Secondary Flame Research Articles

STABILITY AND COMBUSTION/EMISSION CHARACTERISTICS OF STRATIFIED DUAL-SWIRL OXY-METHANE FLAMES: EFFECTS OF OXYGEN FRACTION

Abstract The stability, combustion, and emission features of stratified oxy-methane (CH4/O2/CO2) flames stabilized over a dual annular counter-rotating swirl (DACRS) burner, developed for gas turbine combustion applications, were investigated experimentally. The experiments were performed at fixed velocity ratio (Vr=Vp/VS=3.0) in both the primary and secondary streams at a constant primary stream velocity, Vp of 5 m/s and at fixed primary stream equivalence ratio, φP=0.9, and over ranges of oxygen fractions (OFP for the primary stream, OFS for the secondary stream) and secondary stream equivalence ratios. Measurements of flame macrostructure, temperature profiles, and exhaust emissions were recorded to characterize the flames and validate future numerical models. The testing findings revealed no flame flashback within the operational ranges of OFP and OFS and up to φs = 1.0. However, the near stoichiometric operation of the primary stream (φp = 0.9) at OFp =0.38, permitted the main secondary flame to tolerate exceptionally lean conditions (φs = 0.397 at OFs =0.34 and φs = 0.223 at OFs =0.39), raising the thresholds for flame blowout. Increasing OFP from 0.21 to 0.38 significantly reduced φS at blowout from 0.537 to 0.223, corresponding to a decrease in the combustor's global equivalence ratio (φg) at blowout from 0.554 to 0.254 at global oxygen fraction (OFg) from 0.38 to 0.39. Lower OFp values caused earlier flame lift-off, indicating the greater influence of OFp on flame macrostructures.

Forced response of transverse reacting jet in vitiated crossflow to harmonic velocity disturbances

The orthogonally oriented injector configuration typically used in axial fuel staging-capable gas turbine combustors is known to produce complicated thermoacoustic interactions between two distinct reaction zones. Despite the fundamental importance of transfer functions of transverse reacting jets in vitiated crossflow, however, their distinctive properties remain unknown, and associated low-order modeling combined with two different flame transfer functions remains entirely undemonstrated. From detailed measurements of transfer functions of lean-premixed primary and secondary flames in response to harmonic velocity disturbances, here we show that the transfer function of a transverse reacting jet in high-temperature crossflow is described as having relatively larger gain without undulating patterns, near-linear slow decay in magnitude with respect to the forcing frequency, and remarkably shorter duration response time. By leveraging a reduced-order modeling approach to account for the finite time delay between the two distinct transfer functions and by validating the calculation results against velocity and pressure measurement data, we demonstrate that self-induced instabilities in an axial fuel-staged system can be simultaneously driven by the dynamics of primary and secondary flames, and that the triggering of second-stage-induced instability is characteristically connected to higher-order acoustic modes. Such non-axisymmetric intra-combustor interactions generate a cascade of spectral peaks, including the first and third longitudinal modes and their respective higher harmonics, and additional peaks at the sum and difference of two individual frequencies. While the primary flame's dynamics are capable of selectively exciting the first mode, the secondary jet flame exhibits more complicated modal dynamics, manifested as the L3 mode-coupled jet merging-related flame surface annihilation and the L1 mode-coupled lateral movements of the transverse jet column.

Stability and combustion characteristics of dual annular counter-rotating swirl oxy-methane flames: Effects of equivalence and velocity ratios

An experimental study was carried out to investigate flow/flame interactions, stability, combustion, and emissions properties of CH4/O2/CO2 stratified flames stabilized on a dual annular counter-rotating swirl (DACRS) burner for gas turbine combustion applications. The effects of two targeted parameters, namely equivalence ratios (for primary - ϕp, secondary - ϕs, and global - ϕg streams) and velocity ratio (Vr = Vp/Vs: primary to secondary stream inlet velocity ratio), are studied at fixed volumetric oxygen fraction (OF) in the oxidizer mixture (O2 + CO2) of both primary and secondary streams of 34 % at fixed Vp of 5 m/s. The experimental results indicated that no flame flashback was recorded in the domain of any operational ϕp up to stoichiometric operation of the secondary stream (ϕs = 1.0). At near stoichiometric operation of the primary stream, the main secondary flame can persist in extremely lean conditions (ϕs = 0.45 @ Vr = 3), thereby extending the thresholds of flame blowout. Moreover, raising ϕp from 0.4 to 1.0 results in significant reduction of ϕs at blowout from 0.595 to 0.456, corresponding to reducing the combustor ϕg at blowout from 0.577 to 0.499. At lower ϕp, the oxy-methane flame tends to lift and extinguish earlier.

