Direct Initiation Of Detonation Research Articles

Exploring the chemical kinetic effects on the direct detonation initiation in H2-O2-Ar mixtures

Chemical kinetics plays an important role in the direct detonation initiation (DDI) of various combustible mixtures. However, its impact on detonation dynamics has rarely been studied with detailed mechanisms. This study introduces the active subspace method to systematically explore the chemical kinetics impact on the unsteady detonation dynamics in DDI of H2-O2-Ar mixture, with a 13-species detailed mechanism. The kinetic effects on the ZND structure with sub-CJ shock speed (sub-CJ ZND structure) is first investigated, where three important reactions to the sub-CJ minimum shock speed, H + O2 = O + OH, H + O + M = OH + M and H2O + H2O = H + OH + H2O are identified. Then the active subspace method is further employed to analyze the kinetics impact on the critical initiation energy ( E cr ) predicted with the critical decay rate (CDR) model and critical decay time (CDT) model. Results show that the CDT model, which utilises the sub-CJ ZND model to compute the critical parameters, can properly capture the one-dimensional detonation dynamics. All the important reactions identified with the CDT model agree well with those of one-dimensional simulations. In contrast, the CDR model shares a similar active subspace with the constant volume auto-ignition process and fails to identify the less important reactions. The consistency in the key reactions between one-dimensional simulations and the CDT model implies that it is viable to employ the CDT model as an efficient surrogate for quantifying the kinetic effects in unsteady detonation dynamics.

Study on direct initiation process and initiation mechanism of acetylene/oxygen premixed gas detonation

Studying the process and mechanism of direct initiation of gas detonation is the basis for preventing serious gas explosion disasters and developing detonation propulsion power devices. This article utilizes a combustible gas combustion and explosion experimental system to achieve direct spherical acetylene/oxygen mixture detonation through a high-energy ignition system. The detonation velocity and flame structure evolution process in direct detonation were studied through schlieren images, and a critical detonation model was established to explain the direct detonation process. The critical detonation wave velocity for direct detonation was determined to be half of the CJ detonation velocity. Revealed the mechanism of detonation cell formation during the direct initiation process. The basic criterion for achieving direct initiation of spherical detonation has been obtained: to achieve direct detonation, when the overdrive detonation wave formed by the detonation source attenuates to half the detonation velocity of CJ, the propagation radius must not be less than a full detonation cell size. This article conducts in-depth research on the direct initiation process of acetylene/oxygen mixture detonation. The results obtained have essential value for developing and applying the detonation theory of acetylene fuel. At the same time, they have important practical significance for preventing gas fire and explosion accidents.

Effects of ozone addition on direct detonation initiation in hydrogen/oxygen mixtures

This work computationally studies the influence of ozone addition on direct detonation initiation in stoichiometric H2-O2-O3 mixtures. In addition to greatly reducing the critical initiation energy, Ec, ozone addition is also observed to qualitatively and significantly influence the detonation dynamics in both subcritical and critical regimes. In the planar configuration, an additional critical initiation energy Ec’ ≥ Ec, is identified, indicating the development of a second detonation wave when the initiation energy, E is above Ec’. Mechanistically, the second detonation is initiated through Zeldovich mechanism of reactivity gradient. Instead of artificially setting temperature or composition gradient, the propagating leading shock triggers sequential autoignition events in the unburnt pockets. This detonation evolves into a pressure/shock wave propagating behind the first detonation front, and then either gradually decays, or catches up with the leading detonation, depending on the initiation energy. However, in the spherical configuration, this second shock or pressure wave is suppressed by the curvature loss and is no longer sustainable. Furthermore, the leading shock can be greatly enhanced during the initial stage of failed initiation, which may subsequently lead to re-initiation at larger radius with less curvature loss.

