Combustion Wave Research Articles

Efficient lean combustion in a novel porous medium burner with the integrated of pellets and ceramic foam: Experimental study of flame propagation and stability

Porous medium combustion makes an important contribution to energy saving and emission reduction, and the porous structure significantly affects the combustion characteristics. To combine the advantages of typical ceramic foam and pellets, an integrated structure was designed in the porous medium burner. And the characteristics of flame propagation and stability were studied and compared with the traditional single structure. The experimental results indicated that with the decreasing of pellets diameter, the combustion wave velocity decreased but the combustion temperature increased. The combination of ceramic foam and pellets achieved uniform radial temperature, and the average temperature difference was reduced by nearly 50%. The annular ceramic foam also improved the upstream flame velocity of the pellets burner from −0.045 to −0.238 mm/s. In addition, increasing the equivalence ratio and decreasing the inlet velocity were both beneficial to the propagation of upstream flame. The 20 (8) burner obtained the maximum combustion temperature of 944 K with the preheating efficiency of 23% at φ=0.80 and vin=20 cm/s. Moreover, as the combustion temperature increased, the NOx emission gradually increased and the maximum value reached 19.2 ppm.

Self-propagating high-temperature synthesis of MoAlB boride ceramics based on MAB-phase

This study focuses on the combustion kinetics and mechanisms of reaction mixtures in the Mo–Al–B ternary system taken so that the MoAlB MAB phase was formed. The effect of the initial temperature on the key combustion parameters was demonstrated. Reaction mixture preheating was found to weakly affect the maximum combustion temperature. The effective activation energy of self-propagating high-temperature synthesis (SHS) was calculated. Phase diagrams in the Mo–Al–B system were built using the AFLOW and Materials Project databases. The phase composition and structure of the synthesized ceramics with MoAlB lamellar grains 0.4 μm thick and ~2–10 μm long as a main component were studied. The DXRD lines of MoB and Mo2B5 intermediate borides with their total content of ≤3 % were also identified. Scanning electron microscopy and energy dispersive spectroscopy studies revealed that the Al2O3 phase was present in the intergranular pores. A sequence of chemical transformations in the combustion wave was studied, and a hypothesis about the structure formation mechanism was put forward. MoO2 and Al2O3 can be the primary phases during SHS, and the MoAlB phase is formed from the boron-containing aluminum–molybdenum melt. Submicron-sized MoB precipitates are formed in the post-combustion zone due to the partial oxidation of aluminum by the dispersion strengthening mechanism.

Kinetic Highlights of the Reduction of Silver Tungstate by Mg + C Combined Reducer

The programmed reduction of tungstates and molybdates may yield the production of an intimate mixture of metals, pseudo-alloys or composite powders. As an extension of the study of obtaining powders of tungsten-copper, molybdenum-copper and tungsten-nickel from their respective salts, in the present study the reduction of silver tungstate was performed. Considering the extreme conditions for the synthesis of W-Ag alloys in the combustion wave and the limited toolkit for the study of the associated reduction mechanism, the interaction in the Ag2WO4-Mg-C system was modeled at high heating rates closer to the heating rates of reagents in the combustion wave, namely by the high-speed temperature scanner (HSTS). For the effective study of the interaction mechanism and calculation of the kinetic parameters of the individual stages, the heating rate of the reagents was changed in a wide range (from 100 to 1200 °C min−1). The interaction scheme and the sequence of the reactions along with their starting temperatures were deduced; the nature of intermediates formed during the reduction process and the microstructure evolution were monitored.

Effect of MnO2 morphology on the thermal properties and combustion behavior of nano-Al/MnO2 thermite