In-situ two-dimensional temperature measurements using x-ray fluorescence spectroscopy in laminar flames with high silica particle concentrations

X-ray fluorescence spectroscopy (XRF) was used to measure temperatures and study mixing, for the first time in silica particle synthesis flames. Hexamethyldisiloxane (HMDSO) and trimethylsilanol (TMSO) were the particle precursors. A multi-element diffusion burner was used to produce a flat methane flame, and the precursors, dilute in inert gas, were injected via a central jet. Krypton was the fluorescent medium at 3.2 % concentration by volume. Scans with Kr in the central flow and not in the main flow were made to assess the mixing effects between the central and main gas flows. When HMDSO or TMSO were added, a secondary diffusion flame formed between the jet and the main methane flame. The results revealed a dramatic change in the centerline temperature profile of the jet gases when HMDSO or TMSO were added. The main methane flame stoichiometry also affected the temperature profiles. The results show HMDSO and TMSO reactions are initiated in a low-temperature and low-oxygen concentration region of the jet where thermal decomposition is not expected to be significant. Reaction of the particle precursors is therefore attributed to radical transport from the main methane flame. In the current work, particle number densities of up to 270 g/m3 locally are estimated. Thus, the study also demonstrates the capability of the XRF technique for high spatial fidelity measurements in flames with high concentrations of condensed-phase particles, leveraging the attribute that the XRF signal is generally not impacted by condensed-phase interferences. The observations and data obtained in this study inform likely reaction pathways for this important class of siloxane compounds.

Flame-retardant wire burning behavior by jet flame heating: Ignition, charring, and secondary flame spread

This research presents an in-depth examination of the combustion characteristics of flame-retardant electrical wires, contrasting behavior with non-flame-retardant counterparts under jet flame heating. The study systematically investigates the pyrolysis process, ignition patterns, charring behavior, and the development of secondary flames in wires with different core diameters and insulation materials. Findings indicate the heightened susceptibility of thinner wires to rapid heating, pyrolysis and charring, leading to faster ignition and more intense secondary flame development. This insight is crucial for tailoring flame-retardant formulations to specific wire dimensions. The research also delves into the thermal dynamics within the wires, highlighting how the core diameter influences axial heat conduction and, consequently, the overall flame spread behavior and pyrolysis rate. A critical discovery is the relationship between heating duration and flame sustainability, establishing a specific range of heating times for sustaining secondary flames in flame-retardant wires. Theoretical models used in the study explained the critical heat flux heating time for wire ignition, offering insights into improved fire prevention strategies, particularly in prolonged heat exposure scenarios. These findings not only advance our understanding of flame-retardant wire behavior under fire conditions but also provide guidance for selecting and using electrical wires, thereby optimizing fire safety in diverse applications.

On the intricacies of soot volume fraction measurements in counterflow diffusion flames with light extinction: Effects of curtain flow

Counterflow diffusion flame is an attractive platform for fundamental research on kinetics of soot formation. Accurate determination of soot volume fraction in the flame is a prerequisite for in-depth analysis of the sooting characteristics and assessment of predictive soot models. Light extinction has been proven to be an efficient technique for measuring soot volume fraction thanks to its non-intrusiveness and its simple optical setup. Nevertheless, tomographic inversion needs to be performed if spatially resolved soot volume fraction is to be obtained from the measured light extinction data which is essentially a projection along the line-of-sight. In this regard, radial distribution of soot volume fraction would affect the accuracy of the measurement through its influences on the inversion processes. In this work, we show that the curtain flow, which is necessary to avoid the formation of the undesired secondary diffusion flame and to keep the core counterflow from ambient disturbances, has notable effects on spatially resolved soot volume fraction measurements with line-of-sight measurements. In particular, different flow rate settings of the curtain flow can result in different soot distributions at the edges of soot fields: upwards curved, outwards extended, and downwards curved, which may influence the measurement of centerline soot volume fraction distribution. The necessity of tomographic inversion, the minimal region of the projection image necessary for tomographic inversion (when necessary), the quasi-one-dimensional feature of soot distribution, and the sensitivity of measurement to slight flame asymmetry were investigated where possible to determine the most suitable curtain flow configuration for soot volume fraction measurements by light extinction. Recommendations on curtain flow setting are finally made.

