Chain-Thermal Explosions and the Transition from Deflagration Combustion to Detonation

The transition from combustion to a chain-thermal explosion, a necessary step in the transition from deflagration combustion into detonation, is studied using the example of hydrogen oxidation. Differences between the kinetic modes of ignition and a chain-thermal explosion are discussed.     

Kinetic Factors Determining Specific Features of Solid Phase Formation in Dichlorosilane Oxidation

Experimental data on the kinetic regularities of aerosol SiO₂ formation in the course of dichlorosilane oxidation by oxygen at different initial pressures, compositions of the reaction mixture, and temperatures ranging from 380 to 578 K are presented. It is shown that the regularities of the process, including the specific feature of the transition from the regime of solid phase formation in the form of a film to the regime of aerosol formation can be explained on the basis of the Volmer–Weber–Frenkel–Zeldovich nucleation theory taking into account the branched chain nature of the reaction. The conditions for the transition of chain combustion into the regime of chain–thermal explosion almost coincide with the conditions of intensive formation of aerosol. The SF₆ additives inhibit the process and thereby increase the dispersity of aerosol and the minimal pressure of its formation.     

The Contribution from Quadratic Reactions to the Thermal Regime of Branched Chain Combustion

A mathematical model was developed for the dependence of maximal self-heating in the course of the combustion of a hydrogen–oxygen mixture inside the ignition peninsula. The specific features of the thermal regime, which are related to a dual role of quadratic termination reaction, are discussed.     

Pulsating instabilities in the Zeldovich–Liñán model

In this paper we numerically study the properties and stability of the travelling combustion waves in Zeldovich–Liñán model in the adiabatic limit in one spatial dimension. The structure and the properties of the combustion waves are found to depend on the recombination parameter, showing the relation between the characteristic times of the branching and recombination reactions. For small (large) values of this parameter the slow (fast) recombination regime of flame propagation is observed. The dependence of flame speed on the parameters of the model are studied in detail. It is found that the flame speed is unique, the combustion wave does not exhibit extinction as the activation energy is increased. The flame speed is a monotonically decreasing function of the activation energy. The results are compared to the prediction of the activation energy asymptotic analysis. It is found that the correspondence is good for the fast recombination regime and large activation energies. Stability of combustion waves is studied by using the Evans function method and direct integration of the governing partial differential equations. It is demonstrated that the combustion waves lose stability due to supercritical Hopf bifurcation. The neutral stability boundary is found in the space of parameters. The pulsating solutions emerging as a result of Hopf bifurcation are investigated. The amplitude of pulsations grow in a root type manner as the activation energy is increased beyond the neutral stability boundary.     

THERMAL DEGRADATION AND FLAME RETARDANCY OF CALCIUM ALGINATE FIBERS

Calcium alginate fibers were prepared by wet spinning of sodium alginate into a coagulating bath containing calcium chloride.The thermal degradation and flame retardancy of calcium alginate fibers were investigated with thermal gravimetry(TG),X-ray diffraction(XRD),limiting oxygen index(LOI) and cone calorimeter(CONE).The results show that calcium alginate fibers are inherently flame retardant with a LOI value of 34,and the heat release rate(HRR),total heat release(THR),CO and CO_2 concentrations during ...     

Numerical analysis of detonation combustion wave in pulse detonation combustor with modified ejector with gaseous and liquid fuel mixture

Journal of Thermal Analysis and Calorimetry - The detonation combustion phenomenon is supersonic combustion process and follows on thermal explosion in combustor. Deflagration to detonation...     

