Mathematical simulation of the front processes of combustion of condensed systems in view of the velocity of propagation of heat

It is suggested to use the hyperbolic heat equation for mathematical simulation of the front processes of combustion of energy-intensive condensed systems (CS). The correlation between the velocity of propagation of thermal wave with the velocity of front motion, the thermal effect of the reaction of thermal decomposition, and the value of heat flux delivered to the surface is analytically determined. Numerical solutions of hyperbolic heat equation are obtained for unsteady-state mode of ignition of energy-intensive materials. The agreement between calculated dependences and experimental data on the combustion of energy-intensive compounds is considered.     

Microwave Initiated Self-Propagating High-Temperature Synthesis of SiC

The use of microwave energy to initiate self-propagating, high-temperature synthesis (SHS) of Si + graphite mixtures has been investigated. The results indicate that, unlike with conventional ignition techniques, green densities in excess of 80% of theoretical can be ignited and the combustion wavefront can be crudely controlled. It was found that the induction time for ignition increased with increasing green density and that a higher microwave power level was required with the denser green pellets to achieve the same ignition time. Combustion front velocity increased with green density. The degree of densification was found to decrease with increasing green density. For a given green density, the degree of densification increased with increasing microwave power. The product contained a significant proportion of ultrafine (36–72 nm diameter) SiC whiskers; despite this, final densities as high as 83.6% of theoretical could be obtained without the use of applied pressure. This compares with the 50% densities obtained via conventional ignition techniques.     

Self-Propagating Reactions in the Ti–Si System: A SHS-MASHS Comparative Study

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.     

A numerical study of the heterogeneous porosity effect on self-propagating high-temperature synthesis (SHS)

Self-propagating high-temperature synthesis (SHS), or the so-called micropyretic/combustion synthesis, is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Heterogeneous distributions of porosities are common during self-propagating high-temperature synthesis when powders are pressed and the conventional modeling treatments thus far have only considered uniform systems. Heterogeneities in the porosity are thought to result in local variations of such thermophysical/chemical parameters for the reactants as density and thermal conductivity further changing the combustion temperature, the propagation velocity, and the propagation pattern of a combustion front. This study investigates the impact of porosity variations during self-propagating high-temperature synthesis with Ti + 2B. In addition, the simulations for the propagation of combustion fronts across a non-uniform compact where the porosity is monotonically decreased or increased along the specimens due to die wall friction are also carried out.     

正常燃烧与部分燃烧的离子电流研究

进行了定容燃烧弹正常燃烧和部分燃烧的实验,对测得的这两种燃烧状态的离子电流、压力、温度信号以及用高速摄影同步拍摄的火焰发展的纹影图片进行了对比研究和分析,结果表明,正常燃烧的离子电流信号具有三个明显特征 ,即点火、火焰前锋区和焰后区三个阶段;而部分燃烧由于温度较低,只有点火和火焰前锋区信号;部分燃烧的压力和温度最大值小于正常燃烧的值,并且火焰发展和传播速度缓慢,出现"ω"形火焰.采用离子电流峰值法和积分法分别对正常燃烧和部分燃烧的计算显示,部分燃烧时的离子电流信号峰值总是小于正常燃烧的值,特别是部分燃烧的积分值大大低于正常燃烧的值.     

Volume Combustion Modes in Heterogeneous Reaction Systems

Volume combustion synthesis in metal–metal systems (i.e., Ni-Al and Cu-Al) was investigated. Both thermocouple and infrared imaging techniques were used to study the temperature–time history of the process. It was found that in both systems, volume combustion starts at a temperature near the melting point of aluminum. For the Cu-Al mixture, the reaction essentially occurs uniformly along the sample body; whereas, for Ni-Al, propagation of a rapid reaction wave is typically observed. The characteristic temperature gradient of this wave is more than an order of magnitude lower and the velocity of propagation is even higher, as compared with a conventional combustion wave. An explanation of the observed results based on a new class of wave, the so-called virtual combustion wave, is given.     

