Detonation Initiation Research Articles

A canonical numerical experiment to study detonation initiation from colliding subsonic auto-ignition waves

The collision of two subsonic auto-ignition fronts with initial constant velocity was found to transit to detonation only when the collision angle was acute. The interaction of the reactive phase wave with inert hot layers constituted a singularity providing a continuous source of vorticity due to barocline effect. For an acute angle, this singularity that propagated at supersonic speed induced oblique pressure waves, of which resonance, due to the reactivity gradient geometry, near the center of the channel in the fresh gases accelerated the reactive wave fronts until transition to detonation. The numerical results of the present study, even if based on drastic assumptions, were at least in good qualitative consistency with experiments. The geometry of the reactivity gradients can thus provide another seed for the coupling between gas dynamics and heat release. Continuous pressure fluctuations and oblique shocks coming from vorticity sources and sheets from barocline effects can considerably enhance this transition. This path to transition could be complementary to that invoking mixing burning within premixed non-planar turbulent flame brush.

Deflagration and detonation induced by shock wave focusing at different Mach numbers

Shock wave focusing is an effective way to create a hot spot or a high-pressure and high-temperature region at a certain place, showing its unique usage in detonation initiation, which is beneficial for the development of detonation-based engines. The flame propagation behavior after the autoignition induced by shock wave focusing is crucial to the formation and self-sustaining of the detonation wave. In this study, wedge reflectors with two different angles (60° and 90°) and a planar reflector are employed, and the Mach number of incident shock waves ranging from 2.0 to 2.8 is utilized to trigger different flame propagation modes. Dynamic pressure transducers and the high-speed schlieren imaging system are both employed to investigate the shock-shock collision and ignition procedure. The results reveal a total of four flame propagation modes: deflagration, DDT (Deflagration-to-Detonation Transition), unsteady detonation, and direct detonation. The detonation wave formed in the DDT and unsteady detonation mode is only approximately 75%−85% of the Chapman-Jouguet (C-J) speed; meanwhile, the directly induced detonation wave speed is close to the C-J speed. Transverse waves, which are strong evidence for the existence of detonation waves, are discovered in experiments. The usage of wedge reflectors significantly reduces the initial pressure difference ratio needed for direct detonation ignition. We also provide a practical method for differentiating between detonation and deflagration modes, which involves contrasting the speed of the reflected shock wave with the speed of the theoretically nonreactive reflected shock wave. These findings should serve as a reference for the detonation initiation technique in advanced detonation propulsion engines.

On the critical initiation of planar detonation in Noble-Abel and van der Waals gas

The critical decay rate theory (C.A. Eckett, J.J. Quirk, J.E. Shepherd, Journal of Fluid Mechanics 421 (2000) 147–183) on detonation initiation in unsteady flow was extended from ideal to Noble-Abel and van der Waals gas, in which the finite molecular volume effect (or repulsion force) and the inter-molecular attraction effect (or attraction force) were included. Considering the reactive Euler equations, the reaction zone temperature-gradient equation was established independently of the equation of state and wave geometry, and then solved asymptotically by assuming large activation energy, planar wave front and a one-step irreversible reaction model. A critical decay time was derived as the critical condition for detonation initiation. It was found that the inter-molecular attraction effect makes initiation more difficult by increasing the critical decay time, whereas the finite molecular volume effect promotes initiation. The real gas effects are enhanced when increasing the reduced activation energy and/or decreasing the heat capacity ratio of perfect gas. The applicability and generality of the asymptotic solutions were evaluated by comparison with quasi-unsteady simulation employing detailed reaction models for hydrogen-air and ethylene-air mixtures, and with the further simplified, fully analytical solutions based on strong-shock assumption. The critical conditions of the asymptotic solutions are qualitatively consistent with those obtained with the detailed mechanisms. The asymptotic solutions can also be well reproduced by the simple analytical solutions in most conditions. This study thus establishes a simple, yet reliable, method to evaluate the real gas effect on detonation initiation in high-pressure conditions.