Mitigating thermoacoustic instabilities in premixed hydrogen flames using axial staging

In this paper we demonstrate how axial staging can be utilised to mitigate thermoacoustic instabilities in a combustion system by flame redistribution. This is achieved by moving flow to a second stage flame while keeping the combined thermal power and total flow fixed. The stability of the system was first characterised for premixed H2-CH4-air flames operated with hydrogen volume fraction in the range 80%–100%. As observed in recent work, increasing the hydrogen concentration led to the onset of strong self-excited instabilities. Increasing the flow velocity for a fixed hydrogen concentration was shown to have the same effect. Both of these are linked to a decrease in the convective time scale — causing the flame to become more convectively compact and responsive to higher frequencies. We then show that these instabilities could be reduced for all conditions, by distributing flow to the secondary flame. Two main driving mechanisms causing suppression were observed and analysed. Firstly, a reduction in the flow rate provided to the first stage flame, leading to a corresponding change in the flame structure. This leads to an increase in the convective time scale. This effect is quantified by a reduced gain at higher frequencies, and a change in the time delay of the flame transfer function. Secondly, introducing a second flame with a different convective time scale results in interference between the two flames. This acts to suppress or enhance the global fluctuations in heat release rate and hence, the amplitude of the pressure oscillations in the system.

Effects of the secondary air on the combustion characteristics of turbulent premixed CH4/NH3/air flames in a two-stage swirl combustor

This study experimentally investigates the effects of secondary air injection on the flame structure and CO/NO emission characteristics of turbulent premixed CH4/NH3/air flames in a two-stage swirl combustor. The equivalence ratio (ϕp) and velocity (Up) of the primary fuel/air jet and the injection location (Ls) and velocity (Us) of the secondary air jet are varied. The location of the secondary air injection of 30 and 250 mm are adopted to investigate two interaction cases in two-stage combustion (TSC): 1) a strong interaction where the secondary air is directly injected into the primary premixed flame and strongly affects the flame, and 2) a weak interaction where the secondary air is injected after the primary reaction is completed. A single-stage combustion (SSC) is also tested for comparison. It is found from averaged OH-PLIF images that the strong interaction cases of TSC with ϕp > 1 can produce a significant amount of NO emissions due to the direct interaction between the primary flame and the secondary air, while the weak interaction cases can produce an appreciable amount of CO emissions due to the incomplete combustion of the secondary flames. This result is further confirmed by CO/NO measurements that the strong interaction cases produce higher NO emissions than the weak interaction cases, while the CO emissions of the weak interaction cases become significant and peak at ϕp = 1.1 ∼ 1.2. Based on the OH-PLIF images and CO/NO emission characteristics of TSC, the optimal conditions of the primary and secondary jets are proposed to ensure the reduction of NOx emissions with low level of CO emissions.

Optimization effect of propylene oxide (PO) on evaporation, combustion, and pollutant emissions of high-energy–density JP-10 fuel