(H_2+CO+CH_4+Air)多元爆炸性混合气体爆炸形态与波形的区划

对H2,CO,CH4多元体系支链爆炸的爆炸特性与形态进行了系统的研究.探索了浓度爆炸极限、爆炸形态与波形及其影响因素;测定了爆炸危险度、火焰蔓延极限、最小点火能等爆炸特性参数;根据爆炸形态与波形的不同,提出了爆炸形态与波形的新区划理念,在爆炸极限内,可进一步区划为上下限冷焰区、上下限爆燃区、爆轰区、下爆燃向爆轰转化区等6个爆炸形态区,并探讨了不同爆炸形态压力波的发展机制,对进一步研究相关的多元支链爆炸体系,促进多元支链爆炸理论的发展,具有一定的理论价值.实验测得的爆炸危险度、火焰蔓延极限、最小点火能等特性参数,与引进‘关键组分'的概念,对预防混合气体支链爆炸事故的发生,指导防爆电气设备与阻火器设计,修订相关工业的安全指标,指导支链燃烧与支链爆炸的实践,具有积极的现实意义.     

Changes in the composition of synthesis products upon transitioning from self-ignition to combustion

Changes in the chemical composition of condensed products upon switching from synthesis in the self-ignition mode to combustion synthesis is studied by approximate analytical and numerical means for condensed substances that react via competing reaction pathways. It is shown that these different modes of synthesis produce different compositions of the reaction products. The conditions required for transitioning from one mode of combustion initiation (thermal explosion) to another (ignition) are determined. It is found that this transition can occur upon changing the temperature of a heater by just two characteristic intervals. A scaling procedure that allows the calculation results obtained at zero dimensionless temperature of the heater to be used to determine the effect its non-zero dimensionless temperature has on the ignition mode and the composition of the obtained products is proposed. Calculations show that materials with different distributions of the chemical composition along the sample can be obtained by deliberately changing the temperature of the heater.     

Properties of combustion waves in the model with competitive exo- and endothermic reactions

In this paper we investigate the properties of the travelling combustion wave solutions in a diffusion-thermal model with a two-step competitive exo–endothermic reaction mechanism in one spatial dimension under adiabatic conditions. The model is analysed both numerically and analytically using asymptotic analysis. It is demonstrated that depending on the parameter values, the flame speed as a function of parameters is either a single-valued monotonic function or a double-valued c-shaped function with the turning point type of behaviour. For the case of single-valued flame speed, two flame regimes are identified: the regime with exo- and endothermic reaction domination. Two different routes to extinction are found as well as regions of the existence of combustion waves in the parameter space. Prospects of further work are also discussed.     

2,2,2-三硝基乙基-N-硝基甲胺的热安全性

为评价2,2,2-三硝基乙基-N-硝基甲胺(TNMA)的热安全性, 得到计算TNMA热安全性参数用的基本数据, 用经验式估算了TNMA的比热容(C_p)和热导率(λ). 用键能贡献于生成热Q_f的加和法, 估算了TNMA的标准生成焓Δ_cH_m^θ(TNMA, s, 298.15 K). 用热力学公式计算了TNMA的标准燃烧焓ΔU_m^θ(TNMA, s, 298.15 K)和标准燃烧能Δ_cH_m^θ(TNMA, s, 298.15 K). 用Kamlet-Jacobs 公式估算了爆速、爆压和爆热. 用经验式估算了分解热(Q_d). 通过差示扫描量热(DSC)曲线和高灵敏度布鲁顿玻璃薄膜压力计测得的逸出气体标准体积(V_H)-时间(t)曲线, 得到了TNMA放热分解反应的动力学参数. 用上述基本数据得到了评价TNMA的热安全性参数: 自加速分解温度(T_SADT), 热爆炸临界温度(T_be0和T_bp0), 绝热至爆时间(t_TIad), 撞击感度50%落高(H₅₀), 热点起爆临界温度(T_cr), 被300 K环境包围的半厚和半径为1 m的无限大平板、无限长圆柱和球形TNMA的热感度概率密度函数S(T), 相应于S(T)-T关系曲线最大值的峰温(T_S(T)max), 安全度(SD), 临界热爆炸环境温度(T_acr)和热爆炸概率(P_TE). 结果表明: (1) TNMA有较好的热安全性和对热抵抗能力, 与环三亚甲基三硝胺(RDX)相比, TNMA易从热分解过渡到热爆炸; (2) 不同形状大药量TNMA 热安全性降低的次序为: 球>无限长圆柱>无限大平板; (3)TNMA有高的燃烧能、高的爆轰化学能(爆热)和接近环四亚甲基四硝胺(HMX)的爆炸性能, 其对冲击敏感, 冲击感度与季戊四醇四硝酸酯(PETN)和特屈尔接近, 可用作混合炸药主组分.     