Direct detonation initiation in hydrogen/air mixture: effects of compositional gradient and hotspot condition

Two-dimensional simulations are conducted to investigate the direct initiation of cylindrical detonation in hydrogen/air mixtures with detailed chemistry. The effects of hotspot condition and mixture composition gradient on detonation initiation are studied. Different hotspot pressures and compositions are first considered in the uniform mixture. It is found that detonation initiation fails for low hotspot pressures and the critical regime dominates with high hotspot pressures. Detonation is directly initiated from the reactive hotspot, whilst it is ignited somewhere beyond the non-reactive hotspots. Two cell diverging patterns (i.e. abrupt and gradual) are identified and the detailed mechanisms are analysed. Moreover, cell coalescence occurs if many irregular cells are generated initially, which promotes the local cell growth. We also consider non-uniform detonable mixtures. The results show that the initiated detonation experiences self-sustaining propagation, highly unstable propagation and extinction in mixtures with a linearly decreasing equivalence ratio along the radial direction, i.e. 1 → 0.9, 1 → 0.5 and 1 → 0. Moreover, the hydrodynamic structure analysis shows that, for the self-sustaining detonations, the hydrodynamic thickness increases at the overdriven stage, decreases as the cells are generated and eventually becomes almost constant at the cell diverging stage, within which the sonic plane shows a ‘sawtooth’ pattern. However, in the detonation extinction cases, the hydrodynamic thickness continuously increases, and no ‘sawtooth’ sonic plane can be observed.

Critical Decay Time Model for Direct Detonation Initiation Energy in Gaseous Mixtures

A critical decay time (CDT) model is developed to predict the critical energy of direct detonation initiation in gaseous mixtures. It is based on the global initiation criterion that the energy deposit should allow the decaying shock speed to stay in a specific range below the Chapman–Jouguet (CJ) speed at least for a critical decay time. The speed range is estimated with the sub-CJ Zel’dovich–von Neumann–Döring (ZND) simulations. The critical decay time is calculated as the minimum time to reach unity Mach number in the sub-CJ ZND simulations. The lower-speed bound is taken as a characteristic extinction speed below (which means the lower-speed bound) which the direct initiation should fail. This speed is calibrated using one-dimensional simulations for hydrogen (H2)–oxygen (O2)–argon (Ar) mixtures. The calibrated CDT model is then applied to estimate the critical initiation energy with the point-blast theory. The model yields better agreement with experimental data for hydrogen-fueled mixtures such as H2─O2─Ar and H2─air mixtures than the well-known critical decay rate model. For small hydrocarbon-fueled mixtures such as acetylene (C2H2)─O2─Ar, methane (CH4)–air, ethylene (C2H4)–air, and ethane (C2H6)─air mixtures, the predicted critical energies also agree well with experimental results. The CDT model provides an efficient tool to evaluate the detonability of fuel–oxidizer mixtures, which could be beneficial for ignition initiation in propulsion and power devices such as rotating detonation engines.

Effects of low-temperature chemistry on direct detonation initiation by a hot spot under engine conditions

This study investigates the effect of low-temperature chemistry (LTC) on the minimum runup distance, lm, of direct detonation initiation by an autoignitive hot spot under engine conditions. A newly predictive model is proposed to determine an a priorilm. The temperature and pressure increase and the radical build-up prior to the main ignition resulting from LTC are incorporated in the model that allows a better prediction of ignition modes. A series of 1D simulations are performed by varying the initial temperature, the hot spot size and its temperature difference at constant-volume and engine conditions. DME (ethanol) is used to represent a two (single) stage ignition fuel. It was found that taking the transient evolution of the mixture state as an initial condition to the ZND model results in a better representative exothermic characteristic length scale, which exhibits a strong linear correlation with lm regardless of fuel types over a wide range of conditions. The LTC oxidation is found to reduce lm by approximately a factor of two for the low-temperature cases, leading to a comparable value of lm to that of the high-temperature cases. It was also found that the ignition modes are better predicted by identifying an alternative characteristic length scale based on the variation of ignition delay time τig within its range of monotonic distribution. For a hot spot that has temperature variation spanning across the NTC regime, the runup distance was found to be shortened by approximately a factor of two, such that a longer hot-spot size is required to form detonation. In addition, a better prediction of ignition modes was achieved by defining the normalized front speed as a statistical mean for each runup-distance element than that determined at the midpoint of a hot spot.

