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Combustion, Explosion and Shock Waves

2020 year, number 4

Radiative Fraction and Flame Length of Propane Jet Diffusion Flames in a Crossflow

J.-W. Wang1,2, J. Fang1, J.-F. Guan2, L.-Y. Zhao1, S.-B. Lin1, H. R. Shah1, Y.-M. Zhang1, J.-H. Sun1
1University of Science and Technology of China, Hefei, Anhui, 230026 China
2Tsinghua University, Hefei, Anhui, 230601 China
Keywords: поперечный поток, пропан, тепловое излучение, сажа, диффузионное пламя, crossflow, propane, radiative fraction, soot, turbulent diffusion flame

Abstract >>
Many industrial combustion devices rely on jet flame combustion in the crossflow to achieve mixing and reaction. Previous research offers a limited predictive capability regarding the coupling effects of the crossflow and jet flow on the flame radiative fraction. In this work, a new theoretical equation is derived to relate the radiative fraction to the fuel flow rate and the crossflow velocity. The experimental results show that the flame length increases as the crossflow velocity increases for all considered flames. The results of this work suggest that the stretching factor is 0.08 s. The radiative fraction is almost independent of the nozzle diameter in the case of a low crossflow velocity. The crossflow has the strongest effect on the radiative fraction for a smaller nozzle diameter. This is because of the effect of the crossflow and jet flow velocities on the soot residence time, which is proportional to the radiative fraction.

Laminar Burning Velocity of the Dimethyl Carbonate-Air Mixture Formed by the Li-Ion Electrolyte Solvent

M. Henriksen1, K. Vaagseather1, A. V. Gaathaug1, J. Lundberg1, S. Forseth2, D. Bjerketvedt1
1University of South-Eastern Norway, Porsgrunn, Telemark, Norway
2Norwegian Defence Research Establishment, Oslo, Norway
Keywords: скорость ламинарного пламени, диметилкарбонат, литий-ионный электролит, газовый взрыв, laminar burning velocity, dimethyl carbonate, Li-ion battery electrolyte, gas explosion

Abstract >>
If a Li-ion cell fails and the electrolyte leaks out into air, a flammable premixed gas cloud may be formed. The electrolyte combustion energy is 65 ÷ 70 % of the total energy content of the cell. The main objective of this study is to determine the laminar burning velocity and the Markstein length for dimethyl carbonate and propane in a 20-liter explosion sphere with initial conditions at 100 kPa and 300 K. Five different stretch extrapolation models for the laminar burning velocity give practically the same result. The experimental results agree well with the previously published data and are slightly lower than the theoretical predictions. The laminar burning velocity for dimethyl carbonate is measured close to the saturation point under the initial conditions, which has not been previously reported.

Radiation Extinction of Laminar Diffusion Flame over a Plane Porous Burner in Zero Gravity: Numerical Modeling

E. A. Kuznetsov, A. Yu. Snegirev, E. S. Markus
St. Petersburg Polytechnic University of Peter the Great, St. Petersburg 195251, Russia
Keywords: ламинарное диффузионное пламя, невесомость, тепловое излучение, радиационное погасание, laminar diffusion flame, zero gravity, thermal radiation, radiation extinction

Abstract >>
The dynamics of the formation and extinction of laminar diffusion flames of methane and ethylene formed in an oxidizing medium in zero gravity above the surface of a flat porous burner have been studied numerically. The calculations reproduce the conditions of experiments carried out within the framework of the BRE-Flamenco project of the Advanced Combustion Microgravity Experiment (ACME) program aimed at the study of combustion in zero gravity. A three-dimensional non-stationary model taking into account multistep and multicomponent chemical mechanisms of oxidation of fuels, soot formation and oxidation, and the thermal radiation of combustion products was tested for jet laminar diffusion flame of methane under normal gravity and for ethylene flame inder short-term zero gravity in free fall in the test tower. Calculations for flames formed under long-term zero gravity were performed for the range of fuel consumption typical of combustion of solid and liquid combustible materials. In all cases considered, combustion proceeds in an unsteady mode, despite the constant fuel consumption. The flame growth stage is accompanied by a continuous decrease in temperature in the reaction zone due to heat loss by radiation followed by local extinction and pulsations of the flame and the complete cessation of combustion. The effect of the type and fuel consumption on the duration of the flame and the dynamics of its decay is analyzed. The sensitivity of the calculation results to the chemical mechanism is found. It is established that the proportion of energy emitted by the flame in zero gravity is an order of magnitude higher than that for the flame under normal gravity. Radiant heat loss are shown to cause instability and extinction of the flame.