JP-10 is a high-energy–density fuel that is widely used in aerospace propulsion systems due to its wide range of synthetic raw materials and low production costs. However, it faces challenges related to difficult ignition, low combustion efficiency and high pollutant emissions. To address this, blending JP-10 with propylene oxide (PO), a highly volatile substance, provides a practical solution. This study investigates the propagation and evolution characteristics of JP-10/PO binary fuel in large horizontal pipelines. The results demonstrate a synergistic enhancement effect achieved by the JP-10/PO hybrid fuel. PO, with its lower boiling point, quickly evaporates from the mixed droplets, creating a continuous primary flame. As the droplet temperature increases, JP-10, known for its high combustion heat, starts to evaporate, actively participating in the explosion process and forming a secondary flame that spreads in two directions. The combustion of the JP-10/PO mixed droplets involves both physical and chemical processes, including droplet evaporation and gaseous combustion. A binary droplet evaporation model is developed to quantitatively assess the impact of PO fuel ratio on spray evaporation. Additionally, the study examines steam explosion parameters and reaction product characteristics under different PO fuel ratios. It is observed that the evaporation of JP-10/PO mixed droplets occurs in three stages: transient heating, binary equilibrium evaporation, and single component equilibrium evaporation. Increasing the PO fuel ratio leads to a higher temperature rise rate of the droplets and an increased evaporation reaction rate constant. Moreover, the ignition delay time of JP-10/PO gas-phase mixture fuel decreases, while the adiabatic flame temperature gradually reduces. Furthermore, higher PO fuel ratios help reduce the emissions of unstable products in both mixed fuels due to the inherent oxygen content of PO molecules. The current research results can provide scientific references for the improvement of high-energy density fuels and the development of new propellants.

Effect of Deflector on the Combustion Characteristics of a Micro-Combustor With a Controlled Centrally Slotted Bluff Body

Abstract Flame tip-opening in a micro-combustor with a controlled centrally slotted bluff body adversely affects the combustion characteristics, leading to reduced average combustion efficiency and exhaust gas temperature. To minimize the adverse effects of the flame tip-opening, a deflector is introduced in the micro-combustor, downstream to the bluff body, and its effect on various combustion parameters is studied. The insertion of a deflector significantly increases the exhaust gas temperatures in the central region by establishing a secondary flame root. However, sudden changes in the flow direction caused by the insertion of deflector induce a sudden expansion-compression strain on the flame front, thereby slightly reducing the temperature of the flame zones on either side of the central region. A downstream shift in the position of the deflector marginally mitigates the adverse effects of sudden expansion-compression strain on the exhaust gas temperature, as they are induced within the secondary reaction flame zones. On the other hand, the downstream shift of the deflector negatively impacts the exhaust gas temperature in the central region due to the reduced length available for near-complete combustion downstream of the secondary flame root. In conclusion, the deflector positioned farther from the outlet is found to result in better overall combustion characteristics at higher controllable flow ratios.

The Structure and Stability of Premixed CH4, H2, and NH3/H2 Flames in an Axially Staged Can Combustor

Abstract An experimental investigation of flame structure, stability, and emissions performance was conducted in a two-stage lab-scale generic combustor design operated with CH4, H2, and NH3/H2 fuel blends. The main flame zone features a premixed bluff body stabilized flame, with a secondary flame zone initiated downstream by injecting premixed air and fuel using two opposing radial jets. The total power and air flowrate are kept constant between the different fueling cases, while the air split between stages and equivalence ratios are varied to explore conditions relevant to gas turbine operation. Given the relative novelty of the configuration, special emphasis is given to analyzing the structure of the opposing jet flames in the secondary stage. In contrast to previous literature on reacting jets in cross flow, these interact significantly due to their proximity, leading to a merged flame zone at the impingement location in the center of the combustion chamber, and some flame propagation upstream of the jet location. As the jet-to-crossflow momentum ratio increases, the merged flame zone changes shape, reaching close to the walls for the methane cases but remaining very compact when operating with almost pure hydrogen. For the hydrogen flames, diverting more air to the second stage allows higher total thermal power conditions to be reached, while avoiding flashback, and eliminates thermoacoustic instabilities. For ammonia-hydrogen flames, air is diverted to the second stage, while a constant fuel flow is sent to the primary stage, resulting in some locally rich conditions in the primary flame. A local minima in terms of NOX occurs when the primary flame is operated at an equivalence ratio of 1.15. Analysis of the flame structure suggests that this state corresponds to almost complete combustion or pyrolysis of NH3 in the main flame, with the remaining hydrogen burned in an inverse diffusion flame in the secondary zone.

