Characteristics of exothermic reaction fronts in the gasless combustion system

Gasless combustion model of the self-propagating high-temperature synthesis process was numerically studied in the non-adiabatic cylindrical sample. The model equations, which are very stiff in the dimension of length as well as time, were solved using finite difference method on adaptive meshes. Travelling waves with constant pattern were observed for adiabatic systems. For higher values of heat of reaction and activation energy, combustion fronts started to oscillate for adiabatic and non-adiabatic systems. Simple and complex oscillatory fronts were observed. Multi-peak and irregular oscillations were also detected to presumably result in the gasless chaotic combustion. In oscillatory fronts the temperature can overshoot the adiabatic reaction temperature to result in the complete conversion of solid reactant. In the two dimensional non-adiabatic cylindrical sample in the domain of longitudinal and angular directions, oscillatory piston waves were observed. In addition asymmetrical fingering as well as rotating waves were detected for an asymmetrical perturbation. For the adiabatic annulus cylindrical sample, the velocity of propagating fronts increased with time and the temperature overshooted the adiabatic reaction temperature if the sample were ignited from the inside. If the sample were ignited from the outside, the velocity of propagating fronts decreased with time and the temperature again overshooted the adiabatic reaction temperature. For smaller diameter of sample, the temperature increased very slowly with time for inside ignition. The temperature after ignition increased very fast overshooting the adiabatic reaction temperature for outside ignition. After several oscillations, the reaction rate decreased and the region with very slow reaction was established.     

自蔓延高温合成TaC粉末

以废钽粉为原料,通过自蔓延高温合成技术制备ＴａＣ粉末,阐明了稀释剂加入量、压制压力等因素对燃烧波蔓延速率的影响,得到了合格的ＴａＣ粉末。     

Wellenausbreitung in verfahrenstechnischen Prozessen

Wave propagation phenomena in chemical engineering processes. A phenomenological analysis of the dynamics of a number of different distributed parameter systems in chemical engineering reveals a surprisingly simple dynamic behaviour despite the complexity of the underlying nonlinear process models. Spatial structures or waves are propagating along a spatial coordinate during transients. The dynamics of most of the processes under consideration can be described with sufficient accuracy by three different model structures. These models serve as a basis for a theoretical system analysis. The investigation of characteristic properties of the propagating waves and the mechanisms responsible for wave formation are of central significance. Finally, it is shown how the extended knowledge about the processes may be applied for the solution of problems of technical interest.     

Temperature dependence of crack-propagation parameters and crack morphology of the semicrystalline polyester poly(1,4-dimethylene-rans-cyclohexyl suberate)

Temperature-dependent crack propagation in the semicrystalline polymer poly(1,4-dimethylene-trans-cyclohexyl suberate) was studied as a function of forming temperature (spherulitic morphology) and molecular weight. All experiments were conducted between the glass transition temperature and the crystal melting point. Higher forming temperature (larger spherulitic size) produced lower energy to propagate (G_p). For a given morphology (single forming temperature), increasing the propagation temperature decreased the propagation energy. An equation relating G_p to the microscopic viscosity of the amorphous polymer fraction was derived. Experimental data were fitted to the equation, and Arrhenius activation energies for microscopic flow were obtained. The results show a decrease in activation energy with increased forming temperature and molecular weight, and are discussed in terms of lamellar tie-molecule population. Fracture morphology is related to thermal parameters and chain entanglements. The results indicate that much of G_p is due to plastic deformation.     

Theory of propagation of slow heat waves in a catalytic bed in a reversible reaction

We have performed a qualitative and numerical analysis of the simplest model of slow heat waves propagating in a fixed catalytic bed in the presence of a first-order reversible reaction. A one-parameter family of autowave solutions subject to the boundary conditions of the problem is shown to exist. From the continuum of solutions, one can choose a unique physically reasonable solution that is a slow heat wave. The existence conditions for slow heat waves and the effect of the model parameters on the velocity of their propagation have been studied. Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 5, pp. 52–61, September–October, 1997.     