Ignition Delay of Fluorinated Ethylene Propylene Wire Insulation in a Forced Flow Field in Microgravity

K. Wang1, J. Fang1, J.-W. Wang1, S.-M. Zheng1, J.-F. Guan2, H. R. Shah1, J.-J. Wang1, Y.-M. Zhang1
1University of Science and Technology of China, State Key Laboratory of Fire Science, Hefei, Anhui, P. R. China
2Tsinghua University, Hefei Institute for Public Safety Research, Hefei, Anhui, P. R. China
Keywords: невесомость, зажигание, фтор-этилен-пропиленовая изоляция, взрыв, деформация потока, microgravity, ignition, FEP wire insulation, bursting jet, stretched flow

Abstract >>
Fluorinated ethylene propylene (FEP) wire insulation ignition is investigated in a forced flow field in microgravity and normal gravity with a continuous current. First, FEP insulation melts and decomposes, causing jet bursting in both normal gravity and microgravity. Second, the forced flow and gravity produce minor effects on the core heating and bursting time, while the pyrolysis time increases slightly with increasing air velocity. Third, the positively stretch rates in terms of the velocity gradients are higher in microgravity. Both the forced flow and gravity have significant effects on the induction time, which is dependent on the stretch rate and Damköhler number. The induction time increases with increasing air velocity, and it is higher in microgravity. Finally, the ignition delay time is dominated by the core heating and bursting time, while its bigger value and faster increase in microgravity with increasing air velocity are dominated by the induction time.

Effects of the Particle Size and Agglomeration on the Minimum Explosible Concentration and Flame Propagation Velocity in Dust Clouds

K. Ichinose, T. Mogi, R. Dobashi
The University of Tokyo, Bunkyo-ku, Tokyo, Japan
Keywords: взрыв пыли, размер частиц, агломерация, распространение пламени, dust explosion, particle size, agglomeration, flame propagation

Abstract >>
The flame propagation behavior and the effect of particle agglomeration are examined by changing the size of PMMA particles in laboratory-scale experiments. PMMA particles having a very narrow size distribution are used. We show that the minimum explosible concentration increases as the particle size decreases. On the other hand, the flame propagation velocity also increases as the particle size decreases. Therefore, the minimum explosible concentration and the flame propagation velocity show the opposite dependences on the particle size. It is considered that the minimum explosible concentration is strongly affected by the interparticle distance; meanwhile, the flame propagation velocity strongly depends on the specific surface area. It has to be emphasized that the severity of the explosion can be serious for very fine particles, although the minimum explosible concentration is fairly high.

Effects of Inert Particulate Additives on Ignition and Flame Propagation in Dust/Air Mixtures

K. Huttenbrenner, H. Kern, S. Tomasch, H. Raupenstrauch
Montanuniversitat Leoben, Chair of Thermal Processing Technology, 8700 Leoben, Austria
Keywords: инертные частицы, пылевоздушная смесь, зажигание, пламя, взрывобезопасность, inert particles, dust-air mixture, ignition, flame, explosion safety

Abstract >>
Non-combustible solid materials play a major role as inerting agents in explosion prevention. While the importance and application of these materials are well understood, there is still a lack of information about the actual mechanisms responsible for explosion suppression. Especially, the role of inert materials with a large specific surface area and the influence of the moisture content of inert materials have not been sufficiently investigated. In this work, an experimental and computational study of the effects of inert materials on ignition and flame propagation in lycopodium-air mixtures is undertaken. The influence of a large specific surface area is studied by using Clinoptilolith as an inert material.

Kinetic Simulation of Unsteady Detonation with Thermodynamic Nonequilibrium Effects

C. Lin1,2, K. H. Luo3
1Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082 China
2Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University
3University College London, London WC1E 7JE, United Kingdom
Keywords: нестационарная детонация, термодинамическая неравновесность, unsteady detonation, thermodynamic nonequilibrium

Abstract >>
Thanks to its mesoscopic kinetic nature, the discrete Boltzmann method has a capability of investigating unsteady detonation with essential hydrodynamic and thermodynamic nonequilibrium effects. In this work, an efficient and precise reactive discrete Boltzmann method is employed to investigate the impact of the amplitude and wave length of the initial perturbation, as well as of the chemical heat on the evolution of unsteady detonation with nonequilibrium effects. It is shown that the initial perturbation amplitude only affects the unsteady detonation in the early period, and the detonation becomes self-similar with minor phase differences later on. For small wave lengths, the pressure increases faster with a higher oscillation frequency in the early period, but decreases soon afterwards. With increasing chemical heat release, the pressure and its oscillations increase, and the nonequilibrium effects become more pronounced, but the oscillatory period decreases. When the wave length or chemical heat release is small enough, there is no transverse wave or cellular pattern, and the two-dimensional unsteady detonation reduces to the one-dimensional case.

