The influence of inert retardants on the combustion of hydrogen-oxygen mixtures under elevated temperatures and pressures

The experimental values of the flammability limits in hydrogen-oxygen-inert diluent (helium argon, carbon dioxide, steam) mixtures at temperatures up to 523 K and pressures up to 2 MPa as well as of the burning velocities of H₂–O₂–N₂ mixtures at 293 K and 4 MPa are reported. An anomalous effect of helium on the lower flammability limit in a hydrogen-oxygen-helium mixture is shown. The synergistic effect observed for helium-carbon dioxide and helium-steam mixtures used as inert retardants is noted. The data are interpretea theoretically on the assumption of the important role of the selective diffusion of hydrogen and helium from the initial mixture to the flame.VNIIPO, 143900 Balashikha. Translated from Fizika Goreniya i Vzryva, Vol 30, No. 2, pp. 51–57, March–April, 1994.     

Specific Features of the Mechanism of Flame Propagation in Rich Hydrogen–Propane–Air Mixtures

Propagation of a planar laminar flame in rich homogeneous propane–air and hydrogen–propane–air mixtures is numerically studied. It is shown that the maximum burning temperature is higher than the thermodynamically equilibrium value and is reached when chemical and physical processes have not been yet completed. The degree of superadiabaticity depends on the ratio of hydrogen and propane concentrations. Superadiabaticity in rich hydrogen–propane–air mixtures, in addition to the inhibiting action of propane, determines the deviation from Le Chatelier's principle for flammability limits in these mixtures.     

Chain-thermal explosion and degree of ionization of hydrogen-air flames

Using a passive probe to record the accumulation of electrical noise signals of hydrogen-air flames, it is established that within the concentration limits of intense combustion of these mixtures, which are approximately 18–60% (vol.) hydrogen and are due to the interaction mechanism in the burning mixtures, there is a considerable increase in the degree of flame ionization.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 15–23, January–February, 2005.     

Burning of local hydrogen-air mixtures in an unsealed vessel containing obstacles

Large-scale experiments are reported on the burning of hydrogen-air mixtures in an apparatus having a reaction vessel of volume 20 m³. A fast-burning mixture (30% hydrogen in air) shows a reduction in the pressure rise rate and in the maximum pressure when the discharge hole is close to the region of local burning. For a slow-burning mixture (10% hydrogen in air), the pressure is reduced when the discharge holes are far from the local burning region. A study has been made on the joint effects of immobile alternating obstacles and gas-dynamic perturbation caused by the motion of the medium towards the discharge holes as regards the intensification of the burning for local hydrogen-air mixtures.143900 Balashikha-6. All-Union Industrial Vessels Research Institute. Translated from Fizika Goreniya i Vzryva, No. 1, pp. 17–23, January–February, 1995.     

Filtration combustion of hydrogen-air, propane-air, and methane-air mixtures in inert porous media

The filtration combustion characteristics of hydrogen-air, propane-air, and methane-air mixtures in inert porous media have been studied experimentally. It is shown that the dependences of the combustion wave velocity on the fuel-air equivalence ratio are V-shaped. For hydrogen-air mixtures, the velocity minimum is shifted to the rich region, and for propane-air and methane-air mixtures, it is shifted to the lean region. For lean hydrogen-air and rich propane-air mixtures, the measured maximum temperatures in the combustion wave are found to be reduced relative to those calculated theoretically. For methane-air mixtures, a reduction in the measured temperatures is observed over the entire range of the mixture composition. The results are interpreted within the framework of the hypothesis of selective diffusion of gas mixture components. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 8–20, July–August, 2006.     

Characteristics of the combustion of hydrogen-methane-air mixture in a closed vessel

We experimentally determined the combustion characteristics of hydrogen-methane-air mixtures in a closed vessel. For hydrogen-air mixtures at certain pressures, a sign change of the normal burn velocity pressure exponent is characteristic for S_u>0.5 m/sec. With the addition of methane to the hydrogen-air mixture, a sign change of the pressure exponent from positive to negative can occur even when there is an absolute increase in the normal burn velocity.Balashikha. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 6, pp. 52–56, November–December, 1991.     

Interrelationship of the baric index for the normal burning velocity of gas—Air mixtures and the kinetics of trimolecular reactions in a flame front

Through numerical simulation of flame propagation through a methane- and hydrogen-air mixture, with consideration given to the transfer and chemical reactions, we have investigated the relationship between the kinetics of trimolecular reactions and the baric index of the normal burning velocity. It has been demonstrated that the lower the constants of the trimolecular reaction rates, the higher the magnitude of the baric index. In the case of methane the baric index, although it grows, it nevertheless remains negative, while in the case of hydrogen this index changes sign.Balashikha. Translated from Fizika Goreniya i Vzryva, No. 2, pp. 46–49, March–April, 1991.     

Laminar burning velocities and Markstein numbers of syngas-air mixtures

In the currently reported work, three typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas-air flames. The maximum value of syngas-air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas-air flames are generally unstable. Karlovitz numbers indicates that typical syngas-air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas-air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H₂/95% CO and proved to be similar to methane, although somewhat slower than propane.     

