基于化学爆炸模式分析方法的乙烯对冲扩散火焰熄火机理

重点探讨了化学爆炸模式分析(CEMA)在熄火机理研究中的具体方法理论及可行性,研究了化学反应/热质混合相互作用对熄火的影响,并利用爆炸因子和分叉因子的概念,确定出主导乙烯火焰熄火极限的关键反应动力学因素。结果表明:具有正特征值的CEM首次出现在熄火极限附近火焰化学当量等值面处,因此它可作为判断熄火的重要依据。火焰的熄火极限是放热与链分支、中断反应综合作用的结果,链分支反应R32(H+O_2===O+OH)和放热反应R81(OH+CO===H+CO_2)对乙烯熄火极限的影响最显著,增大这两步反应速率能极大地扩宽燃烧稳定范围;而增大链中断反应R49(H+HCO===H_2+CO)的速率会缩小燃烧稳定范围。基于CEMA方法与爆炸因子、分叉因子的概念,可系统地揭示详细反应动力学对熄火的影响机理。     

甲醇对丙烷/氧气混合气爆炸极限的影响

为探究替代燃料丙烷/甲醇混合气的氧化反应特性,利用爆炸极限开展了甲醇对丙烷/氧气混合气的负温度系数（NTC）响应特性的研究。结果表明：在NTC区域,下拐点的压力随着甲醇摩尔分数的增加而升高,但下拐点的温度几乎保持不变。然而上拐点的温度与压力随着甲醇摩尔分数的增加没有明显变化。整体而言,随着甲醇摩尔分数的增加,丙烷/氧气混合气的NTC区域不断减小并向高压区域移动。对比分析了不同爆炸状态下,即无爆炸、冷焰以及热焰状态,混合气的温度、压力以及主要组分变化,并获得了影响温度变化的主要基元反应。此外,对爆炸极限曲线的NTC区域上、下拐点进行了敏感性分析,确定影响拐点状态的主要基元反应。     

基于化学爆炸模式分析方法的乙烯对冲扩散火焰熄火机理

重点探讨了化学爆炸模式分析(CEMA)在熄火机理研究中的具体方法理论及可行性,研究了化学反应/热质混合相互作用对熄火的影响,并利用爆炸因子和分叉因子的概念,确定出主导乙烯火焰熄火极限的关键反应动力学因素。结果表明：具有正特征值的CEM首次出现在熄火极限附近火焰化学当量等值面处,因此它可作为判断熄火的重要依据。火焰的熄火极限是放热与链分支、中断反应综合作用的结果,链分支反应R32(H+O₂ \stackrel{\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}}{?} O+OH)和放热反应R81(OH+CO \stackrel{\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}}{?} H+CO₂)对乙烯熄火极限的影响最显著,增大这两步反应速率能极大地扩宽燃烧稳定范围;而增大链中断反应R49(H+HCO \stackrel{\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}}{?} H₂+CO)的速率会缩小燃烧稳定范围。基于CEMA方法与爆炸因子、分叉因子的概念,可系统地揭示详细反应动力学对熄火的影响机理。     

T-JUMP/FTIR联用技术研究RDX的热裂解过程

用T-JUMP/FTIR技术研究了模拟燃烧条件下RDX的热裂解过程.通过分析裂解产物随时间的分布规律及温度与压力对裂解过程的影响,提出了RDX可能的快速热裂解机理及影响快速热裂解机理的因素.结果表明,RDX裂解气相产物主要有CO2、N2O、NO2、HNCO、NO、CO和HCN,且在不同的测试条件下产物的相对含量有明显变化:在600℃、0.1～0.4 MPa条件下,裂解5 s以内的NO2与NCN含量比随压力的升高而下降;HCN与N2O含量比随压力的提高而增大;在0.1 MPa压力条件下,随着温度的升高,HCN与N2O的含量比趋向于增大.RDX快速热裂解的初期存在两个竞争反应,即C-N和N-N键的断链竞争反应;提高温度和在0.1～0.4 MPa压力范围内提高压力对生成HCN的N-N键断裂反应有利.     