Characteristics of flame acceleration and deflagration-to-detonation transition enhanced by SF6 jet-in-cross-flow/flame interaction

Jet-in-crossflow (JICF), as a turbulence generator, is proposed to promote the flame acceleration and deflagration-to-detonation transition (DDT) process. Previous work has suggested that JICF using CO2 or Ar was in favor of flame acceleration under optimal conditions. The competition mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative combustion-inhibition caused by non-reactive gas has been demonstrated. It is worth noting that the difference in molar weight between the medium of JICF and the combustible mixture may significantly impact deflagration propagation prior to DDT. Thus, in this work, to investigate the flame propagation perturbed by the enhancement on the difference in molar weight between the JICF medium and combustible mixture, an experimental study of the flame propagation perturbed by SF6 JICF is carried out. By adjusting the injection pressure (Pj) of the SF6 JICF, the characteristics of flame propagation and DDT are analyzed. The results showed that the flame can accelerate to the onset of detonation within a short run-up distance in the cases with SF6 JICF. Schlieren images of flame evolution show that the flame disturbed by SF6 JICF rapidly transitions to a wrinkled flame. It is suggested that the SF6 JICF-enhanced turbulence promotes flame acceleration and DDT. The effect of SF6 JICF on the flame characteristics can be concluded in two aspects: one is that the inhomogeneously distributed vortices that are induced by JICF cause a nonuniform diffusion of SF6 in the tube, resulting in a nonuniform dilution of the combustible mixture near the jet; another one is that the vortices induced by JICF stretch the flame surface to increase the flame area. Increasing the injection pressure could promote the formation of complex wrinkles in the flame, however, it shows few further improvements in promoting DDT. Different jet mediums are also tested, and the results show that SF6 JICF can shorten the run-up distance significantly compared with helium and nitrogen JICF. The small and densely distributed wrinkles significantly increase the flame area to enhance heat release. The flame disturbed by SF6 JICF could accelerate to the local sound speed of the unburnt mixture, which results in the facilitation of the precursor shock wave formation. After the flame passes through the SF6 JICF, it remains a larger acceleration rate and then transitions to quasi-detonation rapidly. The conclusion of this work could help to qualitatively understand the positive effect of JICF-induced turbulence on flame acceleration and DDT. It also provides a relatively efficient and safe technology to promote detonation initiation in the design of detonation engines.

System modeling and simulation of flyer acceleration and explosive detonation in exploding foil initiator

In this theoretical study on an exploding foil initiator (EFI), the sequential events in an EFI system were numerically modeled and investigated from metallic bridge burst by ohmic heating and flyer acceleration by plasma expansion to explosive detonation by high-speed flyer impact. The EFI system modeling employed mass, momentum, and energy conservation coupled with the resistance–inductance–capacitance circuit equation of the initiation circuit and plasma property calculations of highly dense metallic plasma as well as a semi-empirical detonation model. The aim was to predict transient bridge behavior, flyer velocity, and detonation initiation threshold. Through comparisons and verifications with measured data from several existing EFI firing tests, the present system model was shown to predict bridge burst and flyer velocity very well. The numerical results indicated two different thresholds of charging voltage for flyer creation out of plastic film and impact detonation of explosive charge to be quite consistent with the measurements. The system behaviors in three different ranges of charging voltage were discussed in terms of discrete nature of this high-voltage explosive charge initiator.

Effects of Fuel Concentration Gradient on Stabilization of Oblique Detonation Waves in Kerosene–Air Mixtures

