Numerical simulations of autoignition of lean (6% H2), stoichiometric, and rich (90% H2) hydrogen–air mixtures have been performed to examine the influence of third-body efficiency (chaperon efficiency, CE) on the value of ignition delay, τ. The temperature ranges explored in the computations are 850–1000 K for P0 = 1 bar and 1000–1200 K for P0 = 6 bar. By using a detailed kinetic mechanism, it has been found that the sensitivity of ignition delay to CE is the highest for the reaction step H + O2 + M = HO2 + M, which can lead to a variation in τ by a factor of 2 to 3. A pressure increase or deviation from stoichiometry reduces the sensitivity. The influence of CE is qualitatively different and weaker for the reaction step OH + OH + M = H2O2 + M.

Numerical simulations of autoignition of lean (6

Проведено численное моделирование задержек воспламенения и роста давления в процессе самовоспламенения бедной (6% Н2), стехиометрической (29,6% Н2) и богатой (75% Н2) водородно-воздушных смесей при начальных давлениях 1 и 6 атм в диапазоне начальных температур 850- 1700 К. Установлено, что для бедных и богатых смесей рост давления практически не зависит от выбора детального кинетического механизма (ДКМ). В то же время для стехиометрической смеси такая зависимость наблюдается. Выход продуктов горения на параметры термодинамического равновесия (ТР) во временном масштабе задержки воспламенения замедляется с ростом начальной температуры и ускоряется при повышении давления. Для стехиометрической смеси ТР достигается быстрее, чем для бедной и богатой. Показано, что динамика роста давления, определяемая химической кинетикой после индукционного периода самовоспламенения, может быть разной в зависимости от ДКМ и не зависит от задержки воспламенения. Данный факт может иметь существенное значение при относительно высоких температурах.

Simulations of the effect of addition of H, O, OH, HO 2 , and H 2 O 2 on the structure and propagation of laminar flames in lean (12 and 15%) hydrogen-air flames are performed at pressures of 1 and 6 bar. It is found that impurities in concentrations of no more than 0.1% do not have any significant effect on laminar burning velocity. When initial temperature is increased to 400 K, the effect of impurities becomes even weaker. Among the impurities under study, only the addition of OH reduces the laminar flame velocity. The weak effect of the impurities is attributed to fast formation of intermediate products via reactions involving O and H atoms without noticeable change in heat release rate. An increase in initial pressure to 6 bar does not change the effect of impurities.

Проведены численное моделирование структуры и расчеты скорости распространения ламинарного пламени, S L , в бедной смеси водорода (12%) с воздухом при нормальных начальных условиях. Анализ профилей концентраций промежуточных продуктов показывает, что с ростом температуры происходит смена кинетического механизма, определяющего динамику тепловыделения. Таким образом, процесс тепловыделения в пламени состоит из двух этапов. В области максимального градиента температуры концентрации H 2 O 2 и HO 2 достигают пиковых значений. Последующая убыль содержаний H 2 O 2 и HO 2 сопровождается симбатным ростом концентраций H, O и OH. При изменении констант скоростей реакций, определяющих тепловыделение, изменяются скорость нарастания температуры и значение S L . Последнее наиболее чувствительно к реакции молекулярного водорода с гидроксилом с образованием воды.

Проведено численное моделирование влияния примесей атомов, молекул и радикалов на самовоспламенение бедных (14% H 2 ) и ультрабедных (6% H 2 ) смесей водорода с воздухом в диапазоне температур от 800 до 1700 K при давлениях 1 и 6 атм. Расчеты показывают, что добавление H, O, OH, HO 2 и H 2 O 2 сокращает время задержки воспламенения – τ. Выявлены общие тенденции влияния примесей на самовоспламенение смесей в зависимости от температуры. Для каждой примеси наиболее сильное влияние обнаружено при температурах, близких к 900 и 1100 K, при давлениях 1 и 6 атм соответственно. Показано, что степени влияния примесей O и H практически одинаковы. Влияние примеси HO 2 на температурную зависимость задержки воспламенения значительно слабее по сравнению с другими примесями, но эта зависимость качественно совпадает с таковой для H 2 O 2 . Хотя степень сокращения задержки воспламенения и убывает для всех примесей по мере приближения к концам исследованного интервала температур, сохраняется заметное влияние радикала OH в высокотемпературной части и примесей HO 2 и H 2 O 2 в низкотемпературной. Добавление до 1% воды не влияет на величину τ.

Simulations of autoignition and combustion processes in lean and ultra-lean hydrogen–air mixtures are performed in relation to safety aspects of nuclear power plants. Ignition delay times τ and laminar burning velocities SL are evaluated. Comparisons between simulation results obtained for temperatures ranging from 800 to 1700 K at initial pressures of 1 and 6 bar show that both the value of τ and the temperature-dependent behavior of autoignition characteristics vary weakly with hydrogen concentration in air. The largest difference between the values of τ predicted by different detailed kinetic mechanisms (DKMs) is observed at temperatures of 900 and 1100 K for pressures of 1 and 6 bar, respectively. The time to reach peak heat release significantly exceeds τ at initial temperatures above 1250 K. Simulations based on the different DKMs yield similar values of SL. It is concluded that each of the DKMs employed can provide satisfactory accuracy of simulations of autoignition and combustion processes in lean and ultra-lean hydrogen–air mixtures at pressures below 6 bar.

