Explosion limits and runaway criteria: A stretching-based approach

Temperature and concentration evolution in a spatially homogeneous reacting system is described by a dynamical system characterized by a vector field accounting for reaction rates and heat exchange with the surroundings in a lumped way. This article addresses the estimate of the explosion limits and the assessment of runaway criteria in batch reacting systems by focusing on the evolution of the stretching rates along system trajectories. The stretching-based runaway criteria are applied in archetypal systems, such as the Semenov model, and compared with classical sensitivity-based criteria and with other detection methods derived from dynamical system theory. Several other examples are thoroughly addressed, including an isothermal combustion model (the chain-branching model) and the explosion limits in the H₂-O₂ system.     

Detonation synthesis of a new modification of aluminum oxide from gibbsite by explosion

The x-ray diffraction analysis of the condensed products resulting from explosion of gibbsite — hexogen mixtures was carried out. It was established that in explosion of the mixture, along withα-Al₂O₃, another, previously unknown modification of aluminum oxide is formed in nearly the same amount. The x-ray diffractometric data of the new modification are presented.     

Mathematical modeling of the thermal explosion in mechanically activated SiO<Subscript>2</Subscript> + Al mixtures

A mathematical model of synthesis in a mechanically activated SiO₂ + Al mixture is constructed in the macroscopic approximation. It is demonstrated that preliminary mechanical activation makes it possible to obtain solid-phase ignition and to ensure synthesis of Al₂O₃ and Si products in the thermal explosion regime. Based on experimental data, thermophysical and thermokinetic constants of the process are determined.     

Dust cloud explosion characteristics and mechanisms in MgH2-based hydrogen storage materials

The mechanisms involved in premixed magnesium and hydrogen hybrid and synthetic MgH₂ dust cloud explosions were investigated. The results revealed that trace amounts of H₂ in Mg explosions can markedly increase explosion severity. Furthermore, H₂ addition can weaken the influence of oxygen deficiency on Mg explosion. Moreover, the explosion intensity of synthetic MgH₂ was far stronger than that of premixed Mg/H₂ mixture or Mg alone because the vacancy defects in Mg and H atoms can form after dehydrogenation of MgH₂, which caused that Mg and H₂ are prone to oxidation and nitrification in air atmosphere at a low temperature, thereby promoting the explosion. This demonstrates that the explosion risk of MgH₂ (even other H₂ storage materials) is related to its H₂ storage capacity and dehydrogenation temperature. Therefore, for H₂ storage materials, the better H₂ storage performances can exhibit higher explosion risks.     

Inerting of Methane-Air Mixtures by Compositions Based on Carbon Dioxide and Nitrogen with Addition of Halocarbons

Conditions of inerting of methane-air mixtures by carbon dioxide and nitrogen with addition of CF₃Br, C₂F₄Br₂, and CF₃I are examined. The value of the minimum inerting concentration of the inerting substance is significantly reduced by introduction of these substances [up to 15% (vol.)] to N₂ and CO₂. A further increase in CF₃Br and C₂F₄Br₂ concentrations does not reduce the minimum inerting concentration, and an increase in the concentration of CF₃I above 15%(vol.) drastically increases the latter to values higher than the minimum inerting concentration of pure nitrogen. CF₃I is found to be a combustible gas, and its addition to a methane-air mixture substantially expands the flammability limits. The flammability limits of CF₃I in air under standard conditions are determined, and the explosion parameters are measured. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 23–28, September–October, 2005.     

Inhibitory effects of three chemical dust suppressants on nitrocellulose dust cloud explosion

Three types of self-prepared chemical dust suppressants (CDSs) were investigated for their inhibitory effects on nitrocellulose (NC) cloud dust explosion. The results revealed that NC is extremely sensitive to electric sparks and has a high explosion intensity. CaCl₂-CDS effectively increased the particle size to control fly dust substantially inhibiting dust cloud explosions. However, both Na₂SO₄-CDS and MgCl₂-CDS exhibited poor abilities and even promoted explosion. Therefore, neither Na₂SO₄-CDS nor MgCl₂-CDS is recommended as a CDS for NC. Inappropriately using CDSs may engender severe explosions. Furthermore, a mechanism underlying NC dust cloud combustion and explosion was proposed. NC has three stages of heat release: autoxidation, thermal decomposition, and combustion. Thermal decomposition, combustion, and explosion were triggered depending on the energy provided from autoxidation. CaCl₂-CDS inhibited only combustion. This study reveals the mechanism underlying NC dust cloud explosions and provides useful information for the development of more optimized CDSs.     

