Mixing and Combustion of Rich Fireballs

A series of experiments was carried out to investigate the effect of fireball composition on secondary combustion. The fireball was created from a 1.5 liter balloon filled with a propane-oxygen mixture (1 1, the incompletely oxidized products from the primary burn mix with the surrounding air and may be oxidized in a secondary combustion process. The unique feature of the present experiments was a repeatable secondary pressure pulse for sufficiently rich mixtures. The secondary pressure rise was observed repeatably for all initiation configurations. The nature of the secondary pressure pulse is a strong function of the initial equivalence ratio. For [Greek Phi] = 1 and 1.5, no secondary pressure waves are observed. An acoustic analysis of the measured pressure histories has been carried out to infer the rate of volume displacement and the total volume displaced by the secondary combustion. The results of the acoustic analysis are in reasonable agreement with both a simplified thermodynamic model predicting the total volume displacement assuming constant-pressure combustion for the secondary burn and the analysis of the fireball luminosity of the high-speed images. For nearly stoichiometric mixtures, [Greek Phi] = 1 and 1.5, the leading blast wave peak pressures and impulses are comparable with the previously-measured gaseous and high explosive blasts when the energy content of the balloon only is used to formulate Sachs scaling variables. Due to a much slower combustion process than detonation for [Greek Phi] >2 the peak pressure of the leading wave rapidly decreases below the energy-equivalent reference blast values as the equivalence ratio is increased. The Sachs-scaled impulse agrees well with the predictions on the basis of the energy in the balloon alone for 2.75 > [Greek Phi] > 1. One of the key results of the present study has been the documentation of the existence of the secondary pressure wave. The present study has emphasized the acoustic nature of the secondary pressure waves and the origin of these pressure waves due to the processes at the interface between the fireball and the atmosphere. The presence of the secondary pressure peak and the higher impulses indicate that there is the potential for significant enhancement of the blast through secondary combustion

Detonation Diffraction in Mixtures with Various Degrees of Instability

Planar laser induced fluorescence (PLIF) is widely used in combustion diagnostics but has only recently been successfully applied to detonation. The strong spatial variations in temperature, pressure, and background composition under these conditions influence the quantitative link between OH-number density and fluorescence intensity seen on images. Up to now, this has lead to uncertainties in interpreting the features seen on PLIF images obtained in detonations. A one-dimensional fluorescence model has been developed, which takes into account light sheet attenuation by absorption, collisional quenching, and changing absorption line shape. The model predicts the fluorescence profile based on a one-dimensional distribution in pressure, temperature, and mixture composition. The fluorescence profiles based on a calculated ZND detonation profile were found to be in good agreement with experiments. The PLIF technique is used to study the diffraction process of a self-sustained detonation wave into an unconfined space through an abrupt area change. Simultaneous schlieren images enable direct comparison of shock and reaction fronts. Two mixture types of different effective activation energy [theta] are studied in detail, these represent extreme cases in the classification of detonation front instability and cellular regularity. Striking differences are seen in the failure mechanisms for the very regular H2-O2-Ar mixture ([theta] ~ 4.5) and the highly irregular H2-N2O mixture ([theta] ~ 9.4). Detailed image analysis quantifies the observed differences. Stereoscopic imaging reveals the complex three-dimensional structure of the transverse detonation and its location with respect to the shock front. The study is concluded by using the experimentally-obtained shock and reaction front profiles in a simplified model to examine the decoupling of the shock from the chemical reaction. The rapid increase in activation energy for the H2-O2-Ar mixtures with decreasing shock velocity is proposed as an important new element in the analysis of diffraction for these mixture.</p

Stereoscopic Imaging of Transverse Detonations in Diffraction

Diffraction of gaseous detonations has received considerable attention for many years, yet there is limited understanding of the failure and initiation phenomena due to the complex coupling between the combustion and the fluid dynamics. A variety of optical techniques such as streak imaging, open shutter photography, high-speed schlieren imaging, and, more recently, planar laser induced fluorescence (PLIF) has been used to visualize the diffraction process in detonations. To overcome the integrating nature of visualization techniques and also allow for sooted foil records, many diffraction experiments in the past were carried out in narrow channels, studying detonation transition from planar to cylindrical geometry. The experimental investigation on spherically diffracting detonations described in this paper uses stereoscopic image reconstruction of the transverse detonations. The aim is to obtain further insight into the transverse detonations, which are the re-coupling phenomena identified to occur in the critical diffraction regime following a re-initiation event. The 3D reconstruction technique visualizes the transverse detonation as defined by the volume in space with high luminosity. The reconstruction technique is based on gradients, in contrast to those techniques based on target points as used, for example, in 3D particle image velocimetry. Together with a simultaneously obtained schlieren image, the location of the transverse detonation could be determined to be just below the shock surface

Stereoscopic Imaging of Transverse Detonations in Diffraction

Diffraction of gaseous detonations has received considerable attention for many years, yet there is limited understanding of the failure and initiation phenomena due to the complex coupling between the combustion and the fluid dynamics. A variety of optical techniques such as streak imaging, open shutter photography, high-speed schlieren imaging, and, more recently, planar laser induced fluorescence (PLIF) has been used to visualize the diffraction process in detonations. To overcome the integrating nature of visualization techniques and also allow for sooted foil records, many diffraction experiments in the past were carried out in narrow channels, studying detonation transition from planar to cylindrical geometry. The experimental investigation on spherically diffracting detonations described in this paper uses stereoscopic image reconstruction of the transverse detonations. The aim is to obtain further insight into the transverse detonations, which are the re-coupling phenomena identified to occur in the critical diffraction regime following a re-initiation event. The 3D reconstruction technique visualizes the transverse detonation as defined by the volume in space with high luminosity. The reconstruction technique is based on gradients, in contrast to those techniques based on target points as used, for example, in 3D particle image velocimetry. Together with a simultaneously obtained schlieren image, the location of the transverse detonation could be determined to be just below the shock surface