聚乙烯醇缩丁醛薄膜老化机理研究

利用红外光谱和力学性能的变化研究了聚乙烯醇缩丁醛薄膜的热老化和紫外光老化机理,结果表明,在热老化及紫外光老化过程中,PVB薄膜的拉伸强度均是先增大然后再减小。这是由于在热老化过程中PVB薄膜发生了增塑剂的损失及PVB分子主链和侧链的断裂,而在紫外光老化过程中,主要是发生PVB分子链之间的交联反应,仅有少量的分子链发生断裂。PVB薄膜的光老化和热老化都是由树脂中存在少量聚醋酸乙烯分解生成的自由基诱导而发生的,添加适量的热稳定剂和紫外吸收剂可以吸收薄膜加工时生成的自由基,从而提高薄膜的稳定性及粘结性能。     

多马来酰亚胺改性聚苯醚树脂的热分解动力学研究

采用热重分析法对自制的3,3′-二乙基-4,4′-二苯甲烷型多马来酰亚胺(PEDM)改性烯丙基化聚苯醚(PPO)固化树脂的热分解动力学进行了研究。结果表明:在热作用下,共聚交联体系在PPO的苯氧醚键处首先断链,并按无规断链或自由基引发断链方式迅速进行,交联网络被破坏成许多线形分子链,但其他键受影响很小,热分解表现为一级反应,且失重速率很快;随温度升高,当体系的苯氧醚键基本上断裂完毕后,线形分子链上的马来酰亚胺环的羰基或其他能量相近的键也开始逐渐断裂,此时热分解也表现为一级反应,但速率比第1步慢得多。     

覆铜板半固化环氧树脂热反应级数的研究

采用热重（TG）分析以不同的升温速率（5、10、15、20、30℃/min）分别在氮气气氛和空气气氛下对覆铜板中双氰胺固化环氧树脂热分解行为进行研究,根据TG曲线,采用Coats-Redfern和Ozawa热分析处理动力学数据的方法,计算环氧树脂热分解过程中的反应活化能E、反应级数n及频率因子A。结果表明,空气气氛下5%～30%的失重率下反应表观活化能在355.0～366.5 J/mol之间,频率因子A所对应的ln A值在4.987～6.810之间;氮气气氛下5%～30%的失重率下表观活化能在297.0～357.4J/mol之间,频率因子A所对应的ln A值在4.985～7.093之间;环氧树脂的热裂解反应和热氧化分解反应均为一级反应。研究发现,随着环氧树脂失重率的增加,氮气气氛下热分解活化能略有提高,而空气气氛下的活化能几乎不变。该研究为环氧树脂的热分解提供了重要的动力学参数。     

有机锡环氧齐聚物对聚氯乙烯热、光降解的稳定作用

众所周知,可用于聚氯乙烯的不同有机锡和有机金属稳定剂及灭菌剂非常多,但特别重要的是有机锡和环氧化合物。聚氯乙烯的热,光和氧化降解机理是一个复杂的问题。它涉及到聚烯烃当它们断链时所形成的断裂物和随后氧化成羧基。解释聚氯乙烯的降解机理,研究有效的稳定体系以防止上述反应的发生是聚氯乙烯降解和稳定这一领域中的主要课题。     

MH对EBA阻燃及热降解行为影响

采用极限氧指数法、热重分析和差示扫描量热分析等技术和手段研究了氢氧化镁（MH）对乙烯-丙烯酸丁酯共聚物（EBA）的阻燃性能、力学性能及热降解行为的影响。结果表明：EBA／MH复合材料的拉伸强度、断裂伸长率和熔体流动速率均随着MH用量的增加而降低；当MH的质量分数为60％时，复合材料的极限氧指数可达34．3％；MH通过分解吸热可提高EBA的热稳定性；MH表面羟基与EBA的反应性基团发生相互作用，以及EBA发生酯裂解后形成的羧酸与MH反应形成热稳定性更高的离聚物，均可提高EBA在高温下的热稳定性，并改变EBA的热降解历程。MH的“热阱”机理对EBA的阻燃起着很重要的作用。     

重力作用下自由液柱在气相中的断裂特性研究

防止液柱断裂是气液交叉流体系中抑制雾沫夹带的关键问题。通过数值计算并辅以试验的方法,对重力作用下的液柱流断裂特性进行了研究。重点探讨了自由液柱在惯性力与表面张力综合作用下的断裂机理,得到了重力作用下自由液柱无因次液柱长度L/d和孔口韦伯数之间的关系为:Ld=6.23We0.5,并与试验数据进行了对比,研究范围内,数值计算结果和试验数据吻合较好。     

Phenomenological characteristics of cool-flame oxidation of cyclohexane

The phenomenological characteristics of cool flames of cyclohexane are studied. It is shown that cyclohexane flames arise at lower temperatures (beginning at T ≈ 230°C) and lower pressures (lower than 10 torr at T = 290°C) than alkane and alkene flames. The temperature dependence of the lower pressure limit of the cool flame is determined. It is established that, depending on the conditions (p, T, and C₆H₁₂/O₂ ratio), cool-flame ignition is characterized by a rise and drop of the temperature ΔT or by several peaks imposed on each other. The dependence of the cool-flame structure (for ΔT) on the oxygen content in the reaction mixture is studied. It is shown that the cool flame resulting from cyclohexane oxidation is accompanied by ring opening and formation of products with a smaller number of carbon atoms. It is established that cool flames of cyclohexane are characterized by a negative temperature coefficient. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 26–28, January–February, 2008.     

