Normal Burning Velocity Research Articles

Expansion Coefficients and Propagation Speeds of Premixed n-Butane–Air Flames

The propagation speeds of premixed n-butane–air mixtures (2.0–5.7 vol%) were investigated under various initial conditions (pressures of 0.4–1.2 bar; temperatures of 289–500 K). The study consists of both, experimental measurements using two different enclosures (a sphere and a cylinder) and kinetic modeling via a dedicated computing program. The propagation speeds of premixed n-butane–air mixtures were obtained via the adiabatic model of flame propagation, which allows us to obtain these important parameters using the normal burning velocities and expansion coefficients. The expansion coefficients were calculated using thermodynamic data as the ratio of burnt to unburnt gas densities, assuming that an equilibrium was established in the flame front. The propagation speeds obtained based on the experimental burning velocities were analyzed for comparison with the computed velocities. Finally, the dependence of the propagation speed on the initial pressure and temperature was discussed.

The fire safety of infrastructure facilities for hydrogen-powered vehicles

Introduction. The article offers an analytical review of domestic and foreign publications on the fire safety of hydrogen refueling stations and garages for hydrogen-powered vehicles.General characteristics of the fire hazard of infrastructure facilities for hydrogen-powered vehicles. The authors offer the general characteristic of a specific fire hazard from facilities using compressed and liquid hydrogen.Hydrogen refueling stations. Refueling stations using compressed and liquid hydrogen were considered in the article. It was found that compressors are the most hazardous items installed at refueling stations; therefore, the value of potential risks, arising in the area of a refueling station, exceeds 10–4 year–1. Experiments, simulating accidents at hydrogen refueling stations, are described. According to the authors, the minimal distance between the compressor and facilities located outside the station area should exceed 50 m.Garages for hydrogen-powered vehicles. Features of the fire safety of garages for hydrogen-powered vehicles were analyzed. The authors have found that the overpressure inside a small garage (an individual garage) can reach 55 kPa in case of a jet flame caused by the release of hydrogen through the safety valves of fuel tanks. The overpressure of a hydrogen jet can reach 10 kPa in case of the unignited release of hydrogen. High pressure values that accompany the jet formation are triggered by the high normal burning velocity of hydrogen that boosts the velocity of heat release in the flame front, exceeding the same value for flares of hydrocarbon gases. Therefore, requirements, applicable to storage premises designated for vehicles powered by hydrocarbon fuel, may be erroneously extended to garages for hydrogen-powered vehicles (pursuant to NFPA 2).Conclusions. The results of this analysis can be contributed to regulatory documents to be developed in the area of fire safety of infrastructure facilities for hydrogen-powered vehicles.

Experimental and Numerical Study of Ignition and Flame Propagation for Methane–Air Mixtures in Small Vessels

Methane is one of the most common gaseous fuels that also exist in nature as the main part of the natural gas, the flammable part of biogas or as part of the reaction products from biomass pyrolysis. In this respect, the biogas and biomass installations are always subjected to explosion hazards due to methane. Simple methods for evaluating the explosion hazards are of great importance, at least in the preliminary stage. The paper describes such a method based on an elementary analysis of the cubic law of pressure rise during the early stages of flame propagation in a symmetrical cylindrical vessel of small volume (0.17 L). The pressure–time curves for lean, stoichiometric and rich methane–air mixtures were recorded and analyzed. From the early stages of pressure–time history, when the pressure increase is equal to or less than the initial pressure, normal burning velocities were evaluated and discussed. Qualitative experiments were performed in the presence of a radioactive source of 60Co in order to highlight its influence over the explosivity parameters, such as minimum ignition energy, maximum rate of pressure rise, maximum explosion pressure and normal burning velocity. The results are in agreement with the literature data.

Ignition and propagation properties of coal dust – air – methane hybrid mixtures

The presence of a flammable gas in a combustible dust cloud significantly modifies its explosivity parameters. Thus, a hybrid mixture can become flammable even when both the dust-air and gas-air binary mixtures are under the corresponding lower explosion limits. The pressure-time histories of hybrid mixtures formed by coal dust, methane and air at different concentrations, at 1 bar initial pressure and normal temperature, were studied using the standard 20 L spherical explosion vessel and 5kJ chemical igniters or 10 J permanent sparks. From the early stages of the pressure-time history, when the pressure increase is equal to or less than the initial pressure, the normal burning velocities were evaluated and discussed.

