Combustion Wave Propagation Research Articles

Thermally coupled SHS processes in (Ni + Al)/(Co + Ti)/(Ni + Al) layered system: experimental and theoretical study

The paper focuses on the theoretical and experimental study of the mechanisms of reaction mixture combustion in the ≪chemical oven≫ mode in a three-layer Ni–Al/Ti–Co/Ni–Al sample. Experimental studies were carried out in a reactor in an argon atmosphere at atmospheric pressure and an ambient temperature of 298 K on rectangular samples pressed from Ni–Al and Ti–Co powder mixtures in the form of a three-layer package. The Ti–Co acceptor layer was in the middle of the sample, and the Ni–Al donor layer was outside. The acceptor layer thickness was varied from 4.3 to 13 mm, while the donor layer thickness (4.7 mm) remained constant. It was found that as the acceptor layer thickness increases, the combustion wave front propagation velocity and reaction initiation temperature decrease, and the maximum temperature in the front remains constant and equal to the melting point of the final product. The time of acceptor layer heating before the reaction increases. The acceptor mixture reaction proceeds in the thermal explosion mode when the thickness of the acceptor layer exceeds that of the donor one. Maximum temperature in this case is higher than the melting point of the final product. The inner layer synthesis modes change with an increase in the acceptor layer thickness: stationary – pulsating – extinction. The mathematical model of the three-layer sample high-temperature synthesis in dimensional variables is constructed taking into account heat transfer with the environment. As a result of experimental studies and numerical calculations, the critical thickness of the inner layer was found to be 15 mm, at which the inner layer combustion becomes impossible at fixed sizes of donor layers. Critical conditions for the combustion wave propagation along the acceptor layer are weakly dependent on the external heating source. The experimental technique and mathematical model of the layered system combustion can be used to assess the critical conditions for the metal composite synthesis in the frontal combustion mode.

Influence of perforated plate thickness on supersonic combustion wave propagation mechanism in a stoichiometric H2-O2 mixture

The behaviors of a supersonic combustion wave through the perforated plate were studied based on velocity and cellular structures. The perforated plate with the thicknesses of Δ = 40 mm, 70 mm, and 100 mm was placed perpendicular to the direction of combustion wave propagation. Stoichiometric hydrogen-oxygen was used in all tests with initial pressure ranging from 11 kPa to 55 kPa. Soot foils were employed to record the triple-point trajectory of a detonation through the perforated plate. Pressure transducers were used to measure the shock time-of-arrival, based on which average velocity was derived. It was found that the fast flame, entering the perforated plate, either maintains fast flame or accelerates to detonation. For CJ detonation, the diffraction kills the detonation directly, and the decoupled detonation may re-initiate or not, which is up to the initial pressure and the geometry of the perforated plate. However, for an overdriven detonation, it can transmit the perforated plate successfully with no failure. The critical condition for a detonation go or no go requires d/λ (d is the hole diameter and λ is the cell size) to be larger than 2.29.

Role of pressure waves in the heating of the end-gas in HCCI engine with activation by pulsed corona discharge

In this paper we present numerical research of pressure waves influence on the end-gas in HCCI engine with discharge activation. It is shown that there is certain promotion of exothermic chemical reactions by pressure waves. That leads to the slight heating of the end-gas, but mainly the end-gas is heated due to the compression in the process of combustion wave propagation.

Multiscale physics of rotating detonation waves: Autosolitons and modulational instabilities.

Proposed is a phenomenological modeling framework that is capable of reproducing the diverse experimental observations of the nonlinear, combustion wave propagation in a rotating detonation engine (RDE), specifically the nucleation and formation of combustion pulses, the soliton-like interactions between these combustion fronts, and the fundamental, underlying Hopf bifurcation to time-periodic modulation of the waves. In this framework, the mode-locked structures are classified as autosolitons or stably propagating nonlinear waves where the local physics of nonlinearity, gain, and dissipation exactly balance. We find that the global dominant balance physics in the RDE combustion chamber are dissipative and multiscale in nature: The fast combustion physics provide the energy input to form the fundamental mode-locked autosoliton state, while the slow physics of exhaust and propellant recovery shape the waveform and dictate the number of autosolitons. In this manner, the global multiscale balance physics give rise to the stable structures-not exclusively the frontal dynamics prescribed by classical detonation theory. Experimental observations and numerical models of the RDE combustion chamber are in strong qualitative agreement. Moreover, numerical continuation (computational bifurcation tracking) of the RDE analog system indicates that a Hopf bifurcation of the steadily propagating pulse train leads to the fundamental instability of the RDE, or time-periodic modulation of the waves. Along branches of Hopf orbits in parameter space exist a continuum of wave-pair interactions that exhibit solitonic interactions of varying strength.

