Reaction Front Propagation Research Articles

Detonation development from a hot spot in methane/air mixtures: Effects of kinetic models

Natural gas is a promising alternative fuel which can be used in internal combustion engines to achieve low carbon emission and high thermal efficiency. However, at high compression ratio, super-knock due to detonation development might occur. In this study, the autoignitive reaction front propagation and detonation development from a hot spot were investigated numerically and the main component of natural gas, methane, was considered. The objective is to assess the performance of different kinetic models in terms of predicting hot spot–induced detonation development in methane/air mixtures. First, simulations for the constant-volume homogeneous ignition in a stoichiometric methane/air mixture were conducted. The ignition delay time, excitation time, critical temperature gradient, thermal sensitivity and reduced activation energy predicted by different kinetic models were obtained and compared. It was found that there are notable discrepancies among the predictions by different kinetic models. Then, hundreds of one-dimensional simulations were conducted for detonation development from a hot spot in a stoichiometric CH4/air mixture. Different autoignition modes were identified and the detonation regimes were derived based on the peak pressure and reaction front propagation speed. It was found that even at the same conditions, different propagation modes can be predicted by different kinetic models. The broadest detonation development regime was predicted by the reduced GRI mechanism, while a relatively narrow regime was predicted by the recent kinetic models such as FFCM-1 and Aramco 3.0. The present results indicate that super-knock prediction strongly depends on the kinetic model used in simulations. Therefore, significant efforts should be devoted to the development and validation of kinetic models for natural gas at engine conditions.

Catalysis by Imaging: From Meso- to Nano-scale

In-situ imaging of catalytic reactions has provided insights into reaction front propagation, pattern formation and other spatio-temporal effects for decades. Most recently, analysis of the local image intensity opened a way towards evaluation of local reaction kinetics. Herein, our recent studies of catalytic CO oxidation on Pt(hkl) and Rh(hkl) via the kinetics by imaging approach, both on the meso- and nano-scale, are reviewed. Polycrystalline Pt and Rh foils and nanotips were used as µm- and nm-sized surface structure libraries as model systems for reactions in the 10–5–10–6 mbar pressure range. Isobaric light-off and isothermal kinetic transitions were visualized in-situ at µm-resolution by photoemission electron microscopy (PEEM), and at nm-resolution by field emission microscopy (FEM) and field ion microscopy (FIM). The local reaction kinetics of individual Pt(hkl) and Rh(hkl) domains and nanofacets of Pt and Rh nanotips were deduced from the local image intensity analysis. This revealed the structure-sensitivity of CO oxidation, both in the light-off and in the kinetic bistability: for different low-index Pt surfaces, differences of up to 60 K in the critical light-off temperatures and remarkable differences in the bistability ranges of differently oriented stepped Rh surfaces were observed. To prove the spatial coherence of light-off on nanotips, proper orthogonal decomposition (POD) as a spatial correlation analysis was applied to the FIM video-data. The influence of particular configurations of steps and kinks on kinetic transitions were analysed by using the average nearest neighbour number as a common descriptor. Perspectives of nanosized surface structure libraries for future model studies are discussed.

Optical experiments on diesel knock for high altitude engines under spray impingement conditions

In high altitude regions (>4000 m), atmosphere pressure and thus in-cylinder ambient density is decreased significantly, leading to longer spray penetration. Affected by liquid spray impingement on cylinder wall, heavy-duty diesel engines operating at high loads may encounter abnormal combustion like diesel knock. However, with limited research data, the detailed combustion mechanism is not fully understood. To explore the origins of diesel knock and its correlations with spray impingement, optical experiments in a rapid compression machine were employed, and diesel spray impingement and subsequent combustion processes were investigated. High-speed direct photography and simultaneous pressure acquisition were synchronically performed to understand combustion evolutions and pressure characteristics. Meanwhile, the role of injection parameters, ambient conditions, and cylinder sizes were considered. The results show that early injection timing and spray impingement can lead to long ignition delay time. Depending on the premixed mixture formation within longer ignition delay, diesel knock manifesting supersonic detonation-like reaction front propagation can be observed, which results in strong pressure waves with an amplitude approaching hundreds of atmosphere. Further analysis on the critical conditions show that long ignition delay time does not necessarily result in diesel knock. It lies in whether there is sufficient premixed mixture formation before main combustion. The prevalence of diesel knock seems sensitive to large fuel injection pressure and small combustion chamber. Besides, depending on the ambient conditions, there are four kinds of combustion modes under spray impingement conditions.