Al/granular MnO2 and Al/rod MnO2 thermite samples were prepared to investigate the effects of different morphologies of MnO2 on the thermal properties and combustion behavior of nano-Al/MnO2 thermite. The morphology and thermal properties of the thermite were characterized by field emission scanning electron microscopy (FE-SEM), x-ray diffractometry (XRD), and differential scanning calorimetry (DSC). DSC results show that the Al/rod MnO2 releases 1274.39 J·mol−1 heat, which is 292.58 J·mol−1 more than the Al/granular MnO2. The initial reaction temperature of Al/rod MnO2 is 567.39 °C, which is delayed by 17.39 °C versus 550 °C of Al/granular MnO2. Non-isothermal thermodynamic analysis was used to measure the activation energy of Al/rod MnO2 to be 234.36 kJ·mol−1, which is 46.53 kJ·mol−1 higher than that of Al/granular MnO2. This corresponds to an increase in the ignition temperature in the DSC curve and indicating a higher safety profile. In the open burning experiment, the burning time of Al/granular MnO2 was longer and sparks sputtered around the flame. The Al/rod MnO2 has a large combustion flame and a fast combustion rate. The light intensity peaks of the two groups of samples are close in the light intensity test. The light intensity existence time of Al/rod MnO2 is 0.0146 s, which is 0.094 s shorter than Al/granular MnO2. This shows that the combustion rate of Al/rod MnO2 is much faster than that of Al/granular MnO2. The closed-tube combustion experiment shows that the combustion wave velocity of Al/rod MnO2 increases first and then decreases; the maximum wave velocity reaches 339.6 m·s−1, and Al/granular MnO2 cannot self-propagate combustion in the microporous environment. In the constant volume combustion experiment, the peak combustion pressure of Al/rod MnO2 is 0.938 Mpa, and the peak value of Al/granular MnO2 combustion pressure is 0.581 Mpa; the difference is obvious. This shows that the rod MnO2 gas production performance is better. According to the duration of the pressure peak, the burning speed of Al/rod MnO2 in the light intensity test is confirmed again to be much faster than that of Al/granular MnO2. The Al/rod MnO2 is better than Al/granular MnO2 in thermal and combustion performance and is also safer. This provides a basis for future performance and safety research on aluminothermic materials.

Ensemble Combustion of Liquid Oxygen / Gaseous Hydrogen Coaxial Flames with Transverse Acoustics

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames, under the influence of transverse acoustic forcing. The results are systematically compared to the results from a predecessor study of the same ensemble of flames under the same conditions, but without acoustic forcing. Both pressure nodes (PN) and pressure antinodes (PAN) were investigated as a function of the outer-to-inner momentum flux ratio. Shadowgraphs and OH* emission imaging showed that PNs distort the flames more than PANs, similar to previous observations in single flames. A wave amplification mechanism (WAM) identified previously to be present both in single flames and in the unforced ensemble of flames was found to also be present under acoustic forcing. In the predecessor study without acoustics, the WAM tended to self-organize within the ensemble into an out-of-phase varicose nesting that minimized large-scale collisions. In the present study, PN forcing tended to organize the WAM structures into a sinuous nesting, which also minimized collisions. PAN forcing, on the other hand, promoted destructive in-phase varicose nesting that promoted large-scale collisions, causing subsequent breakdown into smaller scales. Dynamic Mode Decomposition revealed natural frequencies and modes in addition to the acoustic quantities. Discrete combustion waves related to the WAM propagated at the same normalized average velocities regardless of whether acoustics were present in the ensemble or not. However, the trajectories are grouped into different curves than for single-element flames.

Hess Law Applicability to Heterogeneous Combustion Reactions Resulting to Non-Monotonicity of Enthalpy Distribution in Them

The definition of reaction irreversibility (reversibility) available now in literature does not allow for determining this reaction peculiarity before a practice. Moreover, in mathematical terms, such a definition is not universal and leads actually to the denial of any similarity in physicochemical processes, in particular in combustion waves. We have considered the irreversibility of reactions from the mathematical viewpoint as independence of a reaction rate on the concentration of products directly. As a result, in terms of Lei numbers, we have formulated conditions sufficient for the enthalpy of reagents to be a complete differential (or a thermodynamic potential) and independent of the reaction path. And conversely, the conditions necessary for that enthalpy of reagents are significantly dependent on the reaction path (Le ≌ 1). A comparison of the conclusions we obtained with the data from experiments available in the literature showed a good agreement with them and put an edge on the question of determining the irreversibility (reversibility) of reactions in the scientific community. We have shown that the small values of the diffusion coefficients of reagents Di compared to the thermal diffusivity coefficient κ, and with them, the corresponding small Lewis numbers (Lei = Di/κ << 1) for the most heterogeneous combustion synthesis systems, are the sufficient conditions for the enthalpy of reactions (H) to be a complete differential of the systems states and, vice versa, a strong similarity appears between the enthalpy and concentration fields at Le ≌ 1.