The transition structures and wave stabilization features of the wedge-induced oblique detonation wave (ODW) affect the combustion of the hypersonic air-breathing detonation engine, which is associated with the inhomogeneity of the inflow mixture. This study numerically investigates the influence of inhomogeneous kerosene–air mixtures on the stabilization of ODW for the first time, considering the inter-phase heat and mass transfers and focusing on the flow structure. The multiphase reacting flows are solved by the two-way coupling Eulerian–Lagrangian method. The inhomogeneous degree of fuel–air premixing is represented by the gradient of the liquid fuel equivalence ratio. A new pattern of transition wave structure from the shock-induced deflagration to oblique detonation is found. Under the fuel-rich condition before the shock-induced deflagration wave, a diamond-shaped wave structure is generated due to the large fuel concentration gradients. This flow structure is formed on the wedge without oscillations and is expected for a well-stabilized ODW. The initiation length of ODW is used to value the combustion performance. Its dependence on the inhomogeneous premixing degree displays a W-shaped curve. The chemical heat release influences the initiation length more obviously than the evaporative cooling in the fuel-lean conditions before the shock-induced deflagration. The ODW stabilization is enhanced by the heat released from the fuel-rich chemical reaction. Generally, the two-phase oblique detonation is determined by the competitiveness between the evaporative heat loss and chemical heat release. A uniform fuel–air mixture may not be optimal for detonation initiation based on the results of the present study.

Criticality for Oblique Detonation Waves Induced by a Finite Wedge in a Hydrogen–Air Mixture

Two-dimensional oblique detonation waves (ODWs) induced by finite wedges in a stoichiometric hydrogen–air mixture have been investigated numerically based on reactive Euler equations with a detailed chemical reaction model. The main zone affected by the expansion wave emanating from the turning point of a wedge is the flowfield downstream of the intersection point of the oblique shock wave (OSW) and the expansion wave. The ODW would be reduced to Chapman–Jouguet (CJ) detonation or decoupled combustion downstream. Three combustion regimes, detonation, decoupled combustion, and no ignition, were observed successively, as the wedge length decreases. It is found that the location of the intersection point is a key parameter for the detonation initiation. When the intersection point is located upstream of the ODW transition point, the expansion wave may quench ODW. Then, the critical wedge length is obtained by theoretical analysis of wave structures and the initiation criterion of ODWs for finite wedges is proposed. When the wedge length is greater than the critical wedge length, ODWs can be initiated. On the contrary, the initiation of ODWs do not occur. For wedge lengths small enough, no ignition occurs. Previously proposed criteria that use the induction length are also examined and compared with the present critical wedge length criterion in this study, and the latter is proven to achieve better results.

A modified Lotka–Volterra oscillating chemical scheme for detonation simulation

Modified Lotka–Volterra chemical schemes were developed with the goal of performing numerical simulation of detonation driven by single- and multi-stage heat release profiles. An initiation reaction was added to the original Lotka–Volterra model to create an induction zone, which is an important characteristic of combustion process. The kinetics parameters of the reaction rates were adjusted, so that up to eight peaks of heat release could be generated in the steady detonation reaction zone. The structure of steady detonations driven by modified Lotka–Volterra models were examined in detail. Interestingly, the stability criteria typically employed to characterize detonations, i.e., the reduced activation energy and the χ parameter, do not seem to be applicable for Lotka–Volterra schemes exhibiting multi-stage heat release profiles. The applicability of these multi-step chemical models to unsteady detonation simulation was verified through preliminary results obtained for one- and two-dimensional numerical simulations. Under super-critical detonation initiation by a point-energy source, some Lotka–Volterra schemes led to an unusual behavior with large velocity oscillations at large over-drive but a rather steady propagation at near Chapman–Jouguet velocity. Two-dimensional cellular detonation driven by Lotka–Volterra scheme with two stages of heat release and a high activation energy of the initiation step demonstrates some features which are characteristic of a double cellular structure. The present work constitutes a first step toward the understanding of the features and dynamics of detonation driven by Lotka–Volterra schemes as further work is needed to fully grasp the complex responses induced by multiple stages of heat release on detonation dynamics.

