Deflagration Process Research Articles

Numerical and experimental study of thermal explosions in LX-10 and PBX 9501: Influence of thermal damage on deflagration processes

We employ in-situ flash x-ray imaging, together with a detailed multiphase convective burn model, to demonstrate how explosives' binder characteristics influence the burning processes in thermal explosions. Our study focuses on the HMX-based explosives LX-10 and PBX 9501. While the HMX (cyclotetramethylene-tetranitramine) crystallite size distributions for these two explosives are nearly identical before heating, our experiments and simulations indicate that after heating, variations result due to differences in binder composition. Post-ignition flash x-ray images reveal that the average density decreases at late times more rapidly in PBX 9501 than LX-10, suggesting a faster conductive burning rate in PBX-9501. Heated permeability measurements in LX-10 and PBX 9501 demonstrate that the binder system characteristics influence the evolution of connected porosity. Once ignited, connected porosity provides pathways for product gas heating ahead of the reaction front and additional surface area for burning, facilitating the transition from conductive to convective burning modes. A multiphase convective burn model implemented in the ALE3D code is used to better understand the influence on burn rates of material properties such as porosity and effective thermally damaged particle size. In this context, particles are defined as gas-impermeable binder-coated crystallites and agglomerations with a set of effective radii reff. Model results demonstrate quantitative agreement with containment wall velocity for confined PBX 9501 and LX-10, and qualitative agreement with density as a function of position in the burning explosive. The model predicts a decrease in post-ignition containment wall velocity with larger radii in reff. These experimental data and model results together provide insight into the initiation and propagation of the reaction wave that defines the convective burn front in HMX-based explosives, a necessary step toward predicting violence under a broad range of conditions.

Group similarity solutions of the lax pair for a generalized Hirota–Satsuma equation

A modified version of generalized Hirota–Satsuma is here analytically solved using a two parameter group transformation method. We here through a Group symmetry transformation reduce its lax pair to a system of ordinary equations and find new solutions. Three similarity transformation variables are investigated. For each case an analytical solution is obtained through a homogenous balance of terms in the reduced lax pair. The obtained results are plotted and show a profile proper to deflagration processes, well described by Degasperis–Procesi equation.

Experimental Characterization of Transient Burning Properties of RDX–XLDB Propellant

The transient burning parameters of an RDX–XLDB propellant were determined experimentally using a strand burner. Experiments were conducted under various conditions ( of 0.1–15 MPa, and of , 25, and 65°C). A few distinct burning regimes were observed. At pressures between 0.1 and 1.5 MPa, the pressure exponent was found to be around 0.7. Between 1.5 and 2.5 MPa, there was a drop in the burn rate that could not be fitted to burn laws. In the range of 2.5–8 MPa, the exponent was 0.4. Between 8 and 12 MPa, a plateau regime was detected, whereas the exponent recovered its positive value above 12 MPa. A stability analysis based on the Zeldovich–Novozhilov approach was performed for various conditions. For operating pressures around 7 MPa, the propellant response was rather low (about 20% of the steady-state value). At lower pressures, the propellant exhibited a higher response (up to 400% of the steady-state value) at acoustic frequencies under 1 kHz. The diversity of burning regimes was attributed to the differences in the deflagration process of XLDB with nitramine oxidizer over a wide pressure range.

COMPUTATIONAL STUDY OF DEFLAGRATION TO DETONATION TRANSITION IN A STRAIGHT DUCT: EFFECT OF ENERGY RELEASE

Numerical simulation based on the Euler equation and one-step reaction model is carried out to investigate the process of deflagration to detonation transition (DDT) occurring in a straight duct. The numerical method used includes a high resolution fifth-order weighted essentially nonoscillatory scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. In particular, effect of energy release on the DDT process is studied. The model parameters used are the heat release at q =50, 30, 25, 20, 15, 10 and 5, the specific heat ratio at 1.2, and the activation temperature at Ti =15, respectively. For all the cases, the initial energy in the spark is about the same compared to the detonation energy at the Chapman-Jouguet (CJ) state. It is found from the simulation that the DDT occurrence strongly depends on the magnitude of the energy release. The run-up distance of DDT occurrence decreases with the increase of the energy release for q =50~20, and increases with the increase of the energy release for q =50~20. It is concluded from the simulations that the interaction of the shock wave and the flame front is the main reason for leading to DDT.