Effect of TiH2 Particle Size and Content on the Underwater Explosion Performance of RDX‐Based Explosives

Experiments were conducted to study the underwater explosion performance of titanium hydride/RDX‐based (TiH₂/RDX) composite explosive. Cylinder charges with different TiH₂ particle sizes and contents were prepared and tested. Explosion parameters like peak overpressure, impulse, shock energy, and bubble energy were analyzed. It was notable that underwater explosion performance of TiH₂/RDX composite explosive was promoted by addition of small particle size TiH₂ (D₅₀=0.96 μm), in which case increasing TiH₂ content also showed a favorable effect. The maximum increments of specific initial shock energy, bubble energy, and total energy were 10.5%, 6.4%, and 7.1% respectively. However, with bigger TiH₂ particle sizes (D₅₀=20.78 μm, D₅₀=136.74 μm), the explosion parameters and the TiH₂ content showed a negative relationship, which reveals that TiH₂ particle size plays an important role in determining the reactivity of TiH₂. Meanwhile, the interaction between TiH₂ particle size and content was significant.     

Modeling of thermal explosion in constrained dies for B4C–Ti and BN–Ti powder blends

The process of reactive in-situ synthesis of dense particulate reinforced TiB₂/TiC and TiB₂/TiN ceramic matrix composites from B₄C–Ti and BN–Ti powder blends with and without the addition of Ni has been modeled. The objective of modeling was the determination of optimal thermal conditions preferable for production of fully dense ceramic matrix composites. Towards this goal heat transfer and combustion in dense and porous ceramic blends were investigated during heating at a constant rate. This process was modeled using a heat transfer–combustion model with kinetic parameters determined from the differential thermal analysis of the experimental data. The kinetic burning parameters and the model developed were further used to describe the process of combustion synthesis in a constrained die under pressure. It has been shown that heat removal from the reaction zone affects the ignition temperature of thermal explosion.     

Explosive Hazard of Vapor–Gas Mixtures of Fluoride‐Containing Saturated Hydrocarbons with Fluoride and Chlorine Trifluoride

The limits of explosion of the CF₃CFH₂–F₂–N₂, C₃F₈–F₂–N₂, C₄F₈–F₂–N₂, C₄F₈–ClF₃–N₂, C₄F₁₀–ClF₃–N₂, and F₂ClCF₂Cl–F₂–N₂ mixtures are determined. For some compositions of the first two mixtures, the maximum pressure of explosion and the maximum pressuregrowth rate are measured. The transition from deflagration to detonation is observed in undiluted mixtures close to stoichiometric compositions. The burning rates of the mixtures examined are comparable with the burning rates of oxygen–hydrogen mixtures.     

Critical evaluation and thermodynamic optimization of the VO–VO2.5 system

A critical evaluation, and thermodynamic optimization of phase equilibrium and thermodynamic properties of the VO–VO_2.5 system are presented. Optimized model parameters for all the oxide phases were obtained so as to reproduce all available and reliable experimental data within experimental error limits. Liquid oxide phase was modeled using the modified quasichemical model in the pair approximation with components representing various valences of vanadium (VO, VO_1.5, VO₂, and VO_2.5) in the liquid oxide. Solid VO and V₂O₃ phases were modeled using simple random mixing models, while all other solid phases were assumed to be stoichiometric compounds. Type of defects in the V₂O₃ solid solution was shown to be extended cluster type defect, based on the available experimental data. Using the presently optimized model parameters, most experimental data has been well reproduced, therefore, the present work can be further extended for the development of thermodynamic database for V oxide containing system.     

Thermodynamic optimization of the K2O-Al2O3-SiO2 system

A critical evaluation and thermodynamic optimization of experimental phase diagrams and thermodynamic properties of the K₂O-Al₂O₃-SiO₂ system was performed at 1?bar total pressure. A set of self-consistent thermodynamic functions of all phases in the K₂O-Al₂O₃-SiO₂ system was obtained. The liquid phase was described using the Modified Quasichemical Model with the KAlO₂ associate component. The set of optimized model parameters obtained for all phases reproduces available and reliable thermodynamic properties and phase diagram data as well as the melt structure of the K₂O-Al₂O₃-SiO₂ system within the experimental error limits.     

