Combustion Wave Velocity Research Articles

Modeling of solid-state combustion in thermally and chemically heterogeneous media

A thermally and chemically heterogeneous medium is modeled by a set of cylinders pressed from a mixture of solid reagents with coaxially aligned cylindrical rods made from an inert material. The change in velocity of combustion waves propagating over one cylinder is studied by numerical methods under the assumption that there is no heat release from the cylinder surface. The mean velocity of the combustion front in the specimen is shown first to decrease and then to increase with increasing thermal conductivity of the inert rod. Spinning waves are obtained in the range of low velocities of the combustion front. The laws of variations of the maximum temperature in the combustion front are determined. It is shown that the inert rod may serve as a heat sink from the hot charge mixture, may be manifested as dilution by an inert component, enhance heat recuperation, and increase the combustion-wave velocity. Introduction of the inert rod may either destabilize combustion-wave propagation in the region of a stable plane front or stabilize the combustion wave in the range of parameters where the plane front is unstable.

Formation of TiAl–Ti 5Si 3 and TiAl–Al 2O 3 in situ composites by combustion synthesis

Preparation of TiAl–Ti 5Si 3 and TiAl–Al 2O 3 in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS). Sample compacts containing Ti, Al, and TiO 2 powders were employed to produce the TiAl–matrix composite reinforced by Al 2O 3, which was yielded through the displacement reaction of Al with TiO 2. On the other hand, elemental powder compacts were used to synthesize the TiAl–matrix composite with Ti 5Si 3 as the reinforcement. Experimental observations show that the addition of Si in the Ti–Al powder mixture enhances the sustainability of the reaction and the increase of Ti 5Si 3 formed in the composite increases the combustion temperature and combustion wave velocity. However, the opposite trends were found in the synthesis of TiAl–Al 2O 3 composites. Not only is a preheating temperature of 300 °C required to achieve self-sustaining combustion, but also the reaction temperature and flame-front propagation rate decrease with Al 2O 3 content. Moreover, it was found that the combustion front propagated in a pulsating mode when the mole fraction of Al 2O 3 higher than 12.5 mol% was produced, because of the lower exothermicity of the reaction between Al and TiO 2 than that of Ti with Al. The Al 2O 3 content of the TiAl–Al 2O 3 composite obtained by this study has an upper limit at 28.6 mol%, beyond which combustion ceased to propagate. The XRD analysis of the end products confirms formation of TiAl–Ti 5Si 3 and TiAl–Al 2O 3 in situ composites. In addition to TiAl, another aluminide compound Ti 3Al known as a major secondary phase in the Ti–Al reaction was detected in both types of the composites.

Combustion synthesis of TiC–TiB2 composites

Abstract An experimental study on formation of TiC–TiB2 in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) using the reactant compacts from different combinations of Ti, B4C, C, and B powders. Direct reaction of Ti with B4C at stoichiometry of Ti:B4C = 3:1 yields a TiB2-rich composite with TiC:TiB2 = 1:2. Formation of the products containing 20, 33.3, and 50 mol% of TiB2 was achieved by the Ti–B4C–C reactants. In addition, the test specimen composed of Ti, B4C, and B was employed for the synthesis of a composite with 80 mol% TiB2. Among three different types of the powder compacts, the boron-containing sample was characterized by the fastest combustion wave and the highest reaction temperature. The lowest combustion temperature and wave velocity were observed in the Ti–B4C compact. When fine Ni particles were added to the Ti–B4C reactant, it was found that the propagation rate of the reaction front was increased and the densification of the end product was enhanced significantly. This was attributed to formation of the Ti–Ni eutectic liquid during the reaction. As a result, the relative density of a TiC + 2TiB2 composite increases from 30 to 86% with the Ni content from 0 to 20 mol%. Based upon the XRD analysis, small amounts of TiNi3 and TiB were detected in the Ni-reinforced TiC–TiB2 composites.