A study of the color diversity around maximum light in Type Ia supernovae

From a sample of 12 well-observed Type Ia supernovae, we find clear evidence of correlations between early phase (U-B), (V-R), and (V-I) colors and the velocity shifts of the [Fe II] \lambda 7155 and [Ni II] \lambda 7378 nebular lines measured from late-phase spectra. As these lines are thought to trace the ashes of the initial deflagration process, our findings provide additional support to the new paradigm of off-center explosions in Type Ia supernovae, and we interpret these correlations as viewing angle effects in the observed colors. We also show that the nebular velocity shifts are related to the strength and width of the Ca II H&K and IR-triplet lines near-maximum light. The evidence presented here implies that the viewing angle must be taken into account when deriving extinction values and distances in future cosmological studies based on Type Ia supernovae.

Kinetic approach to deflagration processes in a recombination reaction

Steady one-dimensional flame structure is investigated in a binary gas mixture made up by diatomic molecules and atoms, which undergo an irreversible exothermic two--steps reaction, a recombination process followed by inelastic scattering (de-excitation). A kinetic model at the Boltzmann level, accounting for chemical encounters as well as for mechanical collisions, is proposed and its main features are analyzed. In the case of collision dominated regime with slow recombination and fast de-excitation, the model is the starting point for a consistent derivation,via suitable asymptotic expansion of Chapman-Enskog type, of reactive fluid-dynamic Navier-Stokes equations. The resulting set of ordinary differential equations for the smooth steady deflagration profile is investigated in the frame of the qualitative theory of dynamical systems, and numerical results for the flame eigenvalue and for the main macroscopic observables are presented and briefly commented on for illustrative purposes.

Exploring the Effects of Nanostructured Particles on Liquid Nitromethane Combustion

Monopropellants consisting of liquid nitromethane and high-surface-area particles of silicon- and aluminum-based oxides were examined to determine the effects of low concentrations of nanostructured materials on deflagration processes. The combustion rates were characterized by measuring linear burning rates in a large pressure vessel filled with argon. Results showed that nitromethane burning rates may be increased at lower pressures with dilute additions of particles. Increases in nitromethane burning rates of greater than 50 % were found with less than 1.0 wt% of particle addition at a nominal pressure of 5.24 MPa. The particle additions were estimated to have only small effects on the equilibrium flame temperatures, density, viscosity, and specific heat. Nitromethane with particle additives displayed a lower burning-rate equation pressure exponent (i.e., reduced pressure sensitivity), which was inversely proportional to particle concentration. Above a pressure of approximately 9 MPa, up to the maximum test pressure (∼14 MPa), the particle additives did not affect the nitromethane burning rate. Condensed-phase temperature profiles of the deflagrating fluid, surface tension, and fluid thermal conductivities were measured in order to elucidate some of the mechanisms causing enhanced burning rates at lower pressures.

Numerical and experimental investigation on the prevention of CO deflagration

The exhaust gases from industrial furnaces contain a huge amount of heat and chemical enthalpy. However, it is hard to recover this energy since exhaust gases invariably contain combustible components such as carbon monoxide (CO). If the CO is unexpectedly ignited during the heat recovery process, deflagration or even detonation could occur, with serious consequences such as complete destruction of the equipment. In order to safely utilize the heat energy contained in exhaust gas, danger of its explosion must be fully avoided. The mechanism of gas deflagration and its prevention must therefore be studied. In this paper, we describe a numerical and experimental investigation of the deflagration process in a semi-opened tube. The results show that, upon ignition, a low-pressure wave initially spreads within the tube and then deflagration begins. For the purpose of preventing deflagration, an appropriate amount of nitrogen was injected into the tube at a fixed position. Both simulation and experimental results have shown that the injection of inert gas can successfully interrupt the deflagration process. The peak value of the deflagration pressure can thereby be reduced by around 50%.