Research on the effects of coal dust parameters on the venting characteristics of gas/dust two-phase explosions

Based on the correlation analysis of venting flame propagation and pressure rise both sides of vessel, the dynamics venting characteristics of gas/dust explosions and influence of dust parameters were studied. Venting parameter appeared a variation process of firstly rising and then reducing with increasing coal dust concentration (q). As q=300 g/m3, venting parameters were the greatest. But it was gradually increased as dust diameter reduced. Venting flame could be affected by dust parameters. As the flame extinguished, an evident oscillation phenomenon emerged and its oscillation amplitude change was similar to venting parameter. A secondary venting flame appeared after extinguishing and got more dramatically with increasing dust diameter. Besides, the pressure at the venting port exhibited an obvious negative value and its appearance moment was corresponding to flame oscillation. And the temperature contour showed the same morphology as flame image. Temperature distribution was related to explosion intensity and affected by dust parameters.

Structure and NOx Emissions of Stratified Hydrogen-Air Flames Stabilized on a Coaxial Injector

Abstract In recent years, the need for low-carbon power has seen hydrogen emerge as a potential fuel to replace conventional hydrocarbons in combustion to limit CO2 emissions in several sectors, including aeronautics. The challenges posed by hydrogen combustion are similar to the issues of kerosene flames but more challenging, like nitrogen oxide (NOx) emissions and flame flashback. One potential solution to address these problems is to burn a rich mixture of hydrogen and air in globally lean conditions on a coaxial injector to obtain a stable and staged combustion and attempt to reduce emissions. In this article, the evolution of NOx production as more air is mixed into the fuel is studied, as well as the changes in flame size and structure. In particular, the appearance of a secondary flame front is observed and increasing the proportion of air in the fuel mixture both shortens the flame and reduces the NOx emission index. Additionally, the effect of the global equivalence ratio and flame thermal power is studied. Finally, existing models for NOx emission of hydrogen flames on a coaxial injector based on average flame residence time and strain rate are tested and shown to have promising results.

A numerical study on the structure and dynamics of fuel-rich premixed H2–air jet flames above a divergent micro-nozzle

In this study, the flame structures of a fuel-rich premixed H2–air mixture issued from a divergent micronozzle were numerically studied and compared with their counterparts. The flame dynamics of the divergent micronozzle at relatively low jet velocities were also analyzed. A flame pattern of “dual flames with a smaller one surrounded by a larger one” was discovered for both the straight and divergent micro-nozzles at high jet velocities. This flame structure began to appear at a lower jet velocity (Vin) as the divergence angle (θ) decreased. Specifically, it first appeared at Vin = 7.5 and 20 m/s for θ = 0° and 5°, respectively. In addition, a flame mode consisting of dual flames with a smaller one inside the nozzle occurred only for divergent micronozzles under low and medium jet velocities. This flame structure could emerge at a higher jet velocity as the divergence angle increased. For example, it could occur until Vin = 5 m/s and 17.5 m/s for θ = 1° and 5°, respectively. A global map of all flame patterns was drawn based on the results obtained by varying the jet velocity and divergence angle. Moreover, periodic flame dynamics appeared when the jet velocity was less than the critical value (<1.7 m/s). Specifically, a small flame (i.e., secondary flame) split from the jet flame (i.e., main flame) and propagated towards the nozzle inlet; however, its propagation direction was inverted and it finally extinguished owing to the heat loss to the nozzle wall. The next cycle was initiated after a short duration. The analysis revealed that the periodic flame dynamics are a result of the thermal interactions between the flame and nozzle wall.

Numerical investigation of the flame suppression mechanism of porous muzzle brake

An excellent flame suppression effect can be achieved using a novel porous brake. To understand the flame-suppression mechanism of a porous brake, combustion using a muzzle brake is investigated. A set of internal ballistic equations is employed to provide accurate velocity and pressure for a projectile moving to the muzzle. The multispecies transport Navier–Stokes equations, which incorporate complex chemical reactions, are solved by coupling a real gas equation of state, the Soave–Redlich–Kwong model, and a detailed chemical reaction kinetic model. The development of muzzle flow with a chemical reaction is simulated, and the interaction between chemical reactions with the muzzle flow field is numerically calculated to explain the muzzle combustion mechanism with a porous brake. The underlying mechanism is analyzed in detail. The results demonstrate that, first, the gas is fully expanded in the brake, leading to a reduction in pressure and temperature at the muzzle, thereby reducing the initial flame. In addition, the shock wave weakens due to the expansion and separation process, leading to a reduction in the mixture of gas and air, ultimately resulting in a reduction in the intermediate and secondary flames.

