Heterogeneous chain propagation in low-pressure flames

An analysis of papers on hydrogen combustion at low pressures is performed, which refines the contribution of the catalytic reactions on the reactor wall to the gas-phase part of the process. A new model for the heterogeneous loss of active reaction centers was proposed and tested experimentally to explain inconsistencies that occur in some papers. In this model, the diffusion region of chain termination is formed under standard experimental conditions in vacuum oxyhydrogen flames at a reactor gas pressure a thousand times lower than the boundary pressure postulated by the previous models as the pressure below which the diffusion region of chain termination cannot be formed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 2, pp. 10–18, March–April, 2006.     

Heterogeneous chain propagation on a quartz surface

Results of experimental and analytical studies of heterogeneous chain processes on quartz are given. A calculation is made of the ignition of an oxyhydrogen mixture in a quartz reactor under conditions of intense heterogeneous negative interaction of chains in the kinetic and diffusion regions. An energy criterion was developed experimentally that allows one to distinguish the range of reactions with an elevated probability of heterogeneous chain propagation on quartz. The possibility of the formation of atoms and radicals on the surface of quartz and conditions for their passage into the bulk are studied. The possibility of passage of hydrogen atoms from a microporous SiO₂ surface into the bulk is analyzed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 42–52, May–June, 2006.     

Kinetic Regimes of Developed Chain Combustion

It is shown that chain thermal explosion is an inherent property of chain branching combustion that is due to simultaneous action of a chain avalanche and self-heating, which under certain conditions also becomes a progressively accelerating process. An attempt to deny this phenomenon is in fact equivalent to denying thermal explosion. This phenomenon is also observed in the absence of convection. The chain branching reaction mechanism is involved in all regimes of gas-phase combustion of hydrogen-containing compounds with oxygen. This results in inhibition or promotion of flame propagation, indicating the necessity of taking into account competition between reaction chain branching and termination.     

Controlling low-pressure flame conditions by affecting the cold service lines located far from the heated reactor

A study was performed of a low-pressure oxyhydrogen flame in an isothermal regime in the diffusion-controlled region of heterogeneous chain termination. The chain propagation and termination sites were widely separated in space. Controlling the chain loss conditions made it possible to carry out isothermal combustion of the mixture in the diffusion-controlled region and to verify the proposition of the Semenov theory that the propagation of a chain explosion in time near the limit can be described by a linear model. From a comparison of calculated and experimental data, gas-surface interaction with a variable rate constant was found and the range of variation of this constant during one ignition was determined. The large differences between the kinetic curves obtained previously for various modes of chain termination on the wall were found to be related primarily to heterogeneous chain interaction, which governs the gas-phase process in the kinetic region and has a weak effect on this process in the diffusion-controlled region of chain termination. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 32–42, March–April, 2007.     

Heterophase combustion of silane near the first ignition limit

Heterophase combustion of silane near the first ignition limit was studied. It was found that the reaction of chain initiation on quartz in the zone of hydrogen and silane combustion manifested itself as positive feedback, which was enhanced during exposure of the reactor walls to the products of the low-pressure flame. It was shown that, in a silane-oxygen flame at a temperature of 350–500°C the quartz surface was activated as a catalyst of heterogeneous chain initiation much more strongly than it was in a hydrogen flame. It was shown that the previously found increase in the concentration of atomic hydrogen during oxidation of silane in oxygen below the first limit was related to the formation of new lattice structures saturated with crystal lattice defects, whose number on the wall increases continuously during condensation of the final reaction products, together with adsorption silicon-containing radicals.     

Critical Conditions of Chain Thermal Explosion

This paper reports results of experimental investigation of the critical conditions of chain thermal explosion of hydrogen–air and methane–air mixtures are introduced. The observed regularities, including the difference in critical conditions between the ignition and explosion of these combustibles, are explained by considering the chemical mechanisms of oxidation of hydrogen and methane in the context of the nonlinear theory of nonisothermal chain processes. Examples of regularities of combustion processes at atmospheric pressure are given that cannot be described without considering the leading role of the competition between the branching and termination of reaction chains. It is shown that in solving the equations used to describe combustion processes, neglect of the variation in the heat effects of reactions with temperature can lead to large errors.     

Kinetic manifestations of low-temperature combustion of hydrocarbons and hydrogen: Cool and intermittent flames

Phenomena inherent in degenerate branched and completely branched chain reactions are considered from a unified viewpoint. In the case of degenerate branched chain reactions, such phenomena include a negative temperature coefficient, cool flames, and oscillations arising in slow combustion of hydrocarbons. Another phenomenon (intermittent flames) is inherent only in completely branched chain reaction of low-temperature combustion of hydrogen at reduced pressures in the presence of SO₂ additives. These kinetic manifestations of chain branching processes are characterized by a variety of elementary reactions with participation of intermediate compounds and free radicals with different structures. A specific kinetic feature of reactions of both types is simultaneous participation of the active center responsible for chain branching in the branched reaction and in the reaction of propagation of an ordinary chain.     