An Experimental Investigation on Combustion Synthesis of Transition Metal Silicides V5Si3, Nb5Si3, and Ta5Si3.

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Numerical Study of the Effect of the Diameter of Alumina Balls on Flame Stabilization in a Porous-Medium Burner

The results of a computer simulation analysis of a porous-medium burner with a double layer of alumina balls are reported. The objectives are the determination of the stabilization ranges, for gas velocity and equivalence ratio, by studying the combustion wave velocity, and optimization of these variables to decrease the formation of pollutants in the combustion of methane. The computer simulation shows the importance of high temperatures in the formation of NO x by Zeldovich's route. The optimum conditions found consider values of the operational variables that provide low temperatures and minimize exposure of the reagents to them.

A comparative study on combustion synthesis of Ta–Si compounds

A comparative study on the preparation of tantalum silicides (including TaSi 2, Ta 5Si 3, Ta 2Si, and Ta 3Si) in the Ta–Si system was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of corresponding stoichiometries. For the powder compacts of Ta:Si = 5:3 and 2:1, upon ignition a planar combustion front traversing the entire sample was easily achieved even without prior heating. In contrast, a preheating temperature of 300 °C was required for the samples of Ta:Si = 3:1 and 1:2 to establish the propagation of a planar reaction front in a self-sustaining manner. It was found that the flame-front propagation velocity and combustion temperature were increased by increasing the preheating temperature and sample compaction density. Among the test samples of different compositions, the reactant compact of Ta:Si = 5:3 exhibited the highest flame speed, followed sequentially by the powder compacts of Ta:Si = 2:1, 3:1, and 1:2. The variation of combustion temperature with starting stoichiometry of the reactant compact was in a manner consistent with that of the flame-front velocity. According to the XRD analysis, a complete conversion yielding single-phase disilicide TaSi 2 was achieved from the sample compact of Ta:Si = 1:2. The silicides phases Ta 5Si 3 and Ta 2Si were obtained in relatively pure form through combustion reactions of the powder compacts with Ta:Si = 5:3 and 2:1, respectively. However, a poor degree of phase transformation was observed in the case of Ta:Si = 3:1, which produced a multiphase product composed mostly of Ta 2Si and Ta 5Si 3, along with small amounts of Ta 3Si and elemental Ta. In addition, based upon the temperature dependence of combustion wave velocity established in this study, the activation energies associated with combustion synthesis of Ta 5Si 3, Ta 2Si, and TaSi 2 were determined to be 105.2, 65.5, and 153.8 kJ/mol, respectively.

Effects of SiC addition on formation of Ti3SiC2 by self-propagating high-temperature synthesis

Formation of Ti 3 SiC 2 was conducted by self-propagating high-temperature synthesis (SHS) from both the elemental powder compact of Ti:Si:C = 3:1:2 and the SiC-containing sample with SiC content ranging from 4.3 to 20 mol%. The effect of SiC addition was studied on combustion characteristics and the degree of phase conversion. In addition, two forms of the elemental carbon, carbon black and graphite, were employed in the starting mixture and their influence was also explored. The synthesis reaction of the powder compact is characterized by a distinct combustion front propagating along the sample in a self-sustaining manner. Due to the significantly higher thermal conductivity of graphite, lower reaction temperatures and combustion wave velocities were observed in the samples with graphite than those using carbon black. For both types of the samples, the addition of SiC reduced the reaction temperature and thus decreased the flame-front propagation velocity. As a result of the increase of reaction time, the degree of phase conversion was enhanced by the addition of SiC and by the use of graphite as the carbon source. For all as-synthesized products, the XRD analysis identifies formation of Ti 3 SiC 2 along with a second phase TiC and a small amount of Ti 5 Si 3 . The resulting Ti 3 SiC 2 is typically elongated grains with a closely packed pattern. Based upon the XRD profile, the products with Ti 3 SiC 2 contents of 71.5 and 82.5 vol.% were obtained from the elemental powder compacts with carbon black and graphite, respectively. For both types of the reactant compacts containing 20 mol% of SiC, an optimum yield about 85 vol.% of Ti 3 SiC 2 was achieved.