Characteristics of dust explosion venting pressure and flame under high activation pressure

A 20 L spherical explosive device with a venting diameter of 110 mm was used to study the vented pressure and flame propagation characteristics of corn dust explosion with an activation pressure of 0.78–2.1 bar and a dust concentration of 400∼900 g/m3. And the formation and prevention of secondary vented flame are analyzed and discussed. The results show that the maximum reduced explosion overpressure increases with the activation pressure, and the vented flame length and propagation speed increase first and then decrease with time. The pressure and flame venting process models are established, and the region where the secondary flame occurs is predicted. Whether there is pressure accompanying or not in the venting process, the flame venting process is divided into two stages: overpressure venting and normal pressure venting. In the overpressure venting stage, the flame shape gradually changes from under-expanded jet flame to turbulent jet flame. In the normal pressure venting stage, the flame form is a turbulent combustion flame, and a secondary flame occurs under certain conditions. The bleed flames within the test range are divided into three regions and four types according to the shape of the flame and whether there is a secondary flame. The analysis found that when the activation pressure is 0.78 bar and the dust concentration is less than 500 g/m3, there will be no secondary flame. Therefore, to prevent secondary flames, it is necessary to reduce the activation pressure and dust concentration. When the dust concentration is greater than 600 g/m3, the critical dust concentration of the secondary flame gradually increases with the increase of the activation pressure. Therefore, when the dust concentration is not controllable, a higher activation pressure can be selected based on comprehensive consideration of the activation pressure and destruction pressure of the device to prevent the occurrence of the secondary flame.

Mode shape-dependent thermoacoustic interactions between a lean-premixed primary flame and an axially-staged transverse reacting jet

Axial fuel staging enables elevation of the turbine inlet temperature in advanced heavy-duty gas turbine combustors to ∼2000 K, ultimately to achieve more than 65% combined cycle efficiency, while limiting excessive nitrogen oxide production. While previous investigations have identified transverse jet-flow-dependent nitrogen oxides emission trends and the topological properties of reacting shear layers in vitiated crossflow environments, there exists a critical gap in the understanding of axial-fuel-staging-induced combustion instabilities. The present study therefore investigates the link between first- and second-stage flame dynamics in a lean-premixed combustor. We demonstrate that the second-stage jet-in-vitiated-crossflow flame is preferentially coupled to higher order acoustic modes than the upstream primary flame with the same characteristic nozzle dimension. The first- and second-stage-driven acoustic modes (two different timescales) are mutually incompatible, and the ensuing selective excitation is critically dependent on the position of the secondary injector relative to the mode shape. When the second-stage flame is located near a pressure node, or a velocity antinode, the transverse jet-stabilized flame is influenced by the primary flame-driven crossflow velocity oscillations, and exhibits conspicuous flapping motions in the crossflow direction accompanied by periodic partial extinction at the windward and leeward-side flame elements. If the second-stage flame is situated near a pressure antinode, on the other hand, the secondary jet flame's dynamics are governed by a combination of jet merging-induced flame surface destruction and the growth of non-axisymmetric coherent structures. In this case, high-intensity pressure oscillations are sustained by the local heat release fluctuations generated solely from the transverse reacting jets, whereas the upstream primary flame remains nearly unperturbed, or decoupled from the underlying feedback processes. This experimental observation is remarkable, and reflects previously unknown properties originating from the complex interactions between the two axially-staged lean-premixed flames.

Burning rate catalysts action on the trinitroresorcinol combustion wave parameters

