Regularities in the consumption of the initial reactants in various kinetic regimes

Using hydrogen oxidation as an example, it is demonstrated that, when gas ignition is prevented by means of inhibition, there is practically no consumption of the initial reactants because reaction chains do not form for lack of time and the rates of intermolecular reactions are insignificant. When the propagating flame and detonation wave are partially suppressed, the inhibitor is consumed only in chain termination reactions involving reactive intermediate species. Oxygen is additionally consumed in its reactions with products of incomplete inhibitor conversion.     

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.     

The chain character of the third self-ignition limit of hydrogen-oxygen mixtures and flame propagation at atmospheric pressure

It is shown that the characteristic time of the hydrogen-oxygen reaction without the participation of reaction chains is thousands of times longer than the characteristic time of heat removal from the reactor under third self-ignition limit conditions. As a result, reaction mixture self-heating does not exceed several degrees and, contrary to commonly accepted views, cannot cause thermal ignition. It is also shown that the reason for self-ignition under these conditions is the excess rate of chain branching compared with chain termination responsible for the formation of chain avalanches. For the same reason, layer-by-layer ignition during laminar flame propagation is also a chain process. Self-heating arises as a result of the development of chain combustion and strengthens chain avalanches. Explosion and detonation inhibition shows that chain avalanches play an important role in these processes.     

Kinetic aspects of chemical control over flame propagation in combustible gases

Kinetic aspects of controlling ignition and flame propagation parameters in the gas phase by chemical methods are considered. The efficiency of the chemical methods is due to the branched chain character of gas-phase combustion reactions and the dominant role of the competition between chain branching and chain termination in these processes.     

Concentration Limits of Flame Propagation in H<Subscript>2</Subscript>—C<Subscript>3</Subscript>H<Subscript>8</Subscript>—Air Mixtures at Elevated Pressures

Upper concentration limits of flame propagation in H₂—C₃H₈—air mixtures at elevated initial pressures are determined. It is revealed that the flame propagation area widens substantially with the initial pressure rise. It is found that the presence of hydrogen promotes combustion of rich propane—air mixtures at concentrations of propane that exceed its upper concentration limit. It is concluded that research results can be explained from consideration of features of chain branching reactions, which are responsible for hydrogen and hydrocarbons combustion in air.     

Branched-chain nature of hydrogen combustion in the detonation mode

Minor admixtures of the simplest hydrocarbons can prevent detonation and break down a steady-state detonation wave in hydrogen-air mixtures at atmospheric and higher pressures. Therefore, the determining role in the appearance and propagation of the detonation wave is played by the branched mechanism and, accordingly, by the competing chain branching and termination reactions. Without taking into account these reactions, combustion theory cannot explain the basic regularities of the process, including the concentration limits of detonation.     

Construction of a Chemical Kinetic Model of Five-Component Gasoline Surrogates under Lean Conditions

The requirements for improving the efficiency of internal combustion engines and reducing emissions have promoted the development of new combustion technologies under extreme operating conditions (e.g., lean combustion), and the ignition and combustion characteristics of fuels are increasingly becoming important. A chemical kinetic reduced mechanism consisting of 115 species and 414 elementary reactions is developed for the prediction of ignition and combustion behaviors of gasoline surrogate fuels composed of five components, namely, isooctane, n-heptane, toluene, diisobutylene, and cyclohexane (CHX). The CHX sub-mechanism is obtained by simplifying the JetSurF2.0 mechanism using direct relationship graph error propagating, rate of production analysis, and temperature sensitivity analysis and CHX is mainly consumed through ring-opening reactions, continuous dehydrogenation, and oxygenation reactions. In addition, kinetic parameter corrections were made for key reactions R14 and R391 based on the accuracy of the ignition delay time and laminar flame velocity predictions. Under a wide range of conditions, the mechanism’s ignition delay time, laminar flame speed, and the experimental and calculated results of multi-component gasoline surrogate fuel and real gasoline are compared. The proposed mechanism can accurately reproduce the combustion and oxidation of each component of the gasoline-surrogate fuel mixture and real gasoline.     

