Molecular-beam mass-spectrometric study of the flame structure of composite propellants based on nitramines and glycidyl azide polymer at a pressure of 1 MPa

A study was performed of the chemical and thermal structure of flames of model composite propellants based on cyclic nitramines (RDX and HMX) and an active binder (glycidyl azide polymer) at a pressure of 1 MPa. Propellant burning rates were measured. The chemical structure of the flame was studied using molecular-beam mass spectrometry, which previously has not been employed at high pressures. Eleven species (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂, NO₂, and nitramine vapor) were identified, and their concentration profiles, including the composition near the burning surface were measured. Two chemical-reaction zones were observed. It was shown that flames of nitramine/glycidyl azide polymer propellants are dominated by the same reactions as in flames of pure nitramines. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 48–57, November–December, 2006.     

Experimental and numerical study of CH4/CH3Cl/O2/N2 premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH₃Cl addition on CH₄/O₂/N₂ premixed flames under oxygen enrichment. The laminar flame speeds of CH₄/CH₃Cl/O₂/N₂ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH₃Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH₃Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH₃Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H₂ → H + H₂O reaction is reduced because of the reduction of OH concentration. However, the function of CH₃Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 103–111, November–December, 2006.     

HMX flame structure for combustion in air at a pressure of 1 atm

The chemical structure of HMX flame during combustion in air at a pressure of 1 atm was calculated using molecular beam mass spectrometric sampling. HMX vapor was recorded for the first time near the burning surface. A total of 11 species were identified in the HMX flame (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂, NO₂, and HMX vapor), and their concentration profiles were measured. The HMX combustion was unstable. The species concentration profiles exhibit periodic pulsations related to variation in the HMX burning rate. The HMX flame structure at various distances to the burning surface was determined using the average value of the burning rate. Two main zones of chemical reactions in the flame were found. In the first zone ≈0.8 mm wide adjacent to the burning surface, HMX vapor decomposes and NO₂, N₂O, and CH₂O react with each other to form HCN and NO. In the second zone ≈0.8–1.5 mm wide, HCN was oxidized by nitric oxide to form the final combustion products. The composition of the final combustion products was analyzed. The global reaction of HMX gasification at a pressure of 1 atm was established. Heat release values in the condensed phase calculated by the global gasification reaction and by the equation of heat balance on the burning surface (using literature data from microthermocouple measurements) were analyzed and compared. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 26–43, November–December, 2008.     

Experimental study of the structure of laminar premixed flames of ethanol/methane/oxygen/argon

The structures of three laminar premixed stoichiometric flames at low pressure (6.7 kPa): a pure methane flame, a pure ethanol flame, and a methane flame doped by 30% of ethanol, have been investigated and compared. The results consist of mole fraction profiles of CH₄, C₂H₅OH, O₂, Ar, CO, CO₂, H₂O, H₂, C₂H₆, C₂H₄, C₂H₂, C₃H₈, C₃H₆, CH₃-C CH (propyne), CH₂ C CH₂ (allene), CH₂O, and CH₃HCO, measured as a function of the height above the burner by probe sampling followed by on-line gas chromatography analyses. Flame temperature profiles have been also obtained by using a PtRh thermocouple. The similarities and differences between the three flames have been analyzed. The results show that, in these three flames, the mole fraction of the intermediates with two carbon atoms is much larger than that of the species with three carbon atoms. In general, the mole fraction of all intermediate species in the pure ethanol flame is the largest, followed by the doped flame, and finally the pure methane flame.     

RDX flame structure at atmospheric pressure

The chemical structure of an RDX flame at a pressure of 1 atm was studied using probing molecular beam mass spectrometry. The flame was found to contain RDX vapor, and its concentration profile was measured in a narrow zone adjacent to the burning surface. In addition to RDX vapor, ten more species were identified (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂ and NO₂), and their concentration profiles were measured. Two main chemical-reaction zones were found in the RDX flame. In the first, narrow, zone 0.15 mm wide adjacent to the burning surface, decomposition of RDX vapor and the reaction of NO₂, N₂O, and CH₂O with the formation of HCN and NO occur. In the second, wide, zone 0.85 mm wide, HCN is oxidized by NO to form the final combustion products. The composition of the final combustion products was analyzed from an energetic point of view. The measured composition of the products near the burning surface was used to determine the global reaction of RDX gasification at a pressure of 1 atm. Values of heat release in the condensed-phase calculated by the global gasification reaction and by the equation of heat balance on the burning surface (using data of microthermocouple measurements) were analyzed and compared. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 49–62, January–February, 2008.     