Influence of an electric field on combustion synthesis process and microstructures of TiC–Al 2O 3–Al composites

In order to achieve combustion synthesis (CS) of high-density and fine-structured TiC–Al 2O 3–Al composites, the activation of combustion reactions of 3TiO 2–3C–(4 + x) Al system with x ≥ 10 mol was conducted by means of external electric field. During the field-activated combustion synthesis (FACS), liquid Al in excess stoichiometric amount was generated and it infiltrated into the synthesized TiC–Al 2O 3 ceramic matrix with porosities, which directly results in a dense TiC–Al 2O 3–Al composites. The influence of electric field on CS process of the reactive system and the resulting microstructures of TiC–Al 2O 3–Al composites was investigated. The results show that an external electric field can effectively improve the adiabatic combustion temperature of the reactive system. Therefore a self-sustaining combustion of the system can be induced thermodynamically. In the experiments, both practical combustion temperature and propagation velocity of combustion wave were obviously enhanced with increasing field strength, whereas sizes of the synthesized TiC and Al 2O 3 particles were significantly decreased at the same time. Under an electric field of 25 V cm −1, TiC–Al 2O 3–Al composites with a relative density of 92.5% was successfully fabricated through the combustion reaction in the system with x = 14 mol, in which TiC and Al 2O 3 particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al.

Low‐gravity combustion synthesis: Theoretical analysis of experimental evidences

AbstractThe results obtained during a parabolic flight campaign sponsored by ESA on March 2002, and related to the combustion synthesis of TiB2 − xTiAl and TiB2 − xTiAl3 composites are reported. Besides classical SHS experiments performed using cylindrical pellets, combustion front quenching experiments using conical samples placed inside a copper block are also conducted. Similar experiments are carried out under terrestrial conditions, for the sake of comparison. As expected, under both low‐gravity and terrestrial conditions, it is found that combustion temperature and front propagation velocity decrease as the system exothermicity is reduced, that is, when the aluminide/diboride molar ratio is augmented. However, it is observed that reaction front propagates relatively slower under low‐gravity conditions. Consistently, the extinction of the combustion front occurs earlier when the reaction is performed under reduced gravity conditions. A theoretical analysis performed by means of appropriate dimensionless numbers is proposed to provide possible explanations of the main experimental evidences reported in the literature on this subject, including those shown in this work. Specifically, this analysis reveals that the relatively higher‐propagation velocity observed under terrestrial conditions may be due to the correspondingly lower‐sample porosity change, which in turn limits the thermal conductivity decreasing in the reaction zone. In addition, free convection phenomena taking place in the molten phase formed at the reaction zone are able to justify the difference in combustion wave velocity experimentally observed under terrestrial conditions between top and bottom ignition configurations. Moreover, it is found that finer microstructure typically observed under low‐gravity is likely a consequence of the reduced coalescence phenomena taking place in such conditions. © 2006 American Institute of Chemical Engineers AIChE J, 2006

An experimental investigation on combustion synthesis of transition metal silicides V 5Si 3, Nb 5Si 3, and Ta 5Si 3

Preparation of transition metal silicides V 5Si 3, Nb 5Si 3, and Ta 5Si 3 was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts at their stoichiometries. The propagation mode of self-sustaining reactions, flame-front velocity, combustion temperature, and phase composition of end products were extensively studied. For the reactant compacts from the mixtures of V:Si = 5:3 and Ta:Si = 5:3, a planar flame front propagating in a steady manner was observed, with or without sample preheating. On the other hand, a preheating temperature of 200 °C or above was required by the Nb–Si powder compact to initiate the self-sustaining combustion. Moreover, it was found that the planar reaction font was not stable in the samples of Nb:Si = 5:3 and was transformed into a localized reaction zone propagating in a spinning mode. For all three types of the samples, as the sample density and preheating temperature increased, the combustion temperature was found to increase and the flame-front velocity was correspondingly enhanced. According to the XRD analysis, formation of single-phase silicides V 5Si 3 and Nb 5Si 3 from the reactant compacts of 5V + 3Si and 5Nb + 3Si, respectively, was confirmed. The samples of 5Ta + 3Si yielded predominantly the silicide Ta 5Si 3, along with an intermediate phase Ta 2Si in a small amount. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of V 5Si 3, Nb 5Si 3, and Ta 5Si 3 were determined to be 111.5, 259.2, and 105.2 kJ/mol, respectively.