Fabrication and combustion behavior of high volumetric energy density core-shell Si/Ta -based nano-energetic composites

This paper reports a group of new energetic materials (EMs) based on Si/Ta coated with recently developed high energetic composites (ECs), such as AP/NC and PVDF/CL-20. These Si/Ta-based nano-energetic composites have been prepared by both mechanical mixing method and an advanced technique of electrostatic self-assembly followed with a spray-drying process. The heat of reaction and combustion behavior of these novel Si/Ta@ECs composites, as well as the morphologies and compositions of their condensed combustion products (CCPs) have been comprehensively studied. It has been shown that the combustion efficiency of Si-Ta could be largely enhanced with the inclusion of two types of ECs. The maximum energy release was 18.39 kJ cm−3 of Si/Ta with the inclusion of ECs prepared by electrostatic self-assembly followed with spray-drying technique, which was 51.6% of the theoretical energy density of the pure Si/Ta and it is about 9.58 kJ cm−3 higher than the mechanically mixed one. It shows that the enhanced flame has improved luminosity and radiation due to increased energy content with higher reaction rate. The maximum flame-front propagation rate and the condensed phase reaction rate of Si/Ta with the addition of 10 wt% of ECs are 30.1 mm s−1 and 120.7 mg s−1, respectively. The maximum combustion wave temperature of Si/Ta-based composite was 2.4 times higher than that of pure Si/Ta without the ECs inclusion. The typical phase composition of the CCPs is composed of TaSi2, which is combined with a minor inclusion of Ta2O5, TaN0.25, TaC and Ta5Si3. The two-step reaction mechanism includes the activation and propagation of the fast combustion wave of the ECs and the initiation of the post-burning reaction.

Influence of the Ar pressure on the structure of the NiTi foams produced by self-propagating high-temperature synthesis

The influence of the Ar pressure in the reactor on the structure and martensitic transformation of the NiTi foams produced by self-propagating high-temperature synthesis (SHS) was studied. It was shown that an increase in Ar pressure significantly affected the porous structure due to the process of combustion wave propagation changed from the stationary to the pulse regime. Regardless of the Ar pressure, the porous partitions included the NiTi phase as well as Ti2Ni, Ni3Ti2, Ni4Ti3 precipitates. An increase in Ar pressure decreased the transformation temperatures, which might be caused by an increase in the Ni concentration in the NiTi phase due to a decrease in the volume fraction of the Ni-rich precipitates.

Thermal effect of hollow spheres in a filtration combustion process

Filtration combustion, or porous media combustion, deals with the partial or total oxidation of a fuel in presence of a solid phase, achieving a more dynamic heat transfer process than conventional technologies. The thermal performance of the solid media has a direct effect on the reaction process, and commonly features a packed bed structure composed of solid spheres. The main objective of this manuscript is to investigate the thermal effect of hollow spheres as an inert solid medium in a filtration combustion process. A heat transfer analysis was developed considering the internal radiation of the hollow spheres and its density reduction in comparison with solid spheres. Then a one-dimensional model based on a two temperatures (gas and solid) approximation was presented using a one-step combustion chemistry for lean methane-air mixtures. Numerical and experimental results with hollow spheres as packed bed are in good agreement and demonstrate higher combustion wave temperatures and propagation rates than solid spheres. Upstream and downstream wave propagations were observed in the range of air–fuel ratios studied. It is shown that hollow spheres as packed bed is a good option for new designs of porous media burners.