Evaluation of physico-chemical properties and antimicrobial synergic effect of ceftazidime-modified chitosan

The aim of this work was to study the effect of tris(3-nitrophenyl) phosphine (NPPh3), which showed a good thermal stability and carbon-forming ability, on the flame retardancy and thermal degradation mechanism of epoxy resins. A series of diglycidyl ether of bisphenol A (DGEBA) loaded with tris(3-nitrophenyl) phosphine (NPPh3) were prepared. It was found that NPPh3 can effectively improve the flame retardancy and thermal stability of the composites. When the loading amount of NPPh3 was 14%, the LOI value of the DGEBA composites was 29.2% (about 1.53 times the corresponding value of the original DGEBA resin). Thermal stability was studied by thermogravimetric analysis, and the results showed that the addition of NPPh3 can improve char formation of this system both in nitrogen and in air atmosphere. Specifically, its combustion residue at 800 °C in nitrogen atmosphere was about 4.26 times of the original resin. Differential scanning calorimetry indicated that NPPh3 slightly decreased the glass transition temperature of epoxy resins. Additionally, the gaseous degradation products were analyzed by thermogravimetric analysis/infrared spectrometry, providing insight into the thermal degradation mechanism. Scanning electron microscopy and Fourier transform infrared were brought together to evaluate the morphology and structure of the residual char obtained after combustion.     

Characteristics of a chain thermal explosion as a function of the kinetic properties of reaction chains

Study of the combustion and explosion of hydrogen?carbon oxide?air mixtures shows that the sharpness of a chain thermal explosion depends on the frequency of branching in a given branch of a reaction chain. It is established that varying the СО: Н₂ concentration allows us to observe and eliminate the degeneration of an explosion while maintaining the regimes of ignition and deflagration.     

低温燃料化学特性对湍流预混火焰传播的影响

大分子碳氢燃料的低温化学反应及两阶段点火特性会显著影响火焰的分区及燃烧情况。本文采用数值模拟的方法探究了正庚烷/空气预混混合气在RATS燃具上的湍流火焰传播,与试验结果具有一致性。模拟使用的是44种物质,112步的正庚烷简化动力学机理。使用Open FOAM的reacting Foam求解器建立了简化模拟流道及出口的三维模型,模拟了在大气环境下,初始反应温度450–700 K、入口速度6 m·s~(-1)与10 m·s~(-1)、焰前流动滞留时间100 ms及60 ms、当量比φ=0.6的正庚烷/空气混合气湍流火焰燃烧情况。结果发现,标准化湍流燃烧速度与混合气初始温度以及流动滞留时间有关。在低温点火阶段,正庚烷氧化程度受到初始温度与速度的影响,燃料分解并在预热区中产生大量中间物质如CH_2O,继而会影响湍流火焰燃烧速度。随着初始反应温度的升高,湍流燃烧火焰逐渐由化学反应冻结区过渡到低温点火区;温度超过一定数值后,燃料不再发生低温反应,此时燃烧位于高温点火区域。     

Simulation of the inhibition of hydrogen-air flame propagation

The results of simulation and experimental data presented here demonstrate that the competition between chain branching and chain termination is the key factor in hydrogen-air flame propagation, including the temperature regime of the process and the formation of concentration limits. Self-heating becomes significant in developed combustion. It enhances the chain avalanche and ensures the temperature necessary for layer-by-layer chain ignition. By varying the ratio between the chain branching and termination rates by means of an inhibitor makes it possible to control the flame propagation process.     

