Reaction Front Propagation Research Articles

Effects of initial temperature on autoignition and detonation development in dimethyl ether/air mixtures with temperature gradient

For large hydrocarbon fuels used in internal combustion engines, different low-temperature and high-temperature chemistries are involved in the autoignition processes under different initial temperatures. As one of the simplest fuels with low-temperature chemistry, dimethyl ether (DME) is considered in this study and one-dimensional autoignitive reaction front propagation induced by temperature gradient is simulated for stoichiometric DME/air mixtures considering detailed chemistry and transport. The emphasis is placed on assessing and interpreting the influence of initial temperature on the detonation development regime. Different initial temperatures below, within and above the negative-temperature coefficient (NTC) region are considered. For each initial temperature, four typical autoignition modes are identified: supersonic autoignitive reaction front (without detonation); detonation development; transonic reaction front; and subsonic reaction front. The detonation development regimes for two fuels, DME and n-heptane, at the same initial temperature and those for the same fuel, DME, at three different initial temperatures respectively below, within and above the NTC region are obtained. Based on these results, the influence of fuel type and initial temperature on detonation development regime are discussed. It is found that the detonation development regime becomes narrower at higher initial temperature. Moreover, the influence of initial temperature on reaction front propagation speed is investigated. The reaction front propagation speed is shown to be strongly affected by different chemistries involved in low and high temperature regions. When only the high-temperature chemistry is involved, the reaction front propagation speed is shown to be less dependent on the initial temperature.

Combustion synthesis in bi-layered (Ti−Al)/(Ni−Al) system

We investigated the peculiarities of the combustion wave propagation as well as the structure and phase formation in bi-layered metal powder compacts of (Ti−Al)/(Ni−Al), with a special emphasis on the use of infrared thermography to monitor the reaction front propagation. The location of the combustion wave front along the direction of propagation at different times was determined to calculate the front velocity. The data of SEM analysis demonstrated the welding of two layers during combustion with the formation of a transition layer with a thickness of about 200μm and a sharp diffusion barrier for Ti in the Ni-Al layer.

In situ TEM probing of crystallization form-dependent sodiation behavior in ZnO nanowires for sodium-ion batteries

Development of sodium-ion battery (SIB) electrode materials currently lags behind electrodes in commercial lithium-ion batteries (LIBs). However, in the long term, development of SIB components is a valuable goal. Their similar, but not identical, chemistries require careful identification of the underlying sodiation mechanism in SIBs. Here, we utilize in situ transmission electron microscopy to explore quite different sodiation behaviors even in similar electrode materials through real-time visualization of microstructure and phase evolution. Upon electrochemical sodiation, single-crystalline ZnO nanowires (sc-ZNWs) are found to undergo a step-by-step electrochemical displacement reaction, forming crystalline NaZn13 nanograins dispersed in a Na2O matrix. This process is characterized by a slowly propagating reaction front and the formation of heterogeneous interfaces inside the ZNWs due to non-uniform sodiation amorphization. In contrast, poly-crystalline ZNWs (pc-ZNWs) exhibited an ultrafast sodiation process, which can partly be ascribed to the availability of unobstructed ionic transport pathways among ZnO nanograins. Thus the reaction front and heterogeneous interfaces disappear. The in situ TEM results, supported by calculation of the ion diffusion coefficient, provide breakthrough insights into the dependence of ion diffusion kinetics on crystallization form. This points toward a goal of optimizing the microstructure of electrode materials in order to develop high performance SIBs.

