Andrey G. Shmakov

Molecular-Beam Mass-Spectrometry to Ammonium Dinitramide Combustion Chemistry Studies

The methods for the study of flame structure and kinetics of the thermal decomposition of solid propellant by probing mass spectrometry are described. The developed methods were applied to the study of ammonium dinitramide (ADN) combustion chemistry. The study has shown that along with ADN decomposition, sublimation takes place to give gaseous ADN followed by dissociation to yielding ammonia and dinitraminic acid (HD). Gaseous ADN has been observed in ADN decomposition products. The structure of ADN combustion zones at 1-6 atm was studied using a molecular-beam mass-spectrometry as well as a microthermocouple technique. Three combustion zones have been observed. Gaseous ADN has been discovered in the first cool flame zone at 3 atm. Gaseous ADN dissociation on NH3 and HD followed by HD decomposition in the near-surface zone are key reactions resulting in a temperature rise of about 150 K. The second high-temperature zone is found within 6-8 mm from the ADN burning surface at 6 atm. The main reaction in this zone is ammonia oxidation by nitric acid and the combustion temperature is 1400 K. The third zone was observed at 40 atm, the measured final temperature was —2000 K. The obtained data form the basis for the development of a chemical mechanism of reactions in both the ADN flame and combustion model.

An Experimental and Kinetic Modeling Study of Premixed Laminar Flames of Methyl Pentanoate and Methyl Hexanoate

Abstract Detailed chemical structures of stoichiometric and rich premixed laminar flames of methyl pentanoate and methyl hexanoate were investigated over a flat burner at 20 Torr and for methyl pentanoate at 1 atm. Molecular beam mass spectrometry was used with tunable synchrotron vacuum ultraviolet (VUV) photoionization for low pressure flames of both methyl pentanoate and methyl hexanoate, and soft electron-impact ionization was used for atmospheric pressure flames of methyl pentanoate. Mole fraction profiles of stable and intermediate species, as well as temperature profiles, were measured in the flames. A detailed chemical kinetic high temperature reaction mechanism for small alkyl ester oxidation was extended to include combustion of methyl pentanoate and methyl hexanoate, and the resulting model was used to compare computed values with experimentally measured values. Reaction pathways for both fuels were identified, with good agreement between measured and computed species profiles. Implications of these results for future studies of larger alkyl ester fuels are discussed.

Diffusion Combustion of a Hydrogen Microjet at Variations of its Velocity Profile and Orientation of the Nozzle in the Field of Gravitation

ABSTRACT Experimental data on diffusion combustion of round hydrogen microjets with a parabolic and a “top-hat” mean velocity profiles at the nozzle exit at different spatial orientation of the microjets are reported. Most of all, we are interested in the behavior of the so-called «bottleneck flame region» of the jet and its contribution to the diffusion combustion. As is found, in the cases of the jet velocity opposite and orthogonal to the direction of the gravitational force, the main features of combustion are practically the same. Otherwise, at the velocity vector matching the direction of the gravitational force, the combustion characteristics become much different. Combustion in the «bottleneck flame region» is found to be more stable at the parabolic profile while the stability of combustion is reduced at the top-hat velocity distribution at the nozzle exit. Then, the flame detachment occurs in the absence of the «bottleneck flame region» and the microjet combustion is terminated at a much higher velocity. An inversion of the dependence l/d = f(U0) is observed at the transition from the parabolic to the top-hat velocity profile of the jet. The ratio of the «bottleneck flame region» size (l) to the nozzle exit diameter (d) is l/d. The nozzle heating is shown to have a profound effect on the microjet combustion.

The Velocity and Structure of the Flame Front at Spread of Fire Across the Pine Needle Bed Depending on the Wind Velocity

The paper addresses a comprehensive experimental laboratory-scale study of the characteristics and regularities of fire spread across a bed of pine needles of Siberian boreal forests (SBF) and the impact of wind velocity on these regularities. We used such precision physical and physicochemical methods as in situ mass spectrometry, PIV, microthermocouple technique, etc. We measured fire spread rates, spatial gas temperature distribution near the surface and inside the pine needle bed, the pine needle temperature distribution on the bed surface, and gas flow velocity fields before and behind the flame front. Data were obtained on concentration profile of ethanol as the main pine needle pyrolysis product, on the concentration profiles of О2 and СО2, on the angle of slope of the flame sheet, and on the impact of the wind velocity on these characteristics. It was established that as the wind velocity changes in the range of 0.15–0.2 m/s, the regularities and characteristics of fire spread drastically change. PIV measurements have demonstrated high turbulence near the flame front. It was established that at the wind velocity of 0.2 m/s, СО2 concentration grows, while О2 drops inside the pine needle bed before the flame front more significantly than at the wind velocity of 0.1 m/s. This is attributed to the increase of turbulent mass transfer before the flame front. At that, the pyrolysis rate of the pine needles slows down, and concentration of the pyrolysis products inside the bed in the flame front decreases. The data obtained throw light on the physicochemical processes taking place during fire spread across the bed of pine needles.

Development and Validation of Skeletal Mechanism for Flame Inhibition by Trimethylphosphate

On the basis of a multistep kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving 9 phosphorus-containing species. Selection of the crucial steps was performed by analyzing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flame zone. The developed mechanism was validated by comparing the modeling results with the measured and simulated (using the starting initial mechanism) speed of H2/O2, CH4/O2, and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H2/O2/N2 flames with various dilution ratios, CH4/air and syngas/air flames doped with trimethylphosphate. Besides, the developed mechanism was used in 2D CFD modeling of diffusion cup-burner methane/air flame and fairly well predicted the minimal extinguished concentration of trimethylphosphate.

Catalytic effect of submicron TiO2 particles on the methane–air flames speed

A PIV study of a conical premixed methane–air Bunsen flame has shown that the inside of the cone has a complex gas-dynamic structure. In this system, the velocity of the gas flow entering the flame front varies in different parts of the flame cone and the stream tubes are not straight. The Landau–Markstein effect is discussed in the interpretation of the experimental data. A method of processing PIV measurement results is proposed that improves the accuracy of determining the burning velocity and allows a quantitative determination of the catalytic effect of submicron TiO2 particles, which is proportional to the particle surface area. The relative increase in the burning velocity is 2% per each ≈0.01 cm2/cm3 (particle surface/gas volume) of the total specific surface area of the particles. The experimental data are well described by modeling using well-known literature data on the detailed mechanism of chemical reactions and the mechanism of catalytic oxidation of methane with oxygen on metal oxides.