DETONATION OF A COMBUSTIBLE GAS MIXTURE UPON THE INTERACTION OF A SHOCK WITH AN ELLIPSOIDAL INERT GAS BUBBLE

Interaction of a shock wave in a combustible gas mixture with an ellipsoidal region of an inert gas of increased density is numerically simulated using the Euler equations in two-dimensional plane and axisymmetric formulations. Four qualitatively different regimes of indirect initiation of detonation have been found: upon reflection of a wave from the gas interface, upon focusing of secondary transverse shock waves on the axis/plane of symmetry, upon amplification of a transverse wave converging to the axis of symmetry, and upon secondary focusing of waves in front of the bubble. It is shown that the mode of detonation initiation significantly depends on both the intensity of the shock wave and the shape of the bubble. Based on a series of simulations, the dependence of the threshold Mach numbers of the incident wave on the shape of the bubble is determined. In the plane flow, a moderate elongation of the bubble leads to a significant decrease in the threshold Mach number. In an axisymmetric flow, the lower threshold Mach number is less sensitive to the shape of the bubble, and the most effective detonation initiation is carried out using a spherical bubble. The effect of shock wave focusing makes it possible to achieve successful initiation of detonation at a fundamentally lower intensity of the incident wave compared to direct initiation.

Universality and Nonuniversality in Distributed Nuclear Burning in Homogeneous Isotropic Turbulence.

Nuclear burning plays a key role in a wide range of astrophysical stellar transients, including thermonuclear, pair instability, and core collapse supernovae, as well as kilonovae and collapsars. Turbulence is now understood to also play a key role in these astrophysical transients. Here, we demonstrate that turbulent nuclear burning may lead to large enhancements above the uniform background burning rate, since turbulent dissipation gives rise to temperature fluctuations, and in general the nuclear burning rates are highly sensitive to temperature. We derive results for the turbulent enhancement of the nuclear burning rate under the influence of strong turbulence in the distributed burning regime in homogeneous isotropic turbulence, using probability distribution function methods. We demonstrate that the turbulent enhancement obeys a universal scaling law in the limit of weak turbulence. We further demonstrate that, for a wide range of key nuclear reactions, such as C^{12}(O^{16},α)Mg^{24} and 3-α, even relatively modest temperature fluctuations, of the order of 10%, can lead to enhancements of 1-3 orders of magnitude in the turbulent nuclear burning rate. We verify the predicted turbulent enhancement directly against numerical simulations, and find very good agreement. We also present an estimation for the onset of turbulent detonation initiation, and discuss implications of our results for stellar transients.

The evolution of fast turbulent deflagrations to detonations

We use advanced experimental techniques to explore turbulence-induced deflagration-to-detonation transition (tDDT) in hydrogen–air mixtures. We analyze the full sequence of turbulent flame evolution from fast deflagration-to-detonation using simultaneous direct measurements of pressure, turbulence, and flame, shock, and flow velocities. We show that fast turbulent flames that accelerate and develop shocks are characterized by turbulent flame speeds that exceed the Chapman–Jouguet deflagration speed in agreement with the tDDT theory and direct numerical simulation (DNS) results. Velocity and pressure evolutions are provided to detail the governing mechanisms that drive turbulent flame acceleration. Turbulent flame speeds and fluctuations are examined to reveal flow field characteristics of the tDDT process. This work contributes to the understanding of fundamental mechanisms responsible for spontaneous initiation of detonations by fast turbulent flames.

Research on Effect of Initiation Detonation Method on Fragment Velocity Distribution

To enhance the damage effect of focusing warhead, the investigation about the scattering behavior of focusing warhead is conducted. Based on the momentum method and the detonation wave theory, the influence of initiation detonation methods and curvature radius of the charge on fragment velocity is analyzed. The fragments velocity distribution models of the end-initiated, the center-initiated and the two ends-initiated are obtained. Numerical simulation shows that fragments velocity distribution and the propagation of the detonation wave of numerical simulation results have a good fit with the theoretical model. It is shown that fragments velocity is higher in the middle and low on both sides. When it was end-initiated, the location where the fragment velocity reaches the maximum was 18.6cm from the initiation end. When it was center-initiated, the maximum fragment velocity appeared at the center of the projectile axis. When it was two ends-initiated, the maximum fragment velocity appeared symmetric about the center line of the projectile axis. With increasing of the curvature radius of the circular charge, the maximum velocity of the fragments increased.