Chain ignition of propane-air and pentane-air mixtures in a heated vessel

The spatial propagation of the chain ignition of propane-air and pentane-air mixtures with oxygen at a pressure of 1 atm and T = 600–800 K is studied. It is established that the features of the spatial propagation of chain ignition process are determined by the conditions of the reactor’s surface. It is shown that the site (or sites) of ignition are located on the surface of the reaction vessel; the flame front propagates from the site into the volume at a normal speed corresponding to the reactor temperature and the composition of the combustible mixture.     

The dependence of the rules governing gas-phase combustion on the competition between chain propagation and chain termination reactions

The competition between chain propagation and chain termination reactions was shown to play an important role in gas-phase combustion. Conditions under which this competition to a substantial extent determined the critical ignition conditions and the rate of the process were analyzed. Examples that showed that ignoring the role played by chain propagation, chain termination, and chain branching reactions in branched-chain processes led to conclusions that contradicted experimental data were considered.     

Thermal explosion characteristics in a reduced kinetics model

The problem of thermal explosion arising from a spatially homogeneous reduced five steps reaction kinetic model, which comprises of the chain initiation, chain propagation/branching and chain termination steps is considered. By assuming realistic approximations, the pertubation technique was used to obtain expressions for thermal ignition time for the adiabatic system. In the non-adiabatic system, expressions for the critical heat loss parameter and the ignition temperature in the line of Semenov theory have been obtained. Analysis of the system involving some parameters, and the contributions of the heat released due to the termination reactions on the behaviour of the ignition times and Semenov parameters have been carried out and expressed graphically. Apart from confirming known results in literature, the results shed more light on hitherto unknown behaviour.     

Synergism in combustion processes

The effect of CF₃H, CF₄, and N₂ on the ignition of methane–air mixtures has been investigated. The effect of trifluoromethane is due to its being involved in reaction chain termination. A nonadditive effect of trifluoromethane and nitrogen on the concentration limits of ignition and flame propagation in methane–air mixtures has been predicted and revealed. The synergistic effect arises from the exponential dependence of the rate of the chain process on the concentrations of the initial components.     

RP-3航空煤油高温骨架机理构建及验证

依据RP-3航空煤油的成分,考虑平均分子量及碳氢摩尔比等性质,本文提出其三组分替代燃料模型,其中正癸烷74.24%、1,3,5-三甲基环己烷14.11%和正丙基苯11.65%(质量分数)。采用机理生成程序ReaxGen得到详细化学反应机理;采用机理简化程序ReaxRed,运用直接关系图法与主成分分析法获得高温骨架机理(79物种,311反应)。该机理针对多个工况进行了点火延迟时间与层流火焰速度的验证,能较好地预测实验结果。路径分析结果表明高温下替代燃料通过氢提取反应、单分子裂解反应及β-断键反应消耗。敏感性分析表明高温点火过程由多种小分子自由基(H、CHO、C2H3等)的氧化及分解反应和大分子燃料的氢提取反应控制;影响火焰传播过程的关键反应来源于C0-C3的小分子核心机理。本文所提出的这个尺寸较小但精度较高的骨架机理可用于发动机燃烧过程的高保真数值模拟。     

正十二烷高温机理简化及验证

由于详细化学反应机理在模拟燃烧室燃烧时,计算量极大,很难被广泛运用。为了满足工程设计要求,采用替代燃料的简化机理进行计算不失为一种行之有效的方法。本文基于误差传播的直接关系图法和敏感性分析法对正十二烷180组分1962步高温机理(温度大于1100 K)进行简化,获得40组分234步化学反应机理。在温度为1100–1650 K,压力为0.1–4 MPa条件下,采用简化机理及详细机理对不同当量比、压力下着火延迟时间进行模拟,模拟结果与实验数据吻合得较好。通过对不同压力及温度下火焰传播速度进行模拟,验证了简化机理能够正确地反映正十二烷的燃烧特性。利用C_(12)H_(26)/OH/H_2O/CO_2等重要组分随时间变化的数据,验证了简化机理能够准确描述燃烧过程反应物消耗、基团变化、生成物产生的过程,并表明该机理具有较高的模拟精度。利用该简化机理对本生灯进行数值分析,结果表明该机理能够准确地反映火焰区温度和组分浓度的变化。紧凑的正十二烷高温简化机理不仅能够正确体现其物理化学特性,而且能够用于三维数值模拟,具有较高的工程运用价值和应用前景。     

Accelerated diffusion of chain carriers and kinetic features of heterogeneous processes in gas-phase chain reactions

In gas-phase combustion processes, the regeneration of free atoms and radicals in chain propagation reactions enhances the diffusion flux of these species from the flame zone. In flame propagation in tubular reactors and in filtration combustion, this effect facilitates the access of chain carriers to the surface even at atmospheric pressure, increases the role of heterogeneous reactions (primarily chain termination), and enhances heat removal due to heterogeneous recombination.     