Different modes of reaction front propagation in n-heptane/air mixture with concentration non-uniformity

According to the reactivity gradient theory of Zel'dovich, the non-uniformity in temperature or concentration can lead to detonation development under certain conditions. In the literature, there are many studies on detonation development caused by temperature gradient or hot spot. However, the modes of supersonic reaction front propagation and detonation development regime caused by concentration non-uniformity have not been investigated previously. In this study, one-dimensional simulations were conducted to investigate the transient autoignition and reaction front propagation processes in n-heptane/air mixture with concentration non-uniformity. With the increase of equivalence ratio gradient, three modes (supersonic autoignitive reaction front, developing detonation and subsonic reaction front) of reaction front propagation induced by concentration non-uniformity were identified. The effects of heat conduction and mass diffusion on these three modes were examined and it was demonstrated that molecular diffusion has little influence on the first two modes. The detonation development regime caused by concentration non-uniformity was reported in this paper. This regime was found to be similar to the one caused by temperature gradient. A non-dimensional parameter was proposed to characterize the lower limit of the detonation regime. Furthermore, the effects of initial temperature on the detonation development regime were examined. It was found that the detonation development regime becomes wider as the initial temperature decreases. The initial temperature was shown to only have great impact on the upper limit of the detonation development regime while it has little influence on the lower limit. The influence of initial temperature was explained using the volumetric energy density of the mixture.

Chemical Reaction Front Propagation between Inert Materials with Different Properties Taking into Account the Reagent Melting

This work presents two-dimensional model of exothermal chemical reaction propagation at the conditions of dissimilar materials conjugation when ignition is carried out from end. The melting of the reagent is taken into account in the model. The convergence of numerical algorithm was studied. It was demonstrated that conversion regimes are different for various properties of conjugated materials.

Rapid fluid flow along fractures at greenschist facies conditions on Syros, Greece

This thesis aims to provide a better understanding of the role of mountain building in the carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations within shear zones and fractures during greenschist- and blueschist-facies metamorphism and 2) the assessment of possible mechanisms of fluid infiltration into rocks during greenschist- to epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors of reaction front propagation.On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological, geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time-averaged fluid flow velocity and flow duration along brittle fractures was calculated by using a governing equation for one-dimensional transport (advection and diffusion) and field-based parameterization. This study shows that fluid flow along fractures on Syros was rapid and short lived.Mechanisms and controlling factors of fluid infiltration were studied in greenschist- to epidote-amphibolite-facies metabasalts in SW Scotland. Fluid infiltration into metabasaltic sills was unassisted by deformation and occurred along grain boundaries of hydrous minerals (e.g. amphibole) while other minerals (e.g. quartz) prevent fluid infiltration. Petrological, mineralogical and chemical studies of the sills show that the availability of reactant minerals and mechanical factors, e.g. volume change in epidote, are primary controls of reaction front propagation.