Surface modification of polypropylene film using N2O-containing flames

Contact-angle measurements and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize polypropylene (PP) films that were exposed to laminar premixed air: natural gas flames containing small quantities of nitrous oxide. During combustion, the nitrous oxide generates gas-phase nitrogen oxides that lead to the affixation of nitrogen-containing functional groups to the PP surfaces. Treatment of PP in nitrous oxide-containing flames also leads to an increase in surface oxidation and markedly improves wettability when compared with standard flame treatments. The chemical form of the nitrogen affixed to the PP surface is strongly dependent on the flame equivalence ratio. Fuel-lean flames tend to affix highly oxidized forms of nitrogen such as nitrate and nitro groups, while fuel-rich flames tend to affix less-oxidized nitrogen groups such as nitroso, oxime, amide, and amine. A computational model, SPIN, was used to elucidate the chemistry of the flame as it impinges upon the cooled PP surface. The SPIN modeling indicates that the principal reactive gas-phase species at or near the PP surface are O₂, OH, H, NO, NO₂, HNO, and N₂O. A number of possible reactions between these species and the PP can account for the formation of the various nitrogen functional groups observed.     

Numerical simulation of the effect of the addition of NO and NO<Subscript>2</Subscript> on a rich hydrogen flame using the tracer method

The effect of the addition of nitric oxides (NO and NO₂) on rich hydrogen-air flames was studied using the tracer method in numerical simulation. It is shown that the effects of these additives are not similar. Both oxides promote the formation of OH and H₂O in the low-temperature zone of the front. The addition of NO reduces the first maximum of the OH profile and the burning velocity. The addition of NO₂ increases the first maximum of the OH profile and does not change the burning velocity. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 19–25, May–June, 2009.     

Comparative study of the influence of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O on the chemical structure of lean and rich methane-air flames at atmospheric pressure

A comparative study of the influence of CO₂ and H₂O on both lean and rich CH₄-air laminar flames is performed. Six premixed flames are stabilized on a flat flame burner at atmospheric pressure: lean (with the equivalence ratio maintained constant at ? = 0.7) and rich (with the equivalence ratio maintained constant at ? = 1.4) CH₄-air, CH₄-CO₂-air, and CH₄-H₂O-air flames. These flames are studied experimentally and numerically. The [CO₂]/[CH₄] and [H₂O]/[CH₄] ratios are kept equal to 0.4 for both flames series. Species mole fraction profiles are measured by gas chromatography and Fourier transform infrared spectroscopy analyses of gas samples withdrawn along the vertical axis by a quartz microprobe. Flames structures are computed by using the ChemkinII/Premix code. Four detailed combustion mechanisms are used to calculate the laminar flame velocities and species mole fraction profiles: GRI-Mech 3.0, Dagaut, UCSD, and GDFkin®3.0.     

Study of the CL-20 flame structure using probing molecular beam mass spectrometry

The CL-20 flame structure at pressures of 0.1 and 0.5 MPa was studied using probing molecular beam mass spectrometry. At a pressure of 0.1 MPa, the gaseous products near the surface of the foamy carbonaceous carcass were found to contain eight species: NO, NO₂, CO, CO₂, N₂, H₂O, HCN, and HNCO. At a pressure of 0.5 MPa, NO₂ was not found and the concentration profiles of the remaining seven species were measured versus the distance to the carbonaceous carcass. In the CL-20 flame at a pressure of 0.5 MPa, consumption of N₂ and CO₂ with the formation of NO and HNCO occurred in a zone of width ≈0.8 mm from the surface of the carbonaceous carcass. At a pressure of 0.5 MPa, the composition of the gaseous combustion products of CL-20 at a distance of 3–4 mm differed from the composition at thermodynamic equilibrium. Key words: flame structure, CL-20, probing molecular beam mass spectrometry. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 58–65, May–June, 2009.     

Investigation of the structure of a CH4/N2-O2/N2 Counterflow diffusion flame using molecular beam and microprobe mass spectrometry

The possibility of using molecular beam mass spectrometry (MBMS) to study the structure of counterflow flames was shown by the example of a CH₄/N₂-O₂/N₂ flame. The thermal structure of the flame was studied, and the CH₄, O₂, and CO₂ concentration distributions were measured using a microprobe technique and MBMS. The results of the measurements performed by the two methods were compared. The MBMS technique was used to study the hydroxyl concentration distribution in the flame. The species concentrations and temperature profiles on the burner axis were calculated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 26–33, July–August, 2006.     