Inert Gas Influence on Propagation Velocity of Methane-air Laminar Flames

The flame propagation in methane-air mixtures diluted by inert additives (He, Ar, N2, CO2) was studied by means of pressure-time records of laminar deflagrations occurring in a spherical vessel with central ignition. Experiments were made using mixtures with various equivalence ratios between 0.610 and 1.310 and various inert concentrations between 5 and 25 vol%, at various initial pressures between 50 and 200 kPa. Examination of pressure-time records in the early stage of explosions delivered the normal burning velocities Su via the coefficients of the cubic law of pressure rise, using a previously described procedure. The propagation velocities (or the flame speed) were calculated from the normal burning velocities using the expansion coefficients of the unburnt gas during the isobaric combustion. The propagation velocities of examined systems obtained from experimental data were examined against the propagation velocities obtained from kinetic modeling of methane-air-inert combustion by means of 1D COSILAB package using the GRI 3.0 mechanism.

Validation of a kinetic scheme for numerical investigation of hydrogen–methanol–air flames

Normal burning velocities in methanol–air mixtures and in the same mixtures with added 4.5 and 7.2% hydrogen as a second fuel were measured over a wide range of equivalence ratio and for initial conditions of 0.16 MPa and 354 K. It has been shown that the mechanism previously proposed for the combustion of mixtures of CO, CH2O and CH3OH with air is applicable to multicomponent mixtures containing hydrogen and methanol.

Thermal and dynamic structure of an ethanol flame in the boundary layer near an obstacle

The distribution of the dynamic and thermal parameters of a gas in the immediate vicinity of a flame stabilizer (behind a rib and a backward-facing step 3 mm high) are studied. Data on the longitudinal and transverse velocity components, velocity pulsations, and correlations of pulsations in the boundary layer with combustion and without combustion are obtained using the PIV equipment. Temperature is measured with a thermocouple. Flame is visualized in the visible and ultraviolet regions. It is shown that in the flow attachment region near the leading edge of the flame, the gas velocity is close to the normal burning velocity. Heat release is estimated from the results of measurements. The presence of regions of volume and frontal combustion and kinetic and diffusion reaction is established. With increasing distance from the stabilizer, the heat-release rate decreases by more than an order of magnitude.

Normal burning velocity and propagation speed of ethane–air: Pressure and temperature dependence

Experiments in a spherical bomb with central ignition using ethane–air mixtures of variable composition ([C2H6]=3.4–7.6vol%), at various initial pressures (p0=30–130kPa) and initial temperatures (T0=298–423K) afforded the calculation of the normal burning velocities Su and propagation velocities Ss. The normal burning velocities (NBV) were determined from the coefficients of the cubic law of pressure rise during the early stage of explosion, using a previously proposed procedure. The propagation velocities (PV) were calculated from the normal burning velocities using the expansion coefficient of the unburnt gas during isobaric combustion. The experimental NBVs are examined against computed values, obtained with the package INSFLA for free laminar premixed flames, using the mechanism CH4–C4 based on 53 chemical species, which participate to 592 elementary reactions. The baric and thermal coefficients of NBVs and of PVs are determined and discussed in comparison with literature data.

Experimental and Numerical Study of Laminar Burning Velocity of Ethane–Air Mixtures of Variable Initial Composition, Temperature and Pressure

Pressure–time records of ethane–air centrally ignited explosions in a spherical closed vessel have been used to study the flame propagation in mixtures with various initial concentrations, pressures and temperatures ([C2H6] = 3.40–7.60 vol %, p0 = 30–130 kPa, T0 = 298–433 K). The normal burning velocities have been calculated from pressure–time records for an extended duration of spherical propagation by means of an improved equation for the burnt mass fraction, recently discussed. For the stoichiometric ethane–air mixture, Su,st = (43.5 ± 1.0) cm/s was obtained, within the range of reported literature data from the spherical bomb technique. The normal burning velocities were examined in correlation with the temperature and pressure reached during flame propagation, in single experiments. A power law has been used to describe the dependency of burning velocities on pressure and temperature. The thermal exponents of burning velocities range between 1.40 and 2.20, revealing the influence of the initial pres...