Numerical simulation of thermal conversion of gaseous products of polypropylene pyrolysis is performed at atmospheric pressure in the temperature range of 700 to 1200 K. The initial mixture compositions are taken from experimental data presented in the literature. Results computed by using several detailed kinetic mechanisms (DKMs) provide qualitatively similar descriptions of thermal conversion but differ significantly in terms of duration of the main stage of the process. As the temperature of the gaseous products of polypropylene pyrolysis increases, production of methane, hydrogen, and C6+ aromatics is observed concurrently with decrease in the concentrations of the C2+ components of the initial mixture. Thermal conversion is found to proceed exothermically, with lower heat release at higher initial temperatures due to an increasing role of the endothermic dissociation of C3H6. The secondary products of thermal conversion predicted by different DKMs differ by concentrations of C6+ aromatics. The accuracy of simulation is supposed to be improved by including C4+ hydrocarbons in the DKM.

Thermal conversion (TC) of polypropylene pyrolysis gaseous products was studied numerically using three detailed kinetic mechanisms (DKMs) for the initial temperatures T0 ranging from 700 K to 1300 K and the initial pressure of 1 atm. Numerical simulations showed that all DKMs predicted similar qualitative behavior of the reacting mixture, however, they gave significantly different rates of the increase of the mole fractions of CH4 and H2 along with simultaneous decrease in C2+ hydrocarbons. The temperature increase in course of TC process was also found to be predicted differently by the three DKMs. Its characteristics depended on the initial temperature, some calculations showed smooth increase while in others explosionlike regimes were observed. The conversion products, along with methane and hydrogen, contained aromatic compounds C6+. Various DKMs gave significantly different TC characteristic times.

A problem formulation is proposed for simulating lean premixed hydrogen–air combustion in a closed volume with numerically generated turbulence. Two-dimensional simulations of flame propagation from a small ignition kernel showed that the following sequence of regimes is observed with increasing turbulence intensity and decreasing mixture strength: a corrugated flame with a continuously connected front, a regime with a local loss of front connectivity, and flame extinction through disintegration of a growing kernel into fragments. The transition from steady self-sustained combustion to extinction corresponds to changes in mixture and turbulence parameters leading to a stronger influence of turbulent velocity field on local flame structure. The proposed approach to numerical simulation of the transition regime characterized by loss of front connectivity and fragmentation of the flame under the action of turbulent eddies can be used to evaluate the effect of turbulence intensity on flammability limits.

Simulations of autoignition of stoichiometric С2Н2/О2 and СН4/О2 mixtures were performed in the temperature and pressure ranges of 800 < T < 2100 K and 0.02 < P < 1.1 MPa using various detailed kinetic mechanisms (DKMs) presented in the literature. The ignition delays predicted by DKMs were compared to choose the optimum value for reduction. The number of species in the DKM was reduced by a trial-and-error method based on an analysis of ignition delay response to the removal of species. The automatic computer code developed in this study made it possible to reduce the number of species more than twofold for C2H2/O2 and threefold for CH4/O2. A comparison of predictions based on the reduced kinetic mechanisms with experimental data presented in the literature showed good agreement for С2Н2/О2 and СН4/О2 mixtures with different argon dilutions.

The values of ignition delay predicted for stoichiometric C 2 H 2 /O 2 and CH4/O2 mixtures by various detailed kinetic mechanisms presented in the literature are compared over the temperature range of 1000 < T < 2100 K and the pressure range of 0.02 < P < 1.1 MPa. The number of species in detailed kinetic mechanisms is reduced by a trial-and-error method while keeping the response of ignition delay within a relative error range of 5 = 30%. The automatic computer code developed in this study has made it possible to reduce the number of species by a factor of more than 2 for the C2H2/O2 mixture and a factor of 3 for the CH4/O2 mixture, respectively. A comparison of predictions based on the reduced mechanisms with experimental data presented in the literature shows good agreement.

Gas-filled elastic bags are a convenient means for storing blast mitigation materials, such as inert gases and two-phase media, and delivering them to protected objects. If the blast wave resulting from an accident propagates through air, it can undergo significant transformations when interacting with various inhomogeneities. This study explores the possibility of blast-wave attenuation by interaction with a thin enclosure (shell) filled with helium, which has a low specific acoustic impedance compared to air. High-speed shadow photography was used to investigate the wave patterns and dynamics of helium compression in the balloons under blast loads resulting from explosions of hydrogen-air mixtures. Blast loads transformations were analyzed. A comparison with three-dimensional numerical simulations showed that the motion of a thin shell can be computed by solving fluid dynamics equations without specifying boundary conditions across the shell.

The objective of this work is to determine experimentally the effectiveness of protective barriers under conditions when blast waves are generated during premixed hydrogen– air combustion in various regimes. Experiments are conducted in a vertical tube having a diameter of 54 mm and a length of up to 2 m. Blast loads are produced by acceleration of premixed hydrogen–air flames in the tube with ring obstacles. Comparative tests are performed between protection barriers made of bulk materials with different densities and aqueous foams with different expansion ratios. It is demonstrated that the degree of blast load attenuation by an aqueous foam barrier increases with decreasing molecular weight of the filling gas and increasing density (decreasing expansion ratio) of the foam. An Aerosil barrier three times thicker than a titanium-dioxide one is found to have a similar attenuating effect on blast action. However, the mass per unit area of an Aerosil barrier is lower than titanium dioxide by a factor of 6 and is comparable to foam. The observed dependence of blast load attenuation on parameters of bulk materials and aqueous foams must be taken into account in systems designed to mitigate the consequences of accidental hydrogen release and combustion.

A technique has been developed for evaluating turbulent combustion characteristics in the presence of microdroplets obtained by vapor condensation in the course of rapid expansion. Experiments have been conducted to visualize spark-initiated flame propagation through hydrogen-air mixtures at various turbulent intensities, water-vapor volume fractions, and microdroplet concentrations. The influence of microdroplet suspensions on iginition and flame propagation is investigated.