SHS reaction and explosive crystallization in thin films: Resemblance and distinction

The SHS waves in multilayer Ni/Al nanofilms and the explosive crystallization waves in Sb films deposited onto a thin copper substrate were comparatively explored by IR imaging and SEM. Despite a markedly lower thermal effect of crystallization, the waves of chemical reaction and crystallization were found to have much in common in their temperature–time histories: warmup rate, temperature gradient in the wave front, and duration of heat release. Observed was a micro-scintillation mode of the propagation of explosive crystallization wave in Sb films.     

A critical review of the complex pressure fluctuation phenomenon in gas-solids fluidized beds

The complex pressure fluctuation phenomenon in gas-solid fluidized beds is systematically examined in this paper based on a comprehensive review of the literature data. The local pressure fluctuations are composed of multiple sources, including local bubble induced fluctuations, global bed oscillations and propagating pressure waves originating in other locations (e.g. bed surface, distributor and windbox). The interaction and coupling among bubble motion, under-damped oscillations of fluidized particles and bed surface, propagating compressible pressure waves and flow pulsation in gas-solid fluidized beds creates the complexity of local pressure fluctuations, and is likely responsible for the formation of complex but unique flow patterns. A few attempts have been reported in the literature on examining the interaction between bed oscillations, plenum chamber air pulsation and propagating pressure waves in fluidized beds, showing some promises on predicting the local pressure fluctuations. Future work should be focused on predicting local and global pressure fluctuations and the formation of unique surface flow patterns by coupling different contributing mechanisms.     

微型定容燃烧腔内C2~C4烷烃/空气火焰传播

在直径35 mm、高度2 mm光学可视的定容燃烧腔内,实验研究了常温常压静止乙烷/空气、丙烷/空气和正丁烷/空气预混气在燃烧腔中心由电火花点燃后向外传播的火焰传播特性。结果表明：3种燃料空气混合气可形成火焰传播的当量比范围不同,范围由大到小排序为乙烷>丙烷>正丁烷;3种燃料均存在由光滑火焰面向褶皱火焰面转变的传播形态;在微型定容燃烧腔内,3种燃料的火焰传播速度均低于常规尺度下定容燃烧弹内火焰传播速度,且火焰传播速度随半径增加而减小;随着当量比增加,火焰锋面容易出现褶皱和断裂现象,在高当量比情况下,火焰传播会出现短暂停滞。     

Modelling thermal degradation of composite materials

A one–dimensional, transient thermal degradation heat transfer model for the response of composite materials when exposed to fire is presented. The model can handle layers of different materials. Material properties are functions of temperature. The reaction can be specified using Arrhenius‐type parameters or by inputting a density–temperature relationship determined by any experimental technique such as thermogravimetric analysis. The model is validated against the experimental data presented in Boyer's 1984 dissertation. Overall, the model provides excellent agreement with the experimental data. It is shown that very little difference is found between results arrived at by Arrhenius kinetics and results obtained by specifying the easier to measure density–temperature relationship. From this it is concluded that this technique is a viable alternative to Arrhenius‐type models. Copyright © 2006 John Wiley & Sons, Ltd.     

Ultrasonic investigations of incommensurate Rb2Zncl4

Elastic properties of Rb₂Zncl₄ were investigated by measuring the velocity and attenuation of both, transversal and longitudinal ultrasonic waves propagating along the crystallographic axes. At the lock-in phase transition temperature only v₄₄ shows a remarkable anomaly. This anomaly has been found to be asymmetric with respect to the interchange of oscillation and propagation direction of the transverse ultrasonic wave. The anomaly of v₄₄ at T_c is proportional to the contents of commensurate regions in the IC structure. From the temperature dependence of ultrasonic velocities at the phase transition from paraelectric to IC phase the critical behaviour of the order parameter should correspond rather to the (n = 2)-Heisenberg-model than to the Ising model.     