Additive effects on the rate of pressure rise for ethylene–air deflagrations in closed vessels

The deflagration of stoichiometric ethylene–air mixtures without or with additives (Ar, N2 or CO2) in spherical and cylindrical closed vessels with central ignition was investigated in order to determine the maximum rates of pressure rise (dp/dt)max and the deflagration indices (severity factors) KG. The influence of initial pressure, additive concentration, shape and size of explosion vessel on (dp/dt)max and KG was studied. A linear correlation between the maximum rate of pressure rise and the total initial pressure was found for all examined systems, at constant initial temperature and fuel/oxygen ratio. The deflagration index KG of centrally ignited explosions, calculated from experimental values of pressure rise rates, was compared with the adiabatic (maximum) deflagration index KG,max calculated from the normal burning velocities and maximum explosion pressures of ethylene–air and ethylene–air-additive mixtures. The differences between experimental and adiabatic deflagration indices highlight the heat losses appearing in the last stage of explosions occurring in cylindrical vessels, strongly influenced by the length to diameter ratio, initial pressure and composition of flammable mixtures.

Ignition dynamics of an annular combustor equipped with multiple swirling injectors

The ignition transient is investigated in this article in a laboratory-scale multiple injector configuration. The analysis specifically focuses on the “light-around” mechanisms that intervene at the start, ensuring flame spreading from one injector to the next, eventually leading to established flames on each injector. The system comprises a plenum feeding premixed gaseous reactants through multiple swirling injectors and an annular combustor formed by two concentric transparent quartz walls that allow full visualization of the flame. Combustion is initiated by a spark igniter located in the neighborhood of one injector. High-speed imaging is used to examine flame propagation and deduce the ignition delay yielding a full light-around of the annular combustor. These data are gathered for a set of operating conditions. Image processing is used to extract the position of the flame front at successive instants. This is then used to identify mechanisms controlling the propagation. It is found that the volumetric expansion across the flame induces an expanding motion that combines with the normal burning velocity and that this defines the flame displacement velocity. In addition, the burned gases formed by ignited injectors convect the flame in the vertical direction, while buoyancy forces accelerate it in the same direction. In azimuthal planes, co-rotating swirlers induce an additional azimuthal velocity component that must be taken into account in the direction of flame propagation. These processes are used to simulate flame spreading from the initial kernel by making use of a modified version of the G-equation. It is found that the simulation provides an approximate description of the evolution of the flame and yields characteristic delay times for light-around, which have the same order of magnitude as those determined experimentally.

Temperature and pressure influence on maximum rates of pressure rise during explosions of propane–air mixtures in a spherical vessel

The maximum rates of pressure rise during closed vessel explosions of propane–air mixtures are reported, for systems with various initial concentrations, pressures and temperatures ([C 3H 8] = 2.50–6.20 vol.%, p 0 = 0.3–1.3 bar; T 0 = 298–423 K). Experiments were performed in a spherical vessel ( Φ = 10 cm) with central ignition. The deflagration (severity) index K G , calculated from experimental values of maximum rates of pressure rise is examined against the adiabatic deflagration index, K G, ad , computed from normal burning velocities and peak explosion pressures. At constant temperature and fuel/oxygen ratio, both the maximum rates of pressure rise and the deflagration indices are linear functions of total initial pressure, as reported for other fuel–air mixtures. At constant initial pressure and composition, the maximum rates of pressure rise and deflagration indices are slightly influenced by the initial temperature; some influence of the initial temperature on maximum rates of pressure rise is observed only for propane–air mixtures far from stoichiometric composition. The differentiated temperature influence on the normal burning velocities and the peak explosion pressures might explain this behaviour.

Experimental Study of Combustion of a Cloud of Boron Particles in Air

Stabilization of a premixed boron—air flame at the Bunsen burner nozzle exit is studied. The normal burning velocity of the gaseous suspension of boron particles is determined by spectral zonal camera recording. Qualitative spectral analysis of the particle-gas flame showed the presence of fluctuation bands in the range of 508–600 nm due to the radiation from BO2 molecules, which confirms gas-phase combustion of the boron particles in the gaseous suspension. Chemical, particle-size, and morphological analyses of the condensed combustion products of powdered boron in air showed the presence of submicron and micron particles with number mean diameters of 100 nm and 14 fum, respectively.

