Observations on combustion front propagation in self-propagating high-temperature synthesis process producing refractory ceramics

High-temperature refractory ceramics can be produced in the combustion regime by using self-propagating, high-temperature synthesis (SHS) processes. The numerical simulation of the SHS process in a simplified diffusion-reaction system is investigated. The SHS process is simplified by the one- and two-dimensional pseudo-homoge-neous environment. The stiff equations of the SHS process are solved by using finite difference methods on two-dimensional adaptive meshes. Travelling waves with constant patterns are observed for adiabatic and nonadiabatic systems. For higher values of heat of reaction and activation energy, the combustion front starts to oscillate. Single and complex oscillating waves are detected. In oscillating combustion fronts, the temperature can overshoot the adiabatic temperature to result in the complete conversion of solid reactants. In two dimensional systems, travelling, fingering, and rotating waves are detected in the combustion synthesis process.     

Spray-combustion synthesis of indium tin oxide nanopowder

Nanocrystalline indium tin oxide (ITO) powders were prepared by a novel spray combustion method. Using single-drop study equipment, we studied the thermodynamics of the combustion reaction. The reaction can be ignited at air temperature as lower as 171.3°C when using urea and glucose as composite fuel. Once the reaction is ignited, the combustion temperature can surge to above 500°C, generating nanocrystalline ITO powders with grain size about 40 nm. Footages from high-speed camera demonstrated that the reaction is in three-step: moderate beginning, violent middle, and decaying end. It is also noticed that the ignition is very sensitive to the air temperature, even 0.2°C minus deviation may fail the combustion. The combustion reaction is self-sustainable, which saves the energy supply. And the low ignition temperature means the combustion reaction can be carried out in a conventional spray dryer. Our results provide a feasible way to mass production of nanocrystalline ITO powders, which as a methodology, may be extended to the production of other oxide nanopowders.     

PROPAGATION OF GASLESS COMBUSTION FRONTS: NONZERO REACTION RATE IN THE WHOLE DOMAIN

This paper deals with the gasless combustion problem in an infinite one-dimensional medium. The possibility of the existence of steadily propagating reaction waves is stated for system models with nonzero reaction rates everywhere but at the boundaries. Two forms for the chemical reaction rate dependence on the temperature are considered: the classical Arrhenius, and a modified Arrhenius incorporating and ignition temperature. The equations for the steadily propagating waves are studied in the phase-plane of the temperature and its derivative. The analysis first addresses the question of whether steadily propagating waves are admissible. Then bounds for the propagation velocity are sought and found. The results of the closed-form analysis are successfully tested against numerical experiments. Non-steady propagation regimes are also found, and regions in the parameter space associated with different asymptotic dynamical behaviour are identified.     

Splitting flames in a narrow channel with a temperature gradient in the walls

A possibility of simultaneous formation of two chemical reaction fronts during nonstationary combustion of a gas in a microchannel with a temperature gradient in the walls is demonstrated. Combustion in a straight tube and in a gap between two disks with radial fuel injection is considered. In both cases, the characteristic transverse size of the channel is smaller than the critical diameter determined for the ambient temperature, and gas combustion occurs in the region where the wall temperature is higher than the ambient temperature. A numerical study of flame repetitive extinction/ignition (FREI) demonstrated a possibility of simultaneous formation of two chemical reaction fronts in the hot region of the channel. One front corresponds to conventional flame propagating upstream from the hot to the cold part of the channel, and the other front moves in the downstream direction and decays as the fuel burns out. Based on this study, a new mechanism of ignition and incomplete combustion of the combustible mixture in microsystems is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 12–19, March–April, 2008.     