One-dimensional Numerical Study on Ignition of the Helium Envelope in Dynamical Accretion during the Double-degenerate Merger

In order for a double-detonation model to be viable for normal type Ia supernovae, the adverse impact of helium-burning ash on early time observables has to be avoided, which requires that the helium envelope mass should be at most 0.02 M ⊙. Most of the previous studies introduced detonation by artificial hot spots, and therefore the robustness of the spontaneous helium detonation remains uncertain. In the present work, we conduct a self-consistent hydrodynamic study on the spontaneous ignition of the helium envelope in the context of the double-degenerate channel, by applying an idealized one-dimensional model and a simplified seven-isotope reaction network. We explore a wide range of the progenitor conditions and demonstrate that the chance of direct initiation of detonation is limited. Especially, the spontaneous detonation requires the primary envelope mass of ≳0.03 M ⊙. Ignition as deflagration is instead far more likely, which is feasible for the lower envelope mass down to ∼0.01 M ⊙, which might lead to subsequent detonation once the deflagration-to-detonation transition (DDT) is realized. High-resolution multidimensional simulations are required to further investigate the DDT possibility, as well as accurately derive the threshold between the spontaneous detonation and deflagration ignition regimes. Another interesting finding is the effect of the composition: while mixing with the core material enhances detonation as previously suggested, it rather narrows the chance for deflagration due to the slower rate of the 12C(α,γ)16O reaction at the lower temperature ∼108K, with the caveat that we presently neglect the proton-catalyzed reaction sequence of 12C(p,γ)13O(α,p)16O.

Phenomenon and analysis of direct initiation of detonation using multiple turbulent flame jets

This work reports experimental investigations on the direct initiation of detonation using multiple turbulent flame jets, with a special focus on the arrangement schemes and fundamental physics in the initiation processes. Results show that the direct initiation of detonation can be achieved using turbulent jets even when the jet tube diameter is much smaller than the empirical critical tube diameter due to flame–shock–wall interactions. Conspicuous evidence has been shown that the probability of the direct initiation increases significantly near the detonatability limit using multi-jets compared to a single jet. These results are found to be closely related to several new phenomena observed when using multiple jets to initiate the detonation. They are: (1) unexpected rapid promotion of the final-stage flame acceleration in ignition tubes by multiple jets, which is attributed to the fact that the expanding precursor shock waves propagate back into the adjacent tube and interact with the flame; (2) enhancement of hot spot generation by multiple jets due to the precursor shock intersection and the formation of an induction zone; (3) obvious velocity loss of impinging jets initiation as a result of induced hot spots propagation in the burned gases.

Effect of ozone addition and ozonolysis reaction on the detonation properties of C2H4/O2/Ar mixtures

Ozone is one of the strongest oxidizers and can be used to enhance detonation. Detonation enhancement by ozone addition is usually attributed to the ozone decomposition reaction which produces reactive atomic oxygen and thereby accelerates the chain branching reaction. Recently, ozonolysis reaction has been found to be another mechanism to enhance combustion for unsaturated hydrocarbons at low temperatures. In this study, the effects of ozone addition and ozonolysis reaction on steady detonation structure and transient detonation initiation and propagation processes in C2H4/O2/O3/Ar mixtures are examined through simulations considering detailed chemistry. Specifically, the homogeneous ignition process, the ZND detonation structure, the transient direct detonation initiation, and pulsating instability of one-dimensional detonation propagation are investigated. It is found that the homogenous ignition process consists of two stages and the first stage is caused by ozonolysis reactions which consume O3 and produces CH2O as well as H and OH radicals. The ozonolysis reaction and ozone decomposition reaction can both reduce the induction length though they have little influence on the Chapman–Jouguet (CJ) detonation speed. The supercritical, critical and subcritical regimes for direct detonation initiation are identified by continuously decreasing the initiation energy or changing the amount of ozone addition. It is found that direct detonation initiation becomes easier at larger amount of ozone addition and/or larger reaction progress variable. This is interpreted based on the change of the induction length of the ZND detonation structure. Furthermore, it is demonstrated that the ozonolysis reaction can reduce pulsating instability and make the one-dimensional detonation propagation more stable. This is mainly due to the reduction in activation energy caused by ozone addition and/or ozonolysis reaction. This work shows that both ozone decomposition reaction and ozonolysis reaction can enhance detonation for unsaturated hydrocarbon fuels.

