A. G. Shmakov

Destruction Chemistry of Dimethyl Methylphosphonate in H2/O2/Ar FlameStudied by Molecular Beam Mass Spectrometry

Molecular beam mass-spectrometry was used to study the structure of a premixed H2/O2/Ar (0.26/0.13/0.61) flame with and without additives (0.1–1.1% DMMP) stabilized on a flat-flame burner at 43–80 Torr(burner temperature 95°C). The behavior of DMMP in the flame has been studied. Mass spectra of samples taken from flames, and intensity profiles of peaks 18 (H20), 32(O2),40(Ar),47(PO), 63(PO2), 647(HOPO), 80,94, 110, 124(DMMP) amu have been measured as a function of the distance from the burner surface to the sampling probe using a quadrupole mass-spectrometer and electron impact ionization at 12.1–21.6 eV with a spread of electron energy ±0.25 eV. Intensity profiles of masses 110,94,80 pass through a maximum. This shows that the species responsible for these masses are intermediates. PO(CH3)(OCH)(OH), P2(OCH3) are possible intermediates. The profile of the temperature in the flame has been determined by using a Pt-PtRh(1O%) thermocouple covered by Ceramobond 569. The effect of promotion on the H2/O2//Ar fl...

Mass spectrometric study of combustion and thermal decomposition of GAP

Abstract Glycidyl azide polymer (GAP) is an active energetic binder in rocket propellants. The main objective of this research was to study the decomposition and combustion chemistry of thoroughly characterized GAP samples to develop a model for the combustion of GAP and propellants based on GAP. The combustion characteristics (burning rates, temperature profiles) and kinetic parameters (order of reaction, activation energy, pre-exponential factor of rate constants) for the thermal decomposition of GAP together with the composition of the products of both the combustion and decomposition of uncrosslinked GAP (with a molecular weight of 350 or 2000) and cured GAP were studied. The flame and thermal decomposition of GAP, as well as the composition of the products were studied using molecular-beam mass-spectrometry (MBMS). The final temperature of a flame of GAP was measured as 1000 to 1100 K. About half of the mass of the combustion products involves large fragments of a polymer without its azide groups. For this reason the mass spectrum obtained on direct MBMS sampling of a flame burning GAP could not be completely interpreted. However, ∼47% of the mass of the combustion products was found to be the volatile gases N 2 , H 2 , CO, CO 2 , CH 4 , C 2 H 4 , C 2 H 6 , NH 3 , H 2 O, acetonitrile, acrylonitrile, and furane, as obtained by mass-spectrometry using freezing/thawing in a liquid nitrogen trap. The thermal decomposition of thin films of GAP at 1 bar was done in a flow reactor with Ar flowing through it. A tungsten plate was used as a sample heater; its temperature was controlled using a chromel-copel (copel is an alloy of 56.5% Cu, 43.0% Ni and 0.5% Mn) or Pt-PtRh (10%) thermocouples. The thermal decomposition of GAP was studied at a high heating rate over a wide temperature range in three ways: (1) the heating rate was changed from the maximal to the minimal one in the course of decomposition (400–100 K/s); (2) at the linear heating rate (50–400 K/s); (3) fast heating (∼400 K/s) to the given temperature and subsequently maintained isothermal. Three stages of thermal decomposition were found. The first stage (yield of nitrogen is ∼15%) is a first order reaction. The second stage (yield of N 2 is ∼25%) is an autocatalytic one; the third stage is first order and is a weakly exothermic one, with a yield of nitrogen of ∼60%. Kinetic parameters (activation energy and pre-exponential factor of rate constants) were found for each stage. The results for both the combustion and thermal decomposition of GAP were compared with literature data and it was concluded that the results strongly depend on the conditions of the experiment and on the source of the GAP.

Kinetics and mechanism of chemical reactions in the H2/O2/N2 flame at atmospheric pressure

The kinetics and mechanism of chemical reactions in the H2/O2/N2 flame were studied experimentally and by simulating the structure of premixed laminar flat atmospheric H2/O2/N2 flames of different initial compositions. The concentration profiles for stable compounds (H2, O2, and H2O), H atoms, and OH• radicals in flames were measured by molecular-beam sampling mass spectrometry using soft electron-impact ionization. The experimental data thus obtained are in good agreement with the results of simulations in terms of three familiar kinetic mechanisms, suggesting that these mechanisms are applicable to the description of the flame structure in hydrogen-oxygen mixtures at atmospheric pressure.

Mass spectrometric study of combustion of GAP- and ADN-based propellants

Abstract The flame structure of composite propellants and sandwiches based on ammonium dinitramide (ADN) and glycidyl azide polymer at 0.015 to 0.3 MPa was studied by molecular beam mass spectrometry. A zone near the surface, ∼1.5 mm wide, was detected, where reactions occur. The gas composition near the surface of burning ADN laminae at 0.1 MPa was close to that near the surface of burning pure ADN at 0.3 MPa. Among the species responsible for reactions in the flame near the surface, the most probable are HNO3, dinitraminic acid, and the vapor of ADN. The luminous zone of the flame extends more than 10 mm from the surface. The composition of the final combustion products has been determined by freezing at the temperature of liquid nitrogen and indicates incomplete combustion. The temperature profiles measured with thin thermocouples confirm the measured widths of the near-surface and luminous zones. The final temperature at the pressure of 0.3 MPa is as high as 2600 K.