Combustion synthesis of MoSi 2 and MoSi 2–Mo 5Si 3 composites

An experimental study on the preparation of molybdenum disilicide (MoSi 2) and MoSi 2–Mo 5Si 3 composites was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of different stoichiometries. Test specimens with six compositions including Mo:Si = 6:5, 1:1, 9:11, 3:4, 2:3, and 1:2 (corresponding to the Si content in the range of 45.5–66.7 at.%) were employed in this study. Experimental evidence shows that the propagation mode of the reaction front, combustion wave velocity, and reaction temperature are significantly influenced by starting stoichiometry and preheating temperature ( T p) of the reactant compact. In the cases of preheating at 200 and 300 °C, self-sustaining combustion featuring a spinning reaction zone was observed for the compacts of Mo:Si = 6:5, 1:1, and 9:11, and the reaction temperatures lower than the melting point of Si (1410 °C) were detected. On the other hand, a planar combustion front propagating in a steady manner was achieved in the samples with Mo:Si = 3:4, 2:3, and 1:2 at T p = 200 and 300 °C, and the reaction temperatures exceeding 1410 °C were found. However, the planar reaction font was no longer stable in the case of without preheating or at T p = 100 °C, and therefore was transformed into a localized reaction zone propagating in a spinning mode. According to the XRD analysis, formation of MoSi 2–Mo 5Si 3 composites from the reactant compacts of Mo:Si = 6:5, 1:1, 9:11, 3:4, and 2:3 was confirmed, but a significant amount of elemental Mo left unreacted was found in the cases of Mo:Si = 6:5 and 1:1. The unreacted Mo was considerably decreased by increasing the Si content in the reactant compact. For the sample of Mo:Si = 2:3, the residual Mo containing in the synthesized composite was in a small quantity. In addition, a single-phase product composed of the disilicide MoSi 2 was yielded from the sample of Mo:Si = 1:2.

Characteristic analysis of low-velocity gas filtration combustion in an inert packed bed

This paper investigates the low-velocity filtration combustion of lean methane–air mixtures occurring in inert packed beds by using a modified one-temperature model, considering the axial thermal diffusion owing to the convective gas–solid heat transfer. Based on the scaling analysis of various transport terms in different conservation equations, a high-activation energy asymptotic method is applied in the flame zone and results in a set of powerful analytical solutions for combustion macrocharacteristics under the fully developed conditions. These are then combined with the eigenvalue method of the modified one-temperature model in the whole flow region to study the flame behaviour analytically and numerically. Our results have shown that the combustion wave velocity is a key characteristic parameter in the filtration combustion process. Compared with other existing theoretical results, the present analytical solutions demonstrate the intricate relationships among the combustion wave velocity, the flame speed, the peak flame temperature and the effects of the variable thermo-physical properties, and show better prediction performance for the combustion wave velocity, the flame speed and the peak flame temperature. Excellent agreements with experimental results have been observed, especially for very lean filtration combustion with stream-wise propagating combustion fronts.