Microwave initiated self-propagating high temperature synthesis of materials

Abstract

The use of microwave energy to initiate self-propagating, high temperature synthesis (SHS) reactions has been reviewed. Microwave initiation usually results in ignition occurring at the centre of the body, with the combustion wavefront propagating radially outwards. This leads to a number of differences compared with conventionally ignited SHS reactions. These include ignition in both weakly exothermic systems and denser green bodies, crude control of the wavefront propagation, and the generation of different microstructures owing to dissimilar time-temperature and spatial temperature profiles. The technology also extends the range of materials and compositions that can be produced in a self-propagating manner. Commercially important developments are likely to be those that utilise these features to produce tailored microstructures for niche applications     

A numerical study of the effect of heterogeneities in composition on micropyretic synthesis

Micropyretic synthesis is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Composition variations in reactants and diluent are common during micropyretic synthesis when powders are mixed and the conventional modeling treatments thus far have only considered uniform systems. Composition variations are thought to result in the local variations of such thermophysical/chemical parameters for the reactant as density, heat capacity, and thermal conductivity; the result is changes in the combustion temperature, propagation velocity, and propagation pattern of a combustion front. This study investigates the impact of composition variations during micropyretic synthesis with Ni + Al. Correlations of variations in the reactants and diluent with the propagation velocity and combustion temperature are both studied by a numerical simulation.     

The energy required to ignite micropyretic synthesis. Part I: stable Ni + Al reaction

The progression of chemical reactions is determined by both thermodynamics and kinetics factors. Micropyretic/combustion reaction is a cascade of many chain chemical reactions and thermodynamics and kinetics of the ignition reaction are expected to greatly affect the overall reaction outcome. Furthermore, the stability of the sequential reaction and its progression are correspondingly changed once micropyretic parameters are changed. Improper ignition of micropyretic reaction provides either excessive or insufficient external energy, thus causes over-heating or extinguishing of the combustion front during propagation and therefore the heterogeneous structures. To achieve the homogeneous micropyretic reaction, it is thought possible to control ignition energy. A numerical study on the correlation of thermodynamics and kinetics factors of ignition on the stable Ni + Al reaction and the required ignition energy is reported in this study. The influences of activation energy (E), enthalpy of the micropyretic reaction (Q), pre-exponential factor (K _o), thermal conductivity (K), heat capacity (C _p ), and thermal activity of the reactants and product, on the temperature/heat loss at the ignition spot and the length of pre-heating zone are respectively studied. It is found that the activation energy and heat capacity have the most significant effects on the ignition energy. The required ignition energy is increased by 44.0% and 23.9%, respectively, when the activation energy and the heat capacity are both increased by 40.0%     

Centrifugal effects on combustion synthesis of (Ti-B-C) compound system

A centrifugal force plays an important role on the control of combustion synthesis. In the present work, the data of reaction propagation rates obtained by changing the direction of reaction propagation and centrifugal force are evaluated in order to make clear the effect of centrifugal force on reaction propagations and product formation. As a result, the reaction propagation rate in the case of the direction of centrifugal force inverse to reaction propagation is larger than that in the case of the same direction, and product grains become smaller in size. It is confirmed that the centrifugal effect is much larger for the present combustion synthesis process in the case that the reaction propagates inversely to the direction of centrifugal force. Since molten titanium near combustion front tends to coalesce into larger drops in that case, reactants of boron and carbon would diffuse more sufficiently into titanium.     

Numerical analysis for micropyretic synthesis of NiAl intermetallic compound

A numerical investigation of the micropyretic synthesis response parameters of the Ni-Al stoichiometric compound was undertaken. The influence of the enthalpy of the combustion reaction,Q, activation ienergy,E, amount of diluent, pre-exponential factor,K ₀, and initial temperatureT ₀, on the combustion velocity, temperature, and mode was studied. The porosity of the unreacted compact, which is related to the initial compaction pressure, was considered in the calculation. It was found that the change in porosity significantly affects the thermal conductivity and the length of the pre-heat zone as also do the temperature patterns and propagation velocities. The combustion front was noted to be extinguished if the temperature in the reaction zone became lower than the melting point of the aluminium phase. This result was obtained simply by considering the changes in the thermal conductivity after the melting of aluminium without having to invoke any changes in the rate of reaction after the melting. A comparison of the numerical data with the experimental and analytical results was also made.     

Porous TiNi Biomaterial by Self‐Propagating High‐Temperature Synthesis

Porous TiNi shape‐memory alloy (TiNi SMA) bodies with controlled pore structure were produced from the (Ti+Ni) powder mixture by self‐propagating high‐temperature synthesis (SHS) method. The effect of processing variables such as the kind of starting powders, ignition temperature and preheating schedule on the behavior of combustion wave propagation, the formation of phases and pore structure was investigated. The relationship between pore structure and mechanical properties was also investigated. An in vivo test was performed to evaluate bone tissue response and histocompatibility of porous TiNi SMA using 15 New Zealand white rabbits. No apparent adverse reactions such as inflammation and foreign body reaction were noted on or around all implanted porous TiNi SMA blocks. Bone ingrowth was found in the pore space of all implanted blocks.     