The use of GA-ANNs in the modelling of compressive strength of cement mortar

In this paper, results of a project aimed at modelling the compressive strength of cement mortar under standard curing conditions are reported. Plant data were collected for 6 months for the chemical and physical properties of the cement that were used in model construction and testing. The training and testing data were separated from the complete original data set by the use of genetic algorithms (GAs). A GA-artificial neural network (ANN) model based on the training data of the cement strength was created. Testing of the model was also done within low average error levels (2.24%). The model was subjected to sensitivity analysis to predict the response of the system to different values of the factors affecting the strength. The plots obtained after sensitivity analysis indicated that increasing the amount of C₃S, SO₃ and surface area led to increased strength within the limits of the model. C₂S decreased the strength whereas C₃A decreased or increased the strength depending on the SO₃ level. Because of the limited data range used for training, the prediction results were good only within the same range. The utility of the model is in the potential ability to control processing parameters to yield the desired strength levels and in providing information regarding the most favourable experimental conditions to obtain maximum compressive strength.     

Thermodynamic modeling of the BaO-SiO<Subscript>2</Subscript> and SrO-SiO<Subscript>2</Subscript> binary melts

The evaluation of the thermodynamic properties and the phase diagrams for the binary BaO-SiO₂ and SrO-SiO₂ systems is carried out using a structural model for silicate melts and glasses. This thermodynamic model is based on the assumption that an addition of metal oxides to silica results in the depolymerization of the silicon-oxygen network, with a characteristic free energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for the above binary systems have been analysed and represented with a small number of parameters. The resulting equations represent the thermodynamic and phase diagram data for the alkaline-earth oxide-silica systems within error limits for most of the experimental data. In particular, the measured limiting liquidus slope, at X _{SiO 2} = 1, is well reproduced.     

The nature of active sites for the oxidation of methane on La-based perovskites

Highly crystalline, monophasic LaFeO₃ and LaCoO₃ perovskites, prepared by the explosion method, are shown to be heterogeneous at surface level. The outmost atomic layers of these perovskites contain high concentrations of carbonate-type species. Their specific activities for methane combustion are in fact identical to La₂O₂CO₃ and air-exposed La₂O₃. These results compared with pertinent data from the literature hint that surface heterogeneity may be often present in mixed oxides catalysts.     

The Effect of the Hydrogen Containing Material TiH2 on the Detonation Characteristics of Emulsion Explosives

In order to improve the detonation performance of emulsion explosives, a new type of emulsion explosives with TiH₂ powders is developed. The influences of the amount of sensitizers GMs and energetic additives TiH₂ on explosion characteristics of emulsion explosives are studied to determine the optimum compositions. Underwater explosion and brisance testing experiments show that, compared to traditional GMs sensitized emulsion explosives, the shock wave specific impulse I and total energy E of GMs‐TiH₂ sensitized emulsion explosives are improved significantly, and the effect of TiH₂ powders on improving the explosion power of emulsion explosives is better than that of Ti powders. The brisance of GMs‐TiH₂ emulsion explosives is 23.80 mm compression of lead block, 7.7 mm more than that of the emulsion explosives sensitized by GMs alone. Therefore, the hydrogen containing material TiH₂ could be a promising energetic additive for developing high‐power emulsion explosives.     

NO reduction by H2 over nano-Ce0.98Pd0.02O2−δ

Development of new catalysts for controlling nitrogen oxides (NO_x) emissions is an important technical challenge as increasingly strict emission limits are being imposed. A new catalyst Pd²⁺ substituted CeO₂ (Ce_0.98Pd_0.02O_2−δ) was synthesized by solution combustion method. The material was characterized by XRD, TEM and XPS and used to investigate the reduction of NO by H₂. The catalyst shows 100% N₂ selectivity at low temperature and thus is superior to other catalysts reported in literature. A bifunctional reaction mechanism has been proposed to model the experimental data.     