Dynamics of phase-formation processes during self-propagating high-temperature synthesis in the thermal explosion regime

Specific features of formation of temperature and concentration fields during self-propagating high-temperature synthesis in the thermal explosion regime in a cylindrical reactor are studied by methods of mathematical modeling. The calculations are performed with allowance for melting of one (chemically active) component in the approximation with the high-melting component being non-soluble in the melt of the low-melting component. It is shown that the conditions of complete conversion of the original components in the volume of the reacting mixture depend on relations between the Biot criterion of the system, the ambient temperature, and the thermal effect of the reaction. After the thermal explosion, which occurs when the melting front reaches the geometric center of the reactor, a front of complete conversion is formed. This front moves from the cylinder centerline to the periphery with a gradually decreasing velocity. The diagram of the critical values of the Biot criterion at which the components burn down completely in the entire reaction volume is calculated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 31–38, July–August, 2008.     

Inhibition of various hydrogen combustion regimes in air by Propylene and Isopropanol

This paper studies the effects of small additives of propylene and isopropanol on the hydrogen-air flame speed in the predetonation regime, deflagration-to-detonation transition, and burning rate. It is shown that the difference in the effects of these additives on the combustion is determined primarily by their ability to terminate reaction chains. In hydrogen flames, the additives are consumed as a result of their reactions with the active intermediate products of H₂ combustion in which these species are replaced by inactive radicals.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 3–14, January–February, 2005.     

长链支化LLDPE的制备及表征

利用预辐照线型低密度聚乙烯(rLLDPE)引发基体LLDPE的长链支化反应,制备出长链支化LLDPE。研究了辐照剂量、rLLDPE用量对长链支化反应的影响,并通过熔体流动速率测试、凝胶渗透色谱、13CNMR和扭矩试验对长链支化LLDPE进行了表征。结果表明:与基体LLDPE相比,长支链化LLDPE的熔体流动速率下降,加工扭矩下降,熔体流动速率比(M10/M2)明显增大;形成长链支化的最佳辐照剂量为15kGy、rLLDPE最佳用量为50%;由13CNMR方法计算的长链支化数达1.4/10000C。     

Increasing the burning velocity of a low-pressure hydrogen-oxygen flame by the addition of trimethyl phosphate in terms of Zel’dovich’s chain mechanism of flame propagation

The chain reaction mechanism and theoretical approach proposed by Zel’dovich for hydrogen flame propagation are used to describe the effect of increasing the burning velocity of a low-pressure hydrogen-oxygen flame by the addition of trimethyl phosphate (TMP), which induce a catalytic recombination of hydrogen atoms. The promotion of a stoichiometric hydrogen-oxygen flame at subatmospheric pressure by the addition of TMP at a low concentration (0.1–0.5%) is described using a model of the catalytic recombination of hydrogen atoms. The results of calculation using Zel’dovich’s theory with the proposed simplified kinetic model are in good agreement with simulation results for the complete kinetic mechanism. It is shown that increasing the recombination rate of hydrogen atoms in a catalytic reaction involving phosphorus-containing species increases the heat release rate and, hence, the flame burning velocity. A kinetic analysis was performed of the dependence of the ratio of the recombination and branching rates, the temperature at the maximum reaction rate, and the maximum mole fraction of hydrogen atoms on the pressure and additive concentration. The study confirmed Zel’dovich’s prediction that the recombination not only has the harmful effect of terminating chains, but it also has the beneficial effect of releasing heat.     

Degenerate thermal explosion regimes of autocatalytic reactions

Conclusions We have analyzed the laws of degeneracy of thermal explosion for autocatalytic reactions close to and remote from the ignition limit over a broad range of variation of the parameters β and γ. The characteristic features of degeneracy for reactions with different autocatalysis numbers η₀ have been noted. Fizika Goreniya i Vzryva, Vol. 4, No. 4, pp. 563–567, 1968     

Role of atomic hydrogen diffusion in a hydrogen flame

A numerical modeling study of the propagation of a laminar flat homogeneous gas flame has shown that in a hydrogen-air flame, a rapid increase in the concentration of OH radicals begins in the range of low temperatures and the concentration profile has two maxima. The first maximum in the low-temperature region of the front is related to the diffusion of H atoms, formation of HO₂ radicals, and the quadratic branching reaction H + HO₂ → OH + OH. The second maximum in the OH concentration profile is due to the classical high-temperature branching reactions H + O₂ → OH + O and O + H₂ → OH + H. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 3–9, March–April, 2007.     

Effect of diluents of various chemical nature on the flammability limits of gas mixtures

The effect of diluents of various chemical nature [halogenated hydrocarbons, inhibitors developed at the Institute of Structural Macrokinetics and Problems of Materials Science (ISMAN), superheated water aerosol] on the flammability limits of hydrogen-methane mixtures in air was investigated experimentally. It was found that ISMAN inhibitors, which are aliphatic hydrocarbons, reduce the upper flammability limit of hydrogen even more effectively than the well-known inhibitor 1,2-dibromotetrafluoroethane. Flammability curves for hydrogen and methane in mixtures with the above-mentioned diluents were obtained. The results are interpreted qualitatively by considering various kinetic regimes of nonisothermal chain branching reactions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 96–102, November–December, 2006.     