Imaging the dynamics of catalysed surface reactions by in situ scanning electron microscopy

Analytical methods that provide direct real-space information about the dynamics of catalysed reactions often require simplified model systems and operate under high-vacuum conditions. There is thus a strong need for the development of methods that enable observation of active catalysts under relevant working conditions. Here, in situ scanning electron microscopy is employed to study reaction dynamics and structure–activity correlations on surfaces. High sensitivity to changes in the work function and surface composition enables the detection of monolayers of adsorbed molecular species on metal surfaces, which is used here to visualize catalytic NO2 hydrogenation on platinum. The initiation of reactive behaviours and propagation of reaction fronts, as well as the spillover of activated species revealed in real-time and across a large pressure range, demonstrate the power of in situ scanning electron microscopy as a surface science tool in the study of gas-phase- and temperature-induced processes. In situ studies of catalytic surface reactions are restricted to a small number of analytical techniques. Here, scanning electron microscopy is utilized to visualize the catalytic hydrogenation of nitrogen dioxide on platinum, showing its potential for monitoring reaction dynamics on surfaces.

Diffusion-controlled coalescence, fragmentation, and collapse of d-dimensional A-particle islands in the B-particle sea.

We present a systematic analysis of diffusion-controlled interaction and collapse of two identical spatially separated d-dimensional A-particle islands in the B-particle sea at propagation of the sharp reaction front A+B→0 at equal species diffusivities. We show that at a sufficiently large initial distance between the centers of islands 2ℓ compared to their characteristic initial size and a relatively large initial ratio of island to sea concentrations, the evolution dynamics of the island-sea-island system is determined unambiguously by the dimensionless parameter Λ=N_{0}/N_{Ω}, where N_{0} is the initial particle number in the island and N_{Ω} is the initial number of sea particles in the volume Ω=(2ℓ)^{d}. It is established that (a) there is a d-dependent critical value Λ_{★} above which island coalescence occurs; (b) regardless of d the centers of each of the islands move toward each other along a universal trajectory merging in a united center at the d-dependent critical value Λ_{s}≥Λ_{★}; (c) in one-dimensional systems Λ_{★}=Λ_{s}, therefore, at Λ<Λ_{★} each of the islands dies individually, whereas at Λ>Λ_{★} coalescence is completed by collapse of a single-centered island in the system center; (d) in two- and three-dimensional systems in the range Λ_{★}<Λ<Λ_{s} coalescence is accompanied by subsequent fragmentation of a two-centered island and is completed by individual collapse of each of the islands. We discuss a detailed picture of coalescence, fragmentation, and collapse of islands focusing on evolution of their shape and on behavior of the relative width of the reaction front at the final collapse stage and in the vicinity of starting coalescence and fragmentation points. We demonstrate that in a wide range of parameters, the front remains sharp up to a narrow vicinity of the coalescence, fragmentation, and collapse points.

Propagation of Reaction Front in Porous Media with Natural Convection

This paper examines the influence of convective instability on the reaction front propagation in porous media. The model includes heat and concentration equations and motion equations under Boussinesq approximation. The non-stationary Darcy equation is adopted and the fluid is supposed to be incompressible. Numerical results are performed via the dispersion relation. The simulations show that the propogating reaction front loses stability as Vadasz number increase, it shows also more stability is gained when Zeldovich number increased.

Numerical studies on autoignition and detonation development from a hot spot in hydrogen/air mixtures

Detonation development inside spark ignition engines can result in the so called super-knock with extremely high pressure oscillation above 200 atm. In this study, numerical simulations of autoignitive reaction front propagation in hydrogen/air mixtures are conducted and the detonation development regime is investigated. A hot spot with linear temperature distribution is used to induce autoignitive reaction front propagation. With the change of temperature gradient or hot spot size, three typical autoignition reaction front modes are identified: supersonic reaction front; detonation development and subsonic reaction front. The effects of initial pressure, initial temperature, fuel type and equivalence ratio on detonation development regime are examined. It is found that the detonation development regime strongly depends on mixture composition (fuel and equivalence ratio) and thermal conditions (initial pressure and temperature). Therefore, to achieve the quantitative prediction of super-knock in engines, we need use the detonation development regime for specific fuel at specific initial temperature, initial pressure, and equivalence ratio.

Front propagation for integro-differential KPP reaction–diffusion equations in periodic media

We study front propagation phenomena for a large class of KPP-type integro-differential reaction–diffusion equations of order $$\alpha \in (0,2)$$ in oscillatory environments, which model various forms of population growth with periodic dependence. We show that, under an exponential rescaling, the solution develops an isotropic advancing front and converges locally uniformly to zero beyond the front and to the periodic stationary state behind the front. The results are the most general available in this general setting.