Simplified Modeling of Deflagration in Vessels

A simplified method that models the deflagration process occurring in closed or vented vessels is described. When combustion occurs within the spherical or cylindrical vessels, the flame moves spherically or segmentally to the vessel periphery. The volume and area of each element along the propagating flame front are calculated by using simple geometrical rules. For instabilities and turbulence resulting in enhanced burning rates, a simple analysis results in reasonable agreement with the experimental pressure transients when two burning rates (a laminar burning rate prior to the onset of instability and an enhanced burning rate) were used. Pressure reduction caused by a vent opening at predetermined pressure was modeled. Parameters examined in the modeling include ignition location, mixture concentration, vented area, and vent opening pressure. It was found that venting was effective in reducing the peak pressure experienced in vessels. The model can be expected to estimate reasonable peak pressures and flame front distances by modeling the enhanced burning rates, that is, turbulent enhancement factor.

CFD simulation of deflagration–detonation processes using vector- and parallel computing systems

In this paper, we describe the state of computational fluid dynamics simulations (CFD) of deflagration and detonation processes in hydrogen–air mixtures, using vector- and parallel computing systems, which have been provided in the Institute for Safety Research and Reactor Technology (ISR) at the Forschungszentrum Jülich (FZJ). The R&D work is performed within the scope of an EC project on hydrogen safety that is addressed to the verification of models and criteria for the prediction of Deflagration-to-Detonation Transition (DDT) in hydrogen–air–steam systems under severe accident conditions. Particularly, we report on the present state and recent progress made in the establishment of the CRAY hardware cluster (T90, T3E, J90) with vector and parallel processing capabilities, as well as on the current achievement of the CFD software cluster (CFX, ERCO, DET, IFSAS), including test cases for verification and validation, with some illustrating examples. Emphasis is put on the multi-dimensional simulation of fast turbulent hydrogen flames, for instance, using the general purpose field code CFX from AEA. The numerical results are compared with experimental results, which have been obtained for various conditions in the Russian large scale RUT test facility. Specifically, we outline deflagration–detonation processes concerning the numerical resolution of reacting flows in complex geometries, applying mesh refinement or massively parallel processing. First test cases indicate that our modern field code cluster (MFCC) with high-performance supercomputer networking (HPCN) will be a suitable constellation to resolve DDT processes in safety enclosures of innovative nuclear reactor containments or other industrial plants, e.g. solar hydrogen demonstration facilities.

Self-ignition of S.I. engine model fuels: A shock tube investigation at high pressure

The self-ignition of several spark-ignition (SI) engine fuels ( iso-octane, methanol, methyl tert-butyl ether and three different mixtures of iso-octane and n-heptane), mixed with air, was investigated experimentally under relevant engine conditions by the shock tube technique. Typical modes of the self-ignition process were registered cinematographically. For temperatures relevant to piston engine combustion, the self-ignition process always starts as an inhomogeneous, deflagrative mild ignition. This instant is defined by the ignition delay time, τ de fl . The deflagration process in most cases is followed by a secondary explosion (DDT). This transition defines a second ignition delay time, τ DDT , which is a suitable approximation for the chemical ignition delay time, if the change of the thermodynamic conditions of the unburned test gas due to deflagration is taken into account. For iso-octane at p = 40 bar, a NTC (negative temperature coefficient), behaviour connected with a two step (cool flame) self-ignition at low temperatures was observed. This process was very pronounced for rich and less pronounced for stoichiometric mixtures. The results of the τ DDT delays of the stoichiometric mixtures were shortened by the primary deflagration process in the temperature range between 800 and 1000 K. Various mixtures of iso-octane and n-heptane were investigated. The results show a strong influence of the n-heptane fraction in the mixture, both on the ignition delay time and on the mode of self-ignition. The self-ignition of methanol and MTBE (methyl tert-butyl ether) is characterized by a very pronounced initial deflagration. For temperatures below 900 K (methanol: 800 K), no secondary explosion occurs. Taking into account the pressure increase due to deflagration, the measured delays τ DDT of the secondary explosion are shortened by up to one order of magnitude.

