Yu. N. Shebeko

An analytical evaluation of flammability limits of gaseous mixtures of combustible–oxidizer–diluent

Abstract This paper is aimed at the creation of new methods for the analytical evaluation of flammability limits of ternary gaseous mixtures of combustible–oxidizer–diluent. The physical nature of flammability limits is considered using some peculiarities of the chemical kinetics of combustion of gaseous organic substances in air. New features of lower flammability limits and compositions of mixtures near peak points of flammability curves are revealed. These features are the following. Firstly, adiabatic flame temperatures, Tad,CO, of combustion of gaseous mixtures up to CO and H2O for mixtures of fuel–air at lower flammability limits and mixtures of fuel–air–diluent at peak points of flammability curves are in rather narrow ranges. Secondly, compositions of ternary gaseous mixtures of fuel–air–diluent are close to stoichiometric in relation to combustion up to CO and H2O. It was found that such parameters as adiabatic flame temperature, Tad,CO, and equivalence ratio in relation to combustion up to CO and H2O are important for evaluation of flammability limits in ternary gaseous mixtures. New analytical methods for the calculation of flammability limits in mixtures of combustible–oxidizer–diluent are proposed, which have acceptable accuracy for practical applications.

Burning velocities and flammability limits of gaseous mixtures at elevated temperatures and pressures

Abstract Burning velocities and flammability limits of gaseous mixtures of combustible gas (hydrogen and methane), oxidizer (oxygen and air), and diluent (nitrogen, argon, helium, carbon dioxide, steam, water aerosol formed by evaporation of superheated water) have been measured at elevated temperatures (up to 250°C) and pressures (up to 4.0 MPa). It was found that with increasing temperature, the flammability region is widened for all the mixtures studied. With increasing pressure, the flammable region for mixtures of hydrogen-oxygen-diluent is narrowed, except where steam or water aerosol is the diluent. A more complicated dependence of flammability limit on pressure is found for mixtures of hydrogen-oxygen-steam. The influence of water aerosol (formed by the rapid evaporation of superheated water having an initial temperature of 150°C) on the flammability limits of methane in air was also investigated. It was found that the inerting action of such an aerosol is mainly due to the steam available within it; water droplets which are present in the mixture do not have a significant influence on the flammability limits. The dependences of burning velocity on temperature and pressure were investigated for stoichiometric hydrogen-air mixtures diluted by nitrogen and steam. It has been shown that, over the ranges of temperature and pressure considered, the temperature index of burning velocity is positive (that is, burning velocity increases with temperature), while the pressure index changes sign from positive to negative on dilution by inert agents. The observed differences in the influences of steam and nitrogen on burning velocity cannot be explained on the basis of a thermal mechanism alone and is mainly due to the more active role of water molecules in enhancing termolecular recombination reactions in the flame front. The influence of small quantities of methane on the burning velocity of hydrogen-air mixtures also was investigated.

The influence of fluorinated hydrocarbons on the combustion of gaseous mixtures in a closed vessel

Abstract The influence of various fluorinated inhibitors (CF 3 H, C 2 F 5 H, C 3 F 7 H, C 3 F 6 H 2 , CF 2 ClH, C 2 F 5 Cl, C 2 F 5 I, C 4 F 8 , C 4 F 10 , C 2 F 4 Br 2 ) on the combustion characteristics of mixtures of hydrogen and air and also of methane and air in a closed vessel has been investigated experimentally. The flammability limits, maximum explosion pressure, and maximum rate of explosion pressure rise have been determined. In some cases, flames of lean mixtures of H 2 or CH 4 with air can be promoted by adding fluorinated hydrocarbons. Thus the maximum explosion pressure, Δ P max , and the maximum rate of pressure rise, ( dP / dt ) max , during the explosion of lean mixtures were both elevated by small additions (several vol. %) of fluorinated hydrocarbons; this is caused by heat release during the chemical conversion of the inhibitors. This heat release is high enough to increase Δ P max and ( dP / dt ) max by 50–100% above the values for mixtures without inhibitors.