Inerting effect of the combustion products on the confined deflagration of liquefied petroleum gas–air mixtures

Measurements on pressure evolution during the deflagrative combustion of liquefied petroleum gas (LPG)–air gaseous mixtures in the presence of their own exhaust gas, made on flammable mixtures with variable initial pressure, fuel/oxygen ratio and exhaust gas concentration, afforded determination of several flammability parameters: the maximum (peak) explosion pressure, p max, the time to peak pressure, θ max, the maximum rate of pressure rise (d p/d t) max and the explosion index, K G . For the examined mixtures, the adiabatic explosion pressures and flame temperatures of isochoric and isobaric combustion were also calculated, assuming chemical equilibrium is reached. Adiabatic values of the explosion index were calculated from peak explosion pressures and normal burning velocities, available from the same sets of data by evaluation of p( t) records in the early stage of propagation ( Δ p ≤ p 0 ), according to a recent model. The scope of experiments and calculations is the examination of the exhaust gas inerting effect on flammability parameters. Their values are basic input values for evaluation of hazards associated to gaseous deflagrations and for design of safety devices.

The Normal Flame Velocity and Analysis of the Effect of the System Parameters on This Velocity

A process chart of flame combustion of gases is suggested, and the region of existence of the normal (or standard) burning velocity U n is given. The results of analysis of numerous experimental data are used to determine the impact made on U n by parameters such as pressure, flame temperature, chemical composition of the mixture, and preheating of the mixture. Generalizing dependences of the normal burning velocity U n on the adiabatic combustion temperature T a and on the preheating temperature T0 are obtained for a wide range of combustible hydrocarbon gases.

The Characteristics Method and Cusped Flame Fronts

The kinematic effects of a space-time forced velocity field upon a thin premixed flame, stabilized above a circular cross-section burner, are studied in order to point out the non-linearities due to a sufficiently high velocity perturbation level whose RMS amplitudes remain nonetheless inferior to the normal burning velocity. The present calculation proposes to seek a solution using the characteristics method, without any linearized calculation, to express these effects. A front evolution equation is interpreted as the differentiated form of a conservation equation of the radial distance between two points of the front. These modelling results are used to interpret experiments of a vibrating flame subjected to a space-time sinusoidal velocity field. In this last case, the limit of cusps formation is represented as a similarity law expressing the nondimensional perturbation amplitude versus a Strouhal number of the aero-acoustic reactive medium.

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.