Relative contributions from the kinetic and thermal characteristics of impurities and the inflammability of air-hydrogen mixtures

The effect of chemically active and inert impurities upon the concentration limits of flame propagation of air-hydrogen mixtures at atmospheric pressure is studied. The effect of impurities as a function of their thermal properties and the mechanism of the action on combustion is discussed. Combustion inhibition is considered a clear sign of the versatility of the one-stage reaction model used as the basis of thermal combustion theory.     

Microencapsulation of bisphenol-A bis (diphenyl phosphate) and influence of particle loading on thermal and fire properties of polypropylene and polyethylene terephtalate

Microencapsulated flame retardant, bisphenol-A bis (diphenyl phosphate) (BDP), with a silane shell was prepared by sol–gel process with the goal of incorporating them in polymeric matrices by melt blending to improve the flame retardancy of isotactic polypropylene (iPP) and polyethylene terephtalate (PET). The influence of the loading content on thermal transitions has been studied by differential scanning calorimetry (DSC), the thermal stability of the polymer/microcapsules composites has been assessed by thermogravimetric analysis (TGA) and cone calorimetry has been used to study the fire reaction. It was noticed that the microcapsules have a limited influence on the thermal transitions of iPP matrix, but a decrease of the melting and glass transition temperatures was detected for the PET microcomposites. TGA results showed that the addition of microcapsules could improve char formation of the PET systems both in nitrogen and in air atmospheres, whereas only a small improvement of the thermal stability was detected in oxidative atmosphere for the iPP samples. Furthermore, cone calorimeter experiments show that the incorporation of microcapsules in the iPP gives almost no improvement in the iPP fire reaction. However, the microcapsules act as flame retardant in PET reducing the heat release rate during the combustion and the total heat evolved. Therefore, microcapsules can act as a char promoter agent to enhance the fire resistance in the case of PET.     

Problems of combustion,explosion, and gas detonation in the theory of nonisothermal chain processes

Results of the studies show that the occurrence of chain avalanches is a necessary condition of gasphase processes of combustion not only at pressures tens of times lower than atmospheric pressure, as was thought earlier, but at almost any higher pressures in any temperature regime. It is concluded that considering the specifics of nonisothermal chain processes over a broad range of pressures allows us to explain and adequately describe observed features of combustion, explosion, and detonation (including some that were unexplained earlier). New laws important in theory and practice are predicted and revealed.     

Synthesis and properties of phosphorus polyesters with systematically altered phosphorus environment

Monomers with phosphorus-containing substituents were incorporated into aromatic-aliphatic polyesters to develop polymeric halogen-free flame retardants as additives for poly(butylene terephthalate) (PBT). They were built into the polyester backbone of PBT substituting 1,4-butane diol as monomer by phosphorus-containing aromatic-aliphatic diols. Starting from 10-(2,5-bis(2-hydroxyethoxy)phenyl)-9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO-HQ-GE), the chemical structure of the phosphorus monomers was systematically varied resulting in new polymers with diphenyl phosphine oxide substituents and bridged phosphine oxide units. The polymers were prepared by transesterification polycondensation in the melt in lab-scale as well as in a 2.4 l-autoclave. The properties of the polyesters were determined and compared to the DOPO-based polyester with respect to the achieved molar mass and polydispersity, solid state structure, glass transition temperature, thermal stability and combustion behavior.It was found that the different phosphorus substituents lead to different glass transition temperatures. The polymers containing bridged phosphorus structural units showed higher glass transition temperatures T_g and resulted in higher char yields after thermal decomposition. Both phosphine oxide structures showed only one-step decomposition with a shoulder at the end of the step. In contrast, two separate steps were observed in the polyesters with DOPO-substituents. The results indicated that the phosphorus polyesters under discussion are suitable to adjust the flame retarding mechanism.     