Dynamic behavior and sensitivity of skeleton thermokinetic model for acetaldehyde oxidation

This work elucidated the dynamic behavior of the Wang-Mou model, a three-variable, skeleton model for acetaldehyde oxidation, in a continuously stirred tank reactor. The characteristics of the five dynamic regimes reported in the literature were numerically explored herein, specifically: (I) low-temperature steady state, (II) oscillatory two-stage ignitions, (III) complex oscillations, (IV) oscillatory cool flames, and (V) high-temperature steady state. Regimes were noted with new dynamic characteristics, which had not previously been reported. Additionally, bifurcations around the bistable and the birhythmic regimes were examined in detail.The sensitivity analysis polynomial approximation method revealed the significance of the kinetic parameters on the system's dynamics. Furthermore, the sequences of the sensitivity coefficients for the five dynamic regimes were reported. The step which initiates oxidation controls the ignition oscillation's dynamics, while the low-temperature branching and high-temperature termination steps control the cool flame.     

Propagation and quenching of premixed flames in narrow channels

The shape and propagation of unsteady premixed flames in narrow channels with adiabatic and isothermal walls are numerically investigated in the present study. The flame chemistry is modelled by an one-step overall reaction, which simulates the reaction of a stoichiometric acetylene-air mixture. The numerical results show that both ignition methods and thermal boundary conditions affect flame formation and its shape. The flame keeps a single tulip shape in the whole process of propagating through the channel if plane ignition is used and a single mushroom shape if spark ignition is used. For isothermal cold walls, the flame cannot keep a single tulip shape or mushroom shape all the time. Under plane ignition, a transition from a tulip-shaped flame to a mushroom-shaped flame occurs as flames propagate from one end of channel to the other. Under spark ignition, the process is just the contrary. It is also shown that the heat loss at the cold walls has a dual effect on the formation of a tulip-shaped flame. Flame propagation and quenching in narrow channels of different heights are analyzed systematically, and a criterion is proposed to judge the flame states of partial extinction and total extinction. It is called the criterion of flame propagation and quenching in narrow channels. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 27–36, May–June, 2006.     

Experimental study on the hazards of the jet diffusion flame of liquefied dimethyl ether

Dimethyl ether (DME) has been considered as a substitute for diesel fuel because it has a low auto-ignition temperature and produces less NO_x, SO_x, and particulate matter. However, the introduction of DME vehicles needs widely available DME supply stations. Moreover, the preparation of safety regulations for DME supply stations is very important, and so safety data is needed. Therefore, the present paper reports the hazards of the DME jet diffusion flame, which is one of several hazardous properties of DME, by studying the results of leaking gas and liquid DME. DME jets were released horizontally from circular nozzles whose diameters were 0.2, 0.4, 0.8 and 2 mm, and the release pressure was varied from the saturated vapor pressure to 2 MPa. When gaseous DME was released at the saturated vapor pressure, the flame was blown out. However, when liquefied DME was released, the flame formed. We obtained the experimental equations for estimating the scale and thermal hazards of DME diffusion flames.     

Unsteady extinction mechanism of nonpremixed flame interacting with a vortex

The extinction mechanism of a CH₄/N₂-air counterflow nonpremixed flame interacting with a single vortex was numerically studied. An augmented reduced mechanism was used to treat the CH₄ oxidation reactions. The contribution of each term in the energy and the OH species equations were evaluated to investigate the unsteady extinction mechanism of nonpremixed flame. The flame temperature began to decrease due to the convection heat loss when the flame interacted with a vortex. The investigation of the radical behavior during the flame-vortex interaction process also provided useful information on the unsteady extinction mechanism. The OH radical concentration could be used as a good tracer of the state of the unsteady extinction of nonpremixed flame. The reduction mechanism of OH concentration was confirmed by analyzing the contribution of each term in the OH species equation. At initial stage of flame-vortex interaction, the OH production and consumption rates increased gradually, while the OH concentration was kept nearly constant. Near the extinction limit, the OH production rate decreased rapidly due to the low flame temperature, and the balance between the OH production and OH consumption by diffusion could not be maintained. The unsteady nonpremixed flame interacting with a vortex under the conditions of regime (V) shown in the spectral combustion diagram [Thévenin, D., Renard, P.H., Fiechtner, G.J., Gord, J.R., Rolon, J.C., 2000. Regimes of non-premixed flame-vortex interactions. Proceedings of the Combustion Institute 28, 2101-2108.] was finally extinguished due to low reactivity, which was induced by the low flame temperature.     