Gaseous detonations: their nature, effects and control

1 Introduction.- 1.1 General remarks.- 1.2 Definitions of commonly-used terms.- 1.3 Unidimensional models of detonations.- 1.4 Structure of a detonation wave.- 1.5 Philosophy of presentation.- 2 Unidimensional models.- 2.1 Introductory remarks.- 2.2 Properties of unidimensional shock waves.- 2.3 Properties of unidimensional shock waves with energy addition.- 2.4 Properties of the Chapman-Jouguet state.- 2.5 Comparison of C-J predictions and experiment.- 2.6 The Zel'dovich, von Neumann, Doring model.- 2.7 Comparison of the ZND model and experiment.- 2.8 The Taylor expansion wave.- 2.9 Concluding remarks.- 3 Structure of detonation fronts.- 3.1 General remarks.- 3.2 'Spinning' detonation fronts.- 3.3 'Galloping' fronts.- 3.4 Experimental studies of multi-headed fronts.- 3.5 Theoretical treatments of multi-headed fronts.- 3.6 Concluding remarks.- 4 Detonable media.- 4.1 General remarks.- 4.2 Confined and unconfined detonations.- 4.3 Gases and vapours which are detonable in the absence of an oxidant.- 4.4 Comparison of detonation limits for confined and unconfined detonations with flammability limits for mixtures of hydrocarbons with oxygen and air.- 4.5 Homology hypothesis for predicting detonation limits.- 4.6 Detonations with oxidants other than oxygen.- 4.7 Influence of initial pressure and temperature on detonability.- 4.8 Influence of additives on detonability.- 4.9 Detonations in suspensions of dusts and droplet mists in oxidizing atmospheres.- 5 Initiation of a detonation wave.- 5.1 General remarks.- 5.2 Initiation of confined detonations by shock waves.- 5.3 Initiation by blast waves from electrical and laser sparks and charges of conventional explosives.- 5.4 Detonations in large unconfined clouds of vapour.- 5.5 Minimum ignition energies.- 5.6 Laminar burning velocities.- 5.7 Expansion ratios.- 5.8 Detonations arising from accelerating flames.- 5.9 Influence of initial temperature and pressure of the medium on run-up distances.- 5.10 Influence of diameter of pipeline on run-up distances.- 5.11 Effect of additives on pre-detonation distances.- 5.12 Effects of surface roughness and obstacles on the acceleration of confined flames.- 5.13 Pressure piling (cascading).- 5.14 Concluding remarks.- 6 Interaction of a detonation with confinement.- 6.1 Introductory remarks.- 6.2 Diffraction at an isolated wall, ?w < 0 .- 6.3 Diffraction at an isolated wall, 0 < ?w < ?crit.- 6.4 Diffraction at an isolated wall, ?crit < ?w < 90 standard two- and three-shock theory for non-reactive media and the effects of reaction.- 6.5 Normal reflection of a detonation wave.- 6.6 Transmission of a planar detonation wave through an abrupt expansion in area.- 6.7 Propagation of detonations through bends and junctions.- 6.8 Interaction of a detonation with an inert surrounding gas.- 6.9 Refraction of detonations in mixtures of different composition.- 6.10 Concluding remarks.- 7 Damage caused by detonations.- 7.1 Introduction.- 7.2 Early experiments on effective pressures generated by detonations.- 7.3 Damage produced by detonations in chemical plant.- 7.4 Experimental studies of failure resulting from detonations.- 7.5 Concluding remarks.- 8 Prevention and mitigation of detonations.- 8.1 Introductory remarks.- 8.2 Inhibition of flames of normal burning velocity.- 8.3 Venting in the early stages of an explosion.- 8.4 Quenching of flame-shock complexes.- 8.5 Suppression of detonations.- 8.6 Mitigating the effects of detonations.- 8.7 Concluding remarks.- 9 Concluding recommendations.- 9.1 Introductory remarks.- 9.2 Stress waves in confining walls.- 9.3 Planned deformations as safety measures.- 9.4 Designing to minimize the effects of local peaks in pressure.- 9.5 Suggestions for further studies of detonations.- References.- Author index.

Effect of variable heat loss intensities on the dynamics of a premixed flame front

The theoretical study concerning premixed flame fronts in nonuniform and unsteady flows was developed for an arbitrary gas expansion coefficient for small amplitudes of front corrugation and then extended to the finite amplitude case in the frame of an asymptotic analysis for large values of the Zeldovich number ..beta.. (..beta.. ..-->.. infinity) and in peturbation of the relative stretch flame intensity epsilon. As was anticipated by early semiphenomenological analyses, these theoretical studies point out that two effects, front curvature and flow inhomogeneities, influence the normal burning velocity U/sub n/. But to leading order in the power expansion in epsilon of the relative modification in normal burning velocity, these two effects appear through only one geometric scalar expressing the nondimensional flame stretch tau (d Insigma/dt) experienced by the curved fronts in a nonuniform flow.

Characteristics of the combustion of mixtures of combustible gases with refrigerants during downward flame propagation

An investigation of the limiting conditions of combustion in mixtures of hydrocarbons with refrigerants during downward flame propagation is described. The task undertaken was twofold: to determine whether mixtures at the extreme points of the ignition regions are stoichiometric during combustion to CO and H/sub 2/O, and to investigate the normal burning velocity of limiting mixtures as a function of the refrigerant content, including those refrigerants having an inhibiting effect on combustion. Propane and propylene were each mixed with 1,1,2-trifluorotrichloroethane and with 1,2-dibromotetrafluoroethane and ignited in a combustion chamber with a stabilizing screen and with photodiodes to record the normal burning velocity. The ignition region during downward propagation of the flame was found to be considerably narrower than in the case of upward propagation. Analysis of the compositions of the limiting mixtures corresponding to the stabilization points revealed that the concentrations of combustibles in them are close to stoichiometric. The investigation indicates that the mechanisms of flame extinguishment are probably different for downward and upward propagation, but further investigations are necessary to clarify this problem.