Reaction waves in solid fuels for adiabatic competitive exothermic reactions

We investigate travelling premixed reaction waves in a diffusional-thermal model with a two-step competitive reaction mechanism where both reactions are exothermic. Travelling waves are assumed to propagate at constant speed. An approximation of the Arrhenius reaction rate is adopted to simplify the combustion model. Based on this assumption, an asymptotic theory is presented for solid fuels under adiabatic conditions. This approach provides a convenient way to analyse the system in the phase plane. The asymptotic speeds for the flame fronts are compared with numerical solutions by solving the governing partial differential equations. In addition, piecewise approximate solutions for the temperature and fuel mass fraction profiles are presented and compared with those obtained numerically. Our results can be applied to combustion synthesis in the production of advanced materials. References Martirosyan, N. A., Dolukhanyan, S. K., and Merzhanov, A. G., Experimental observation of the nonuniqueness of stationary combustion in systems with parallel reactions. Combust. Explos. Shock Waves 19(6):711–712, 1983. doi:10.1007/BF00750777 Martirosyan, N. A., Dolukhanyan, S. K., and Merzhanov, A. G., Nonuniqueness of stationary states in combustion of mixtures of zirconium and soot powders in hydrogen. Combust. Explos. Shock Waves 19(5):569–571, 1983. doi:10.1007/BF00750423 Sidhu, H. S., Towers, I. N., Gubernov, V. V., Kolobov, A. V., and Polezhaev, A. A., Investigation of flame propagation in a model with competing exothermic reactions. Chemeca 2013 , Brisbane, Australia, 29 September–2 October, 2013. http://www.conference.net.au/chemeca2013/papers/29350.pdf Towers, I. N., Gubernov, V. V., Kolobov, A. V., Polezhaev, A. A., and Sidhu, H. S., Bistability of flame propagation in a model with competing exothermic reactions. P. R. Soc. Lond. A Mat. 469:2158, 2013. doi:10.1098/rspa.2013.0315 Hmaidi, A., McIntosh, A. C., and Brindley, J., A mathematical model of hotspot condensed phase ignition in the presence of a competitive endothermic reaction. Combust. Theory Model. 14(6):893–920, 2010. doi:10.1080/13647830.2010.519050 Matkowsky, B. J., and Sivashinsky, G. I., Propagation of a pulsating reaction front in solid fuel combustion. SIAM J. Appl. Math. 35(3):465–478, 1978. doi:10.1137/0135038 Forbes, L. K., A note on travelling waves in competitive reaction systems. ANZIAM J. 55(1):1–13, 2013. doi:10.1017/S1446181113000333 Barenblatt, G., The Mathematical theory of combustion and explosions . Springer, 1985. http://www.springer.com/us/book/9781461294399 Jovanoski, Z., and Robinson, G., Piecewise linear approximation to Fisher's equation. ANZIAM J. 53:C465–C477, 2011. http://journal.austms.org.au/ojs/index.php/ANZIAMJ/article/view/5129 Schiesser, W. E., The numerical method of lines: integration of partial differential equations . Academic Press, 1991.

Supersonic reaction front propagation initiated by a hot spot in n-heptane/air mixture with multistage ignition

For large hydrocarbon fuels such as n-heptane, multistage ignition occurs at low initial temperature. Therefore, multiple pressure pulses produced by multistage ignition and complicated reaction–pressure wave interactions are expected to happen during autoignition and reaction front propagation initiated by a hot spot. In this study, 1D simulations are conducted for n-heptane/air mixture with three ignition stages respectively caused by low-, intermediate- and high-temperature chemistries. Multiple pressure waves, shock waves, and detonation waves are identified and they are found to be generated by heat release from different ignition stages and reaction–pressure wave interactions. The thermal states of flow particles at different initial locations are tracked and analyzed; and the mechanism for the development of multiple shock waves and detonation waves is discussed. With the change of temperature gradient inside the hot spot or the hot spot size, such interactions can be strengthened or weakened and thereby the mode of supersonic reaction front propagation changes. Furthermore, both planar and spherical configurations are considered and the curvature effects are examined. It is found that in spherical configuration, the pressure wave caused by intermediate-temperature ignition is not strong enough to induce a second detonation wave as that in planar configuration.

Hydrogen addition effect on a reaction front propagation in NTC-affected auto-igniting mixture

Synergic effect of fuels with different ignition properties on engine combustion process has been followed with interest recently. To further reveal their interaction mechanism during combustion, numerical simulations have been performed in this study, addressing hydrogen addition effect on a reaction front propagation in negative temperature coefficient (NTC) –affected auto-igniting dimethyl ether (DME) and air mixture. Firstly, homogeneous auto-ignition responses show that auto-ignition delay is obviously retarded in NTC regime while advanced beyond the regime as hydrogen addition due to the decreasing heat release during cool flame ignition and increasing heat release during final hot ignition. Chemical analysis indicates that the variation in this polarized heat release distribution is attributed to NTC-affected chemistry and total equivalence ratio. Secondly, the results of a reaction front propagation in non-reacting mixture show that the NTC-affected reaction front is also influenced by hydrogen addition, characterized as the delayed cool flame ignition and increasing maximum temperature gradient of a reaction front as well as linearly increasing reaction front speed. Finally, when the reaction front propagates in NTC-affected partially auto-igniting mixture, it shows that both reaction progress and hydrogen addition significantly increase transport-controlled deflagration front speed. However, contrary to the reaction progress, hydrogen addition depresses NTC-affected cool flame and combustion regime transitions from transport-controlled deflagration towards reaction-controlled spontaneous ignition. And meanwhile, slight flame acceleration is also observed when the reaction progress crosses over the corresponding temperature region dominated by NTC chemistry.