Mechanism of inhibition of hydrogen/oxygen flames of various compositions by trimethyl phosphate

The effect of the catalytic recombination reactions of H and OH− involving phosphorus-containing products of trimethyl phosphate (TMP) combustion on the burning velocity and the structure of H2/O2/N2 flames at atmospheric pressure has been investigated. An earlier mechanism for inhibition of rich hydrogen/oxygen flames by organophosphorus compounds has been tested and modified by comparing experimental data with the results of simulation. The sensitivity analysis of the calculated flame speed to the rate constants of chain branching reactions and chain termination reactions involving phosphorus-containing compounds has revealed significant specific features of the inhibition mechanism of hydrogen flames with various stoichiometries and dilution ratios. Unlike the inhibition efficiency of hydrocarbon flames, in which the reactions of H and OH− radicals with PO, PO2, HOPO, and HOPO2 play the key role, the inhibition efficiency of hydrogen flames at atmospheric pressure is determined by the interaction of hydrogen and oxygen atoms with TMP and with organophosphorus products of its decomposition in the low-temperature zone of the flame. The sensitivity analysis has demonstrated that, as the equivalence ratio (ϕ) or the dilution ratio is increased, the ratio of the chain branching rate to the rate of chain termination via reactions involving phosphorus compounds decreases. As a consequence, the efficiency of inhibition of H2/O2/N2 flames, as distinct from that of hydrocarbon flames, increases as ϕ is raised from 1.1 to 3.0 and as the mixture is progressively diluted with nitrogen.

Combustion Chemistry of Energetic Materials Studied by Probing Mass Spectrometry

The methods of probing mass spectrometry (PMS) for diagnostic of flames and for the study of kinetics and mechanism of the thermal decomposition products of energetic materials (EM) are described. Several types of instruments based on microprobe and molecular beam mass spectrometric sampling have been developed. Time of flight mass spectrometer has been used. Apparatuses for high (10 atm) and low (

Promoting effect of halogen- and phosphorus-containing flame retardants on the autoignition of a methane–oxygen mixture

This paper presents a numerical and experimental study of the effect of flame-retardant additives on the autoignition of methane behind shock waves. It is shown that at a temperature of 1300–1900 K, the compounds CCl4, CF3H, and (CH3O)3PO not only do not suppress ignition but significantly reduce the induction time of methane–oxygen mixtures. A kinetic mechanism is proposed which relates the promoting effect to the reactivity of the pyrolysis products of the additives.

Spatial and temporal resolution of the particle image velocimetry technique in flame speed measurements

Limitations of the spatial and temporal resolution of the particle image velocimetry (PIV) technique in velocity field measurements in a laminar flame have been investigated. The limitations are due to the need to introduce a suspension of tracer particles into the flow. For a methane-air mixture with a stoichiometry coefficient of 0.9, it is determined that at a mass fraction of TiO2 solid particles over 0.08%, the change in the flame propagation velocity by the particles exceeds 5%. The maximum spatial resolution of PIV for which the influence of the particles is insignificant corresponds to a concentration of 0.03%; in this case, the minimum resolvable scale is limited by a value 200 times larger than the size of tracer particles. Based on analytical estimates and a comparison of measured and numerically calculated particle velocities in the flame, it is concluded that particles smaller than 2 µm adequately track the flow velocity. Under these conditions, the error of the velocity measurement is mainly determined by the limited spatial resolution of PIV. The results of the work can be used to evaluate PIV measurement errors in other experimental studies of flames.

Terahertz free-electron laser radiation to determine water concentration in flames

The possibility of measuring the concentration of H2O molecules in flames based on the absorption of terahertz free-electron laser radiation was studied. These measurements were performed using the 177.32 cm−1 absorption line in the rotational spectrum of H2O. This line has a low intensity at room temperature, and at about 1000 K, its intensity is comparable to that of the strongest lines. The temperature dependence of the radiation absorption coefficient at a frequency of 77.32 cm−1 was studied theoretically and experimentally. It is shown that the method can be used for measurements in a sooty C2H4/O2/Ar flame, which strongly scatters visible and UV radiation.

Autoignition mechanism of dimethyl ether–air mixtures in the presence of atomic iron

The search for reactive additives capable of reducing the combustibility of dimethyl ether is an important problem due to the widening use of ether as an alternative environmentally friendly motor fuel. This paper presents a numerical study of the autoignition chemistry of mixtures of dimethyl ether with air in the presence of atomic iron. Atomic iron, which is an effective inhibitor of premixed laminar hydrocarbon flames, was found to shorten the induction period. However, the additive affects only the first stage of the induction period. The mechanism of promotion of the low-temperature oxidation of dimethyl ether–air mixtures by atomic iron is the formation of hydroxyls in reactions involving iron compounds. Since the additive hardly changes the duration of the second stage of the induction period, it can be suggested that OH radicals play an insignificant role in the low-temperature oxidation of dimethyl ether at this stage.