Traveling-wave Phenomena in a Model of Autocrine Signaling Coupled with Dynamics of the MAPK Cascade

Recently we have revealed a minimal reaction subnetwork in the MAPK (mitogen-activated protein kinase) cascade that is responsible for the emergence of bistable and oscillatory behavior. Here we examine a possible mechanism that provides for the propagation of increased MAPK activity in cell populations by interconnecting the intracellular MAPK subnetwork with the ligand-receptor signaling machinery. Such approach allows for significant reduction of the dimensionality of the parameter space on one hand and the conservation of dynamical complexity of the system on the other hand. The coupled model predicts coexistence of one, two or three different stable steady states, or the coexistence of a stable steady state and periodic solution. We found two robust and physiologically relevant characteristics of the proposed model: (i) There is a very large region of coexistence of at least one stable steady state with non-zero MAPK activity and one steady state with zero MAPK activity in the parameter space. (ii) Spontaneous traveling front waves always switching originally inactive cells into ligand releasing cells emerge in adjacent cell populations, e. g. in healthy and injured tissues. Moreover, the formation of composite traveling front waves and spatial oscillatory patterns of MAPK activity are predicted and discussed.     

Critical gel phenomena in the rheology of epoxide network formation

Multiple frequency dynamic mechanical experiments are generated by the linear addition of harmonic sinusoidal waves in order to study the gelation behavior of a crosslinking epoxy–bisphenol-A copolymerization. The following three criteria are investigated as kinetic parameters for the network formation: equivalence of the moduli increase, the critical gel condition, and the mechanical gel condition. The effect of strain magnitude is investigated for both the gelation phenomena and the scaling exponent of the critical gel behavior. The exponent is found to vary from 0.85 to 0.65, depending on the reaction temperature and waveform of the strain input. When Arrhenius type behavior is found, the activation energy of the gelation process is found to be 21 ± 1 kcal/mol. The effects of vitrification, network rupture, and nonlinear viscoelastic response were found with respect to the frequency probe. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 593–600, 1999     

Hot Spots in Energetic Materials

Porosity in high explosives increases the sensitivity against shock initiation and fast heating. An idealized energetic material is studied 3-dimensionally assuming that some mechanism like adiabatic heating of the pores by compression waves or the percolation of hot gases propagate in one direction like a plane wave with a constant or varying velocity. It induces successively hot spots distributed randomly or in regular structures. A conversion mechanism is included by an exothermal zero order reaction of Arrhenius type. The spreading of the heat input and the subsequent reactions depend on the energy input to the hot spots, the distance of the hot spots and the propagation velocity.     

聚乙烯压力管道裂纹快速扩展性及止裂性测试

聚乙烯压力管道在低温时易出现裂纹快速扩展，其破坏一般都具有突发性，难以预防。本文采用小尺寸稳态试验方法，测试了在不同温度下，裂纹扩展长度随压强的变化情况，同时也测试了在不同压强下，裂纹扩展长度随温度的变化情况。通过计算分析，确定了裂纹快速扩展的临界压强和临界温度。给出了影响该试验结果的因素如温度、压力、介质、裂纹扩展速度及裂纹尖端的流体减压等。并通过试验测试了焊缝以及套筒对塑料管道开裂的影响。最后给出了塑料管道在实际运行中的一些具体的止裂措施。     

Kinetic study on free radical polymerization of alkyl acylamidoacrylates by pulsed laser polymerization using nitrogen laser

Radical polymerization of captodatively substituted alkyl acylamidoacrylates was studied by pulsed laser polymerization using a N₂ laser. Propagation rate coefficient of the acrylates was varied with solvents used in spite of a radical mechanism, but little varied with methyl, n-propyl, and isopropyl substituents on the acrylates. Arrhenius parameters for the propagation indicated that a solvent affected mostly on a frequency factor rather than a activation energy. In addition, it was suggested that these captodatively substituted propagating radicals were thermodynamically persistent but kinetically active in propagation, which resulted in rather smooth propagation in spite of bulky 1,1-disubstituted olefins. The mechanism of the polymerizations was discussed in detail on the basis of the kinetic studies and ESR spectroscopy.     