Spin instability of technological combustion mode in a wave of self-propagating high-temperature synthesis

The results of a computer experiment to determine the invariant properties of the Trace transform of a two-dimensional chronogram of wave propagation in the spin combustion mode of the Ni-Al system are presented. The chronogram model was chosen in the form of four sets of isocline lines, the angle of inclination of which corresponded to the value of the tangential velocity from 16.5 to 41.8 m/s. A high degree of shift invariance of the Trace-transformation nodes was found, the error in determining the coordinates of which is limited only by the presence of the initial additive geometric noise in the image of the differential chronogram of the combustion wave. Scale invariance properties in practice provide a dynamic magnification range of 40 dB.

Effect of chemical reaction on mixing transition and turbulent statistics of cylindrical Richtmyer–Meshkov instability

Direct numerical simulations of a three-dimensional cylindrical Richtmyer–Meshkov instability with and without chemical reactions are carried out to explore the chemical reaction effects on the statistical characteristics of transition and turbulent mixing. We adopt 9-species and 19-reaction models of non-premixed hydrogen and oxygen separated by a multimode perturbed cylindrical interface. A new definition of mixing width suitable for a chemical reaction is introduced, and we investigate the spatio-temporal evolution of typical flow parameters within the mixing regions. After reshock with a fuller mixing of fuels and oxygen, the chemical reaction becomes sufficiently apparent at affecting the evolution of the flow fields. Because of the generation of a combustion wave within the combustion regions and propagation, the growth of the mixing width with a chemical reaction is accelerated, especially around the outer radius with large temperature gradient profiles. However, the viscous dissipation rate in the early stage of the chemical reaction is greater because of heat release, which results in weakened turbulent mixing within the mixing regions. We confirm that small-scale structures begin to develop after reshock and then decay over time. During the developing process, helicity also begins to develop, in addition to kinetic energy, viscous dissipation rate, enstrophy, etc. In the present numerical simulations with cylindrical geometry, the fluctuating flow fields evolve from quasi-two-dimensional perturbations, and the generations of helicity can capture this transition process. The weakened fluctuations during shock compression can be explained as the inverse energy cascade, and the chemical reaction can promote this inverse energy cascade process.

Counterflow combustion waves in short samples of metal powders at natural filtration of oxygen

Combustion of a porous solid fuel is considered. An exothermic reaction takes place between the fuel and a gaseous oxidiser which is delivered to the reaction zone by filtration through the pores in the sample from an open end toward which the combustion wave propagates (counterflow filtration). The gas reacts with the solid fuel to form a solid product. The gas filtration is due to the pressure difference between the ambient pressure at the open end and the pressure in the reaction zone where the gas is being consumed (referred to as natural filtration). A 1D mathematical model based on equations describing conservation of energy, gas mass, solid reactant mass, and gas momentum, as well as an equation of state, and appropriate boundary and initial conditions is formulated and analytically studied taking advantage of the separation of length scales in the process. When the reaction zone is sufficiently far from the open end, the combustion wave propagates at a constant speed and has a time-independent structure, while when the reaction is close to the open end (closer than the filtration length), the structure of the combustion wave and its speed become time dependent. Both cases are discussed in the paper though the main emphasis is on short samples, in which the combustion wave is affected by the gas flow from the open end during the entire propagation process. A specific example of interest involves magnesium as the solid fuel and oxygen as the gaseous oxidiser.

Burning characteristics and combustion wave model of AP/AN-based laser-controlled solid propellant

Laser-controlled solid propellant (LCSP) is a novel composite propellant that can achieve non-self-sustaining combustion under laser irradiation. Accordingly, a laser-controlled solid propellant based on AP/AN was designed. The thermal decomposition of LCSP was identified by TG/DSC coupled with MS/FTIR. It was found that the pyrolyzed gas products of LCSP were mainly composed of N2, N2O, NH3, CO2, H2O and HCl, etc. Furthermore, the combustion performance parameters including burning rate, ignition delay time, chamber pressure and conducting current of combustion products were tested. As the laser power density increased from 0.42 W/mm2 to 1.06 W/mm2, the burning rate increased from 0.51 mm/s to 0.88 mm/s and the platform pressure increased from 0.47 KPa to 0.78 KPa, while the ignition delay time decreased from 0.76 s to 0.23 s. Combining with the thermocouple, the combustion wave structure of LCSP can be divided into: pre-heating zone, condensed phase, optical-thermal reaction zone, dark zone and flame zone. Meanwhile, the high laser power density reduced the thickness of condensed phase and increased the thickness of dark zone. Finally, we inferred the combustion model of LCSP and proposed the hypothesis of critical combustion energy to explain the mechanism of non-self-sustaining combustion.