Theoretical and experimental investigation of combustion to detonation transition in chemically active gas mixtures in closed vessels

This paper presents the results of numerical and experimental investigations of the process of deflagration to detonation transition. It is shown that both regimes of combustion to detonation transition and regimes including deflagration waves lagging behind the shock waves are possible. Several types of flow patterns of deflagration to detonation transitions are determined theoretically and experimentally. Stability of one- and two-dimensional detonation waves is investigated. It is shown that one-dimensional pulsations in the symmetrical case with free boundaries cause strong transverse pulsations.

Investigation of the Process of Deflagration-to-Detonation Transition (DDT) in Granular Secondary Explosives with High-Speed Photography.

The deflagration-to-detonation transition (DDT) in confined charges of granular secondary explosives has been investigated with high-speed photography. Specially designed steel confinements, which incorporate slit windows to allow streak photography, were constructed to house pressed pentaerythritol tetranitrate (PETN) charges of known density. The results indicate that convective burning is only important during the initial stages of the DDTprocess in causing rapid pressure build-up, and does not govern the deflagration process up to the point of DDT. Following convection, the propagation of the combustion front was found to be governed by the compaction wave generation of hot-spot ignition sites, The transition to detonation is finally attained when the front of a plug of highly compacted material, essentially a second compaction waveformed at the combustion front, overtakes the first compaction wave and causes shock initiation of the uncompacted, and therefore. more highly shock sensitive, original-density PETN. The detonation then propagates to the end of the charge at constant velocity and a rearward shock propagates upstream through the highly compacted plug material and intersects the combustion front. The proposed mechanism is supported by the observed deformation of the internal surface of the steel confinement.

Deflagration to detonation transition in combustible gas mixtures

This paper presents the results of a computational investigation of the process of deflagration to detonation transition in a combustible gas mixture. The type of combustion (i.e., deflagration or detonation) supported by a two-step reaction scheme is studied as a function of the activation energies. It is shown that both a deflagration to detonation transition and a deflagration wave that lags behind a leading shock are possible. Two types of deflagration to detonation transitions are found theoretically: initiation of detonation from the flame zone and initiation of detonation along a contact discontinuity in the compressed gas near the primary shock wave.

Ignition and growth of a detonation by a high energy plasma

Many experimental and numerical studies have been achieved to describe the transition process of deflagration to detonation when a projectile impacts an explosive. Also a large work has been done for the determination of various parameters — such as the impact pressure, the efficiency factors, etc. of the laser — material interaction. When a laser beam impacts an explosive, the P2τ criterion, characteristic of shock initiated detonations, is no longer valid due to the generated hot plasma whose effect is to decrease the DDT (Deflagration to Detonation Transition) duration. The present paper deals with a modelling of the plasma-explosive medium allowing the determination of distances and times of the DDT process. The two phase modelling of the granular explosive takes into account the creation of hot spots. The pressure of the plasma is computed using a semi empirical model, while the temperature is obtained from Maxwell Boltzmann statistics. The authors focused their attention on the equation of state for the detonation products and the numerical process.

Vented gas deflagration modeling: A simplified approach

Starting from a detailed mathematical model previously developed, which accounts for each physicochemical phenomenon involved in a vented deflagration process, a simplified (or short-cut) model has been derived based on suitable assumptions and algebraic manipulations. The simplified model reduces to a system of three ordinary differential equations, the solution of which can be readily achieved on a small personal computer. The reliability of such a simple model has been tested by comparison with a collection of published experimental data (the same one considered when developing the original detailed model), covering a wide range of values for vessel volume (0.001–199 m 3), initial pressure (0.1–0.4 MPa), and bursting pressure (0.1–2.96 MPa) and including various vassel shapes and fuel-air compositions. The error in the maximum absolute pressure values, averaged over 91 experimental data relative to vented explosions, is equal to 28%. The proposed model can be regarded as a simple tool for predicting with reasonable accuracy the maximum pressure value during a vented deflagration in a wide range of operating conditions, thus providing a significant contribution to the venting area design.