Propagation of a gasless combustion wave in a narrow cylindrical duct without wall separation

The influence of the method of ignition of a gasless medium on the limiting sample radius and on the dynamics of the pulsations and average value of the thermal wave propagation velocity in the steady state at the combustion limit is established on the basis of a numerical investigation. The influence of mild heat losses on the temperature gradients and reaction depth is traced in a gasless combustion wave propagating without separation from the walls of a narrow cylindrical duct.A. V. Lykov Institute of Heat and Mass Transfer. Translated from Fizika Goreniya i Vzryva, No. 1, pp. 65–70, January–February, 1995.     

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.     

Effect of gas pressure on the laws of propagation of spinning waves during filtration combustion

A three-dimensional mathematical model of filtration combustion with the combustion front propagating over a cylindrical specimen pressed from a powdered solid reagent and placed into an oxidizing medium is considered. Characteristics of spinning waves are studied for different pressures of the ambient gas. Steady waves of surface combustion are demonstrated to propagate over the specimen at low ambient pressures. For spinning waves formed at higher pressures, the characteristics may change nonmonotonically with increasing pressure, and the point with the maximum temperature may be located in the depth of the specimen.     

Experimental study of traveling waves in nonadiabatic fixed bed reactors for the oxidation of carbon monoxide

An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occuring on a Pt/alumina catalyst has been used. Unlike the adiabatic conditions the velocity of a traveling temperature wave in a nonadiabatical arrangement depends on its axial position. In certain regions of inlet concentration multiple temperature fronts have been observed. For low inlet CO concentration a downstream temperature wave results and the lower (kinetic) steady state is dominant. For high inlet CO concentration an upstream propagating front results and the upper steady state is dominant. For a downstream moving wave oscillations of wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions periodic behavior of temperature profiles in the reactor has been observed.     

Monoliths in catalytic oxidation

Catalytic combustion is useful to avoid emission of nitrogen oxides, to combust fuel gas of different calorific levels, and to combust low contents of badly smelling or hazardous gaseous compounds. After dealing with some characteristics of catalytic combustion it is argued that catalytic combustion to a final temperature lower than about 800°C calls for a rapid transport of thermal energy out of the reactor. A fluidized bed in which combustion has been successfully performed is dealt with as well as a reactor filled with metal bodies sintered to each other and to the wall of the reactor. To achieve a sufficiently high catalytically active surface area a thin layer of silicone rubber is applied to the surface of the metal bodies and subsequently pyrolyzed to a highly porous layer of silica. To raise the thermostability alumina can be added to the silica layer.

To establish a final temperature above 900°C the homogeneous gas-phase combustion can be ignited by a solid catalyst or the reaction can be performed completely catalytically. Since the combustion reaction proceeds very rapidly at elevated temperatures, a large gas flow can be utilized, which calls for a reactor exhibiting a low-pressure drop. For catalytic combustion monoliths and gauzes are appropriate. The chemical composition of ceramic and metallic monoliths is dealt with as well as the cell densities and wall thicknesses of commercial monoliths. The application of active components to the surface of the walls of monoliths is subsequently discussed. Since monoliths do not allow radial mixing, a homogeneous gas mixture has to be fed to the monolith to prevent very high temperature levels moving randomly over the channels of the monolith and deactivating the catalyst.

With monoliths in gas turbines often catalytic ignition is used. To limit the temperature a fraction of the fuel feed is injected into the homogeneous combustion chamber. A number of alternatives of transporting the fresh fuel to the homogeneous combustion zone is mentioned. The cause of the catalyst temperature being higher than that of the gas flow is dealt with as well as the low volatility at elevated temperatures required for the catalytic components. Selection of the catalytically active materials is discussed and the procedure to bring the gas flow at the light-off temperature of the catalyst.

Monolithic combustors used in radiant heaters display often an oscillatory behavior. After dealing with the cause of the oscillations, prevention by means of a flame arrestor is mentioned.     

