Reaction Front Propagation Research Articles

An optical study of in-cylinder CH2O and OH chemiluminescence in flame-induced reaction front propagation using high speed imaging

Flame-induced auto-ignition (FIAI), where spark-triggered flame propagation causes auto-ignition without engine knock, is one of the promising high-efficiency combustion modes. FIAI and SI knocking combustion have several similarities; however, the former is beneficial to thermal efficiency, and the latter should be avoided for engine safety. This study investigates the flame structures, intermediate species and pressure evolution during FIAI and SI knocking combustion using an optical engine with ultra-high speed imaging of CH2O and OH chemiluminescence. The flame structures are analyzed together with cylinder pressure and intermediate species to understand their dynamic interactions. Findings reveal that auto-ignition in FIAI begins from the outer rim of the SI flame front; the reaction front propagates at a speed of approximately 160m/s, which is more than one order higher than the speed of normal flame propagation. The reaction front propagates in subsonic speed. The propagation regime changes from normal flame propagation to reaction front propagation at approximately 795K for n-heptane. The temperature of the unburned zone and the remaining energy are the key factors for the occurrence of knocking; they also differentiate FIAI from typical knocking.

Surficial weathering of iron sulfide mine tailings under semi-arid climate

Mine wastes introduce anthropogenic weathering profiles to the critical zone that often remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide-rich ore tailings, propagation to depth of the oxidative weathering front controls the depth-variation in speciation of major and trace elements. Despite the prevalence of surficial mine waste deposits in arid regions of the globe, few prior studies have been conducted to resolve the near-surface profile of sulfide ore tailings weathered under semi-arid climate. We investigated relations between gossan oxidative reaction-front propagation and the molecular speciation of iron and sulfur in tailings subjected to weathering in a semi-arid climate at an EPA Superfund Site in central Arizona (USA). Here we report a multi-method data set combining wet chemical and synchrotron-based X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) methods to resolve the tight coupling of iron (Fe) and sulfur (S) geochemical changes in the top 2m of tailings. Despite nearly invariant Fe and S concentration with depth (130–140 and 100–120gkg−1, respectively), a sharp redox gradient and distinct morphological change was observed within the top 0.5m, associated with a progressive oxidative alteration of ferrous sulfides to (oxyhydr)oxides and (hydroxy)sulfates. Transformation is nearly complete in surficial samples. Trends in molecular-scale alteration were co-located with a decrease in pH from 7.3 to 2.3, and shifts in Fe and S lability as measured via chemical extraction. Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosite-group minerals with an accompanying decrease in pH. Interestingly, thermodynamically stable phases such as goethite and hematite were not detected in any samples, but ferrihydrite was observed even in samples with the lowest pH, indicating its metastable persistence in these semiarid tailings. The resulting sharp geochemical speciation gradients in close proximity to the tailings surface have important implications for plant colonization, as well as mobility and bioavailability of co-associated toxic metal(loid)s.

Stress-assist chemical reactions front propagation in deformable solids

We consider a stress-assist chemical reaction front propagation in a deformable solid undergoing a localized chemical reaction between solid and gas constituents. The reaction is sustained by the diffusion of the gas constituent through the transformed solid material. The transformation strain produced by the chemical reaction lead to internal stresses which in turn affect the chemical reaction front kinetics. Basing on the notion of the chemical affinity tensor we formulate a kinetic equation in a form of the dependence of the front velocity on the normal component of the chemical affinity tensor. As an example we consider a planar chemical reaction front propagation in an elastic plate subjected to uniaxial tension or compression. We demonstrate the possibility of the locking effects – blocking the reaction by stresses at the reaction front. We study in details how the velocity of the chemical reaction front and the locking effect depend on chemical reaction parameters and external loading.

Hydrodynamic instability of thermal fronts in reactive porous media: spinning patterns.

The interaction of convection and reaction front propagation is known to exhibit a nonlinear phenomenons in the case of an adverse flow, i.e., when the gas velocity (V=[u,v]') and the front velocity (Vf) are of opposite directions. This was demonstrated both experimentally and analytically under isothermal conditions with Vf constant independent of V. Here we analyze thermal reaction front propagation in a porous medium in which an exothermic reaction of Arrhenius kinetics occurs. Numerical simulations of a packed-bed reactor model accounting for variable temperature, concentration, and hydrodynamic fields following the Euler-Darcy equations revealed emergence of spinning transversal patterns. Such solution cannot emerge in a two-variable (C,T) model, assuming "frozen" hydrodynamics, within a feasible domain of parameters. Two models are employed for analysis: a qualitative model based on a learning one-tube and two-tube consideration and an extended Landau-Darries instability analysis to account for the momentum losses and for a variable front propagation velocity. Both models revealed the important role of the Vf(u) dependence which can be presented as Vf=u-Vch, where the chemical component of the front velocity (Vch) depends on the main governing parameters such as the adiabatic temperature rise (ΔTa) and the inlet velocity (uin). The instability takes place if the parameter β=∂Vch/∂u<1. The effect of ΔTa, uin on the instability domain obtained in simulations can be translated to the effect of the parameter β.