Exploring the controlling mechanisms for gradient evolution in unsteady detonation flows

Proper characterization of the transient interaction between shock front motion and chemical energy release is the key for further understanding of unsteady detonation dynamics. We have shown that the gradients could be the adequate intermediate quantities to reflect the transient interaction between shock front motion and chemical energy release through the Lagrangian particle analysis of the simulated direct detonation initiation (DDI) process in H2–O2–Ar mixtures. Specifically, the “shock change equations” are verified to describe the direct relation between the shock front motion and the gradients immediately behind the shock. Moreover, given the time derivatives of shock speed, the gradient evolution in the induction zone can be reproduced by the gradient evolution equations that are deduced from the Euler equations, no matter if the shock front undergoes rapid deceleration or acceleration. While in the reaction zone where the heat release is significant, it is demonstrated that the evolutions of velocity, pressure, and their gradients can be described by the Zel'dovich–von Neumann–Döring model with a constant wave speed that is below the Chapman–Jouguet speed, no matter if the shock front undergoes rapid deceleration or acceleration. These two distinct controlling mechanisms are verified in both planar and spherical DDI processes, showing their general applicability. This work suggests a new perspective, in terms of gradient evolution, for further understanding the unsteady detonation dynamics.

Asymptotic Solutions of Two Fundamental Problems in Gaseous Detonations

ABSTRACT Two recent analytical results concerning i) the direct initiation of gaseous detonations in free space and ii) the deflagration to detonation transition in tubes are revisited in this article. The first problem is treated by an asymptotic analysis in the limit of small heat release, enlightening the mechanisms controlling the direct initiation process of real detonations. The second one is limited to a one-dimensional mechanism of transition at work on the tip of elongated flames in tubes. Particular attention is paid to the physical mechanisms more than to the technical details of the analyses. However, the mathematical formulation as well as the different steps of the analysis are clearly presented in a synthetic manner.*

Onset of Oblique Detonation Waves for a Cavity-Based Wedge

To shorten the ignition length of the wedge-induced oblique detonation wave, a geometric setup in which the leading and aft wedges are connected by a cavity is proposed. High-resolution viscous computations to solve the reactive Reynolds-averaged Navier–Stokes equations have been performed, which examine the onset and evolution of combustion and flow configuration with premixed mixtures of inflow Mach numbers ranging from 4.0 to 6.0. The unreactive flowfield of the cavity-based wedge is analogous to that of a sole cavity with the exclusion of shock/shock interaction downstream. For and 5.0, no sustainable oblique detonation wave is achieved, and the flame holds in the cavity and near-wall area of the aft wedge; however, a short-lived detonation wave triggered by the shock focuses is observed during the combustion procedure with . For , nearly direct detonation initiation of postshock occurs, and an oblique detonation wave is established above the fore edge of the cavity. Unexpectedly, a delayed ignition downstream contributes to the development of a recirculation zone, which results in a detached oblique detonation wave for .