Chemical reactions in spherically symmetric problems of mechanochemistry

A stress-assisted chemical reaction front propagation in a linear elastic solid is considered. The reaction between gas and solid constituents is sustained by the diffusion of the gas through the transformed material. The consideration is based on the kinetic equation in a form of the dependence of the reaction front velocity on the normal component of the chemical affinity tensor that in turn depends on stresses and gas concentration. Spherically symmetric problems of mechanochemistry are solved for the reaction front propagation in a sphere, in a body with a spherical hole and in an inclusion placed into an infinite medium. It is demonstrated how stresses can enhance, retard and even lock the reaction. The effects of the sign and value of the reaction front curvature are also examined.

Effects of pressure, temperature and atomic exchanges on phase separation dynamics in Au/Ni(111) surface alloy: Kinetic Monte Carlo study

Instability of the Au/Ni(111) surface alloy is studied in different CO gas pressure, p, and temperature limits using kinetic Monte Carlo simulations. We analyze the reaction front dynamics and formation of Au clusters using the model which takes into account surface adatom pair and three-body interactions, CO adsorption and desorption, catalytic carbonyl formation reaction, Au and Ni adatom diffusion and their concerted exchange. Variation of interaction parameters allows us to identify three possible reaction front propagation limits with different pressure dependencies: (i) slow channel-like flow in agreement with experimental data [1] (step flow rate, R, increases with p), (ii) intermediate regime (weak p–dependence), and (iii) fast homogeneous flow (R decreases with p). We find that only Au–Ni exchange, contrary to both Ni–CO and Au–CO exchanges, significantly reduces the number of screened Ni atoms inside the Au clusters and stimulates the occurrence of Ni-free Au clusters. The size of Au islands depends on both pressure and temperature. At a fixed temperature it decreases with pressure due to an increased step flow rate. In the high temperature limit, despite the step flow rate exponential increase with temperature, the cluster size increases due to an enhanced Au mobility.

Experimental studies of coherent structures in an advection-reaction-diffusion system.

We present experimental studies of reaction front propagation in a single vortex flow with an imposed external wind. The fronts are produced by the excitable, ferroin-catalyzed Belousov-Zhabotinsky chemical reaction. The flow is generated using an electromagnetic forcing technique: an almost-radial electrical current interacts with a magnetic field from a magnet below the fluid layer to produce the vortex. The magnet is mounted on crossed translation stages allowing for movement of the vortex through the flow. Reaction fronts triggered in or in front of the moving vortex form persistent structures that are seen experimentally for time-independent (constant motion), time-periodic, and time-aperiodic flows. These results are examined with the use of burning invariant manifolds that act as one-way barriers to front motion in the flows. We also explore the usefulness of finite-time Lyapunov exponent fields as an instrument for analyzing front propagation behavior in a fluid flow.

Self‐Alignment of Beads and Cell Trapping in Precipitate Tubes

Propagating reaction fronts allow the formation of materials in self-sustained, steep concentration gradients, which would otherwise rapidly decay. These conditions can result in macroscopic, noncrystallographic structures, such as tubes with large aspect ratios. For hollow silica/Zn(OH)2 tubes, we report the inclusion of diverse mesoscopic building blocks ranging from polymer beads to biological cells. For agarose beads, we observe spontaneous alignment along vertical tracks; the nearly periodic spacing of the beads along these tracks follows a log-normal distribution. We interpret this patterning in terms of hydrodynamic recruitment and discuss similarities to the adhesion dynamics of leukocytes in blood vessels. For diatoms and other cells, we observe novel surface textures, and yeast tagged with a green fluorescent protein shows strong fluorescence activity after trapping. The inclusion of these guest units should improve the possibilities for the application of these tubes in microfluidics and biotechnology.