Numerical modeling of heterogeneous combustion with phase transitions in porous metal-containing media

In this paper, the heterogeneous combustion of porous metal-containing media is studied taking into account the phase transitions of metal in condensed and gas phases. Such process takes place when applying a novel promising method for the extraction of rare metals from various combustible materials, such as coal and petroleum wastes, by means of filtration combustion. A novel mathematical model, based on the conservation laws, is developed for investigating the physicochemical and gas-dynamic processes during the propagation of combustion waves in porous media with admixtures of metals. Using numerical experiments, the possibilities of metal mass transfer and accumulation of condensed metal in a narrow zone of the porous reactor are shown. The effect of the gas velocity at the reactor inlet and initial metal concentration on combustion of porous metal-containing media is studied. It is revealed that by changing the gas velocity at the reactor inlet, it is possible to control the process of metal transfer and accumulation during the extraction of rare metals from various combustible media. It is shown that the investigated method of rare metals extraction can be very effective for combustible media with a low initial metal concentration.

Chemical Condensation Wave Initiating Oxygen-Free Combustion and Detonation

It is well known that the propagation of traditional combustion and detonation waves was determined by the branched chain reactions discovered by Academician N.N. Semenov. This article discusses a new type of detonation waves initiated by chemical condensation processes. Chemical condensation waves arise as a result of the heat released during the explosive condensation of a highly supersaturated carbon vapor formed as a result of the dissociation of the initial carbon-containing molecules behind the front of the initiating shock wave. Unlike traditional combustion and detonation waves, the mechanism of chemical condensation does not include branched chain reactions; nevertheless, the laws of propagation of detonation waves of condensation are clearly described by the Zel’dovich–Neumann–Döring theory.

Study of combustion wave propagation in linear charges from mechanically activated thermite mixtures

The combustion wave propagation in Al–CuO and Al–Bi2O3 termite mixtures was experimentally investigated. Waveforms of electric current through the reaction area between electrodes with a given potential difference, chronograms of piezo- and photodiode response, the data of pyrometric measurements, as well as photos of the glowing area were obtained and analyzed. There were revealed the advance filtration of the gas component through pores of mixture, the generation of charges providing considerable electrical conductivity of chemical reaction products, the connection of electrical conductivity dynamics with the optical radiation of the chemical conversion region.

Experimental investigation of flame stability in the premixed propane-air combustion in two-section porous media burner

The characteristics of combustion and the flame stabilization within a two-layer porous media burner were investigated experimentally in the present study. Lean propane-air mixtures were used with the equivalence ratio values ϕ in the range between 0.1 and 0.5 and the flow rate Q of the mixture in the range between 10 and 45 l/min. It was experimented with three porous ceramics: reticulated foam alumina, honeycomb foam alumina and SiC foam. The axial temperature distribution, reaction zone, maximum temperature, CO and NOx emissions, and combustion wave propagation rate were analyzed. The experimental results showed that, in the three ceramics studied, the combustion wave propagation rate increases with increased inlet velocity and decreased equivalence ratio of premixed gases. Result of the analysis of the information obtained, a porous burner with flame stabilized between two porous ceramics was built that works permanently with the equivalence ratio ϕ = 0.5 and the flow rate of the mixture in the range between 20 and 35 l/min. The honeycomb foam was placed on top of the burner and the reticulated foam on the bottom. The combustion temperatures were in the range between 1160 and 1380 K, where a higher flow rate resulted in higher temperatures. The CO and NOX emissions did not exceed a few ppm showing that combustion was almost complete.

Combustion Wave Propagation in between Two Reactive Layers: Mathematical Modeling with Account for Melting and Interdiffusion

Propagation of combustion wave in a two-layer system was numerically modelled with account for melting, interdiffusion, and heat losses. Combustion wave propagation in between two reactive layers of a bilayer structure was found capable of steady propagating until some critical bilayer thickness (lcr). An oscillation combustion mode arising above lcr was found to yield a product with periodic inhomogeneities in its structure. Strong heat losses led to encapsulation of combustion process into a shell of unreacted mixtures.

Autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method

Abstract Studies on autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method were conducted to obtain a new understanding of combustion modes and their mutual transition mechanism. Autoignition induced by the converging cylindrical shock wave and weak detonation can be observed by experiment and numerical simulation. Research indicates that for an appropriate spatial reactivity gradient, the velocity of the weak detonation wave can decrease and adjust to the local sound speed. This leads to the transformation of the weak detonation into the self-propagating Chapman-Jouguet (CJ) detonation. The speed of the spontaneous reaction wave characterizing the end of the chemical reaction is never less than the local sound speed; hence, there is no overdriven phenomenon throughout the weak-to-CJ detonation transition. This conforms to the fact that the overdriven phenomenon is not a necessary condition to initiate the detonation, and the combustion has higher thermal efficiency compared with isochoric combustion. Further research indicates that CJ detonation propagating in the preheated reactant can also transform into weak detonation for an appropriate spatial reactivity gradient. The initial CJ detonation degrades into a shock that decays and finally disappears. The wave-eliminating effect of the CJ-to-weak detonation transition can ensure the flow field quality and thermal efficiency of the combustion. In summary, performing a controllable weak detonation between isochoric combustion and CJ detonation in an engine has significant practical importance for improving combustion thermal efficiency and knock suppression.