Sodium-chloride-assisted synthesis of nitrogen-doped porous carbon shells via one-step combustion waves for supercapacitor electrodes

• Sodium-chloride-assisted porous carbon synthesis is developed by combustion waves. • Thermochemical reactions pass through core–shell hybrids of NaCl-nitrocellulose. • Nitrogen-doped cube-like hierarchical porous carbon shells are obtained in seconds. • Multi-scale pores and high N-doping ratio improve active site area and conductivity. • Supercapacitors using the electrodes exhibit outstanding capacitance and stability. Heteroatom-doped, multiporous carbon structures are of considerable interest as high-performance electrochemical electrodes. However, their complex and time-consuming synthetic procedures impede a scalable production. Herein, a combustion-driven sodium-chloride-assisted synthesis route of nitrogen-doped, cube-like hierarchical porous carbon shells (N-C-HPCS) is developed for the electrode materials of supercapacitors. Free-standing hybrid films composed of nitrocellulose and NaCl particles serving as the chemical fuel layer and templates are prepared, and self-propagating combustion waves passing through the films within a few seconds fabricate controllable nitrogen-doped porous carbon (N-PC) after the simple removal of the templates by washing. The optimal tuning of thermochemical reactions through the nitrocellulose loadings leads to synthesizing the N-C-HPCS, while other precursors produce sparse or dense N-PC structures. Supercapacitor electrodes using the developed N-C-HPCS exhibit an outstanding specific capacitance (305F/g at 0.5 A/g) and retention at a high current density (∼78 % at 16 A/g), as well as long-term cyclic stability (∼116% after 10,000 cycles). The symmetric two-electrode cell exhibited high power and energy densities (8 kW/kg and 10.1 Wh/kg) and superb cycling stability (107.7 % after 10,000 cycles at 5 A/g). This work will inspire rational synthesis strategies for versatile N-PCs, useful for supercapacitors, batteries, catalysts, filters, and CO 2 adsorption.

Combustion Wave Propagation in Conjugated Systems of a Powder Mixture of Ni + Al + Al2O3 and a Metal Plate

Combustion Wave Propagation in Conjugated Systems of a Powder Mixture of Ni + Al + Al2O3 and a Metal Plate

Synchronized Two-Camera Laser Monitor for Studying Combusting Powder Systems

In this paper, we offer a laboratory facility for in situ visualization of the combustion of ultrafine metal powders, which combines laser initiation and simultaneous high-speed recording of images of the flame of a burning material and a surface covered by a flame. Visualization of the surface through the flame is realized using a laser monitor—an optical projection system with brightness amplification. The proposed imaging system makes it possible to get more detailed information about the combustion process, in particular, to study the change in the surface through the flame in the area of laser initiation, and the propagation of heating and combustion waves over the sample, as well as to study the change in the surface reflectance during combustion. To study the area of laser initiation, it is proposed to simultaneously record images of a laser monitor with two cameras. The symmetry of the combustion wave front propagation and the combustion products’ formation during laser initiation of the nanoAl + Fe3O4 thermite mixture was demonstrated. The nature of propagation in the form of a ring is a consequence of the symmetry of the properties of the system under study, at the micro and macro levels.

Measurement and analysis of species distribution in laser-induced ablation plasma of an aluminum–magnesium alloy

We proposed a theoretical spatio-temporal imaging method, which was based on the thermal model of laser ablation and the two-dimensional axisymmetric multi-species hydrodynamics model. By using the intensity formula, the integral intensity of spectral lines could be calculated and the corresponding images of intensity distribution could be drawn. Through further image processing such as normalization, determination of minimum intensity, combination and color filtering, a relatively clear species distribution image in the plasma could be obtained. Using the above method, we simulated the plasma ablated from Al–Mg alloy by different laser energies under 1 atm argon, and obtained the theoretical spatio-temporal distributions of Mg I, Mg II, Al I, Al II and Ar I species, which are almost consistent with the experimental results by differential imaging. Compared with the experimental decay time constants, the consistency is higher at low laser energy, indicating that our theoretical model is more suitable for the plasma dominated by laser-supported combustion wave.