Kinetics of the chemical reaction front in spherically symmetric problems of mechanochemistry

The establishment of relations between chemicalreactions and processes of deformation and destruction is of importance from the viewpoint of both fundamental science and engineering applications. Theeffect of mechanical stresses on the reaction rate isconventionally taken into account heuristically—bypostulating the dependences of the diffusion processand the reaction constant on certain characteristics ofthe stressed state. At the same time, Gibbs [1] and DeDonder [2] laid the foundation for the theory of chemical reactions based on the concept of chemical affinity, which is a linear combination of the chemicalpotentials of substances participating in the reaction.In the classical physical chemistry dealing with gasesand ideal liquids, the chemical potential and, consequently, the chemical affinity are scalar values dependent, in particular, on pressure. However, in studiesdevoted to investigation of phase transformations indeformable materials, it was shown that the chemicalpotential should be a tensor (see, for example, [3–7]).Recently in [8, 9] for the case of a reaction between gasand deformable solid components, the expression forthe chemicalaffinity tensor was obtained as a result ofthe analysis of balances of mass, momentum, energy,and the second principle of thermodynamics. In turn,this has made it possible to obtain in a natural way thedependence of the reaction rate on stresses.Previously, we investigated the propagation of reaction fronts in the axisymmetrical problems [10] and fora plane front [11]. In this study, we investigated thekinetics of the reaction front in spherically symmetricproblems for linearly elastic solid components of thereaction.BASIC RELATIONSWe consider the chemical reaction of the type ofwhere , and are the chemical formulas ofmaterials of reacting components, and are thesolid elastic components, and is the gas component. We assume that the reaction is localized at thefront separating the regions occupied with materials and and supported by the diffusion of the component through the formed material . We assumealso that the all the gas that came to the reaction frontis spent for the reaction. An example of such a reactionis the oxidation of silicon .We consider the solid component as a solidsceleton for the diffusing gas component assumingthat the gas component causes additional deformations of the frame. Also we disregard the effects ofinternal friction related to the relative motion of thegas and solid components, the thermal effect of thereaction considering the temperature

Analysing the Reaction Kinetics for Individual Catalytically Active Components: CO Oxidation on a Pd Powder Supported by Pt Foil

Spatially- and component-resolved kinetic measurements and resulting phase diagrams for CO oxidation on a model catalyst consisting of Pd powder agglomerates supported by a polycrystalline Pt foil are reported. The kinetic data for the micrometer-sized Pt(100) and Pt(110) domains and for the different Pd powder ag- glomerates of similar dimensions were obtained by local analysis of PEEM video-sequences recorded in situ during the ongoing CO oxidation reaction. Individual domains of the supporting Pt foil as well as the lm-sized Pd powder agglomerates behave in the combined Pdpowder/Ptfoil sam- ple independently from each other with respect to CO oxidation, at least in the 10 -5 mbar pressure range. The propagating reaction fronts move within grain boundaries for Pt domains and also remain confined to the Pd agglomerates.

A front propagation formulation for under-resolved reaction fronts

A method to simulate propagating reaction fronts on under-resolved grids is presented. The proposed method prevents the spurious propagation of under-resolved reaction fronts by introducing a front propagation form of the chemical reaction term. The front propagation formulation adopts a regularized Delta function to discretely preserve the total reaction rates on computational grids. By introducing the regularized Delta function that reproduces the physical mechanism of front propagation, the present method allows for the simulation of reaction fronts with variable thicknesses on under-resolved grids. Applications to isothermal reaction fronts, laminar premixed flames, and large eddy simulation of turbulent premixed flames are presented.