Direct Electrochemical Reduction of Hematite Pellets in Alkaline Solutions

Our previous studies on the electrowinning of iron from hematite particles suspended in sodium hydroxide solutions demonstrated that iron was produced through a direct solid phase electrochemical reduction of the oxide particles. This suggests that the electro-deoxidation of a hematite powder pellet by the FFC type process should be possible in aqueous solutions at a low temperature. We investigated the electrochemical reduction of a solid hematite pellet cathode manufactured by conventional mechanical pressing followed by sintering process in concentrated sodium hydroxide solutions using a nickel inert anode at 110 °C. The experimental results indicated that hematite was reduced to iron metal. Propagation of the reduction reaction front through the oxide phase occurred at a high speed, and the rate-determining step was the diffusion of the oxygen ion through the shrinking cores of the oxide particle.

F211 ソルトフィンガー生成を伴う発熱中和反応の可視化に関する研究

The propagation of reaction front in acids (CH_3COOH) aqueous solution sodium hydroxide (NaOH) aqueous solution has been investigated using Hele-Shaw cell. The reaction front didn't have any acute angled protuberances which were appeared during chemical reaction with common Rayleigh Taylor instability. In this case, some semicylindrical protuberances were formed at the reaction front due to the generation of salt fmger between sodium acetate aqueous solution and NaOH solution. There were three mass transfer modes which had the stable reaction front, unstable reaction front with salt finger and unstable reaction front with Rayleigh Taylor instability and salt finger.

Convective dynamics of traveling autocatalytic fronts in a modulated gravity field.

When traveling in thin solution layers, autocatalytic chemical fronts may be deformed and accelerated by convective currents that develop because of density and surface tension gradients related to concentration and thermal gradients across the front. On earth, both buoyancy and Marangoni related flows can act in solution layers open to the air while only buoyancy effects operate in covered liquid layers. The respective effects of density and surface tension induced convective motions are analysed here by studying experimentally the propagation of autocatalytic fronts in uncovered and covered liquid layers during parabolic flights in which the gravity field is modulated periodically. We find that the velocity and deformation of the front are increased during hyper-gravity phases and reduced in the micro-gravity phase. The experimental results compare well with numerical simulations of the evolution of the concentration of the autocatalytic product coupled to the flow field dynamics described by Navier-Stokes equations.

STRESS-MODULATED DRIVING FORCE FOR LITHIATION REACTION IN HOLLOW NANO-SPHERICAL ANODES

ABSTRACTRecent experimental evidence on nano-particle and nano-wire silicon anodes showed an initial rapid velocity of reaction front at the initial stage of lithiation, followed by an apparent slowing or even halting of the reaction front propagation. This intriguing phenomenon is attributed to the lithiation-induced mechanical stresses across the reaction front which is believed to play an important role in the kinetics of reaction at the front. Here, through theoretical formulation, we presented a comprehensive study on lithiation-induced stress field and its contribution to the driving force of lithiation in hollow spherical anodes with different boundary conditions at the inner surface of the particle. Our results reveal that hollow spherical silicon anodes can be lithiated more easily than solid spherical silicon particles and thus may serve as an optimal design of high performance anodes of lithium-ion battery.

A reactive-transport model for examining tectonic and climatic controls on chemical weathering and atmospheric CO2 consumption in granitic regolith