VAc聚合过程工艺条件对链支化的影响

研究了以甲醇为溶剂、偶氮二异丁腈(AIBN)为引发剂的VAc聚合过程,通过比较醇解前后PVAc和PVA聚合度的变化研究过程参数如溶剂、引发剂量和分段加入方式以及反应时间对PVAc链支化程度的影响。结果表明,在溶剂总量为20%(wt)、引发剂总量为0.00013%~0.0013%范围内,分段加入引发剂使PVAc链支化程度增加,醇解后PVA聚合度与PVAc聚合度相比有所下降;过低的引发剂浓度虽然可以获得比较高的PVAc聚合度,但醇解后的PVA聚合度相比PVAc下降可达27.0%。在引发剂总量为0.000 63%情况下分批加入溶剂,在合适的反应时间内可控制PVAc支链化程度,缩小PVAc和PVA聚合度的差异并获得相对较高PVA聚合度。无论是一次性加入溶剂或分段加入溶剂,过长的反应时间都使PVAc的支链化程度增加,醇解后PVA聚合度与PVAc聚合度相比差异增大。     

Effect of heat transfer on the distributions of OH and CH radicals in the boundary layer with ethanol combustion

The reasons for formation of superequilibrium concentrations of radicals are studied by means of joint consideration of experimental data on the distributions of CH and OH molecules formed during diffusion combustion of ethanol and data on heat transfer in the chemical reaction region. The air flow velocity near the stagnation point in experiments with combustion is 0.7 m/sec, and the flow velocity along a flat plate is 10 m/sec (the turbulence levels are 1 and 18%). Mutual locations of specific features in the distributions of the heat-release rate and temperature are analyzed and compared with the distributions of OH and CH radicals. For all turbulence levels and flow velocities considered, the maximum concentration of radicals is reached on the boundaries of the heat-release region, whose locations are determined by molecular transport mechanisms. It is demonstrated that this conclusion is applicable to experimental data on diffusion combustion of a submerged hydrogen jet in air. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 3–11, November–December, 2008.     

Role of the intermolecular interaction in polymer fibre chemistry and technology

The existence of a specific intermolecular interaction of a hydrogen bond affects many polymerization processes and the occurrence of reactions in the chains of macromolecules. The features of the hydrogen bond in monomeric and oligomeric compounds are revealed. In production of aromatic polyamides, polyamido acids, polyimides, and the fibres based on them, the role of the hydrogen bond is reduced to the possibility of deliberate regulation of the chain rigidity. __________ Translated from Khimicheskie Volokna, No. 6, pp. 44–46, November–December, 2006.     

Activated combustion of the SiO2-Al-C system and synthesis of SiC/Al2O3 composite powders

The mechanism of initiation and combustion of the 3SiO₂-4Al-3C system is studied under conditions of activation of the combustion reaction by a high-energy additive of Al+αKNO₃. It is shown that combustion of this system is possible only when a certain quantity of the additive is introduced into the starting mixture. The mechanism of activation of the reaction is established, and the processes involved in the reduction stage of the combustion are shown to play a leading role in the reaction. Optimal conditions for synthesis of a SiC/Al₂O₃ composite powder containing SiC fibers are determined. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 51–55, March–April, 2000.     

Gap effect on the initiation of detonation in gas mixtures in cylindrical combustion chambers. II. Triggering of detonation within a turbulent flame

Schlieren moving-picture photography is used to study the burnup of oxygen gaseous mixtures in a cylindrical chamber with a gap at its periphery. It is found that a flame penetrating from the chamber into the gap can accelerate up to detonation speeds. The reaction wave in the gap precedes the primary combustion front propagating through the chamber and the reaction products escaping the gap create secondary combustion sources in the chamber. A process occurs in which a detonation wave that appears in the gap near one flank of the flame enters the main volume through the opposite flank, first triggering an explosion in the turbulent combustion zone (“an explosion within an explosion”) and then a detonation wave in the unreacted gas charge (“knock” in an engine). An interpretation is provided for the gas-dynamic structure of the secondary combustion source which is created in the cylindrical combustion chamber by a detonation wave propagating in the gap. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 4, pp. 77–87, July–August 1998     

Dynamics of thermal explosion in the postinduction period

The dynamics of thermal self-acceleration of the reaction and heating is studied for gasless condensed compositions in the induction and postinduction periods up to complete transformation of the substance. It is shown that the propagation of the reaction over the sample is of a distinct frontal character in a broad range of values of the Biot criterion Bi and within the framework of the macrokinetics of “weak” deceleration.” Two qualitatively different mechanisms of front propagation are revealed. This is the normal propagation of the combustion front initiated by the ignition zone for large values of Bi. For small values of Bi, propagation of the front is the apparent effect, which is due to subsequent adiabatic self-ignition of separate portions of the substance uniformly heated during the induction period. In the second case, by the propagation velocity, we mean the “phase” velocity. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 65–70, November–December 1999.