Synthesis of Inorganic Cobalt-Containing Spinel-Type Pigments by Self-Propagating Synthesis

Cobalt-containing ultramarine spinel-type pigments are fabricated in the ZnO–MgO–CoO–Al(OH)3–Al system by self-propagating high-temperature synthesis (SHS). the initial components are oxides of cobalt (Co3O4) and zinc (ZnO), aluminum hydroxide (Al(OH)3), and magnesium nitrate hexahydrate (Mg(NO3)2 · 6H2O). An aluminum powder of the ASD-4 brand is used as the reducer metal. The synthesis is performed using samples 40 mm in diameter. The combustion wave propagation velocity is 1–2 mm/s, and the maximal synthesis temperature is 1180°C. Parallel processes of oxidation of aluminum and aluminothermic reactions are leading reactions that provide the synthesis of spinel-based ceramic pigments in the layer-by-layer combustion. These processes result in charge self-heating to the synthesis temperature of spinels, which are also formed with heat liberation. The rapid destruction of Al(OH)3 upon heating leads to the formation of active submicron γ-Al2O3 participating in the subsequent synthesis of finely dispersed spinel structure. Endotherms associated with the decomposition of Al(OH)3 lead to cooling of combusting sample, which complicates the SHS implementation are requires an additional heat supply. Gases liberated during thermal decomposition loosen the charge in the heating zone and decrease the maximal combustion temperature, which makes it possible to perform the synthesis in the solid phase without product fusion and acquire it in the finely dispersed state. Microstructural investigations into the samples by means of scanning electron microscopy confirmed the finely dispersed structure of pigments. IR spectroscopic and X-ray phase analyses revealed a spinel structure. Particle-size-distribution histograms for the initial Al(OH)3 and after its heating, as well as for synthesized spinels, are presented in the work. It is shown that the content of particles 903 nm in diameter is maximal in pigment. Thus, the fabrication of spinel-type pigments in the finely dispersed state by solid-phase synthesis immediately in the combustion wave considerably simplifies the process flowsheet of their production due to the absence of a grinding state.

Intensification mechanisms of the lean hydrogen-air combustion via addition of suspended micro-droplets of water

The present study is devoted to the detailed numerical analysis of the combustion wave propagation in the confined vessel filled with the lean 15% hydrogen-air mixture containing suspended micro-droplets of water. Considered gaseous mixtures possesses a relatively low reactivity, and so the intensity of the combustion is moderate. Herewith, it is found that the flame can be noticeably accelerated in the presence of suspended micro-droplets with a diameter larger than 50 μm. Numerical analysis of the flame structure performed via in-house computational package developed by authors showed that the interaction between the flame and the droplets results in the intensification of the flame instability and small scale flame front wrinkling. It is demonstrated that along with the scales associated with intrinsic hydrodynamic and thermo-diffusive modes of flame front instability, smaller wrinkles are excited as a result of the local effect of micro-droplets on the flame front. The rapid growth of the corrugated flame surface leads to the combustion intensification, and in the considered system, the burnout process can proceed up to 2.2 times faster in the presence of micro-droplets of 200 μm diameter than in the pure gaseous mixture. It is expected that distinguished mechanisms of the influence of water droplets on the flame front structure are relevant for the other two-phase combustible systems, such as reactive gaseous mixtures with suspended fuel droplets or solid particles.

Gasless Combustion in Partially Mechanoactivated Binary Mixtures: Mathematical Model

In this communication, we report on the mathematical modeling of gasless combustion in partially mechanoactivated binary mixtures. The model involves the equations of heat balance, chemical kinetics, and boundary conditions, with special emphasis on the influence of mechanoactivation parameters on the process of combustion wave propagation over the system under study.