Effects of the quantity and arrangement of reactive jet obstacles on flame acceleration and transition to detonation: A numerical study

Solid obstacles induce the detonation in premixed gases while causing significant thrust losses. Jet obstacles can alleviate this deficiency, but there have been few studies on multi-group reactive jet obstacles. In this study, a detailed numerical simulation is performed to investigate the selection of the quantity of reactive jet obstacles and examine the effect of the arrangement of different jet obstacles on flame acceleration and the deflagration to detonation transition (DDT) processes. The results show that there is an optimal quantity of jet obstacles that can rapidly trigger detonation while reducing thrust loss and combustion chamber weight. In terms of flame acceleration, the mechanisms of flame acceleration by reactive jet obstacles with various arrangement types are complex. Specifically, the initial flame acceleration effect of reactive jet obstacles with different arrangement types is similar. However, the staggered arrangement has a longer-lasting vortex structure, stronger leading compression wave and flow field intensity, which provides favorable conditions for the ensuing flame acceleration. When the flame interacts with the jet, the larger virtual blockage ratio and stronger combustion heat release effect further amplify the flame acceleration discrepancy. In DDT, compared with solid obstacles, reactive jet obstacles have more advantages. Different types of jet obstacles arrangement differ in their ability and method in triggering DDT, with the staggered arrangement having the best DDT effect, and detonation initiation modes are divided into two categories: i) the detonation induced by the coupling of the flame surface and the high-pressure region; ii) the reflected wave impinges on the flame surface after the compression waves in front of the flame collide with the wall, resulting in a detonation. Although the quantity and arrangement of jet obstacles vary in this study, all the cases where detonation occurs are consistent with the flame acceleration model of Liberman and similar SWACER mechanism.

NUMERICAL STUDY OF DETONATION INITIATION BY SPARK DISCHARGE IN A HYDROGEN-OXYGEN HIGH-PRESSURE GAS MIXTURE

The influence of the initial pressure of the combustible gas mixture on the processes that occur during the initiation of detonation by a spark discharge in a high-pressure hydrogen-oxygen mixture was estimated. A onedimensional gas-dynamic model in a chemically reacting gas with separation of the vibrational temperature of diatomic molecules from the kinetic temperature of the gas was used for the numerical study. It was simulated that an intensity of the shock wave causing the detonation initiation by a critical regime is below of the detonation wave intensity when the initial gas pressure is 1.013 MPa and the intensity of the shock wave exceeds the detonation wave intensity when the initial gas pressure is 0.304 MPa. It was found out that the process of the detonation initiation falling by a near-critical regime is more longitudinal when the gas pressure is low. Energy deposition history into the spark channel when detonation is initiated at different initial gas pressures was calculated.

Strong External Electric Fields Reduce Explosive Sensitivity: A Theoretical Investigation into the Reaction Selectivity in NH2NO2∙∙∙NH3

Controlling the selectivity of a detonation initiation reaction of explosive is essential to reduce sensitivity, and it seems impossible to reduce it by strengthening the external electric field. To verify this, the effects of external electric fields on the initiation reactions in NH2NO2∙∙∙NH3, a model system of the nitroamine explosive with alkaline additive, were investigated at the MP2/6-311++G(2d,p) and CCSD(T)/6-311++G(2d,p) levels. The concerted effect in the intermolecular hydrogen exchange is characterized by an index of the imaginary vibrations. Due to the weakened concerted effects by the electric field along the -x-direction opposite to the "reaction axis", the dominant reaction changes from the intermolecular hydrogen exchange to 1,3-intramolecular hydrogen transference with the increase in the field strengths. Furthermore, the stronger the field strengths, the higher the barrier heights become, indicating the lower sensitivities. Therefore, by increasing the field strength and adjusting the orientation between the field and "reaction axis", not only can the reaction selectivity be controlled, but the sensitivity can also be reduced, in particular under a super-strong field. Thus, a traditional concept, in which the explosive is dangerous under the super-strong external electric field, is theoretically broken. Compared to the neutral medium, a low sensitivity of the explosive with alkaline can be achieved under the stronger field. Employing atoms in molecules, reduced density gradient, and surface electrostatic potentials, the origin of the reaction selectivity and sensitivity change is revealed. This work provides a new idea for the technical improvement regarding adding the external electric field into the explosive system.