AbstractThe effect of burning rate catalyst—3% nickel salicylate (NS) with 1% carbon nanotubes (CNT)—on the combustion wave parameters of trinitroresorcinol (TNR) at 2 MPa as well as on the structure and elemental composition of the carbon frame on the surface of quenched TNR samples at 2 and 15 MPa were studied. The catalyst itself increases the burning rate by ∼4.3 times. It is shown that for the sample with NS and CNT, the accumulation of nickel particles formed during the decomposition of the catalyst occurred (∼110 times) at 2 MPa on the carbon frame. The degree of catalyst particles accumulation at 15 MPa is ∼60 times lower than at 2 MPa, so the burning rate increases by only 1.3 times. At 2 MPa the catalyst significantly (by ∼68 K) increases the combustion surface temperature, by ∼2.7 times increases the temperature gradient in the carbon frame zone, and reduces the length of the primary (fizz zone) and secondary flames. As a result, the heat release rate on the frame is 13.5 times higher than on the carbon frame of the sample without a catalyst. For TNR samples the thermal conductivity coefficient, based on the characteristics of the framework obtained, was calculated, and it was shown that the thermal conductivity coefficient of the carbon frame for a sample with NS and CNT is significantly (∼8 times) higher than for a sample without a catalyst. From the calculation of the heat balance of the c‐phase at 2 MPa it follows that the combustion leading zone of the sample with a catalyst is the carbon frame, from which ∼98% of the necessary for combustion propagation heat enters the c‐phase; for a sample without a catalyst, the heat gain from the gas zone is ∼20%; the leading zone is the reaction layer of the condensed phase. Thus, the mechanism of combustion catalysis of aromatic nitro compounds is the same as for double‐base propellants. Two conditions are necessary for combustion catalysis: the formation of a carbon frame on the combustion surface, on which catalyst particles accumulate, and the carbon frame thermal conductivity coefficient must be significantly higher than that of the gas zone above combustion surface of the sample without a catalyst.

Combustion of suspensions of mixed aluminum and silicon carbide in the products of hydrocarbon flames

A stabilized methane/air Bunsen flame is seeded with powder fuel mixtures containing various combinations of spherical, micron-sized aluminum (Valimet H-2 with nominal d50 = 3.5 µm and H-15 with nominal d50 = 20 µm) and silicon carbide (SiC) at different mass ratios over a ∼0-400 g/m3 concentration range. It is observed that in dispersions of only H-2 aluminum, a bright white aluminum flame front forms and couples to the methane-air flame, resulting in a sustained flame speed even at concentrations beyond 400 g/m3. In contrast, dispersions of only H-15 powder decrease flame speed rapidly and cause an open-tipped methane-air flame at concentrations beyond 200 g/m3, similar to inert SiC powder. When blended together into H-2:H-15 1:1 (mass ratio) mixture dispersions, an aluminum flame front forms and couples to the methane-air flame, with a flame speed comparable to unitary H-2 aluminum in contrast to mixtures of H-2:SiC 3:1 which produce no noticeable difference in flame speed from purely inert mixtures of SiC at nearly 200 g/m3. Mixtures of H-2:SiC 3:1 demonstrate an aluminum flame coupling to the methane flame, but with an increased separation between the fronts that was not observed in previous hybrid flames studies. The flame temperatures, flame coupling, and aluminum combustion efficiency behaviors are attributed to the effective amount of slowly reacting or inert solid material in the mixed powder fuels. The behaviors are consistent with a simple hybrid flames model, developed in previous work, where the effectively reduced heat of reaction of the powder fuel is unable to support sufficient heat feedback to the methane-air flame permitting effective secondary flame front formation and flame-coupling. From this understanding, a method for determining the relative energy contribution of a lower reactivity component in a fuel mixture when it is thermally driven by a higher reactivity fuel is demonstrated using the secondary front formation and flame coupling as a benchmark for reaching a certain heat of reaction of the fuel solid mixture. It is estimated that the H-15 component of a H-2:H-15 1:2 mixture contributes to approximately 55% of the thermal energy required to achieve the same behavior as a H-2:SiC 3:1 fuel mixture.

ЛАЗЕРНОЕ ЗАЖИГАНИЕ ПИРОТЕХНИЧЕСКИХ СОСТАВОВ – ПОРИСТЫЙ КРЕМНИЙ - ФТОРПОЛИМЕР СКФ-32 - МНОГОСЛОЙНЫЙ ГРАФЕН

The article presents the results of studies on laser ignition of films of pyrotechnic compositions based on porous silicon, fluorine-containing polymer and graphene. The effect of graphene additives on the ignition process and the process of film combustion was analyzed. It was shown that in a number of cases the process of film combustion is accompanied by the appearance of a secondary flame zone and smoke formation.