Lead Shock Oscillation and Decoupling in Propagating Detonations

Experimental images of propagating detonation waves provide lead shock velocity measurements through the cell cycle. We examine the issue of local decoupling of the shock and reaction front using these data. In highly unstable mixtures with high reduced activation energy, experimental images and analysis suggest that local decoupling occurs at the end of the cell cycle. Regions of high fluorescence intensity are observed in shear layers in apparently decoupled portions of the detonation front

Detonation wave diffraction in H₂-O₂-Ar mixtures

In the present study, we have examined the diffraction of detonation in weakly unstable hydrogen–oxygen–argon mixtures. High accuracy and computational efficiency are obtained using a high-order WENO scheme together with adaptive mesh refinement, which enables handling realistic geometries with resolution at the micrometer level. Both detailed chemistry and spectroscopic models of laser induced fluorescence and chemiluminescence were included to enable a direct comparison with experimental data. Agreement was found between the experiments and the simulations in terms of detonation diffraction structure both for sub-critical and super-critical regimes. The predicted wall reflection distance is about 12–14 cell widths, in accordance with previous experimental studies. Computations show that the re-initiation distance is relatively constant, at about 12–15 cell widths, slightly above the experimental value of 11 cell widths. The predicted critical channel height is 10–11 cell widths, which differs from experiments in circular tubes but is consistent with rectangular channel results

Numerical Study of the Detonation Wave Structure in Ethylene-oxygen Mixtures

We examine a transition from a weakly to a highly unstable regime of a cellular detonation in stoichiometric ethylene-oxygen systems with varied dilution. The structure and propagation of cellular detonations is calculated using two-dimensional, time-dependent, reactive Euler fluid-dynamics algorithm. A dynamically adapting mesh is used to resolve reaction zones, shocks, contact surfaces, and vortices in flow. A simplified chemical model with Arrhenius kinetics is used for all mixtures. Effects of dilution are modeled by varying the adiabatic index γ and molecular weight of matter. Results are compared to experimental data obtained in a detonation tube by simultaneous visualization of a chemical species (OH), density gradients in the reaction zone, and to soot foil records. Due to a strong sensitivity of the post-shock temperature to variations of γ, the degree of chemical - fluid dynamics coupling inside the detonation structure varies significantly with dilution. Variations of the equation of state with dilution can account for and could be the main mechanism explaining a wide range of detonation behavior observed in the experiments

Direct observations of reaction zone structure in propagating detonations

We report experimental observations of the reaction zone structure of self-sustaining, cellular detonations propagating near the Chapman-Jouguet state in hydrogen-oxygen-argon/nitrogen mixtures. Two-dimensional cross sections perpendicular to the propagation direction were imaged using the technique of planar laser induced fluorescence (PLIF) and, in some cases, compared to simultaneously acquired schlieren images. Images are obtained which clearly show the nature of the disturbances in an intermediate chemical species (OH) created by the variations in the strength of the leading shock front associated with the transverse wave instability of a propagating detonation. The images are compared to 2-D, unsteady simulations with a reduced model of the chemical reaction processes in the hydrogen-oxygen-argon system. We interpret the experimental and numerical images using simple models of the detonation front structure based on the “weak” version of the flow near the triple point or intersection of three shock waves, two of which make up the shock front and the third corresponding to the wave propagating transversely to the front. Both the unsteady simulations and the triple point calculations are consistent with the creation of keystone-shaped regions of low reactivity behind the incident shock near the end of the oscillation cycle within the “cell.

Reactant jetting in unstable detonation

We note the common existence of a supersonic jet structure locally embedded within a surrounding transonic flow field in the hitherto unrelated phenomena of unstable gaseous detonation and hypervelocity blunt body shock wave interaction. Extending prior results that demonstrate the consequences of reduced endothermic reaction rate for the supersonic jet fluid in the blunt body case, we provide an explanation for observations of locally reduced OH PLIF signal in images of the keystone reaction zone structure of weakly unstable detonations. Modeling these flow features as exothermically reacting jets with similarly reduced reaction rates, we demonstrate a mechanism for jetting of bulk pockets of unreacted fluid with potentially differing kinetic pathways into the region behind the primary detonation front of strongly unstable mixtures. We examine the impact of mono-atomic and diatomic diluents on transverse structure. The results yield insight into the mechanisms of transition and characteristic features of both weakly and strongly unstable mixtures

Application of a laser induced fluorescence model to the numerical simulation of detonation waves in hydrogen-oxygen-diluent mixtures

A laser-induced-fluorescence model has been implemented and used to post-process detonation wave numerical simulation results to allow a direct comparison with previous experimental visualizations of detonations in hydrogen–oxygen–diluent mixtures. The model is first applied to steady one-dimensional simulation results obtained with detailed chemistry. The effects on the fluorescence intensity of the model parameters are examined to explore the dominant processes. The dominant interference process in the experiments carried out to date is the absorption of incident laser light by the high concentration of OH in and behind the reaction zone. The model is then applied to unsteady two-dimensional simulation results obtained with reduced chemical schemes to obtain synthetic PLIF image. The results demonstrate good qualitative agreement between the experimental and calculated laser-induced-fluorescence intensities. The model limitations and the experimental uncertainties are discussed together with a critical evaluation of the modeling approach