Upward Flame Propagation Research Articles

Experimental measurement and numerical analysis of binary hydrocarbon mixture flammability limits

The flammability limits of binary hydrocarbon mixtures in air were measured in a combustion apparatus using an innovative method developed for this apparatus. The experimental results were obtained at standard conditions (room temperature and ambient atmospheric pressure) with upward flame propagation. The experimentally determined flammability limits for pure hydrocarbons (methane and ethylene) were compared with existing data reported in the literature. Le Chatelier's Law was fit to all experimental data to obtain LFLs and UFLs for various two-component combinations of saturated and unsaturated hydrocarbons (methane, ethylene, acetylene, propane, propylene, and n-butane). A modification of this law was used if experimental observations showed large deviations from Le Chatelier's predictions. Also, experimentally measured flammability limit data of the binary hydrocarbon mixtures were analytically related to the stoichiometric concentrations.

The relationship between flash point and LFL with application to hybrid mixtures

AbstractThere usually is a significant difference between the flash point and the equilibrium liquid/vapor (“saturation”) temperature that corresponds to the lower flammable limit (LFL). This occurs because (1) the LFL is frequently not measured at the flash point temperature and (2) the LFL concentrations are typically for upward propagation of flame, whereas the flash point is obtained for downward propagation of flame, and upward propagation occurs at lower (“conservative”) concentrations. In this paper, guidance is given for “correcting” this difference. Since many “hybrid” mixtures of flammable vapors and combustible dusts occur at elevated temperatures (as in driers), it may be important that the effect of temperature on the LFL be taken into account.For several hybrid mixtures, there is a linear relationship between the dust concentration and the vapor concentration that is required to make the mixture explosible. This relationship is more readily visualized if vapor concentrations are expressed in terms of grams per cubic meter (g/m3) like dust concentrations. A linear relationship is observed when the deflagration indices (Kst and Kg) are about equal, with strong deviations if the indices differ significantly. Testing is advisable to establish the relationship with confidence. © 2007 American Institute of Chemical Engineers Process Saf Prog 2008

Evaluation of Polyimide Foam as a Fire Barrier for Spacecraft Cushion Materials

Polyimide foam is an intrinsically flame resistant foam material. This study uses the NASA upward flame propagation test and the cone calorimeter test to evaluate the application of polyimide foam as a fire barrier for spacecraft cushion materials. The flame propagation test results demonstrate that a thin-layer of polyimide foam (3-mm thick) could totally stop the flame spread on the underneath polyurethane foam in an environment containing 30% oxygen. The cone calorimeter test results show that polyimide foam increased the minimum heat flux for ignition of a cushion sample from 27 to 48kW/m2. This suggests that application of polyimide foam can significantly reduce the ignition risk. It was also found that polyimide foam significantly reduced the peak heat release rate, mass burning rate, and the generation of carbon monoxide and smoke in the flaming combustion. For polyimide foam-covered polyurethane, smoldering combustion was insignificant at 25 and 35kW/m 2 incident heat fluxes; it became more important when the incident heat flux reached 45 kW/m2. The smoldering combustion of polyurethane foam and polyimide foam-covered polyurethane could not be self-sustaining if the external heat source was removed.

Transmission of an explosion between linked vessels

Small scale experiments on explosion transmission through a narrow cylindrical tube (diameter: 3.0–8.3 mm) between two cylindrical vessels were performed, using a stoichiometric C 3H 6/air mixture at various initial pressures within the range 20–120 kPa. The successful explosion transmission between the linked vessels occurred above a critical initial pressure, which increases when the tube diameter decreases. The critical pressure of transmission measured for upward flame propagation is lower than the one for downward propagation, at each value of the tube diameter. Different patterns of pressure evolution were observed for upward and downward flame propagation. The influence of the tube diameter and initial pressure on the characteristic features of explosion evolution in each vessel (the peak pressures, the maximum rates of pressure rise and the time delay between ignition in the primary and secondary vessel) are discussed.

An experimental study of flammability limits of LPG/air mixtures ☆

The liquefied petroleum gas (LPG) is generally considered to be eco-friendly viable fuel not only in domestic sector but also for transport sector. The inhibition of LPG–air premixed flames is a very important practical problem that has received relatively little attention. This paper is concerned with experimental determination of the flammability limits of LPG–air mixture. The standard procedure suggested by US Bureau of mines has been adopted for the present studies for determining the flammability limit of LPG–air mixture. The lower flammability limit (LFL) is found to be 1.81% and upper flammability limit (UFL) is 8.86% of LPG for upward propagation of flame. Whereas, for downward propagation of flame, the LFL and UFL are 1.87 and 7.69% of LPG, respectively. The nitrogen dilution effects on the flammability limits have been explored, which is presented on a flammability limit plot. It is believed that these data will be very useful for developing fire extinguishers and other combustion devices.