A study of the behaviour of a protected vessel containing LPG during pool fire engulfment

Theoretical and experimental investigations of various methods for protection against fires of vessels containing liquefied petroleum gases (LPG) (safety relief valves, intumescent fire retardant coatings, thermal isolation) have been carried out. A simple mathematical model has been proposed, which describes dependences of various parameters on time. These parameters are temperature, pressure and mass of LPG, temperatures of the vessels walls and thermal protection layer. The case of total fire engulfment of the vessel with LPG was considered. Experiments have been executed, which were aimed on the investigation of the behaviour of vessels with LPG (50 l), equipped with protective devices during total fire engulfment. It was found out that the safety valve prevented an explosion of the vessels without any other protective measures. The presence of the intumescent fire retardant coating caused a significant delay in operation of the safety valve. A rather good agreement between the theoretical and experimental data was obtained. It has been revealed that the considered methods for protection of LPG vessels are promising in regard to prevention of explosions in these vessels at the fire engulfment.

Effect of an electric field on concentration limits for propane flame propagation in air

An experimental study is performed of the effect of dc and ac electric fields on the concentration limits for propagation of a propane flame in air within a vertical tube with closed lower and open upper ends. It is established that both limits for upward propagation and the lower limit for downward propagation remain unchanged upon field application. A qualitative interpretation of the results obtained is offered.

Effect of an AC electric field on normal combustion rate of organic compounds in air

The present study attempts to develop a probable mechanism for the effect of a high frequency electric field on normal combustion rate of organic compounds in air. Several previous experiments were analyzed to concretize the mechanism by which the rate of reaction is changed. The experiments were performed under conditions of electric field strength and nitrogen molecule density such that highly efficient oscillatory excitation is realized. Replacement of nitrogen with argon in the original fuel mixture led to a significant decrease in the rise in combustion rate. Methane/air and methane/oxygen mixtures were tested in an electric field of frequency 6 mHz and intensity 1.67 kV/cm. Selective excitation of oscillatory levels of nitrogen molecules by electrons, with further transfer of oscillatory energy to oxygen molecules explains, in principle, the effect of an AC electric field on normal combustion rate. However, the required experimental and theoretical data for complete proof of the proposed mechanism are absent at present.

Critical Conditions of Chain Thermal Explosion

This paper reports results of experimental investigation of the critical conditions of chain thermal explosion of hydrogen–air and methane–air mixtures are introduced. The observed regularities, including the difference in critical conditions between the ignition and explosion of these combustibles, are explained by considering the chemical mechanisms of oxidation of hydrogen and methane in the context of the nonlinear theory of nonisothermal chain processes. Examples of regularities of combustion processes at atmospheric pressure are given that cannot be described without considering the leading role of the competition between the branching and termination of reaction chains. It is shown that in solving the equations used to describe combustion processes, neglect of the variation in the heat effects of reactions with temperature can lead to large errors.

Investigation of concentration limits of flame propagation in ammonia-based gas mixtures

The concentration limits of flame propagation inNH3-O2,NH3-H2-O2,NH3-O2-N2,and NH3-H2-O2-N2 mixtures at temperatures of up to70°C and pressures of up to1.0 MPa were studied experimentally. The lower concentration limit of propagation of ammonia flame in oxygen decreases significantly at elevated temperature and pressure, the effect of temperature being stronger than for organic combustible substances. It is shown that the Le Chatelier rule for the limits ofN2-NH3-O2 mixtures is satisfied with sufficient accuracy.

Some aspects of fire and explosion hazards of large LPG storage vessels

Results of experimental and theoretical investigations of fire and explosion hazards of liquefied petroleum gas (LPG) storage vessels are presented. The evaporation intensity from LPG pools and the thermal radiation intensity distribution on the surface of cylindrical vessels containing LPG as a result of pool fires placed near the vessel were measured. Available mathematical models were verified and new models were proposed for processes such as evaporation from the pool, dispersion of vapour clouds, vapour cloud explosion, the behaviour of the vessel containing LPG placed near pool fires, boiling liquid expanding vapour explosions, and thermal radiation from the pool fire. The results can be used for ensuring fire and explosion safety with respect to LPG storage vessels.

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.