Model of Cellular Instability of Flames of Ternary Mixtures

A simple phenomenological diffusive-thermal model of cellular instability of premixed flames of ternary mixtures is developed and presented. The model shows that preferential diffusion can alter stoichiometry of the mixture, i.e., the ratio of the fuel and oxidizer concentrations, and also its effective dilution by an inert. Key parameters of the model are estimated using numerical modeling of burning velocities. Laminar burning velocities are calculated for hydrogen-oxygen-nitrogen, methane-oxygen-nitrogen, and propane-oxygen-nitrogen mixtures. Conditions for the appearance of cellular instability in ternary mixtures are determined and compared with experiments. In good agreement with experimental observations, the diffusive-thermal instability is predicted in hydrogen flames with equivalence ratios φ ≲ 1.45, in lean methane flames with φ ≲ 1.02, and in rich propane flames with φ ≳ 1.03. The magnitude of the change in the local flame velocity due to preferential diffusion is evaluated. It is demonstrated that nitrogen diffuses faster than oxygen in hydrogen-air and methane-air flames, while oxygen diffuses faster than nitrogen in flames of propane and other heavier hydrocarbons. In mixtures of air with propane or heavier hydrocarbons, the transition between stable and unstable regimes is predicted in mixtures that are leaner than the mixture corresponding to the peak of the burning velocity curve, in agreement with experimental observations. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 14–22, September–October, 2005.     

Selective Diffusion during Flame Propagation and Quenching in a Porous Medium

The propagation of propane-air flames in an inert high-porosity medium with nitrogen dilution and oxygen enrichment of the mixture was studied experimentally. It is shown that variation in the nitrogen or oxygen concentration (in the gas phase) leads to a more significant variation in the flame propagation velocity than in the laminar burning velocity; with the addition of nitrogen, the rate of increase in the flame velocity with the initial pressure becomes lower and the concentration range of flame propagation becomes narrower. At the flame propagation limit, the Peclet number obtained from the laminar burning velocity of the initial mixture is not constant but depends on the fuel-to-oxidizer ratio and the nitrogen content in the mixture. The results are interpreted from a physical point of view based on the hypothesis of selective diffusion. It is shown that accounting for the effects of the Lewis numbers of the fuel and oxidizer allows flame propagation in inert porous media to be described quantitatively over wide parameter ranges using a unified relation. At the flame propagation limit, the Peclet number constructed from the laminar burning velocity taking into account these effects is a constant.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 50–59, July–August, 2005.     

The effect of density on the limits and regularities of spin combustion of titanium in nitrogen

A region of spin combustion in the coordinate system nitrogen-pressure (100–1800 torr)-burning sample-density (2.0–3.3 g/cm ³) has been determined experimentally. The effect of density on the spin combustion characteristics (velocity, frequency, and step) has been studied at various nitrogen pressures. A transition of the spin mode of combustion to stationary with decreasing burning velocity has been observed for the first time.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 1, pp. 53–57, January–February, 1996.     

Explosive Hazard of Vapor–Gas Mixtures of Fluoride‐Containing Saturated Hydrocarbons with Fluoride and Chlorine Trifluoride

The limits of explosion of the CF₃CFH₂–F₂–N₂, C₃F₈–F₂–N₂, C₄F₈–F₂–N₂, C₄F₈–ClF₃–N₂, C₄F₁₀–ClF₃–N₂, and F₂ClCF₂Cl–F₂–N₂ mixtures are determined. For some compositions of the first two mixtures, the maximum pressure of explosion and the maximum pressuregrowth rate are measured. The transition from deflagration to detonation is observed in undiluted mixtures close to stoichiometric compositions. The burning rates of the mixtures examined are comparable with the burning rates of oxygen–hydrogen mixtures.     

Gas combustion in a narrow channel at high pressure

This paper presents experimental results on the effect of pressure on the flame propagation velocity in a tube with diameter close to the critical diameter. An important feature of the investigated combustion regime is heat transfer along the tube wall from the combustion products to the fresh mixture. Methane-air and hydrogen-air mixtures were used. The experiments show that with increasing pressure, the burning velocity of methane-air flames decreases whereas the burning velocity of hydrogen-air flames is almost unchanged. This behavior is explained by the pressure dependence of the laminar burning velocity.     

Inhibition of various hydrogen combustion regimes in air by Propylene and Isopropanol

This paper studies the effects of small additives of propylene and isopropanol on the hydrogen-air flame speed in the predetonation regime, deflagration-to-detonation transition, and burning rate. It is shown that the difference in the effects of these additives on the combustion is determined primarily by their ability to terminate reaction chains. In hydrogen flames, the additives are consumed as a result of their reactions with the active intermediate products of H₂ combustion in which these species are replaced by inactive radicals.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 3–14, January–February, 2005.     