Isothermal waves of hydrogen oxidation

It is analytically shown that the chain ignition region is extended in the reaction of hydrogen oxidation due to the nonlinear interaction H + HO₂ = 2 OH. As a result, the reaction propagates isothermally under the isothermal conditions outside the ignition region, which is calculated by the scheme taking into account only linear reactions with respect to radicals, if the reaction is initiated by additives of hydrogen atoms. The ignition limits were calculated with allowance for the above interaction, and the mathematical modeling of propagation of the hydrogen oxidation reaction under the isothermal conditions was performed.     

Spatially Directed Pyrolysis via Thermally Morphing Surface Adducts

Combustion is often difficult to spatially direct or tune associated kinetics—hence a run-away reaction. Coupling pyrolytic chemical transformation to mass transport and reaction rates (Damköhler number), however, we spatially directed ignition with concomitant switch from combustion to pyrolysis (low oxidant). A ‘surface-then-core’ order in ignition, with concomitant change in burning rate,is therefore established. Herein, alkysilanes grafted onto cellulose fibers are pyrolyzed into non-flammable SiO₂ terminating surface ignition propagation, hence stalling flame propagating. Sustaining high temperatures, however, triggers ignition in the bulk of the fibers but under restricted gas flow (oxidant and/or waste) hence significantly low rate of ignition propagation and pyrolysis compared to open flame (Liñán's equation). This leads to inside-out thermal degradation and, with felicitous choice of conditions, formation of graphitic tubes. Given the temperature dependence, imbibing fibers with an exothermically oxidizing synthon (MnCl₂) or a heat sink (KCl) abets or inhibits pyrolysis leading to tuneable wall thickness. We apply this approach to create magnetic, paramagnetic, or oxide containing carbon fibers. Given the surface sensitivity, we illustrate fabrication of nm- and μm-diameter tubes from appropriately sized fibers.     

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。     

The thermal explosion of the detonating mixture is impossible without a chain avalanche

The fundamental regularities of hydrogen/oxygen combustion are considered, which unambiguously indicate the branched chain character of the process at atmospheric pressure. It is noted that, in the general case, the ignition conditions are determined by the competition between chain termination and both chain branching and chain propagation reactions. Some publications ignoring this important point are considered.     

聚乙烯薄膜等离子体接枝甲基丙烯酸及阻燃性能的研究

以聚乙烯为基础,通过等离子体接枝方法接枝甲基丙烯酸.研究了接枝反应条件对接枝率的影响,并对接枝样品的阻燃性能进行了表征.接枝后样品的点燃时间明显延长,极限氧指数明显提高,成炭量明显增加,说明接枝后的侧基(-COOH,-COO^-Na⁺和-COO^-K⁺)在热降解过程中不仅自身参与成炭,而且大大促进了基体聚合物的成炭过程.     

Quenching of Flame Propagation Through Endothermic Reaction

The propagation of a premixed laminar flame supported by an exothermic chemical reaction under adiabatic conditions but subject to inhibition through a parallel endothermic chemical process is considered. The temperature dependence of the reaction rates is assumed to have a generalised Arrhenius type form with an ignition temperature, below which there is no reaction. The heat loss through the endothermic reaction, represented by the dimensionless parameter , has a strong quenching effect on wave initiation and propagation. The temperature profile can have a front or a pulse structure depending on the relative value of the ignition temperatures and on the value of the parameters and , the latter represents the rate at which inhibitor is consumed relative to the consumption of fuel. The wave speed-cooling parameter () curves are determined for various values of the other parameters. These curves can have three different shapes: monotone decreasing, -shaped or S-shaped, with the possibility of having one, two or three different flame velocities for the same value of the cooling parameter .