Synthesis of narrow distribution polytetrahydrofuran

Bulk polymerizations of tetrahydrofuran (THF) have been studied kinetically at reaction temperatures in the range ?10 to +80°C using p-chlorophenyldiazonium hexafluorophosphate initiator. Initiation has been studied to enable selection of a ‘clean’ initiation condition (95°C for 4 min). Factors causing broadening of the molecular weight distribution are discussed, the main causes of such broadening being chain transfer reactions and concurrent initiation with propagation. These could be minimized by using a low reaction temperature (?10°C). Molecular weight distributions were measured by gel permeation chromatography. Propagation rate constants were determined and found to increase with increasing temperature according to an Arrhenius expression giving an activation energy of 51 kJ/mol. The method will produce monodisperse samples of THF polymer over a wide molecular weight range from 5 × 10³ to 10⁶.     

Filtration combustion characteristics of low calorific gas in SiC foams

This study investigated the combustion characteristics of low calorific gas in silicon carbide (SiC) foam. The temperature distribution, reaction zone, maximum temperature, and combustion wave propagation velocity were analysed at different inlet velocities, equivalence ratios of premixed gases, and pore densities. The temperature distribution near the reaction zone was determined by a time-based method. Super-adiabatic combustion was obtained in porous media under different conditions. The experimental results showed that higher temperatures were obtained in SiC foams of 30 pores per inch (PPI) than those measured for foam of 20 PPI. Increased equivalence ratio of premixed gases and pore density led to a thicker reaction zone and a higher preheating efficiency in the preheating zone. The combustion wave propagation velocity, which was less than 2 mm/s under these experimental conditions, was increased with increased inlet velocity and decreased equivalence ratio of premixed gases. The combustion wave propagation in foams of 20 PPI had the lowest velocity because of the good match of convection and radiation.     

Macrokinetics of Solid-Phase Synthesis of an Activated 3Ni + Al Mixture in the Thermal Explosion Mode

Specific features of heating of a mechanically activated 3Ni + Al mixture during synthesis in the thermal explosion mode are considered. Based on the authors’ criterion of the minimum curvature of the heating rate logarithm as a function of inverse temperature, the activation energies of the synthesis process in the above-mentioned system are determined and the temperature intervals of the Arrhenius dependence and self-deceleration segments on thermograms are found. A kinetic function structure analysis shows that synthesis follows the laws of homogeneous kinetics with the reaction order close to unity. The calculated activation energies are anomalously low, as compared with conventional self-propagating high-temperature synthesis in this system.     

Cellular filtration combustion of porous layers

The cellular filtration combustion of porous energetic compositions that interact with an active gas reagent and form condensed reaction products is studied for titanium powder combustion as an example. A simple and visually accessible quasi-experimental analysis of the nonlinear propagation of the combustion front with adjustable filtration transport of gas is proposed. Under conditions of a deficiency of the gas reagent and instability, the flat front of filtration combustion is divided into isolated cells of exothermic chemical reaction propagating in a pulsating mode. The results obtained are interpreted within a model of thermal filtration stability of the filtration combustion front.     

The mechanical response characteristics of sapphire under dynamic and quasi-static indentation loading

Sapphire is widely used as optical materials and substrate materials due to its excellent physical and chemical properties. The mechanism of crack propagation and fracture damage evolution has important significance for improving the manufacturing quality and application performance of sapphire parts. In this study, dynamic and quasi-static indentation tests have been performed on the c-plane and a-plane of sapphires by Hopkinson pressure bar tester and continuous indentation tester, respectively. The crack propagation path in sapphire has been captured by High-speed camera and the crack velocity has been calculated. The crack propagation and fracture damage evolution has been analyzed based on the fracture morphology of specimen. It was found that the bearing capacity of sapphire is related to the loading velocity, while the crack propagation is affected by the crystal orientation. Under the indentation loading, the cracks in sapphire first propagate steadily, and then the cracks begin to propagate uncontrollably after reaching the critical conditions, where the crack propagation velocity obviously increases, typically from 204?m/s to 1006?m/s (dynamic indentation) or from 0.0032?m/s to 820?m/s (quasi-static indentation). And the crack propagation velocity depends on the loading speed at stable stage. The r-planes of sapphire are weaker than other crystal planes and are prone to crack propagation.