Combustion Measurements of Fuel‐Rich Aluminum and Molybdenum Oxide Nano‐Composite Mixtures

Fuel rich nano‐composite powders of aluminum and molybdenum oxide were tested for ignition and combustion behind the incident and reflected shock waves in a shock tube. The powders consisted of approximately 10 μm particles, each of which contained Al and MoO₃ mixed by mechanical alloying on the nano‐scale. These powders were aluminum rich with composition ratios of 4 : 1, 8 : 1, and 16 : 1 Al : MoO₃ by mass. Ignition tests were performed behind incident shocks for temperatures in the range of 900 to 1500 K. From these tests, ignition delay times were obtained, and some information on combustion duration was also derived. Samples were tested in air at 0.2 MPa, and compared against nano‐Al, 2.7 μm Al, and 10 μm Al baselines. Ignition results for the baseline Al cases were as expected: 10 μm Al not igniting until 2000 K, 2 μm Al igniting down to ∼1400 K, and n‐Al igniting as low as 1150 K. The thermite samples showed considerable improvement in ignition characteristics. At the lowest temperature tested (900 K), both the 8 : 1 and 4 : 1 samples ignited within 250 μs. The 16 : 1 sample (94% Al) ignited down to 1050 K – which represents an improvement of roughly 1000 K over baseline Al with only a small energetic penalty. In all cases, the ignition delay increased as the amount of MoO₃ in the composite was reduced. The 4 : 1 nano‐composite material ignited as fast or faster than the n‐Al samples. Ignition delay increased with decreasing temperature, as expected. Emission spectra and temperature data were also taken for all samples using high‐speed pyrometry and time‐integrated spectroscopy. In these cases, measurements were made behind the reflected shock using end‐wall loading, though the conditions (temperature, pressure, and gas composition) were identical to the incident shock tests. Spectroscopy showed strong AlO features in all the samples, and the spectra fit well to an equilibrium temperature. Broadband, low resolution spectra were also fit to continuum, gray body temperatures. In general, the observed temperatures were reasonably close to 3500 K, which is similar to the combustion temperatures of pure aluminum under these conditions.     

炸药激光起爆过程的准三维有限差分数值模拟

根据含锌材料的热起爆机理，建立了在激光作用下炸药点火过程的三维(二维轴对称)有限差分模型，运用此模型对RDX、PETN、HMX和改性B炸药在激光作用下的温度成长过程、温度场分布及点火延迟时间进行了数值模拟。结果表明，三维模型延迟时间计算结果与实验结果较为符合；药柱温度增长主要是在激光照射面上很薄一层药剂内发生；激光的光斑直径、脉宽和炸药的激光吸收系数对点火能量阈值有较大的影响。     

Transversal patterns in three‐dimensional packed bed reactors: Oscillatory kinetics

Formation of transversal patterns in a 3D cylindrical reactor is studied with a catalytic reactor model in which an exothermic first‐order reaction of Arrhenius kinetics occurs with a variable catalytic activity. Under these oscillatory kinetics, the system exhibits a planar front (1D) solution with the front position oscillating in the axial direction. Three types of patterns were simulated in the 3D system: rotating fronts, oscillating fronts with superimposed transversal (nonrotating) oscillations, and mixed rotating–oscillating fronts. These solutions coexist with the planar front solution and require special initial conditions. We map bifurcation diagrams showing domains of different modes using the reactor radius as a bifurcation parameter. The possible reduction of the 3D model to the 2D cylindrical shell model is discussed. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

An experimental study of instability in a non-adiabatic multi-reaction tubular reactor

An experimental study of instability in a non-adiabatic packed bed reactor using NO reduction with CO over a fiber glass supported Ru catalyst is presented. A 2-reaction kinetic model is used. The results are compared to the predictions of two different TRAM ignition criteria which are expanded to cover the non-adiabatic, single reactant but multireaction case. Although there were difficulties in experimentally evaluating some of the transport parameters, the results show that one of the expanded ignition criteria in particular does satisfactorily model the data. Data are also presented which show how the temperature profile in an ignited bed moves as the inlet temperature is lowered to the quench point. Additional quench data are provided which show that the quench point is strongly affected by the feed concentration of the reactant and only a mild function of gas velocity.     