Upward buoyant filtration combustion

Heterogeneous combustion in a porous sample with only the top and bottom ends of the sample open to gas flow is considered. Gas enters the sample due to buoyant upward convection. That is, ignition at the bottom produces an upwardly propagating filtration combustion wave which induces hot gas to rise, thus pulling cool, fresh gas containing oxidizer in through the bottom of the sample. The gas moves through the solid products to reach the reaction zone just as in forced forward filtration combustion. In contrast to forced forward filtration combustion, in which the incoming gas flux is fixed by an external source, here the incoming gas flux is determined by the combustion process itself. That is, the incoming gas flux is determined by the burning temperature which in turn is affected by the incoming gas flux. Thus, a feedback mechanism exists which hinders ignition of the samples, but also makes the wave hard to extinguish, once it has formed. A one-dimensional model is analyzed and two types of wave structure, termed reaction-leading and reaction-trailing according as the reaction occurs at the leading or trailing edge of the heated region of the sample, respectively, are determined. For each structure, two solution modes are described, termed stoichiometric and kinetically controlled, according as the rate of oxygen supply or the kinetics controls propagation of the wave. In each of these four situations, expressions are derived for the evolution of the burning temperature, propagation velocity, incoming gas flux, degree of oxidizer consumption and degree of fuel conversion as the wave moves through the sample. In addition, profiles for the temperature are described. Analysis of the case where significant heat is lost through the sides of the sample leads to extinction limits and demonstrates the sensitivity of the wave structure to changes in external heat losses.     

Reaction process during relative sintering of NiAl

Nickel aluminide compounds were synthesized by the thermal explosion mode of the self-propagating high temperature synthesis (SHS). The effects of green density and heating rate on the combustion characteristics and the microstructure of the products were studied. It was found that the combustion can not be ignited with heating rates lower than 5 K min-1. In this case, the formation of NiAl can be achieved by classic reactive sintering. At heating rates higher than 5 K min-1, a precombustion stage exists for compacted samples allowing the ignition of the reaction at low temperature. Combustion temperature were found to be higher than thermodynamic predictions in argon and, moreover in air, because of the formation of aluminium oxide which triggers the NiAl synthesis reaction. © 1998 Chapman & Hall     

B/CuO复合燃料的制备及其对Mg/PTFE富燃料推进剂的影响

为了研究纳米氧化铜(CuO)改性硼(B)对镁/聚四氧乙烯(Mg/PTFE)富燃料推进剂的影响,利用球磨法制备了B/CuO复合燃料,将其添加到Mg/PTFE富燃料推进剂中,利用混合模压成型法制备含有不同比例的复合燃料的推进剂药柱。利用扫描电镜、TG-DSC分别测试了B/CuO复合燃料的微观形貌和热反应性能;利用红外测温仪、X射线衍射、TG-DSC分别测试了推进剂的燃烧速度、燃烧温度、反应产物以及热反应性能。结果表明:复合燃料混合较为均匀,局部有团聚;n(B)∶n(CuO)=32∶3的复合燃料的放热量高于B的放热量,燃烧效率最高,为73.1%,点火温度比B低66 ℃。含此复合燃料的推进剂的燃烧速度和质量燃烧速度均高于Mg/PTFE,分别提高了25.6%和3.1%,平均燃烧温度降低了16 ℃,最高燃烧温度则提高了6 ℃,但是相对于含B的Mg/PTFE推进剂,含此复合燃料的推进剂的燃烧速度和质量燃烧速度分别下降2.91%和19.51%,平均燃烧温度下降了94 ℃和121 ℃;复合燃料推进剂一次燃烧的凝聚相产物主要有MgF₂、MgO、C、Cu以及Mg₃F₃（BO₃）;一次燃烧反应过程主要是PTFE的分解以及F₂和Mg的反应,二次燃烧反应过程则主要为C、Mg以及复合燃料的氧化。     

预混层流燃烧中离子电流与火焰传播速度的相关性研究

在当量空燃比φa=1、初始压力P0=0.1MPa、初始温度T0=300K、离子电流装置偏置电压U=400V的条件下,对压缩天然气掺氢(CNG+H2)、液化石油气(LPG)、二甲醚(DME)在定容燃烧弹中的球形火焰燃烧过程进行了分析,分别测量了三种燃料的层流火焰传播速度、离子电流以及燃烧压力,利用相关性原理探讨了预混层流火焰传播速度与离子电流之间的关系,研究结果表明:正常燃烧的离子电流通常由点火、火焰前锋以及焰后区三部分组成,而火焰传播速度与焰后区离子电流峰值的样本相关系数在0.51～0.95之间.因此,三种燃料焰后区的离子电流峰值与层流火焰传播速度之间均存在一定的相关性.     