乙醇和二甲醚微射流火焰燃烧特性研究

采用实验及数值计算研究了乙醇和二甲醚微圆管射流火焰燃烧特性。通过实验观察到不同燃料流速下乙醇和二甲醚火焰都具有四种典型的火焰形态；使用平面激光诱导荧光测试系统获得了微射流火焰的OH基元分布，实验结果表明在较高流速下稳定燃烧的乙醇火焰比二甲醚火焰直径小，且略高于二甲醚火焰；采用考虑详细化学反应机理的数值计算对乙醇和二甲醚火焰进行了数值模拟，计算结果与实验现象吻合较好；利用一维非预混对冲火焰计算进一步研究了这两种燃料的化学反应路径，分析结果表明乙醇和二甲醚火焰的中间产物有显著差异，两种燃料化学反应特性的差异导致了不同的微火焰结构。     

Unsteady auto-ignition of hydrogen in a perfectly stirred reactor with oscillating residence times

The ignition characteristics of a hydrogen–air mixture in a perfectly stirred reactor (PSR) with an oscillating residence time were investigated numerically. An unsteady numerical algorithm was developed and solved using a stiff-equation solver in order to investigate the unsteady auto-ignition behavior of the fuel/air mixture. The amplitude, frequency, and phase of the residence time oscillations were varied, and the effects on the simulated ignition behavior were recorded. Under small amplitude oscillations of the residence time, once ignited, the temperature in the reactor varied following the phase of the oscillations. Under larger amplitude variations, periodic ignition, and extinction events were observed. A critical frequency was observed, where the ignition delay time became significantly large than at the other frequencies. The existence of this critical frequency was found to depend on the phase of the residence time oscillation, and only occurred when the phase was such that the residence time decreased from the initial conditions. Ignition did not occur for frequencies of the oscillation in the residence time beyond 2.84 kHz, regardless of the phase. The physics of ignition delay for the case where the oscillatory residence time decreased initially could be clarified by investigating the time variation of characteristic chemical times of important reactions to ignition.At low frequencies of the residence time oscillation, similar behavior to that of the steady state was observed. However, the ignition delay time was found to be significantly different at high frequencies, especially for larger amplitude fluctuations in the residence time. Combustion of the fuel/air mixture could be sustained at shorter residence times under the oscillating residence time conditions than under the steady-state conditions. The reaction could not be sustained at high frequencies, and a pulsed-mode flame was observed, where the period of the ignition and extinction events was the same as the period of the oscillations in the residence time.The concentration of free radicals was found to increase with time prior to ignition, and the H radical concentration saturated at a maximum at the ignition time, indicating that the H radical concentration is a good indicator of ignition time under oscillating residence times.     

The relationship of knock during controlled autoignition to temperature inhomogeneities and fuel reactivity

Experiments have been performed in a rapid compression machine (RCM), to investigate the conditions for and the origins of ‘knock’ in controlled autoignition (CAI), or homogeneous charge compression ignition (HCCI). Ignition in an RCM is the closest approach to that in a CAI engine without engendering the full complexity of reciprocating motion and fuel+air charge induction. As a representative fuel of intermediate reactivity, the combustion of n-pentane in air was studied at the compositions φ=1.0, 0.75 and 0.6 at end-of-compression pressures of 0.80-0.86 MPa (7.9-8.4 bar) and 1.4-1.5 MPa (13.8-14.8 bar), respectively, over the compressed gas temperature range 690-820 K. Autoignition is characterised by a two-stage development in these ranges of conditions, a ‘cool flame’ being followed by hot stage combustion.Filtered Rayleigh scattering from a planar laser sheet was used to characterise the temperature field that developed in the combustion chamber following rapid compression. High resolution pressure records, combined with image intensified, natural light output originating from chemiluminescence, were used to characterise the transition from non-knocking to knocking reaction and the evolution of the spatial development of chemical activity in this temperature field. It appears that knock originates from a localised development of the incandescent hot stage of ignition. Even though non-homogeneities prevail in the non-knocking reaction, it is associated with a relatively benign development, in which the cool flame is followed by a second stage, blue flame rather than the normal incandescent hot flame. The kinetics that may contribute to this distinction are discussed.