Burning rate modulation for composite propellants by interfacial control of Al@AP with precise catalysis of CuO

Burning rate modulation is a critical but challenging technology for solid propellants’ design. Great efforts have been made to synthesize and utilize catalysts to modify the pressure exponent (n), but poor catalytic efficiency and lower energy level of conventional catalysts limit their further applications. In this research, interfacial control of Al@AP together with precise catalysis of CuO is represented for the purpose of pressure exponent reduction and combustion performance improvement. The core-shell structured Al@AP and Al@AP/CuO composites were prepared and applied into HTPB/Al/AP composite propellants. These prepared propellants have the same formula but different micro-structures of ingredients, leading to varied combustion performance, in terms of the burning rates, combustion wave structures, flame spectra, agglomeration processes of Al particles and condensed combustion products. The investigation results show that by using interfacial control of Al@AP and precise catalysis of CuO, the n of involved propellants over 1–4 MPa can be greatly decreased from 0.42 to 0.21 and the agglomeration of Al could be effectively inhibited. In addition, propellants based on Al@AP and Al@AP/CuO interfacial controls of this research are summarized and compared together with those based on Al@RDX published recently, to present overall characteristics of interfacial control techniques, which could lead to the development of new smart controllable propellants.

The behaviors of supersonic combustion wave through a perforated plate in a stoichiometric mixtures of H2/CH4/O2 and H2/O2

In this study, we reported the supersonic combustion wave behaviors as it propagated through a perforated plate in stoichiometric mixtures of H2-CH4-2.5O2, H2-CH4-4O2 and 2H2-O2. The perforated plate maintained an identical thickness of 10 mm with different hole diameters of 4 mm, 8 mm and 10 mm, respectively. Explosion pressures were recorded using pressure transducers flushed-mounted on the top wall, based on which the average velocity was calculated. Meanwhile, the cellular structures of the supersonic combustion waves were recorded by soot foils. According to the average velocity and soot foils, the propagation modes were classified into four regimes including Fast-flame to fast-flame, Fast-flame to detonation, Detonation to fast-flame, Detonation to re-initiation and Overdriven detonation to detonation. The perforated plate exerts a positive effort on fast-flame by disturbing the flow flied, therefore the fast-flame does not decelerate downstream of the perforated plate. However, the effect of the perforated plate is opposite for detonation, and it weakens the detonation and even causes detonation failure. And this effect is weakened with the hole diameter increasing, which is proven by the limit of re-initiation decreases from d/λ = 0.98(d = 4 mm) to d/λ = 0.71(d = 10 mm). However, the re-initiation mechanisms are different at various initial pressures. At higher initial pressure, the detonation front is elongated dramatically but not failed and it recovers downstream. However, the failed detonation needs the aid of the shock refection off the tube wall to re-initiate at lower initial pressure. With increasing the sensitivity of the mixture, the overdriven detonation characterized by d/λ > 1 was found to transmit the holes with no failure. By comparing with the failing cases owning small cells, d/λ > 1 is not the sole condition to ensure a detonation traveling through with no failure.

Characteristics of phase formation during combustion of the MgO-Al2O3-Mg(NO3)2·6H2O-Al-B system

The research revealed the carbon micro traces in the synthesis products of magnesium aluminate spinel produced by the method of self-propagating high-temperature synthesis (SHS) in the MgO-Al2O3-Mg(NO3)2·6H2O-Al system with boron additives. The phase composition, structure, and morphological features of the surface of the samples were examined. The gases evolving during combustion were analyzed. IR spectroscopic analysis showed that carbon has a diamond-like lattice similar to the lattice of detonation nanodiamonds. It was shown that low-energy nuclear reactions (boron-proton reaction) proceed in the combustion wave during high-speed SHS processes under certain conditions. Based on the experimental data, the most probable mechanisms of carbon formation in the synthesized products were formulated and proposed.