Numerical and experimental analysis of detonation induced by shock wave focusing

Previous studies have demonstrated that shock wave focusing is an efficient method for detonation initiation by creating hot spots in the interaction of shock-shock or the collision of shock with the boundary layer. The velocity of the incident shock wave and the shape of the reflector are both crucial to the self-ignition process and the flame propagation mode. This study adopts both numerical and experimental methods to investigate the ignition modes and detonation propagation induced by shock wave focusing. Two reflectors, a 60° wedge reflector and a 90° wedge reflector, and four incident shock wave velocities are employed as parametric variables, and their effects on the detonation initiation behavior are systematically investigated. The results show that three ignition modes, namely, deflagration, quasi-detonation, and direct detonation, occur with increasing intensity of the incident shock wave. The comparison between different cases suggests that the formation of new hot spots and transverse waves is of great importance to the initiation and maintenance of detonation waves. The Richtmyer-Meshkov instability (RMI) is demonstrated to be the reason for the generation of a mushroom-shaped jet, which can be observed both in simulations and experiments and represents the successful formation of detonation. This study offers new insight into the ignition behavior in wedge reflectors, supporting the crucial role of shock wave focusing in self-ignition and detonation initiation.

An experimental study of detonation initiation in supersonic flow using a hot jet

An experimental study of detonation initiation in supersonic flow using a hot jet

Mathematical Modeling of Detonation Initiation in the Channel with a Profiled End Using Parallel Computations

Mathematical Modeling of Detonation Initiation in the Channel with a Profiled End Using Parallel Computations

DEVELOPMENT OF A MODEL OF HYBRID DETONATION IN A MIXTURE OF OXYGEN-HYDROGEN-ARGON WITH ALUMINUM PARTICLES

A simple mathematical model of hybrid detonation in oxygen-hydrogen-argon mixtures with suspended aluminum particles is suggested. The model is based on the approaches of the mechanics of multiphase media and equations of reduced chemical kinetics to describe gaseous and heterogeneous reactions. Production of alumina in the form of nanoparticles is considered. The problem of initiation and propagation of cellular detonation in a planar channel is analyzed. The results of calculations show that addition of small amount of combustible particles leads to increase in the detonation velocity and transformation of the cellular structure.

Reduction and applicability of chemical kinetic model for ethylene detonation simulation

The selection of chemical reaction kinetic model is essential in detonation simulation. However, how to obtain an appropriate kinetic mechanism to describe chemical reaction process is still an open question, even for the common ethylene detonation simulation. Since the complete detailed mechanism is extremely time consuming in detonation simulation, single-step or several-step simplified models are widely used, although not capable to realistically reproduce unsteady processes in some detonation studies, especially for detonation initiation and failure. Therefore, a series of studies on the reduction, validation and applicability of chemical reaction kinetic model for ethylene detonation are carried out. First, a reduced kinetic model with 55 reactions and 26 species is proposed for ethylene, based on the detailed kinetic model of USC 2.0. A combination method of path flux analysis and sensitivity analysis is used to initially develop a skeleton model, which is employed to acquire a reduced kinetic model by the sensitivity analysis method. Then, predictive performance of the proposed reduced kinetic model is extensively investigated for one-dimensional steady detonation, one-dimensional and two-dimensional unsteady detonation, as well as zero-dimensional constant volume explosion. The results show that the present reduced model is well suitable for detonation simulations within a broad range of conditions. Compared with three typical reduced models in the literature, the present model performs much better overall, particularly in unsteady ethylene detonation simulation. This work provides a valuable roadmap and solution for the development and selection of chemical reaction kinetic models in detonation simulation.