Combustion synthesis of advanced materials: Part I. Reaction parameters

An explanation of combustion (self propagating high temperature) synthesis (SHS) is given together with a historical perspective of the examination of such exothermic reactions. The application of thermochemical functions has been used to predict theoretically the maximum adiabatic temperature, T_ad. This, combined with a knowledge of the ignition temperature, T_ig, and the actual combustion temperature, T_c, has been used to determine the heat loss from the SHS reaction and the amount of heat needed to raise the adjacent, cold, reactant layer to the ignition temperature in order to maintain the self sustaining nature of the propagating mode of the reaction. The pertinent reaction parameters that control self propagating high temperature (combustion) synthesis reactions have been examined. These include: reactant particle size and shape; powder mixing and compaction; green density; reaction stoichiometry; impurities; volatiles and diluents; reaction environment; mode and technique of ignition; heating rate; and the effect of these parameters on the generation of heat, exothermicity and control of the SHS reaction.     

Laser‐Ignited Relay‐Domino‐Like Reactions in Graphene Oxide/CL‐20 Films for High‐Temperature Pulse Preparation of Bi‐Layered Photothermal Membranes

Light‐ignited combustions have been proposed for a variety of industrial and scientific applications. They suffer, however, from ultrahigh light ignition thresholds and poor self‐propagating combustion of typical high‐energy density materials, e.g., 2,4,6,8,10,12‐(hexanitrohexaaza)cyclododecane (CL‐20). Here, reported is that both light ignition and combustion performance of CL‐20 are greatly enhanced by embedding ε‐CL‐20 particles in a graphene oxide (GO) matrix. The GO matrix yields a drastic temperature rise that is sufficient to trigger the combustion of GO/CL‐20 under low laser irradiation (35.6 mJ) with only 6 wt% of GO. The domino‐like reduction‐combustion of the GO matrix can serve as a relay and deliver the decomposition‐combustion of CL‐20 to its neighbor sites, forming a relay‐domino‐like reaction. In particular, a synergistic reaction between GO and CL‐20 occurrs, facilitating more energy release of the GO/CL‐20 composite. The novel relay‐domino‐like reaction coupled with the synergistic reaction of CL‐20 and GO results in a deflagration of the material, which generates a high‐temperature pulse (HTP) that can be guided to produce advanced functional materials. As a proof of concept, a bi‐layered photothermal membrane is prepared by HTP treatment in an extremely simple and fast way, which can serve as a model architecture for efficient interfacial water evaporation.     

Influence of surface microwave discharge on ignition of high-speed propane-air flows

Ignition of supersonic propane-air flow under conditions of low-temperature plasma of surface microwave discharge is experimentally studied. We show that both rich and poor mixtures ignite, and the combustion intensity is maximal for the stoichiometric mixture. The dependence of the supersonic propaneair flow ignition time on the reduced electric field, E/n, under conditions of nonequilibrium gas-discharge plasma is experimentally obtained. The induction period is shown to decrease from 1 ms to 5 μs with the increase in E/n from 40 to 200 Td. The propagation velocity of the combustion front boundary (depending on the equivalent mixture ratio and the input microwave power) is maximal for the stoichiometric mixture and reaches 160 m/s at E/n = 150 Td. Under these conditions, the combustion temperature is about 3000 K.     

Linear stability analysis of spherically propagating thermal frontal polymerization waves

Spherically propagating frontal polymerization (FP) waves were observed for the first time in condensed media. After a brief period of ignition in a spherical domain by an external UV source, the front began to expand radially. Once the front attained a critical size, it became unstable, resulting in so-called ‘spin modes’. These spin modes are manifested as slightly raised regions that travel on the surface of the expanding spherical front. The onset of these instabilities from a stable, uniformly propagating spherical front can be described by a linear stability analysis. The bifurcation parameter is the Zeldovich number which is related to the activation energy of the reaction. A basic solution was constructed which describes a spherically symmetric outward propagating front of radius R. An asymptotic analysis was then employed under the assumption that R is large. This corresponds to the case where the conditions of ignition do not affect front propagation. It was found to leading order that the front propagates at a constant velocity and corrections to velocity due to curvature have been determined. The linear stability analysis shows that for the Zeldovich number in a certain range, there exists a situation in which the sphere will be unstable but will recover its stability in time as it expands.