Calculation of Shock Waves in an Explosion of a Liquid Gas Pressure Reservoir

S. E. Yakush
Institute for Problems in Mechanics, Russian Academy of Science, Moscow, 119526 Russia
Keywords: взрыв расширяющихся паров, ударная волна, быстрый фазовой переход, численное моделирование, explosion of expanding vapors, shock wave, fast phase transition, numerical simulation

Abstract >>
Rapid phase transitions occurring with a sharp increase in a specific volume can be accompanied by explosive gas-dynamic phenomena. A model is presented for calculating shock waves generated in the atmosphere during an explosion of a liquid gas pressure reservoir, based on the assumption of a thermodynamically equilibrium state of a vapor-liquid mixture in which both vapor and liquid have equal velocities and are in a state of saturation at local pressure. The spherically symmetric scattering of a boiling liquid cloud is calculated, pressure profiles under various initial conditions are compared, and the primary shock wave parameters are validated according to the results of available experimental data. Two-dimensional calculations of shock waves during the fracture of a cylindrical tank near the underlying surface at various degrees of filling are presented.

On the Nature of the Synergetic Effect in Flames of Methane- and Formaldehyde-Air Mixtures

V. M. Shvartsberg, V. A. Bunev
Voevodsky Institute of Chemical Kinetics and Combustion, Novosibirsk, 630090 Russia
Keywords: метан, формальдегид, синергизм, пределы распространения пламени, численное моделирование, селективное окисление, methane, formaldehyde, synergism, flammability limits, numerical simulation, selective oxidation

Abstract >>
This paper is devoted to the study of the nature of the synergistic effect in flames of methane- and formaldehyde-air mixtures. Combustion of mixtures of various fuels is of great practical and fundamental interest. It is found that the addition of formaldehyde to a rich methane/air flame at a constant concentration of methane first reduces its propagation speed and then begins to increase it. The synergism mechanism in this case is due to the predominant and complete formaldehyde consumption due to its greater reactivity and its negative impact on the rate of methane consumption. As a result of predominant combustion of one of the fuels, there are two spatially separated heat release zones in the flame. Heat release in the first the zone is mainly due to the oxidation of formaldehyde and formyl radical, and in the second zone to in the recombination of methyl radicals. An analysis of the flame speed sensitivity shows that the key reactions affecting the flame speed are the stages of formation of radicals (mainly hydroxyl) or products that lead to their formation. Reactions that make the main contribution to heat release generally do not affect the flame speed. It is found that the interaction of two fuels CH4 and CH2O in the mixture with air leads to a marked increase in the super-adiabatic temperature effect.

Thermochemical and Energy Characteristics of N-(2,2-Bis(Methoxy-NNO-Azoxy) Ethyl)Nitramine

I. N. Zyuzin, D. B. Lempert, A. V. Nabatova, A. I. Kazakov
Institute for Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, 142432 Russia
Keywords: 1,1-бис(метокси-NNO-азокси)-3-нитро-3-азабутан, 1,1,8,8-тетракис(метокси-NNO-азокси)-3,6-динитро-3,6-диахаоктан, алкокси-NNO-азоксисоединения, нитрамины, энтальпия сгорания, энтальпия образования, смесевые твердые ракетные топлива, удельный импульс, 1-bis(metho-xy-NNO-azoxy)-3-nitro-3-azabutane, 1,1,8,8-tetrakis (methoxy-NNO-azoxy)-3,6-dinitro-3,6-diazaoctane, alkoxy-NNO-azoxy compounds, nitramines, enthalpy of combustion, enthalpy of formation, solid composite propellants, specific impulse

Abstract >>
The standard enthalpies of formation of 1,1-bis(methoxy-NNO-azoxy)-3-nitro-3-azabutane and 1,1,8,8-tetrakis (methoxy-NNO-azoxy)-3,6-dinitro-3,6-diazaoctane were experimentally determined to be 87.7 ± 3.9 and 283.8 ± 6.2 kJ/mol, respectively. Calculations have shown that solid composite propellants containing these two compounds as gasifying components in metal-free compositions based on an active binder and ammonium perchlorate are inferior in the highest achievable effective impulse at the third stage of the rocket system Ief(3) to compositions based on HMX, but in designing special compositions with a limited content of organic explosive (not higher than 30 ÷ 35 %), these two compounds provide 5-10 s higher values of Ief (3) than when using HMX.