Vented gas deflagrations a detailed mathematical model tuned on a large set of experimental data

A detailed mathematical model based on conservation laws and physicochemical relationships has been developed to simulate vented gas deflagrations. The main phenomena involved are described using a priori relationships whenever well-established quantitative theories are available. In the case of flame front acceleration due to flame wrinkling in closed vessels or to turbulence induced by vent opening, where such theories do not exist two empirical relationships have been introduced. These have been turned by comparison with a collection of about 160 literature experimental data, covering a wide range of values for vessel volume (0.001–199 m 3), initial pressure (0.1–0.4 MPa), and bursting pressure (0.1–2.96 MPa) and including various vessel shapes and fuel-air compositions. The obtained average relative error in the maximum explosion pressure values is equal to 10.4%, comparable with the uncertainty inherent to the vented deflagration data considered. A dedicated computer algorithm has been developed to provide a fast running simulation program (i.e., about 10 seconds of CPU time on a VAX 8650 for a typical deflagration process).

Deuterium Isotope Effects during HMX Combustion: Chemical kinetic burn rate control mechanism verified

AbstractThe appearance of a significant deuterium isotope effect during the combustion of the solid HMX compound verifies that the chemical reaction kinetics is a major contributor in determining the experimentally observed or global burn rate. Burn rate comparison of HMX and its deuterium labeled HMX‐d8 analogue reveals a primary kinetic deuterium isotope effect (1° KDIE) at 500 psig (3.55 MPa) and l000 psig (6.99 MPa) pressures and selectively identifies covalent carbon‐hydrogen bond rupture as the mechanistic step which ultimately controls the HMX bum rate under the static combustion conditions of this experiment. The 1° KDIE value further suggests the rate‐limiting CH bond rupture occurs during the solid state HMX decomposition/deflagration portion of the overall combustion event and is supported by other independently published studies. A possible anomalous KDIE result at 1500 psig (10.4 MPa) is addressed. This condensed phase KDIE approach illustrates a direct link between lower temperature/pressure thermal decomposition and deflagration processes and their potential applicability to the combustion regime. Most importantly, a new general method is demonstrated for mechanistic combustion investigations which selectively permits an in‐situ identification of the compound's burn rate‐controlling step.

The deflagration-to-detonation transition process for high-energy propellants - A review

Introduction T HE deflagration-to-detonation transition (DDT) in solid energetic materials has been of interest to researchers for many decades since it has a variety of areas of applicability, ranging from industrial to military. A recent example of the former is the detonation that occurred during the production of propelling charges for hunting ammunition. In the military area, the applicability extends from gun systems and projectile impact hazards to rocket motors. Hence, the DDT process has been investigated for both voidless (cast) and porous systems. We shall use the rocket motor environment to assess the DDT hazards associated with high-energy propellants. In the area of solid propellant rocket motors, an oft-asked question is Will a cast, well-manufactured rocket propellant grain undergo a transition to detonation from the burning mode? The answer is no, to the best of our knowledge. Although there are few journal articles directly providing this assessment, our knowledge of the DDT mechanism in gases, liquids, and solids provides a rationale for reaching this conclusion. The rationale is based on the thesis that the deflagration process must ultimately produce a shock wave to drive the system to detonation.' That is, the shock-to-detonation transition (SDT) is the final stage in any DDT process. Consequently, in the solid-propellant rocket motor situation, the confinement provided by the motor case must be sufficient to allow the pressure from deflagration to build up to a sufficiently high shock pressure to initiate the cast propellant. As will be shown below, these shock amplitudes cannot be reached for cast propellant systems confined in rocket motor cases.

Deflagrations and detonations as a mechanism of hadron bubble growth in supercooled quark-gluon plasmas

We discuss the possibility that hadron bubbles formed in quark-gluon plasmas below or slightly above the critical temperature start growing by explosive deflagration or detonation processes. In these the phase transition takes place in a thin layer of discontinuity propagating outward from the point of bubble formation. Combustion theory is written in relativistic form, and possible physical deflagration and detonation bubble solutions conserving energy and momentum, producing entropy, and satisfying correct boundary conditions are classified and numerically discussed using the bag equation of state for quark matter. The implications of these solutions to ultrarelativistic nucleus-nucleus collisions and early cosmology are discussed.