Mesoscopic simulation of laser initiation in pentaerythritol tetranitrate

A numerical simulation of laser initiation in a pentaerythritol tetranitrate is presented. A contact smoothed particle hydrodynamics modeling with an explicit simplified-geometry of the microstructure is defined. A sequence of spheres in simple cubic and face-centred cubic lattices is considered. Different types of PETN crystal particles compression by high-enthalpy products expansion are simulated in the hydrodynamic approach. The compactification of spheres and formation of a shock wave in a porous material is observed. Compactification of spherical particles stops the flow of hot products and shock wave is able to override the reaction front propagation. The simulation demonstrates a shock-driven mechanism in the detonation initiation processes.

Modeling subgrid combustion processes in simulations of thermonuclear supernovae

Supernovae of type Ia are thought to arise from the thermonuclear incineration of a carbon-oxygen white dwarf stellar remnant. However, the detailed explosion scenario and stellar evolutionary origin scenario – or scenarios – which lead to observed supernovae are still quite uncertain. One of the principal tests of proposed scenarios is comparison with the explosion products inferred, for example, from the spectrum of the supernovae. Making this comparison requires computation of the combustion dynamics and products through simulation of proposed scenarios. Here we discuss two specific proposed explosion scenarios, the deflagration-detonation transition and the helium shell double detonation, With these two examples in mind, we proceed to discuss challenges to computational modeling of the combustion taking place in these explosions. Both subsonically and supersonically propagating reaction fronts are discussed, called deflagrations and detonations respectively. Several major stages of the combustion occur on length and time scales that are many orders of magnitude smaller than those accessible in simulations of the explosion. Models which attempt to capture this sub-grid behavior and the verification of those models is briefly discussed.

Investigation of the mechanism of emergence of autowave structures at the reaction front.

A qualitative mechanism for autowave pattern formation at the reaction front, observed in certain chemical systems including combustion, is suggested. It is assumed that patterns are formed as a result of interaction of two subsystems, one of which is responsible for the reaction front propagation while the other determines the formation of waves at the front. A corresponding phenomenological model is constructed in which reaction front propagation is described by a submodel of the Fisher-Kolmogorov-Petrovskii-Piskunov type and waves on the front are described by a submodel of the FitzHugh-Nagumo type. In the three-dimensional numerical analysis, it is demonstrated that the model is able to qualitatively explain the emergence of wave patterns of both spiral and target types, which are experimentally observed at the reaction front. The dependence of these patterns on the velocity and thickness of the front is examined.

Non-uniform ignition behind a reflected shock and its influence on ignition delay measured in a shock tube

Accurate measurement of ignition delay in a shock tube is extremely important for developing chemical mechanisms of different fuels. In ideal shock-tube experiments, the test gas behind the reflected shock is expected to ignite uniformly. However, in practical shock-tube experiments, non-uniform ignition occurs due to different factors such as boundary layer growth and boundary layer–reflected shock interaction. Such non-uniform ignition greatly complicates the interpretation of measurements and affects the accuracy of ignition delay measurement. Even without boundary layer and multi-dimensional effects, non-uniform ignition can still happen since the reflected shock itself can induce the spatial gradient of ignition delay. This was first studied numerically by Melguizo-Gavilanes and Bauwens (Shock Waves 23(3):221–231, 2013) using a simplified three-step chemical mechanism. They found that non-uniform ignition can greatly affect the determination of ignition delay in a shock tube. As an extension of Melguizo-Gavilanes and Bauwens’ work, in the present study we conduct simulations considering detailed chemistry for two fuels, hydrogen and n-heptane, without and with low-temperature chemistry. Moreover, the detonation development for different mixtures is interpreted through comparing the local sound speed and reaction front propagation speed. The ignition delay recorded at different positions away from the end wall is compared to that in a homogeneous system. A large deviation in ignition delay is observed. The deviation is shown to change linearly with the distance away from the end wall, and a correlation is proposed to accurately describe such deviations.