Dissipative structures in a reaction-diffusion system

A transient analysis of reaction-diffusion equations associated with the model reaction of Prigogine and Lefever (Brusselator model), has been performed. For low system lengths and for fixed boundary conditions, steady state solutions with the low amplitude are unstable. For zero flux boundary conditions the multiplicity of symmetric solutions with the same wave number may exist and the majority of them are unstable. The diffusion of initial components induces relaxation oscillations in space for fixed as well as zero flux boundary conditions. The amplitude of the oscillations increases as the diffusion coefficient of the initial component decreases. For conditions of relaxation oscillations the spatial profiles result in single or multiple propagating fronts. High system lengths for both zero flux and periodic boundary conditions, may give rise to a multipeak incoherent wave pattern. For periodic boundary conditions the multiplicity of waves has been observed. Numerical simulation of two-dimensional spatial structures reveals the existence of certain similarities between the one- and two-dimensional cases.     

KF对微米铝粉在水蒸气中着火燃烧特性的影响

为改善微米铝粉在水蒸气中的着火特性和燃烧效率,采用自行设计的管式炉实验平台研究了KF对30 ?m铝粉在1000℃水蒸气中着火燃烧特性的影响。用高速摄影系统记录了样品着火燃烧过程,并通过X射线衍射、扫描电镜技术和化学分析方法分析了产物组分、形貌和燃烧效率。结果表明,加入KF可显著降低30 ?m铝粉的点火延迟时间,与加入5wt% (0.003 g) KF相比,加入15wt% (0.009 g) KF后,样品的点火延迟时间减少了47.58 s;微米铝粉在1000℃水蒸气中不能着火,加入KF后能着火,这是因为KF与水蒸气反应生成KOH,KOH与Al2O3反应会破坏铝粉的氧化壳,加快铝与水蒸气的反应,促进铝粉着火。随KF加入量提高,样品的燃烧效率显著上升,最高为82.24%,比未添加KF样品的燃烧效率提升了38.75%。提高KF加入量,可产生更多的KOH,对氧化壳的破坏效果更显著,进一步促进铝与水蒸气反应,提高铝粉燃烧效率。     

Investigation of the Burning Properties of Slow‐Propagation Tungsten Type Delay Compositions

Twenty‐eight delay compositions with a variation of the W content of ±20% wt, on the basis of the primitive formula, W/BaCrO₄/KClO₄/SiO₂/Viton=27 : 54 : 11 : 6 : 2, were designed for delay ignition in self‐destruction application of fuse. They were processed finally to be cylindrical pellets and were then characterized by measuring their burning properties including igniting ability, burning rate, and output flame temperature. In the experiments, the cylindrical pellets were packed into a heat‐resisting quartz tube and were ignited by an electrical‐heating wire. A high‐speed video camera and a digital‐image processing device were used for simultaneously recording the combustion phenomena such as ignition and flame propagation. In addition, the output flame temperature was detected by placing a thermocouple at the end of the quartz tube. The delay compositions were also loaded in a cylindrical aluminum tube to measure burning properties as compared to naked cylindrical pellets. According to these experimental results, this work discussed the effect of composition on burning properties. The consequences of these analyses will be very important and useful for defining criteria in formulating the tungsten type slow‐propagation delay compositions, as well as for improving self‐destruction designs of fuse.     