We developed a 1D reactive-transport model for examining how tectonic and climatic parameters, namely uplift rate, water seepage velocity, and temperature, control chemical weathering and atmospheric CO2 consumption rates during regolith development. The model consists of mass-conservation equations describing how mineral and solute concentrations change temporally and spatially (vertically) during weathering of granite containing 30% plagioclase (An20) and up to 3% accessory calcite. The equations are coupled by volumetric weathering rates, which depend on mineral abundances, intrinsic dissolution rate constants, and surface areas as well as the departure of pore water from thermodynamic equilibrium. We numerically solve a non-steady-state, non-dimensional version of the model. By eliminating the need to prescribe regolith thickness, non-dimensionalization introduces two key scaling parameters that drive variations in the model results: the rock residence time (τr, the time for rock to travel upward through the model domain) and the water residence time (τw, the time for water to travel downward through the model domain). We use the model to examine fundamental properties of chemical weathering, especially reaction front propagation, with the primary aim of characterizing weathering regimes and identifying factors that maximize the dissolution of plagioclase because only silicate weathering regulates atmospheric CO2 levels over geological timescales.Weathering regimes are commonly classified as transport (or supply)-limited versus weathering (or kinetically)-limited, but as outgrowths of geomorphology, these terms primarily refer to the role of τr in regulating regolith thickness. Our findings suggest that the paradigm should be expanded to include kinetic controls determined by τw. Both regimes can experience far-from- and near-equilibrium mineral dissolution. However, transport-limited regimes generally have lower τw/τr ratios compared to kinetically-limited regimes. Under transport limitation, thick to thin reaction fronts propagate downward indefinitely yielding extensive mineral depletion, deeply weathered regolith, and high ratios of silicate-to-carbonate weathering. Under kinetic limitation, thin to non-existent reaction fronts evolve to steady-state yielding minimal mineral depletion, no regolith development, and low ratios of silicate-to-carbonate weathering. Kinetically-limited regimes typifying tectonically active mountain ranges have low long-term atmospheric CO2 consumption rates. Transport-limited regimes typifying tectonically stable cratons also have low long-term atmospheric CO2 consumption rates, but because much of the Earth's surface is transport-limited, we infer that such environments have the greatest impact on the global atmospheric CO2 consumption flux. The model output leads us to hypothesize that the maximum contribution should occur for extensive regions of silicate bedrock consolidated in warm, wet climates because the combination of tectonic stability, high temperatures, and rapid seepage velocities accelerates reaction front propagation, facilitates calcite depletion, and sustains deep regolith development over long timescales. Our findings point to the importance of internal climate feedbacks for stabilizing long-term atmospheric CO2 levels.

The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation

The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by fluid–rock interaction. In this model, grain boundaries present in the rock as well as the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until transformation is complete. Hydrothermal treatment of the marble using phosphate bearing solutions led to the formation of hydroxylapatite and β-TCP; the formation of the latter phase was probably promoted by the presence of ∼0.6 wt.% Mg in the parent carbonate phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxylapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of β-TCP and a very fine porous microstructure. If fluorine was added as an additional solution component, up to 3 wt.% of F were incorporated into the product apatite and the formation of β-TCP was avoided. The use of the isotope 18O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of 18O in the PO4 tetrahedron of hydroxylapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the 18O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the rock more than one order of magnitude faster compared to the newly developing channel-like porosity structures, which act as pathways towards the center of single mineral grains. Thus, after only short reaction durations, it may be possible for the fluid to progress relatively large distances along the grain boundaries without developing broad reaction fronts along the path.

Universal Flame Propagation Behavior in Packed Bed of Biomass

This article aims at seeking the universal behavior of propagation rate variation with air superficial velocity (V s ) in a packed bed of a range of biomass particles in reverse downdraft mode while also resolving the differing and conflicting explanations in the literature. Toward this, measurements are made of exit gas composition, gas phase and condensed phase surface temperature (T g and T s ), and reaction zone thickness for a number of biomass with a range of properties. Based on these data, two regimes are identified: gasification—volatile oxidation accompanied by char reduction reactions up to 16 ± 1 cm/s of V s and above this, and char oxidation—simultaneous char oxidation and gas phase combustion. In the gasification regime, the measured T s is less than T g ; a surface heat balance incorporating a diffusion controlled model for flaming combustion gives and matches with the experimental results to within 5%. In the char oxidation regime, T g and T s are nearly equal and match with the equilibrium temperature at that equivalence ratio. Drawing from a recent study of the authors, the ash layer over the oxidizing char particle is shown to play a critical role in regulating the radiation heat transfer to fresh biomass in this regime and is shown to be crucial in explaining the observed propagation behavior. A simple model based on radiation–convection balance that tracks the temperature–time evolution of a fresh biomass particle is shown to support the universal behavior of the experimental data on reaction front propagation rate from earlier literature and the present work for biomass with ash content up to 10% and moisture fraction up to 10%. Upstream radiant heat transfer from the ash-laden hot char modulated by the air flow is shown to be the dominant feature of this model.