HCCI and SICI combustion modes analysis with simultaneous PLIF imaging of formaldehyde and high-speed chemiluminescence in a rapid compression machine

Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM). A radiofrequency igniter with a high energy deposit (305 mJ) is used to investigate the SICI combustion phenomena in lean conditions (Φ = 0.5), relatively close to the frontiers of the SICI regime. Fluorescence images enable to monitor both the development of the cool flame process and the topology and dynamics of reaction fronts during the second stage of ignition. The results are first analyzed from a phenomenological point of view, bringing insights into the understanding of the both HCCI and SICI combustion processes as they take place in the RCM. Additional data are gathered from double-pulse 355 nm PLIF imaging, with focus on the temporal evolution of the cool flame and on the reaction front propagation during hot ignition. From a more quantitative point of view, an analysis of apparent velocities of the reaction zones is then presented, and large variations of these values are observed depending on the experimental conditions. These local quantities are closely related to the global heat release rate which is a key parameter for practical applications of HCCI and SICI combustion modes. The proposed simultaneous diagnostics finally lead to a better understanding in the local reaction modes, namely deflagration, spontaneous ignition fronts and bulk auto-ignition – e.g. volumetric auto-ignition -, which are implied in the combustion processes. The results highlight the complex aerothermal interactions taking place in the RCM vessel, in particular through the pre-ignition thermal stratification. The results suggest the latter strongly affects the HCCI combustion process, but also drives the heat release rate during the second stage of the SICI combustion mode. Furthermore, deflagration fronts are found to be significantly affected by cool flame chemistry, as well as by the large and small scale structures of the fluid flow.

УСТОЙЧИВОСТЬ ФРОНТОВ ХИМИЧЕСКИХ ПРЕВРАЩЕНИЙ ВБЛИЗИ СОСТОЯНИЯ БЛОКИРОВАНИЯ

In the current work we consider a chemical reaction localized on the sharp interface between a solid and a diffusive constituent. The driving force for the reaction front propagation is a normal component of the chemical affinity tensor. The configuration of the deformed state where the driving force is zero corresponds to the reaction blocking state. The paper is aimed at studying this configuration. The utilized approach to analyze the stability of the equilibrium interphase between two deformable domains involves a linearized analysis of the kinetic equation for the perturbed interface. Previously this method was applied for an interface stability analysis in the case of phase transformations. The advantage of this method is that it also shows how the loss of stability occurs. An analytical solution for the perturbed kinetic equation is only possible for some simple configurations. Numerical procedures were applied in order to solve these types of problems. As an example, we used the numerical procedure, and the problem of the chemical reaction front propagation in a hollow cylinder is solved. For the unstable configuration we analyzed a stress state in the vicinity of the reaction blocking.

Determinants of drug-target interactions at the single cell level.

The physiochemical determinants of drug-target interactions in the microenvironment of the cell are complex and generally not defined by simple diffusion and intrinsic chemical reactivity. Non-specific interactions of drugs and macromolecules in cells are rarely considered formally in assessing pharmacodynamics. Here, we demonstrate that non-specific interactions lead to very slow incorporation kinetics of DNA binding drugs. We observe a rate of drug incorporation in cell nuclei three orders of magnitude slower than in vitro due to anomalous drug diffusion within cells. This slow diffusion, however, has an advantageous consequence: it leads to virtually irreversible binding of the drug to specific DNA targets in cells. We show that non-specific interactions drive slow drug diffusion manifesting as slow reaction front propagation. We study the effect of non-specific interactions in different cellular compartments by permeabilization of plasma and nuclear membranes in order to pinpoint differential compartment effects on variability in intracellular drug kinetics. These results provide the basis for a comprehensive model of the determinants of intracellular diffusion of small-molecule drugs, their target-seeking trajectories, and the consequences of these processes on the apparent kinetics of drug-target interactions.

Surface-Tension-Driven Dynamic Contact Line in Microgravity.

We study the effect of Marangoni flow on a dynamic contact line formed by a propagating reaction front and a liquid-air interface. The self-sustained iodate-arsenous acid reaction maintains the production of the weakly surface active iodine leading to an unbalanced surface force along the tip of the reaction front. The experiments, performed in microgravity to exclude the contribution of buoyancy, reveal that the fluid flow generated by the surface tension gradient is localized to the contact line. The penetration depth of the surface stress is measured as 1-2 mm; therefore, with greater fluid height the liquid advancement on the upper surface does not lead to enhanced mixing in the bulk. Because the propagation velocity of the reactive interface remains at that of reaction-diffusion, the leading edge consists of two straight lines; a tilted segment connects the contact line on the surface with the vertical segment on bottom. Modeling calculations of the reaction-diffusion-advection system in three dimensions reconstruct the experimental observations and along with the experiments validate a model based on geometric spreading. According to the calculated flow field, the direction of significant fluid flow follows the concentration gradients and hence coincides with the propagation of the reaction front, allowing only negligible transverse flow in the upper fluid layer.

Effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer

Hydrogen addition is widely used to improve the combustion performance of single-component fuel. In this study, the effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer were examined and interpreted numerically. Detailed chemistry and transport were considered in simulation. The non-premixed ignition delay times at different hydrogen blending levels were obtained and analyzed. It was found that hydrogen addition greatly reduces the ignition delay. This is mainly due to the fact that the preferential mass diffusion of hydrogen over iso-octane significantly increases the local hydrogen blending level at the ignition kernel. Besides, for the non-premixed ignition process, two modes of reaction front propagation were identified through the analysis based on Damköhler number and consumption speeds. One is the reaction-driven mode characterized by local or sequential homogeneous autoignition; and the other is the diffusion-driven mode, which depends on the balance of mass diffusion, heat transfer and chemical reaction. These two modes lead to different ignition behaviors. For pure iso-octane with low mass diffusivity, ignition is mainly caused by local homogeneous reaction occurring at the most reactive position. With the increase of diffusion layer thickness, the local temperature at the most reactive position increases and therefore the non-premixed ignition delay time of pure iso-octane decreases. However, when hydrogen with high mass diffusivity is added into iso-octane, the non-premixed ignition is controlled by fuel diffusion. With the increase of diffusion layer thickness, the concentration gradient becomes smaller and thereby less hydrogen diffuses into the ignition kernel. Consequently, unlike pure iso-octane, the non-premixed ignition delay time of hydrogen/iso-octane blends increases with the diffusion layer thickness.

Mathematical modelling of porous materials synthesis supported by the filtration combustion of gas mixture

This paper proposes a mathematical model for the synthesis of porous materials from a granular low-calorie gas-free powder mixture, the reaction of which is supported by the filtration combustion of a gas mixture. The dynamics of synthesis is described by the equations that comprise the heat balance in the condensed and gas phases, the motion of the gas phase, the rate of reactions in phases, the conservation of momentum with additional relations. A numerical study was carried out using finite-difference methods. Mechanisms and modes of the propagation of reaction fronts in phases are determined depending on the characteristics of gas flow during co-current and counter filtration, the ratio of the gas-phase and solid-phase reaction constants, and the parameters of interphase heat transfer

Effects of low temperature heat release on the aerodynamics of a flat piston rapid compression machine: Impact on velocity and temperature fields

The study of auto-ignition under temperature stratification is of great interest. Indeed, further understanding of the thermo-kinetic interactions and its influence on the combustion propagation regime is needed. In a previous work [1], experiments in a flat piston Rapid Compression Machine (RCM) demonstrated that the apparent propagation of reaction fronts is highly influenced by the typical temperature stratification observed at inert conditions. Nevertheless, the influence of low temperature heat release (LTHR) on the internal aerodynamics and temperature of the RCM is not well understood. In the present study, we first address the LTHR-flow interaction then address the LTHR-temperature interaction. We performed 2D-PIV experiments at 10 kHz for inert and reactive lean isooctane mixtures. We averaged spatially the acceleration to present the time evolution during the cool flame period. We found that the normalized acceleration has a decreasing trend in both inert and reactive tests. No significant effect of the cool flame was observed on the trend. We performed temperature measurements using thin wire (7.6 µm) type K thermocouples at inert and reactive n-hexane mixtures (same test conditions of Fig. 7 in [1]). The temperature evolution of the hot (adiabatically compressed) and the colder gases were recorded when cool flame occurs. The corrected gas temperature showed good agreement with the theoretical adiabatic core temperature as well as previous measurements with toluene LIF. In the tested case, we found that the cool flame induces an equal temperature rise of approximately 110 K in both the adiabatically compressed and the colder vortex gases. These results confirm quantitatively that LTHR does not significantly affect the mixing of the temperature stratification of our flat piston RCM. In the studied test conditions, the temperature stratification is conserved globally despite the LTHR.

Acceleration of chemical reaction fronts

The propagation of reaction-diffusion fronts in an open liquid solution layer is critically affected by mass transfer between the liquid solution and the adjacent gas phase. This is the case in the iodate–arsenous acid (IAA) reaction when run under stoichiometric excess of iodate. Here, iodine is the reaction product, which has a low solubility in the liquid phase, hence, excess iodine rapidly evaporates. In the gas phase, it diffuses and overtakes the reaction front propagating in the liquid medium because its diffusion coefficient in the gas phase is considerably larger than that in aqueous solution. Evaporated iodine is re-dissolved into the reaction medium ahead of the reaction front. Since iodine is the autocatalytic species of the IAA reaction, this additional gas-phase transport may lead to an acceleration of the propagating reaction front.