高浓度丙酮VOCs高温预热近极限贫燃预混火焰特性的数值分析

为进一步对一种丙酮挥发性有机化合物（VOCs）焚烧炉进行设计优化和运行参数调节,本文对其在不同的燃料当量比、预热温度下的火焰特性进行了数值模拟,分析了其绝热火焰温度、着火延迟时间、火焰传播速度和一维火焰产物分布特性。研究结果表明：典型当量比（约0.113）下的绝热火焰温度为850~900℃,属于中低温燃烧,绝热火焰温度随预热温度和当量比（0.06~0.4）的升高均线性升高。预热温度和化学当量比对着火延迟时间的影响十分敏感。在其典型贫燃条件下,层流火焰传播速度随预热温度升高呈指数函数关系增大,随化学当量比增大而缓慢升高,且其层流火焰传播速度不超过150cm/s。反应过程首先发生丙酮的分解和部分氧化,并持续时间较长,仅当混合物的温度升高一定程度后才发生较剧烈的CO氧化。     

Ignition and combustion of a hydrogen-air mixture in a rapid-compression combustor

In a rapid-compression combustor with a freely moving piston, the efficiency of thermomechanical conversion of energy was determined in the detonation combustion regime of a stoichiometric hydrogen-air mixture under conditions close to those observed during operation of a piston engine of internal combustion in the starting regime. It is shown that this regime of heat release is characterized not only by a dramatic pressure increase in the combustion chamber, but also by its subsequent rapid decrease caused by heat transfer to the cylinder walls and partial condensation of water vapors. The intensity of these components of the thermal process depends progressively on the pressure and temperature of the combustion products, which, in turn, depend on the parameters of the mixture immediately before its ignition. However, the relative increase in combustion pressure turns out to be minimum when the ignition is initiated near the top dead center. It is also shown that the coefficient of thermomechanical conversion of energy (an analog of the indicated efficiency of an internal combustion engine) reaches the maximum value of 31% if the mixture is ignited at a time of approximately 3/4 of a period of oscillations of the piston group after the beginning of compression of an air charge. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 3–14, November–December 1999.     

Transport limited pattern formation in catalytic fluid-particle systems

We develop and analyze a two-phase model of a system of n catalyst particles in an adiabatic reactor where the fluid phase is well mixed and a single exothermic reaction of the type A→B occurs on the solid phase. In contrast to prior models, we include the flow rate or velocity dependence of the inter-phase heat and mass transport coefficients. This eliminates many non-physical solutions and leads to a unique and globally stable uniform state in the limit of very small or large residence time. The stability of the uniform state of the particles with respect to non-uniform perturbations in the solid and fluid phases was examined to determine the possibility of non-uniform states in the solid phase (for finite residence times). Linear instability theory revealed that when transport resistances are significant, steady non-uniform patterned states emerge from the unstable middle branch of the uniform state of the particles. In contrast to the Turing patterns, these transport limited patterns exist only in the region of multiple homogeneous states and always emerge as unstable branches but become stable after a limit point bifurcation. Moreover, the stable branches emerge from the unstable branches after going through a hysteresis on the patterned branch. For n=2, the non-uniform states appear sub-critically near the ignition point and super-critically near the extinction point, while, for n>2, the bifurcation to non-uniform state is either super-critical or transcritical near the ignition point and sub-critical or transcritical near the extinction point. For a fixed set of kinetic constants, the range of Damköhler numbers over which stable patterned states exist increases with increased heat and mass-transfer resistances between the phases or with decreased heat conduction between the solid particles. For the case of n particles (n?2), the number of patterned branches emerging from the unstable homogeneous branch can be as large as (n-1) while the number of stable patterned states can be even larger (and increases exponentially with n). Some major results of this work are: (i) development of a model that tends to the thermodynamic and flow limits for the case of large and small residence times, respectively; (ii) physical interpretation of the meaning of unstable and stable patterned states (e.g. unstable patterned states describe the non-uniform perturbations that lead to transients with propagating temperature and/or concentration fronts where the entire system attains the ignited or quenched homogeneous state while stable patterned states correspond to “propagation failure” of localized perturbations); (iii) for Le_f<1, the temperature of the solid in the patterned state can exceed not only the adiabatic temperature but also the temperature of the solid in the uniform case; and (iv) estimation of the critical heat conduction values in the solid at which stable and unstable patterned states disappear.