laser Shadowgraph of a detonation Mach Reflection in a C2H2 + 2.5 O2 + 14 Ar mixture (at 50 kPa initial pressure) travelling from left to right. The change in transverse wave spacing behind the incident wave and Mach-stem, the shear-layer produced by the wave interaction and the curved triple-wave interaction region, are clearly noticable in the image.

The phenomenon of Mach Reflection has been researched in numerous efforts in conjuction with shock waves in non-reacting and relaxing gases. The present study is primarily an experimental one, involving detonable mixtures of gases. Self-sustaining detonation waves in these mixtures are made to interact with a ramp that spans the width of the test-section and thus presents an essentially 2-d change in geometry. This produces a Mach reflection configuration that consists of a reactive Mach stem, and a non-reactive reflected wave. Shadowgraphs of this interaction are made at various positions along the wedge, using a pulsed ruby laser as the light source. Effective triple point trajectory angles, and wavefront contours are obtained from digitized images. The triple-point trajectories are compared with calculated values from shock-wave theories extended to Chapman Jouguet detonations. Wave contours are scaled by their position on the wedge and compared to investigate self-similarity.

Shadowgraph of critical detonation diffraction in 100 kPa 2H2+O2. The re-initiated detonation is propagating spherically outward and sweeping back into the shocked reactants.

An in-depth scientific investigation of the fundamental problem of detonation diffraction is underway in the Detonation Physics Laboratory. The outcome of a detonation wave propagating from a confined to an unconfined volume through an abrupt area change will fall into one of three regimes, depending primarily on the combustible gas composition, initial pressure, and geometry of the confining area. The detonation is able to continuously transit the area change without failure in the super-critical regime. The sudden expansion from confinement results in shock wave separation from the reaction zone and complete failure of the detonation wave in the sub-critical regime. The critical regime is characterized by partial failure of the diffracting wave, followed by re-initiation leading to the detonation propagating throughout the unconfined volume. The physical mechanisms which govern the failure and re-initiation of a diffracting detonation wave will be fully understood through the use of several flow visualization techniques, multiple confinement geometries, and other detonation diagnostics. This will assist in the derivation of an analytical model for the prediction of the transmission regime given initial and boundary conditions. Many aspects of the diffraction problem are similar to processes observed in critical ignition energy and deflagration to detonation transition phenomena, and thus we hope to shed light on these problems as well.

An analytical model is presented for the direct initiation of gaseous detonations by a blast wave. For stable or weakly unstable mixtures, local analysis of the one-dimensional unsteady reaction zone structure identifies a competition between heat release, wave front curvature and unsteadiness. The primary failure mechanism is found to be unsteadiness in the induction zone arising from the deceleration of the wave front. A critical shock decay rate is determined in terms of the other fundamental dynamic parameters of the detonation wave, and hence this model is referred to as the critical decay rate (CDR) model. The local analysis is validated by integration of quasi-unsteady reaction zone structure equations with real gas kinetics, as well as numerical simulations of the complete unsteady point blast problem with simplified Arrhenius kinetics. The CDR model is then applied to the global initiation problem to produce an analytical equation for the critical energy. Unlike previous phenomenological models of the critical energy, this equation is not dependent on other experimentally determined parameters and for evaluation requires only an appropriate reaction mechanism for the given gas mixture. It is found to give satisfactory agreement with experimental data for a number of fuel--oxidizer mixtures.

Measurement of detonation properties in mixtures with ammonia and nitrous oxide. Application to hazards analysis for Hanford waste tanks.

Analysis of detonation properties of benzene-air mixtures and application to hazards analysis at Savannah River Site.

Study of resonant elastic response to shock and detonation wave loading. Experiments have been carried out in shock tubes (Burcsu, Zuhal) and detonation tubes (Beltmann) to determine the deflection of the tubes for various speed waves. Extensive comparisons with the elastic theory and finite elements have been carried out. The effect of detonation cell width on the loading has also been investigated. Further experiments are in progress to examine plastic loading and failure due to fracture

A 1-m grating spectrometer has been used to measure emission spectra over 700-250 nm range for various fuel-oxygen-diluent mixtures. Comparison with low-pressure flame spectra demonstrate the existence of excited states after the detonation.

Experiments on detonation initiation with application to pulse detonation engines (PDEs) have been performed. These experiments and modeling activities examine the mechanisms of initiation and techniques for initiation of insensitive fuels of practical interest to engine designers.