Filtration combustion of hydrogen-air, propane-air, and methane-air mixtures in inert porous media

The filtration combustion characteristics of hydrogen-air, propane-air, and methane-air mixtures in inert porous media have been studied experimentally. It is shown that the dependences of the combustion wave velocity on the fuel-air equivalence ratio are V-shaped. For hydrogen-air mixtures, the velocity minimum is shifted to the rich region, and for propane-air and methane-air mixtures, it is shifted to the lean region. For lean hydrogen-air and rich propane-air mixtures, the measured maximum temperatures in the combustion wave are found to be reduced relative to those calculated theoretically. For methane-air mixtures, a reduction in the measured temperatures is observed over the entire range of the mixture composition. The results are interpreted within the framework of the hypothesis of selective diffusion of gas mixture components.

Analytical modelling of filtration combustion in inert porous media

A study is made of the self-sustaining combustion waves during the filtration of lean methane–air mixtures in inert porous media using the two-temperature approximation. Such processes are characterized by an intense thermal interaction between the gas and the porous material. Due to interfacial heat transfer, the solid phase is able to redistribute heat absorbed from reaction products to the unburned mixture. The analytical solution is built in three different regions: the pre-heating region, the reaction region and the region occupied by the combustion products. Analytic expressions predicting the temperature and methane mass fraction profiles in the wave, as well as the combustion wave velocity and the longitudinal extension of the reaction region are derived. The results are confirmed by numerical calculations using the finite difference method and a full set of basic equations.

A comparative study on combustion synthesis of Nb–Si compounds

A comparative study on the preparation of niobium silicides (such as Nb 3Si, Nb 5Si 3, and NbSi 2) in the Nb–Si system was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of different stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Test specimens with five different compositions including Nb:Si = 3:1, 5:3, 3:2, 1:1, and 1:2 were employed in this study. Experimental observations indicated that except for the sample of Nb:Si = 3:1, upon ignition self-sustained combustion was well established and proceeded throughout the entire sample. Measured results showed that the reactant compact of Nb:Si = 5:3 had the highest flame-front propagation velocity, followed sequentially by the powder compacts with Nb:Si = 3:2, 1:1, and 1:2. Variation of the combustion temperature with sample initial stoichiometry is in a manner consistent with that of the flame-front velocity. On account of the close atomic ratios, the test specimens made up of either Nb:Si = 5:3 or 3:2 yielded a single-phase silicide Nb 5Si 3 in both α and β forms. As identified by the XRD analysis, it was found that α-Nb 5Si 3 was dominant in the final product from the Nb:Si = 5:3 compact, but β-Nb 5Si 3 was the major composition formed by the Nb:Si = 3:2 compact. Because of the nonexistence of a silicide phase with the atomic ratio Nb/Si = 1, a multiphase product consisting of α-Nb 5Si 3 and NbSi 2 was synthesized from the reactant compact of Nb:Si = 1:1. For the samples with Nb:Si = 1:2, the disilicide NbSi 2 was monolithically produced along with trivial amounts of unreacted Nb and Si. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of Nb 5Si 3 and NbSi 2 were determined to be 259.2 and 160.9 kJ/mol, respectively.

A comparative study on combustion synthesis of Nb–B compounds

A comparative study on the preparation of specific niobium borides (including Nb 3B 2, NbB, Nb 5B 6, Nb 3B 4, and NbB 2) in the Nb–B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of their corresponding stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Experimental evidence indicated that except for the samples of Nb:B = 3:2, upon ignition a planar and self-sustained combustion front was established and proceeded throughout the entire sample in a steady manner. However, combustion of the samples with Nb:B = 3:2 was characterized by a localized reaction zone propagating along a spiral trajectory, due largely to the low combustion temperatures which further resulted in a poor degree of phase conversion with a significant amount of Nb left unreacted. The incomplete reaction in the Nb:B = 3:2 powder compact produced boride compounds Nb 3B 4 and NbB in minor quantities. Reactant compacts of Nb:B = 1:1 and 1:2 were shown to yield practically single-phase monoboride NbB and diboride NbB 2, respectively. In contrast, multiphase compounds consisting of Nb 3B 4, NbB, and NbB 2 were synthesized from the powder compacts with starting stoichiometries Nb:B = 3:4 and 5:6. However, it was found that two boride phases Nb 3B 2 and Nb 5B 6 did not appear in the end products from any of the reactant compacts. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of NbB and NbB 2 were determined to be 151.8 and 132.4 kJ/mol, respectively.