Species distribution in laser-induced plasma on the surface of binary miscible alloy

The spatiotemporal species distribution and dynamics of plasma play a crucially important role in improving the quality of the deposited film fabricated by pulsed laser deposition (PLD) and optimizing the spectral acquisition to enhance the quantitative performance of spatial-resolved laser-induced breakdown spectroscopy (LIBS). In this work, the monochrome imaging is used to analyze the plasmas induced from binary miscible alloys and explore the dependence of species distribution in plasma on laser-supported combustion (LSC) and laser-supported detonation (LSD) wave propagation regimes. This paper includes three conclusions: 1) the species distribution in LSC-wave dominated plasma induced from immiscible alloy is heavily influenced by the melting points of constituted elements, while in LSC-wave dominated plasma induced from miscible alloy, it is related to the boiling points; 2) whether the alloy is miscible or immiscible, the species distribution in LSD-wave dominated plasma is only related to the atomic masses; 3) to a great extent, the reduction rate of species in LSC-wave dominated plasma is associated with the transition probability, while that in LSD-wave dominated plasma is associated with the number density of species in the upper energy level.

Stability of combustion waves propagating in two thermally coupled thin solid fuel layers

In this work we investigate the linear stability of combustion waves propagating in a system of two layers of different exothermic reacting materials under conditions of thermal contact between them through the common interface using the reaction sheet approximation. The dispersion relations are found and the dependence of the stability boundary on the parameters of the model is studied. It is demonstrated that the boundaries of stable combustion wave propagation in such system can be significantly extended if the properties of the layers are properly tailored.

Cellulose assisted combustion synthesis of high surface area Ni-MgO catalysts: Mechanistic studies

The objective of this study was to determine the combustion mechanism of cellulose paper impregnated with Mg(NO3)2, Ni(NO3)2, glycine solutions, and their different combinations. It was established that the combustion mechanism changes as a function of the impregnated media composition. In the Mg(NO3)2-cellulose system, Mg2+ ions strongly catalyze cellulose pyrolysis; hence no atmospheric oxygen is needed for the self-sustained combustion reaction. In the Ni(NO3)2-cellulose system, Ni2+ ions catalyze pyrolysis at a lower rate, and atmospheric oxygen assists the combustion wave propagation. In glycine containing systems, glycine blocks the ability of metal cations to catalyze the early cellulose pyrolysis, and the combustion front propagates due to the exothermic reactions between metal nitrates and glycine. The above mechanisms influence the microstructure of the combustion products. Due to the near-complete degradation of cellulose fibers during the Mg2+ catalyzed pyrolytic combustion, the resulting materials had a highly porous, sponge-like microstructure with a BET surface area of up to 152 m2/g. After the reduction in hydrogen, Ni segregates to the surface of NiO-MgO solid solution, and the resulting catalysts exhibited near-equilibrium methane conversion during the dry reforming of methane reaction at 600°C with low carbon formation and no deactivation for 24 h of time on stream.

On the Role of Peroxide Compounds in the Cold Zone Heating of Combustion Waves of Methane–Air Mixtures at Atmospheric Pressure

To study the role of the reactions of CH3O2 radicals in the unsteady process of formation and propagation of methane combustion waves in air at P = 1 atm, the reactions were mathematically simulated using a kinetic model complemented with a block of these reactions. The influence of this block is limited by the decomposition reaction СН3О2 → СН3 + О2(II); therefore, the standard calculations were performed for a wide range of compositions at a smaller value of the reaction rate constant k2∞, and additional calculations were performed at a larger value of this constant. For mixtures containing 5, 9, and 16% CH4, the calculations were performed without taking into account the CH3O2 radicals. All the calculated data (according to flame velocities and the structure of their preheating zones) were compared with one another and with the experimental values. Based on the results of standard calculations, the space profiles of temperature and substances were presented for mixtures containing 5, 9, and 16% CH4, and time profiles were additionally given for the case with 9% CH4. To characterize the structure of the combustion wave using these data, two small kinetic schemes were chosen, which are determined by the low-temperature kinetics of the HO2 and CH3O2 radicals. Analytical criteria were obtained based on these schemes; the calculations of self-ignition temperatures using these criteria (including the state of various points of the preheating zone) characterize the change in the activity of the system up to its coldest point.