Long-time solutions of scalar hyperbolic reaction equations incorporating relaxation and the Arrhenius combustion nonlinearity

Abstract We consider an initial-value problem based on a class of scalar nonlinear hyperbolic reaction–diffusion equations of the general form $$\begin{align*} & u_{\tau\tau}+u_{\tau}=u_{{xx}}+\varepsilon (F(u)+F(u)_{\tau} ), \end{align*}$$in which ${x}$ and $\tau $ represent dimensionless distance and time, respectively, and $\varepsilon>0$ is a parameter related to the relaxation time. Furthermore, the reaction function, $F(u)$, is given by the Arrhenius combustion nonlinearity, $$\begin{align*} & F(u)=e^{-{E}/{u}}(1-u), \end{align*}$$in which $E>0$ is a parameter related to the activation energy. The initial data are given by a simple step function with $u({x},0)=1$ for ${x} \le 0$ and $u({x},0)=0$ for ${x}> 0$. The above initial-value problem models, under certain simplifying assumptions, combustion waves in premixed gaseous fuels; here, the variable $u$ represents the non-dimensional temperature. It is established that the large-time structure of the solution to the initial-value problem involves the evolution of a propagating wave front, which is of reaction–diffusion or reaction–relaxation type depending on the values of the problem parameters $E$ and $\varepsilon $.

Tuning the reactivity of Al–Ni by fine coating of halogen-containing energetic composites

In this paper, various core-shell structured Al–Ni@ECs composites have been prepared by a spray-drying technique. The involved ECs refer to the energetic composites (ECs) of ammonium perchlorate/nitrocellulose (AP/NC, NA) and polyvinylidene fluoride/hexanitrohexaazaisowurtzitane (PVDF/CL-20, PC). Two Al–Ni mixtures were prepared at atomic ratios of 1:1 and 1:3 and named as Al/Ni and Al/3Ni, respectively. The thermal reactivity and combustion behaviors of Al–Ni@ECs composites have been comprehensively investigated. Results showed that the reactivity and combustion performance of Al–Ni could be enhanced by introducing both NA and PC energetic composites. Among which the Al/Ni@NA composite exhibited higher reactivity and improved combustion performance. The measured flame propagation rate (v = 20.6 mm/s), average combustion wave temperature (Tmax = 1567.0 °C) and maximum temperature rise rate (γt = 1633.6 °C/s) of Al/Ni@NA are higher than that of the Al/Ni (v = 15.8 mm/s, Tmax = 858.0 °C, and γt = 143.5 °C/s). The enhancement in combustion properties could be due to presence of the acidic gaseous products from ECs, which could etch the Al2O3 shell on the surface of Al particles, and make the inner active Al to be easier transported, so that an intimate and faster intermetallic reaction between Al and Ni would be realized. Furthermore, the morphologies and chemical compositions of the condensed combustion products (CCPs) of Al–Ni@ECs composites were found to be different depending on the types of ECs. The compositions of CCPs are dominated with the Al–Ni intermetallics, combining with a trace amount of Al5O6N and Al2O3.

Infiltration-controlled combustion of magnesium for power generation in space

Chemical heat integrated power systems can potentially provide high energy densities and long lifetimes compared to the best batteries. They are of interest for space missions where the use of solar or nuclear energy is impractical. In a new concept of such a system, a metal combustor is coupled with a chemical oxygen generator, and the metal powder bed inside the combustor burns with the infiltrated oxygen. However, the infiltration-controlled combustion of metal powders is not well understood. In the present work, a magnesium powder poured in a vertical quartz tube was ignited by a laser inside a chamber filled with oxygen at pressures of 44 – 90 kPa. The ignition resulted in the propagation of a thermal wave followed by a second ignition at the other end and propagation of a thermal wave backward. High-speed and infrared video recordings as well as thermocouple measurements were used for diagnostics. The results demonstrate that if a metal with a low Pilling-Bedworth ratio (0.81 for magnesium) is used, coflow combustion is possible. The coflow combustion wave propagates at incomplete conversion, with an axial velocity being as low as 0.1 mm/s and the zone of the highest temperature traveling along a helical path. An increase in the axial velocity of the combustion wave is accompanied by an increase in the flame thickness and a decrease in the extent of conversion.