Combustion modes and flame propagation criteria for an encumbered space

The topic is analyzed. Experiments have been performed with fuel—air mixtures in a porous medium. Measurements have been made on the velocity and pressure in the detonation and combustion waves, together with the critical initial parameters and the limits to the Peclet number. The mean detonation rate has been found to fall and the ignition has been found to be retarded in a porous medium when the wave passes through a free gap.Combustion and Flame Institute, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 4, pp. 52–60, July–August, 1994.     

Effect of diluents of various chemical nature on the flammability limits of gas mixtures

The effect of diluents of various chemical nature [halogenated hydrocarbons, inhibitors developed at the Institute of Structural Macrokinetics and Problems of Materials Science (ISMAN), superheated water aerosol] on the flammability limits of hydrogen-methane mixtures in air was investigated experimentally. It was found that ISMAN inhibitors, which are aliphatic hydrocarbons, reduce the upper flammability limit of hydrogen even more effectively than the well-known inhibitor 1,2-dibromotetrafluoroethane. Flammability curves for hydrogen and methane in mixtures with the above-mentioned diluents were obtained. The results are interpreted qualitatively by considering various kinetic regimes of nonisothermal chain branching reactions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 96–102, November–December, 2006.     

Discrete-continual model of flame propagation in a gas suspension of metal particles. I. One-dimensional approximation

Experimental data on combustion of magnesium particles in oxygen-containing gas mixtures are analyzed and generalized, which allowed us to develop a semi-empirical mathematical model that takes into account the integral effect of the initial particle size, pressure, and velocity of the oxidizing flow on combustion. Flame propagation in an air suspension of fine magnesium particles is considered in a one-dimensional approximation. For this purpose, a discrete-continual model of flame propagation in a gas suspension of metal particles is developed. The flammability limits in a vessel, caused by heat losses into the ambient medium and by spatial nonuniformity of the distribution of disperse-phase particles, are numerically determined, and the influence of nonuniformity and bidispersion of a one-dimensional ensemble of particles on flame characteristics is studied.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 2, pp. 81–93, March–April, 2005.     

Flammability of Vapor–Gas Mixtures C2F4, SO3, and C2F4OSO2

Flammability limits and conditions of flegmatization of vapor–gas mixtures by helium and perfluoropropane are determined. For most mixtures, the maximum pressure of explosion and the maximum growth rate of pressure in explosion are determined. Addition of sulfuric anhydride to tetrafluoroethylene significantly decreases the minimum energy of flammability as compared to pure tetrafluoroethylene but weakly affects the maximum pressure of explosion and pressure growth rate in explosion. It is found that pure vapors of tetrafluoroethanesultone present an explosive hazard, but its mixtures with sulfuric anhydride are most dangerous.     

Possible mechanism of nonsteady-state erosion combustion of composite solid propellants

It has been demonstrated that in the combustion of composite solid propellants the instantaneous mass burning rate can depend on variations in the velocity of cross-flow of gases even if in the steady-state regime the combustion is insentitive to cross-flow velocity. A method has been proposed for studying the nonsteady-state erosion combustion in experiments with composite systems.Institute of Chemical Kinetics and Combustion, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 4, pp. 40–43, July–August, 1995.     

Technological characterization and ceramic application of gravel pit by-products from middle-course Jarama river deposits (central Spain)

This work, unlike previous others, deals with ceramic products obtained exclusively from gravel pit by-products. The residues are originated from the sand and gravel washing process of middle-course Jarama river Quaternary sediments, located in Madrid region, central Spain. These natural aggregates allow an intense exploitation, because of quality (well-rounded quartzite) and reserves (up to 700×10⁶ m³), generating considerable waste volumes.Thirty silty–clay by-products, collected from seven gravel pits, have been tested at a laboratory scale. The mineralogical composition of these materials is mainly represented by phyllosilicates (muscovite-illite, smectite, kaolinite and chlorite), quartz and feldspars.For the technological characterization, six representative mixtures (M1 to M6) were designed combining suitable mineralogy and grain-size distribution of the 30 raw samples. A wide range of values was measured on mixture powders, determining specific surface (13–76 m² g⁻¹, BET method), methylene blue index (5–20) and Atterberg limits (W_l: 43–90; W_p: 25–38).Ceramic characterization was performed on extruded bodies, testing drying sensitivity (Bigot curves and Ratzemberger test). Smectitic content influences directly the parameters measured on dried bodies. This influence can be controlled by increasing the silty component, and the ceramic properties measured on these mixtures (M3, M4 and M5) get better: hygroscopicity (less than 2%), drying shrinkage (4–7 cm m⁻¹), keeping the bending strength above 5 MPa.Three maximum firing temperatures were studied: 850, 950 and 1050 °C. On mixtures fired at 950 °C, firing shrinkage is less than 4 cm m⁻¹, bending strength may reach 69 MPa and water absorption range from 23% to 0.5%. Efflorescence susceptibility is faint, and colour is red for all the samples, getting darker for increasing temperature.The positive results obtained in this set of preliminary tests drive to envisage new research programs, focused on testing these by-products on a semi-industrial scale, assessing the effective possibilities of using them as ceramic raw materials.