Nitrite in soils: accumulation and role in the formation of gaseous N compounds

Nitrite is an intermediary compound formed during nitrification as well as denitrifiication. It occasionally accumulates in soils and drainage water. The nitrite can then undergo transformations to gaseous nitrogen compounds such as NO and NO₂. Soil pH controls the abiotic nitrite decomposition to a large extent. Under acidic conditions(pH <5.5), nitrous acid spontaneously decomposes preferentially to NO and NO₂. Nitrite also undergoes reactions with metallic cations (especially ferrous iron) and with organic matter. As a result of these reactions gaseous compounds such as NO, NO₂, N₂O and CH₃ONO can be formed. Through reaction of nitrite with phenolic compounds nitroand nitrosocompounds can be formed, building up organic N. With normal agricultural practices on slightly acidic soils, the nitrite instability usually does not lead to economically important N losses from soils. However, the compounds formed through its degradation or interaction with other soil constituents are linked to environmental problems such as tropospheric ozone formation, acid rain, the greenhouse effect and the destruction of the stratospheric ozone.     

On the role of C<Subscript>2</Subscript>O radicals in the prompt-NO mechanism

The measurements of NO concentrations in the post-flame zone of different hydrocarbon + O₂ + N₂ flames at standard temperature and atmospheric pressure available in the literature are compared with predictions of the original Konnov reaction mechanism and with the same mechanism extended by the reaction of C₂O with N₂. The goal was to investigate the possible role of this reaction proposed by Williams and Fleming [Proc. Combust. Inst., Vol. 31 (2007), pp. 1109–1117]. This new reaction of C₂O with N₂ seems to be a reasonable explanation of the deficiencies in the prompt-NO route. Direct comparisons of the experimental measurements performed in different flames with the modeling strongly suggests that the upper limit of this reaction rate constant is k = 7 · 10¹¹ exp(−17,000/RT) [cm³/(mole · sec)]. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 3–7, September–October, 2008.     

NO reduction by CH4over La2O3: temperature‐programmed reaction and in situ DRIFTS studies

The chemistry between NO _x species adsorbed on La₂O₃ and CH₄ was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH₄ in the presence of O₂, NO ₃ ^- does not appear to activate CH₄, thus either an adsorbed O species or an NO ₂ ^- species is more likely to activate CH₄. In the absence of O₂, a different reaction pathway occurs and NO^- or (N₂O₂)^2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH₄ unidentate NO ₃ ^- , which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO₂ or H₂O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N₂ and N₂O from NO was observed at much lower temperature during the TPR experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H₂ as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H₂ and CH₄, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H₂/50%CO and 45%H₂/45%CO/10%CH₄ by volume, and three diluents, N₂, H₂O, and CO₂. The effectiveness of these diluents is characterized in terms of their ability to reduce NO_x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO_x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H₂/50%CO mixture, it is due to the prompt mechanism for the 45%H₂/45%CO/10%CH₄ mixture. For both mixtures, CO₂ and H₂O are more effective than N₂ in reducing NO_x in syngas flames. H₂O is the most effective diluent on a mass basis, while CO₂ is more effective than N₂. The effectiveness of H₂O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.     

Chemical processes in the HNF flame

Results of modeling the HNF flame structure are presented. From an analysis of literature data on the thermal decomposition and combustion of HNF, it is concluded that the dissociative vaporization of HNF proceeds via the route HNF_liq → (N₂H₄)_g + (HC(NO₂)₃)_g. The flame structure is modeled using a detailed kinetic mechanism consisting of 47 species and 283 elementary reactions. Its constituents are the decomposition mechanisms of hydrazine (N₂H₄)_g and trinitromethane (HC(NO₂)₃)_g (nitroform, NF_g). The latter come from the burning surface by dissociative vaporization. The modeling was performed for different routes of NF_g decomposition involving HC(NO₂)₂, HCNO₂, and HC(O)NO₂ radicals. The HNF flame structure was calculated for pressures of 0.4, 1, and 5 atm using data on the product composition on the burning surface that correspond to the developed reaction in the condensed phase and are consistent with the chemical composition and enthalpy of formation of HNF. As follows from the calculations, the heat release in the gas-phase reaction of nitroform with hydrazine (and partially with ammonia) leads to a temperature increase in the flame zone adjacent to the burning surface from its value on the surface to ≈1300 K. A further increase in the flame temperature is related to the reaction in the H₂O/N₂/N₂O/NH₃/NO/NO₂/HNO₂/CO/CO₂/HCNO/HCN mixture. The calculation results are compared with experimental data on the thermal and chemical structure of the HNF flame. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 20–31, September–October, 2006.     