Preparation of niobium borides NbB and NbB 2 by self-propagating combustion synthesis

Preparation of niobium borides NbB and NbB 2 was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of the sample green density, preheating temperature and starting stoichiometry on combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained reaction zone propagating along a spiral trajectory for the reactant compacts without prior heating or preheated at 100 °C. The increase of initial sample temperature to 200 and 300 °C by prior heating brings about a planar flame-front propagating in a steady mode. As the preheating temperature or sample green density increased, the combustion temperature was found to increase and the propagation rate of combustion wave was correspondingly enhanced. According to the temperature dependence of combustion wave velocity, the activation energies associated with the Nb + B and Nb + 2B reactions were determined to be 151.8 and 132.4 kJ/mol, respectively. As indicated by the XRD analysis, the final composition of burned products was essentially governed by the starting stoichiometry of reactant compacts. Synthesized products composed of a single boride phase NbB and a small amount of unreacted Nb were obtained from the reactant compacts of Nb:B = 1:1. In addition, the SHS reaction of powder compacts with an initial composition Nb:B = 1:2 yielded niobium diboride NbB 2 as the dominant phase, along with another boride phase Nb 3B 4 in a minor quantity.

Effects of Silica on the Combustion Synthesis and Glass Formation of TiB2‐Containing Calcium Aluminate Matrix Composites

Combustion synthesis has been used to study the combustion characteristics and microstructure in the TiB2–CaO–Al2O3–SiO2 system. Both the combustion temperature and wave velocity decreased with an increase in the amount of silica. The reacted product consisted of solid crystalline TiB2 phase and CaO–Al2O3–SiO2 matrix. No significant reaction between the TiB2 and the matrix was observed. It was found that the binary calcium aluminate matrix (Ca12Al7O33 and CaAl2O4 compounds) could form amorphous phase (glass) at a high cooling rate. Under a normal cooling rate, significant formation of glass in the matrix was found possible only by adding another glass‐former, e.g., silica (SiO2). The addition of SiO2 led to the formation of Gehelenite, to the reduction of the other crystal phases and to an increase in the glass phase in the matrix. A pure glass matrix was obtained for all compositions in the composition range from Ca12Al7O33 to CaAl2O4 compounds provided that an adequate amount of SiO2 was added. The resulting glass matrix can be expressed by the formula Ca2(SixAl1−x)3O, where x=0.32–0.58, depending on the composition. Effects of the TiB2 phase on the combustion synthesis process and on the microstructure are also discussed. TiB2 increased the reactivity and led to the formation of the molten matrix which was critical for glass formation. However, its effects on the vitrification of the matrix was considered to be minimum. The mechanism of glass formation in the matrix is discussed using the Zachariasen–Warren network theory.

Ranges of Rates of Supersonic Combustion in a Porous Medium

The results of investigations of the propagation of supersonic combustion in a tube completely or partially filled with a porous medium are presented. The relationship between the combustion-wave velocity, the medium parameters, and the properties of the combustible mixture has been established.

Effect of random internal structure on combustion of binary powder mixtures

Combustion of a mixture of two solid reactants is considered. A combined geometrical and physicochemical model of mixing of these reactants is proposed. The model takes into account random distribution of reactants and voids over the system. This allows description of incomplete burning of reactants. The model is used for studying the combustion wave propagation in a binary heterogeneous mixture. The obtained results are compared with those calculated from the perfect-mixing model, which implies that the deficient (with respect to the stoichiometry) component burns completely. It is shown that micrononuniformity of mixing not only reduces the combustion wave temperature and velocity but also can lead to a shift of the maximal wave velocity away from the stoichiometric composition of the mixture. The results of this study allow us to suggest that one of the main reasons for such a shift, which was observed in a number of experimental works, is structural disorder of binary mixtures.