Characteristics of Syngas Auto-ignition at High Pressure and Low Temperature Conditions with Thermal Inhomogeneities

Abstract Effects of thermal inhomogeneities on syngas auto-ignition at high-pressure low-temperature conditions, relevant to gas turbine operation, are investigated using detailed one-dimensional numerical simulations. Parametric tests are carried out for a range of thermodynamic conditions ( T = 890-1100 K, P = 3-20 atm) and composition ( Ф = 0.1, 0.5). Effects of global thermal gradients and localized thermal hot spots are studied. In the presence of a thermal gradient, the propagating reaction front transitions from spontaneous ignition to deflagration mode as the initial mean temperature decreases. The critical mean temperature separating the two distinct auto-ignition modes is computed using a predictive criterion and found to be consistent with front speed and Damkohler number analyses. The hot spot study reveals that compression heating of end-gas mixture by the propagating front is more pronounced at lower mean temperatures, significantly advancing the ignition delay. Moreover, the compression heating effect is dependent on the domain size.

Stress-modulated driving force for lithiation reaction in hollow nano-anodes

Lithiation of a crystalline silicon anode proceeds by the movement of an atomically sharp reaction front that separates a pristine crystalline phase and a fully-lithiated amorphous phase. The velocity of the reaction front is limited rather by the reaction rate at the lithiation front than by the diffusivity of lithium ions in the amorphous lithiated phase. Experiments on solid nanoparticle/nanowire silicon anodes show an initial rapid advancing of reaction front at the initial stage of lithiation, followed by an apparent slowing or even halting of the reaction front propagation. Lithiation-induced stresses during lithiation are attributed to alter the driving force of lithiation and thus result in the observed slowing of reaction front. Recent experiments on lithiation of hollow silicon nanowires reveals similar slowing of reaction front, however, quantitative study of the effect of lithiation-associated stress on the driving force of lithiation still lacks so far. Here, through chemo-mechanical modeling and theoretical formulation, we present a comprehensive study on lithiation-induced stress field and its contribution to the driving force of lithiation reaction in hollow silicon nanowire anodes. We show that hollow silicon nano-anodes could be fully lithiated with lower stress-induced energy barrier than solid silicon nano-anodes. As a result, it is expected that the hollow nanowires and nanoparticles may serve as an optimal structural design for high-performance anodes of lithium-ion batteries. Results from the present study shed light on a number of open questions of lithiation kinetics of silicon-based anodes in recent literature and offer insight on developing silicon-based anodes with high charging capacity and high charging rate.

Imaging Unsteady Propagation of Reaction Fronts in Reactive Multilayer Foils with Multi-Frame Dynamic TEM

Imaging Unsteady Propagation of Reaction Fronts in Reactive Multilayer Foils with Multi-Frame Dynamic TEM

Numerical experiments on reaction front propagation in n-heptane/air mixture with temperature gradient

Usually different autoignition modes can be generated by a hot spot in which ignition occurs earlier than that in the surrounding mixture. However, for large hydrocarbon fuels with negative temperature coefficient (NTC) behavior, ignition happens earlier at lower temperature than that at higher temperature when the temperature is within the NTC regime. Consequently, a cool spot may also result in different autoignition modes. In this study, the modes of reaction front propagation caused by temperature gradient in a one dimensional planar configuration are investigated numerically for n-heptane/air mixture at initial temperature within and below the NTC regime. For the first time, different supersonic autoignition modes caused by a cool spot with positive temperature gradient are identified. It is found that the initial temperature gradient has strong impact on autoignition modes. With the increase of the positive temperature gradient of the cool spot, supersonic autoignitive deflagration, detonation, shock-detonation, and shock-deflagration are sequentially observed. It is found that shock compression of the mixture between the deflagration wave and leading shock wave produces an additional ignition kernel, which determines the autoignition modes. Furthermore, the cool spot is compared with the hot spot with temperature below the NTC regime. Similar autoignition modes are observed for the hot and cool spots. Different autoignition modes in the considered simplified configuration are summarized in terms of the normalized temperature gradient and acoustic-to-excitation time scale ratio. It is shown that the transition between different autoignition modes is not greatly affected by the NTC behavior. Therefore, our 1-D simulation indicates that like hot spot, the cool spot may also generate knock in engines when fuels with NTC behavior is used and the temperature is within the NTC regime.