Application of global kinetic models to HMX beta-delta transition and cookoff processes

The reduction of the number of reactions in kinetic models for both the HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) beta-delta phase transition and thermal cookoff provides an attractive alternative to traditional multi-stage kinetic models due to reduced calibration effort requirements. In this study, we use the LLNL code ALE3D to provide calibrated kinetic parameters for a two-reaction bidirectional beta-delta HMX phase transition model based on Sandia instrumented thermal ignition (SITI) and scaled thermal explosion (STEX) temperature history curves, and a Prout-Tompkins cookoff model based on one-dimensional time to explosion (ODTX) data. Results show that the two-reaction bidirectional beta-delta transition model presented here agrees as well with STEX and SITI temperature history curves as a reversible four-reaction Arrhenius model yet requires an order of magnitude less computational effort. In addition, a single-reaction Prout-Tompkins model calibrated to ODTX data provides better agreement with ODTX data than a traditional multistep Arrhenius model and can contain up to 90% fewer chemistry-limited time steps for low-temperature ODTX simulations. Manual calibration methods for the Prout-Tompkins kinetics provide much better agreement with ODTX experimental data than parameters derived from differential scanning calorimetry (DSC) measurements at atmospheric pressure. The predicted surface temperature at explosion for STEX cookoff simulations is a weak function of the cookoff model used, and a reduction of up to 15% of chemistry-limited time steps can be achieved by neglecting the beta-delta transition for this type of simulation. Finally, the inclusion of the beta-delta transition model in the overall kinetics model can affect the predicted time to explosion by 1% for the traditional multistep Arrhenius approach, and up to 11% using a Prout-Tompkins cookoff model.     

Synthesis of a novel phosphorus‐containing curing agent and its effects on the flame retardancy,thermal degradation and moisture resistance of epoxy resins

A novel phosphorus‐containing compound diphenyl‐(1, 2‐dicarboxylethyl)‐phosphine oxide defined as DPDCEPO was synthesized and used as a flame retardant curing agent for epoxy resins (EP). The chemical structure of the prepared DPDCEPO was well characterized by Fourier transform infrared spectroscopy, and ¹H, ¹³C and ³¹P nuclear magnetic resonance. The DPDCEPO was mixed with curing agent of phthalic anhydride (PA) with various weight ratios into epoxy resins to prepare flame retardant EP thermosets. The flame retardant properties, combustion behavior and thermal analysis of the EP thermosets were respectively investigated by limiting oxygen index (LOI), vertical burning tests (UL‐94), cone calorimeter measurement, dynamic mechanical thermal analysis and thermogravimetric analysis (TGA) tests. The surface morphologies and chemical compositions of the char residues for EP thermosets were respectively investigated by scanning electron microscopy and X‐ray photoelectron spectroscopy (XPS). The water resistant properties of the cured EP were evaluated by putting the samples into distilled water at 70°C for 168 hr. The results revealed that the EP/20 wt% DPDCEPO/80 wt% PA thermosets successfully passed UL‐94 V‐0 flammability rating and the LOI value was as high as 33.2%. The cone test results revealed that the incorporation of DPDCEPO effectively reduced the combustion parameters of the epoxy resin thermosets, such as heat release rate and total heat release. The dynamic mechanical thermal analysis test demonstrated that the glass transition temperature (Tg) decreased with the increase of DPDCEPO content. The TGA results indicated that the incorporation of DPDCEPO promoted the decomposition of epoxy resin matrix ahead of time and led to a higher char yield and thermal stability at high temperatures. The surface morphological structures and analysis of the XPS of the char residues of EP thermosets revealed that the introduction of DPDCEPO benefited the formation of a sufficient, compact and homogeneous char layer with rich flame retardant elements on the epoxy resin material surface during combustion. The mechanical properties and water resistance of the cured epoxy resins were also measured. After water resistance tests, the EP/20 wt% DPDCEPO/80 wt% PA thermosets retained excellent flame retardancy, and the moisture adsorption of the EP thermosets decreased with the increase of DPDCEPO content in EP thermosets because of the existence of the P–C bonds and the rigid aromatic hydrophobic structure in DPDCEPO. Copyright © 2015 John Wiley & Sons, Ltd.