Numerical investigation of free oblique detonation wave induced by non-intrusive energy deposition

To validate the concept of free oblique detonation waves (ODWs) induced by non-intrusive energy deposition applied in ODW engines (ODWEs), numerical simulations are performed by modeling the laser spark as a high-temperature and high-pressure zone embedding in the high-speed incoming flow and by solving the non-dimensional reactive Euler equations with one-step chemistry. The initiation process, flow structures, and self-adapted characteristics of such kinds of free ODWs in an open space are discussed with varying dimensionless source energy and inflow Mach number. The results show that free ODWs can be initiated successfully by local non-intrusive energy deposition. In particular, four ignition regimes, namely, the subcritical regime, the critical I regime, the critical II regime, and the supercritical regime, which are analogous to the one-dimensional direct initiation of detonation, are distinguished by varying the dimensionless source energy. However, the typical energy required by each ignition regime is higher than that in the one-dimensional case due to multi-dimensional energy transmission and instabilities. Moreover, the dimensionless source energy should be moderate to obtain a stable free ODW flow field because extremely low source energy leads to ignition failure, while extremely high source energy causes premature ignition. When the inflow Mach number varies, it is found that free ODWs can adjust automatically to a nearly Chapman–Jouguet state, indicating the self-sustaining nature of free detonation waves. Due to this feature, ODWE performance will be relatively stable despite the change in inflow Mach numbers if free ODWs are utilized in the combustor.

Direct detonation initiation: A comparison between the critical curvature and critical decay rate models

The present study compares the critical initiation energy predicted by the critical curvature (CC) and critical decay rate (CDR) models. To ensure a fair and meaningful comparison between these two theoretical approaches, the Taylor–Sedov blast wave model, which enables us to relate the critical state to the energy of the point source, has been employed in both models. Simplified as well as detailed chemical mechanisms were employed for the comparison. By using the same blast wave model, the ratio of critical initiation energy calculated with the CC and CDR models was found to be one to two orders of magnitude smaller than the results in previous studies. Although the choice of the blast wave model is important, the critical energy predicted by the CC model is invariably larger than the one predicted by the CDR model. This was explained by analyzing the relationship between the shock front radius and decay time, as well as the ignition delay-time around the critical conditions of the two models. It was demonstrated that the critical conditions of the CDR model can be fulfilled more easily than those of the CC model. As a result, the main source of discrepancy between the CDR and CC models is that they adopt different initiation failure mechanisms, namely, curvature for the CC model against unsteadiness for the CDR model.

Asymptotic analysis of the critical dynamics of spherical gaseous detonations

Abstract

Effect of 2-step energy release on direct detonation initiation by a point energy source in a rich H2–NO2/N2O4 mixture

In the present study, direct detonation initiation by a point energy source in a rich H2–NO2/N2O4–Ar mixture has been investigated using high-resolution one-dimensional numerical simulations performed with detailed chemistry. In particular, the effect of the 2-step energy release profile on the critical initiation energy and the dynamics of the failure process has been studied for the first time. Three reduced reaction models exhibiting a similar first step of energy release but a second step of different strengths have been employed. As the second step of heat release is strengthened, the overall behavior gets closer and closer to a 1-step process. It was found that the critical initiation energy for successful detonation initiation decreases as the second step of heat release is strengthened and the width of the overall heat release pulse is reduced. This feature has been investigated by studying the competition between chemical heat release and unsteadiness along the path of Lagrangian particles. The decrease of the critical initiation energy for stronger second step of heat release seems to be a result of the decrease of the sensitivity of this second step to unsteadiness-induced quenching behind a decaying shock wave. In view of the present results and previous experimental observations on propagating and diffracting detonation in H2–NO2/N2O4(–Ar) mixtures, it seems that the 2-step energy release presenting the specific characteristics observed in rich H2–NO2/N2O4(–Ar) mixtures, i.e. a strong and rapid first step and a weak and slow second step, constitutes a feature which is not essential for detonation propagation but makes detonation direct initiation and transmission to open space more difficult.