Reaction-subdiffusion front propagation in a comblike model of spiny dendrites

Fractional reaction-diffusion equations are derived by exploiting the geometrical similarities between a comb structure and a spiny dendrite. In the framework of the obtained equations, two scenarios of reaction transport in spiny dendrites are explored, where both a linear reaction in spines and nonlinear Fisher-Kolmogorov-Petrovskii-Piskunov reactions along dendrites are considered. In the framework of fractional subdiffusive comb model, we develop a Hamilton-Jacobi approach to estimate the overall velocity of the reaction front propagation. One of the main effects observed is the failure of the front propagation for both scenarios due to either the reaction inside the spines or the interaction of the reaction with the spines. In the first case the spines are the source of reactions, while in the latter case, the spines are a source of a damping mechanism.

Hydrodynamic instability of the coflow filtration combustion: Numerical simulation

The stability of the front to infinitely small distor� tions was analyzed under the assumption that the reac� tor channel width is infinitely large in comparison with the combustion zone thickness (5). It was concluded that the plane filtration combustion wave may be unstable. The experimental investigation of the filtra� tion combustion wave stability confirmed (6) that the propagation of a plane filtration combustion front may be unstable and may be accompanied by the formation of structures similar to Saffman-Taylor fingers, which were observed as a highviscosity liquid was displaced by a lowviscosity liquid (7). In this work, for the first time, we made mathemat� ical modeling of twodimensional unsteady filtration combustion during the forced oxidant filtration in the combustion wave direction. The purposes of the study was to determine the conditions for the occurrence of filtration combustion with a stable plane front in chan� nels of finite width and investigate the filtration com� bustion wave propagation modes in the range of insta� bility of the plane reaction front. The physical formulation of the problem is the fol� lowing. A gas mixture with oxidant concentration a0 is fed to a reactor filled with a porous mixture containing a fuel component. The gas flow rate at the inlet (x = 0) is maintained constant (G0). Reaction initiation with a hightemperature source leads to reaction front prop� agation in the х direction at rate u. As the fuel is burned out, the structure of the porous medium changes, because of which the porosity, thermal diffusivity, and permeability of the combustion products may differ from the respective characteristics of the initial reac� tion mixture. The problem is considered in the plane geometry under the assumption that the reactor is shaped as a thin slitlike channel. If the reaction front propagation velocities are low, which is characteristic of filtration combustion waves, the difference between the temperatures of the parti� cles and the gas can be ignored and the porous medium can be considered locally uniform in temper� ature and concentration. Under this assumption, the dimensionless system of equations describing filtra� tion combustion has the form (1, 2)

Phase diagram of sustained wave fronts opposing the flow in disordered porous media

Using lattice Boltzmann simulations, we analyze the different regimes of propagation of an autocatalytic reaction front in heterogenous porous media. The heterogeneities of the porous medium are characterized by the standard deviation of its log-normal distribution of permeability and its correlation length. We focus on the situation where chemical reaction and flow field act in opposite directions. In agreement with previous experiments we observe upstream, downstream fronts as well as static, frozen ones over a range of flow velocity which depends drastically on the heterogeneities of the flow field. The transition between the static regime and the downstream one account for large enough low-velocity zones, whereas the transition from static to upstream regime is found to be given by a kind of percolation path.

An exactly solvable model of subdiffusion–reaction front propagation

The propagation of a reaction front is considered in the framework of the Seki–Lindenberg reaction–subdiffusion model, which is appropriate in the case of a diffusion-limited reaction. Typically, this kind of problem is solved using approximate approaches or numerical methods. We apply a model piecewise linear reaction function, which allows for exact analytical solutions. For a front between two stable homogeneous states, a unique value of the front velocity is found. In the case of fronts between a stable and an unstable state, a family of traveling wave solutions is revealed. The conditions for the existence of a special ‘pushed’ front are considered. The influence of advection on the front velocity is also analyzed.

Analysis of statistical repeatability of a fixed bed downdraft biomass gasification facility

In this work a statistical repeatability study of the main biomass gasification process parameters in a laboratory-scale fixed bed downdraft gasifier on batch operation was carried out using analysis of variance. To achieve this, a multi-factorial experimental design of biomass gasification was carried out keeping the main input operational parameters constant (air flow, moisture, and size and shape of biomass). Repeatability was evaluated by carrying out the tests by the same user in the same test facility but in different days under different atmospheric conditions.The results allowed to highlight that parameters such as flow of gasifying agent, temperature field, flame front velocity (or reaction front propagation velocity), biomass consumption rate, and fuel/air equivalence ratio were repeatable with a reliability of 95%. Nevertheless, two temperature signals out of nine registered, and the H2 concentration in producer gas were not repeatable. So, 81.25% of the variables evaluated meet repeatability, while the remaining 18.75% vary due to the randomness of wood packing in the bed. Therefore, it is possible to affirm that the tests were repeatable to a satisfactory extent. The results showed the reliability of the experimental facility, and the suitability of the methodology developed for biomass gasification studies.