Effect of Particle Size on Boron Combustion in Air

A. P. Shpara, D. A. Yagodnikov, A. V. Sukhov
Bauman Moscow State Technical University, Moscow, 105005Russia
Keywords: бор, частица, горение, моделирование, микро- и наноразмеры, boron, particle, combustion, modeling, micron size and nanosize

Abstract >>
Mathematical modeling of combustion of micron-sized and nanometer boron particles in air with allowance for changes in the heat and mass transfer mechanism and a decreasing particle size is carried out. The Knudsen number is taken as an indicator of the transition from one regime to another: a continuous medium assumption is valid for describing the heat and mass transfer mechanisms in the case where Kn < 0.01, the free molecular regime takes place for Kn > 10, and a transition regime is realized for 0.01 < Kn < 10. Particle sizes at which different types of heat and mass transfer occurs with respect to boron combustion in air at pressures of 0.1 ÷ 4 MPa are estimated. The time it takes for boron particles to combust within the framework of the assumption of a continuous medium and in a free molecular mode is determined. It is shown that calculation models for determining the burning time of boron particles with initial sizes close to micron-sized and nanodispersed ones should take into account a change in the heat and mass transfer mechanism with variation in the current particle radius during burnout.

Study of Shock-Compressed Argon Plasma using Microwave Diagnostics

E. N. Bogdanov, M. V. Zhernokletov, G. A. Kozlov, A. V. Rodionov
All-Russian Research Institute of Experimental Physics, Sarov, 607188 Russia
Keywords: аргон, неидеальная плазма, микроволновая диагностика, argon, collisional plasma, microwave diagnostics

Abstract >>
A microwave diagnostics method and radio interferometers with wavelengths of 3.2 and 2.1 mm are used to study the kinematic and electrophysical characteristics of shock-compressed argon plasma, which is initially at atmospheric pressure. This research study is carried out at pressures of 12 ÷ 56 MPa, shock wave velocities of 3.1 ÷ 6.2 km/s, temperatures 9 000 ÷ 19 000 K, and densities 0.006 ÷ 0.012 g/cm3 for powers of Coulomb nonideality from 10-4 to 0.2. The data obtained on the shock-wave compressibility of argon are consistent with the known measurement results and calculations using the modified Van der Waals model and the chemical plasma model. A set of values of the reflection coefficient of electromagnetic radiation from the shock wave front at wavelengths of 3.2 and 2.1 mm is obtained, which serves as a basis for estimating the conductivity and electron concentration behind the shock wave front. The experimental data are consistent with calculation results in a wave velocity range of 3.1 ÷ 3.6 km/s. It is established that with a further increase in the velocity has no effect on the reflection coefficient.

Explosive Cumulative Projectors for Forming High-Velocity Compact Elements

S. I. Gerasimov1,2,3,4, D. V. Malyarov1, A. G. Sirotkina2, S. A. Kapinos1, A. P. Kalmykov1,4, A. S. Knyazev1
1All-Russian Research Institute of Experimental Physics, Sarov, 607188 Russia
2Sarov State Physics and Technical Institute, Sarov, 607186 Russia
3Nizhny Novgorod State Technical University n.a. R.E. Alekseev (NNSTU), Nizhny Novgorod, 603950 Russia
4Institute for Problems in Mechanical Engineering, Nizhny Novgorod, 603024 Russia
Keywords: взрывное метательное устройство, компактный элемент, кумулятивная облицовка, кавитационный заряд, детонационная волна, explosive projector, compact element, cumulative lining, cavitation charge, detonation wave

Abstract >>
This study presents a review of high-velocity projection methods and devices intended for the experimental study of the protection of equipment and structures from a high-velocity impact by compact elements, particularly protecting spacecrafts from collision with natural meteorite particles. Explosive projectors used in experiments to test protective structures at high impact velocities are shown. The results of numerical and experimental studies of these schemes are presented.