Analytical study of the direct initiation of gaseous detonations for small heat release

An analysis of the direct initiation of gaseous detonations in a spherical geometry is presented. The full set of constitutive equations is analysed by an asymptotic analysis in the double limit of Mach number close to unity (small heat release) and large thermal sensitivity. The quasi-steady curvature-induced quenching phenomenon is first revisited in this limit. Considering a realistic decrease rate of the rarefaction wave, the unsteady problem is reduced to a single nonlinear hyperbolic equation. The time-dependent velocity of the lead shock is an eigenfunction of the problem when two boundary conditions are imposed to the flow at the lead shock and at the burnt gas side. Following (Linan et al. , C. R. Mec. , vol. 340, 2012, pp. 829–844), the boundary condition in the quasi-transonic flow of burnt gas is expressed in terms of the curvature. Focusing our attention on successful initiation, the time-dependent velocity of the lead shock of a detonation approaching the Chapman–Jouguet regime is the solution of a nonlinear integral equation investigated for stable and marginally unstable detonations. By comparison with the quasi-steady trajectories in the phase space ‘propagation velocity versus radius’, the solution exhibits the unsteady effect produced upon the detonation decay by the long time delay of the upstream-running mode for transferring the rarefaction-wave-induced deceleration across the inner detonation structure from the burnt gas to the lead shock. In addition, a new and intriguing phenomenon concerning pulsating detonations is described. Even if the results are not quantitatively accurate, they are qualitatively relevant for real detonations.

Combustible Gas Cylinder Detonation upon Incident Shock Focusing

Two-dimensional interaction of a shock in air with elliptic area (two-dimensional gas bubble) filled with propane-oxygen mixture with addition of heavy gas is numerically studied using Euler’s equations. Propane combustion is modeled with one-stage Arrhenius kinetics. Three different ignition regimes are found: direct detonation initiation by sufficiently strong shock, detonation near the triple point formed during weaker shock refraction and detonation at the focusing point of even weaker shock. The latter regime is observed only for significantly elongated bubbles. Detonation initiation regime dependence on shock Mach number and bubble diameter ratio is determined. It is shown that due to bubble elongation, critical Mach number may be significantly lowered in comparison with direct initiation.

Advances in the analytical study of the dynamics of gaseous detonation waves

Recent theoretical results on the dynamics of gaseous detonations are presented. An asymptotic analysis is performed, retaining the physical mechanisms controlling the modifications to the inner structure of the detonation. As a result, the system of hyperbolic equations for the compressible fluid mechanics coupled with a detailed chemical kinetics of heat release is reduced to a single integral equation for the propagation velocity of the combustion wave versus time. Concerning the direct initiation of spherical detonations by a blast wave, curvature effects are shown to be responsible for a critical condition of initiation. Near criticality, the role of the unsteadiness of the inner structure is pointed out. The whole complexity of the critical dynamics is reproduced and explained by the integral equation. The necessary background knowledge in gaseous detonation is recalled in the two first sections of the article in order to facilitate the reading by non-specialists.

Shock focusing and detonation initiation at a flame front

Multidimensional numerical simulations were performed to study the interaction of focused shock waves and a flame front leading to detonation initiation. The fully compressible Navier–Stokes equations, coupled with a chemical-diffusive model for energy release and conversion of fuel to product in a stoichiometric hydrogen–air mixture, were solved using a third-order method on a dynamically adapting mesh. Preliminary simulations of deflagration-to-detonation transition (DDT) in an obstructed channel, when compared to previous experiments, point to a DDT scenario where detonation initiation arises from multi-shock focusing at a flame front. A detailed examination of an idealized problem showed two mechanisms of detonation formation: (1) direct detonation initiation triggered at the collision spot by focusing shocks at the flame front, and (2) focusing of relatively weak shocks leading to a delayed transition to detonation through the reactivity-gradient mechanism. Comparisons between the detailed analysis of shock-focusing and experimentally observed DDT phenomena suggests that shock focusing plays an important role in the occurrence of DDT for this problem.