Studies on Energetic Compounds Part 27: Kinetics and Mechanism of Thermolysis of Bis(Ethylenediamine)Metal Nitrates and Their Role in the Burning Rate of Solid Propellants

Four bis(ethylenediamine)metal(II) nitrate (BEMN) complexes, i.e. [M(EDA)₂](NO₃)₂, where M=Cu, Co, Ni and Zn, have been prepared and characterized. Thermolysis of these complexes induced by heat and drop‐weight impact has been investigated by TG‐DTG, DTA, explosion delay (D_E), explosion temperature (T_E) and impact sensitivity measurement. The kinetics of early thermolysis reaction prior to fast decomposition have been evaluated. Contracting area (CA, n=2) and contracting cube (CC, n=3) equations were found to give the best fits in isothermal TG data among all tested nine mechanism‐based kinetic models. The values of activation energy (E_a), T_E, D_E and activation energy for explosion (E*) have been found to be quite lower for the copper complex as compared to cobalt, nickel and zinc complexes. A mechanism of thermolysis has also been proposed. All these complexes were found to be insensitive towards impact of 2 kg weight up to the height of 110 cm. These complexes were used as energetic burning rate modifiers in the combustion of hydroxy‐terminated polybutadiene (HTPB)‐ammonium perchlorate (AP) composite solid propellants. A two‐fold increase in burning rate was observed with copper and cobalt complexes at low concentration (2% by wt.). The in situ freshly formed metal oxides with large number of active sites in their crystallites seem to be better additives for combustion of propellants.     

Explosive synthesis of ultrafine Al2O3 and effect of temperature of explosion

Synthesis of ultrafine Al₂O₃ is considered. An X-ray phase analysis indicates that ultrafine γ-Al₂O₃, (θ + α)-Al₂O₃, and α-Al₂O₃ are synthesized by explosion of water-gel explosives with a zero oxygen balance, prepared by mixing aluminum nitrate, RDX, and carbamide. All granules of ultrafine Al₂O₃ are spherical and homogeneous, and the granule size varies from 10 to 30 nm. The average crystal sizes of ultrafine Al₂O₃ are calculated by the Scherrer equation. The temperatures of explosion of water-gel explosives are found by a simplified approach. A comparison indicates that the higher the temperature of explosion, the greater the average nanocrystal size. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 127–131, September–October, 2006.     

Modelling the effect of particle size on dust explosions

The recent concept of inherent safety uses the properties of a material or process to eliminate or reduce the risk thus removing or minimizing the hazard at the source as opposed to accept the hazard and looking to mitigate the effects. In this framework the control of particle size in dust explosion prevention and mitigation is recognized as a major inherent safety methodology. Indeed, the increase of particle size may allow significant reduction of particle reaction rate eventually reducing the risk.In this paper a novel model is developed to quantify the effect of particle size on dust reactivity in an explosion phenomenon. The model takes into account all of the steps involved in a dust explosion: internal and external heating, devolatilization reaction and volatiles combustion. Varying the dust size can establish different regimes depending on the values of the characteristic time of each step and of several dimensionless numbers (Damköhler number, Da; Biot number, Bi; thermal Thiele number, Th). Results from the model are reported in terms of the deflagration index (K_St) as a function of dust diameter in all regimes and at varying Da, Th and Bi. Comparison with experimental data from polyethylene explosion tests shows promising results. Finally, the results of the model are presented in the form of a dust explosion regime diagram, which is helpful to make a draft evaluation of the role of dust size on explosion behavior and severity.     

Full‐field Peak Pressure Prediction of Shock Waves from Underwater Explosion of Cylindrical Charges

Cylindrical charge is a main form in most application of explosives. By employing numerical calculation and an indirect mapping method, the relation between peak pressures from underwater explosion of cylindrical and spherical charges is investigated, and further a model to predict full‐field peak pressures by the former one is developed and then verified by experiments. Besides, a method to intuitively sketch the corresponding orientational performance is proposed and discussed. The results indicate the following: when R /R ₀ ranges from 12 to 100, the mapping relation is nearly dominated by the amplitude similarity constant K_p , and an approximate shifted sine relation between K_p /K ₀ and action angle exists; in cases that L /d is over 1.5, the prediction model shows a good agreement with experimental data, with most errors below 10% and it is also universally applicable for different explosives; the directivity of peak pressure distribution in underwater explosion is continuously improved along the increasing L /d but no significant influence appears after L /d becomes over 5.