Influence of organophosphorus inhibitors on the structure of atmospheric lean and rich methane-oxygen flames

The inhibition of atmospheric laminar methane-oxygen flames of various compositions by trimethyl phosphate was studied experimentally and by numerical modeling using mechanisms based on detailed kinetics. The H and OH concentration profiles in flames with and without the addition of trimethyl phosphate were measured and calculated. It was shown that the addition of the inhibitor reduced the maximum (in the reaction zone) concentrations of H and OH in lean and rich flames. The concentration reduction was higher in rich flames than in lean flames. The concentration profiles of the phosphorus-containing products PO, PO₂, HOPO, HOPO₂, and (HO)₃PO in lean and rich flames stabilized on a flat burner were measured and calculated. Tests of the previously developed model of flame inhibition by phosphorus compounds showed that the model provides adequate predictions of many experimental results. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 23–31, March–April, 2007.     

Numerical analysis of combustion kinetics for hydrogen—air mixtures with NH3, CH4, and C2H6 additives behind shock waves

The possibility of reducing the concentration of nitrogen oxides and HNO_x-group components with simultaneous shortening of the reaction-zone length during combustion of hydrogen-air mixtures in a supersonic flow behind an oblique shock wave by introducing NH₃, CH₄, and C₂H₆ additives into the mixture is analyzed. A numerical study shows that a small (up to 5%) amount of these additives substantially changes the combustion kinetics behind the shock-wave front, shortens the flame length, and diminishes the NO and NO₂ content in the combustion products. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 3, pp. 31–38, May–June, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 96-02-18377).     

Effect of the addition of triphenylphosphine oxide,hexabromocyclododecane, and ethyl bromide on a CH<Subscript>4</Subscript>/O<Subscript>2</Subscript>/N<Subscript>2</Subscript> flame at atmospheric pressure

The chemical and thermal structure of a Mache-Hebra burner stabilized premixed rich CH₄/O₂/N₂ flame with additives of vapors of triphenylphosphine oxide [(C₆H₅)₃PO], hexabromocyclododecane (C₁₂H₁₈Br₆), and ethyl bromide (C₂H₅Br) was studied experimentally using molecular beam mass spectrometry (MBMS) and a microthermocouple method. The concentration profiles of stable and active species, including atoms and free radicals, and flame temperature pro.les were determined at a pressure of 1 atm. A comparison of the experimental and modeling results on the flame structure shows that MBMS is a suitable method for studying the structure of flames stabilized on a Mache-Hebra burner under near-adiabatic conditions. The relative flame inhibition effectiveness of the added compounds is estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the flame propagation velocity. The results of the investigation suggest that place of action of the examined flame retardants is the gas phase. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 12–20, September–October, 2007.     

Promotion and inhibition of a hydrogen—oxygen flame by the addition of trimethyl phosphate

This paper gives results of numerical modeling of a laminar hydrogen—oxygen flame doped with trimethyl phosphate at various pressures and compositions of the combustible mixture. The calculations were performed using the PREMIX and CHEMKIN-II software packages. It was found that phosphorus-containing additives promoted the flame at subatmospheric pressures and inhibited it at atmospheric pressure. Kinetic analysis showed that catalytic recombination reactions were responsible for both phenomena. In the case of subatmospheric pressures, the promoting effect and its enhancement with increasing additive concentration were related to a flame temperature rise in the chemical-reaction zone due to catalysis of the recombination reactions by phosphorus-containing compounds. Increasing the additive concentration led to an increase in both the rate of the branching reaction H + O₂ = OH + O and the rate of the chain termination reaction, but the increase in branching reaction rate prevails, resulting in an increase in the flame velocity. In the case of atmospheric-pressure flames, where the reaction-zone temperature is close to the adiabatic equilibrium value, the additive led to an increase in the rate of decay of active flame species and, hence, to a decrease in the flame propagation velocity with increasing additive concentration. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 3–13, September–October, 2006.