A computational study and correlation of premixed isooctane air laminar reaction fronts diluted with EGR

The current work investigates the propagation of premixed laminar reaction fronts for mixtures of isooctane–air and recirculated combustion products (or EGR) under high pressure and temperature conditions. The work uses a transient one-dimensional flame simulation with a skeletal 215 species chemical kinetic mechanism to generate laminar burning velocity and front thickness predictions. The simulation was exercised over fuel–air equivalence ratios, unburned gas temperatures, pressures and EGR levels ranging from 0.1 to 1.0, 400 to 1000K, 1 to 250bar, and 0% to 60% (by mass) respectively, a range extending beyond that of previous researchers. Steady reaction fronts with burning velocities in excess of 5cm/s could not be established under all of these conditions, especially when burned gas temperatures were below 1450K and/or when characteristic reaction front propagation times were on the order of the unburned gas ignition delay. For a given pressure, Tu and Tb, the burning velocity of an EGR dilute mixture was found to be lower than that of an air dilute mixture, with the decrease in burning velocity attributed primarily to the reduced oxygen concentration’s effect on chemistry. Steady premixed laminar burning velocities were correlated using a modified two-equation form based on the asymptotic structure of a laminar flame, which produced an average error of 3.4% between the simulated and correlated laminar burning velocities, with a standard deviation of 4.3%, while additional correlations were constructed for reaction front thickness and adiabatic flame temperature. Correlations are presented based on a non-product equivalence ratio φ and a fraction of stoichiometric combustion products XSCP. Conversion factors are provided to facilitate application to modern direct injection internal combustion engines with inherent charge stratification where the local global Φ is different from the global Φ of the residual gas.

Reactive-infiltration instabilities in rocks. Fracture dissolution

AbstractA reactive fluid dissolving the surface of a uniform fracture will trigger an instability in the dissolution front, leading to spontaneous formation of pronounced well-spaced channels in the surrounding rock matrix. Although the underlying mechanism is similar to the wormhole instability in porous rocks there are significant differences in the physics, due to the absence of a steadily propagating reaction front. In previous work we have described the geophysical implications of this instability in regard to the formation of long conduits in soluble rocks. Here we describe a more general linear stability analysis, including axial diffusion, transport-limited dissolution, nonlinear kinetics, and a finite-length system.

The propagation of a laminar reaction front during end-gas auto-ignition

A transient, one dimensional premixed laminar reaction front is used as a model problem to further understand the physical processes influencing reaction front propagation during the various stages of spark-assisted compression ignition (SACI) combustion for both constant and variable domain pressures. This approach is consistent with the wrinkled laminar flame representation of turbulent, spark ignited engine combustion. With the proper choice of timescales and pressure rise rate, it applies to the interaction of the flame with auto-igniting end-gas in a typical automotive engine. Under the conditions simulated by a transient flame code, the reaction front begins as a deflagration, propagating into an end-gas with an initially negligible level of reaction progress. The diffusive–reactive nature of the front is maintained until significant levels of end-gas reaction progress, where the burning velocity depends upon the degree of pre-reaction. At the time of the end-gas maximum chemical power, the maximum temperature gradient and peak rate of heat conduction within the front diminish to the point where combustion becomes chemically controlled. Although significant increases in burning velocity are observed at the onset of chemically controlled combustion within the front, the end-gas is within one front time from the completion of combustion. As a result, no more than one front thickness is consumed by the apparent propagation of the spontaneous ignition front.

A kinetic eikonal equation

We analyse the linear kinetic transport equation with a BGK relaxation operator. We study the large scale hyperbolic limit (t,x)→(t/ε,x/ε). We derive a new type of limiting Hamilton–Jacobi equation, which is analogous to the classical eikonal equation derived from the heat equation with small diffusivity. Interestingly, the hydrodynamic limit and the large deviation approach do not commute. We prove well-posedness of the phase problem and convergence towards the viscosity solution of the Hamilton–Jacobi equation. This is a preliminary work before analyzing the propagation of reaction fronts in kinetic equations.