Flammability limits of hydrogen–carbon monoxide mixtures at moderately elevated temperatures

Consistent data for the flammability limits of different hydrogen–carbon monoxide mixtures were determined experimentally over a wide range of compositions and initial temperatures up to 573 K (300°C) and at atmospheric pressure for upward flame propagation in a stainless-steel test tube apparatus. Experimental results also showed that the lean flammability limits of hydrogen–carbon monoxide mixtures obeyed Le Chatelier's Rule over the entire range of temperature considered when the corresponding individual limit values of the components were employed. However, the rich flammability limits deviated very significantly from the corresponding values calculated using this Rule, especially for mixtures containing small concentrations of hydrogen. The rich limits predicted by the Rule were narrower than those obtained experimentally. To investigate the existence of pre-ignition reactions at these levels of temperature and their effect on the flammability limits, the hydrogen–carbon monoxide–air mixtures were exposed to elevated temperatures over different periods of time varying from 5 to 30 min before spark ignition was employed to establish whether the mixture is flammable or not. It was shown that duration of the waiting time before spark ignition affected significantly the values of the flammability limits especially at higher temperatures and for rich mixtures. Additionally, the effect of the presence of water vapour in the fuel–air mixture was investigated.

The Effect of Time of Exposure to Elevated Temperatures on the Flammability Limits of Some Common Gaseous Fuels in Air

The flammability limits of methane, ethylene, propane, and hydrogen were experimentally determined at elevated initial mixture temperatures up to 350°C at atmospheric pressure for upward flame propagation in a steel test tube apparatus. The existence of preignition reactions at these levels of temperatures that may influence the value of the flammability limits was also investigated. The fuel-air mixtures were exposed to elevated temperatures over different periods of time before spark ignition (up to 2 h). It was shown that the flammability limits for methane widened approximately linearly with an increase in the initial mixture temperature over the entire range of temperatures tested and were not affected by the length of the exposure time to these temperatures before spark ignition. However, different behaviour was observed for the flammability limits of the other tested fuels—ethylene, propane, and hydrogen. At higher temperatures the flammability limits narrowed and were very significantly affected by the exposure time. The longer was the exposure time of fuel-air mixtures to the elevated temperatures, the narrower were their flammability limits.

Effects of temperature and time of exposure on the flammability limits of HYDROGEN–AIR mixtures

The flammability limits of hydrogen in air were determined for upward flame propagation at elevated temperatures up to 350°C and atmospheric pressure in a conventional stainless steel (316) test tube apparatus. The effect of the extent to which the residence time, i.e. time interval between the time of acquiring the desirable temperature by the hydrogen–air mixture and initiation of spark ignition, and, consequently, the existence of preignition reactions that may influence the value of the flammability limits was also investigated. The flammability limits of hydrogen in air were widened with an increase in the initial temperature up to 200°C. In this initial temperature range the limits were not affected by the length of the residence time. However, at initial temperatures exceeding 200°C the flammability limits, especially, the rich limits narrowed with an increase in the temperature and were very significantly affected by the length of the residence time before spark ignition. This means that the time of exposure of the mixture to elevated temperatures before initiation of ignition should be taken into consideration when determining the flammability limits and should be reported together with the obtained values of the limits. It was suggested that the substantial drop in the value of the rich limit and increase in the value of the lean limit with the increase in the residence time was caused by the relatively low temperature catalytic reactions on the stainless steel surface of the flame tube. A simple method for calculating the hydrogen conversion catalytically to water was proposed. The results of calculations are in fair agreement with the experimental evidence.

Flammability Testing of Flame-retarded Epoxy Composites and Phenolic Composites

Flame-retarded epoxy composites and phenolic composites containing fiberglass, aramid (Kevlar® 49), and graphite fiber-reinforcements were tested using the NASA upward flame propagation test, the controlled-atmosphere cone calorimeter test, and the liquid oxygen (LOX) mechanical impact test. The upward flame propagation test showed that phenolic/graphite had the highest flame resistance and epoxy/graphite had the lowest flame resistance. The controlled-atmosphere cone calorimeter was used to investigate the effect of oxygen concentration and fiber reinforcement on the burning behavior of composites. The LOX mechanical impact test showed that epoxy/fiberglass had the lowest ignition resistance and phenolic/aramid had the highest ignition resistance in LOX. The composites containing epoxy resin and/or aramid fiber reinforcement reacted very violently in LOX upon mechanical impact. © 1997 by John Wiley & Sons, Ltd.

Flame Propagation and Lean-Limit Extinction Within Stratified Mixtures Involving a Diluent Gas

Flame propagation within confined, stratified gaseous environments were investigated experimentally. The diluents nitrogen and helium were used in turn to overlay initially combustible methane-air or hydrogen-air mixtures. Gas stratification was achieved by allowing the two initially homogeneous gases to interdiffuse for a certain period of time at constant temperature and pressure within a long, vertical, smooth, closed, circular tube. Upward flame propagation was examined following spark ignition while the tube was closed at the top but open at the bottom. Near-extinction flame speeds, lower than those predicted according to Davies and Taylor (1950), were obtained with helium dilution. Moreover, estimated reactant concentrations at the observed location of flame extinction indicated, in specific instances, that mixture stratification appears to slightly enhance locally the lean flammability limit. Nonuniform, stratified combustible gaseous mixtures and flame propagation within such mixtures are found in many situations, including in numerous technical applications, as well as in various potentially hazardous circumstances. The leakage of a fuel from storage tanks or pipelines, the formation of layered combustible mixtures within rooms, corridors, or elevator shafts of buildings, the formation of gas pockets in coal mine galleries, and the inerting of flammable mixtures through diluent gas addition are some examples. Some relevant information about the stratification process and the flame propagation characteristics in these specific circumstances has already been reported in the literature by Bakke and Leach (1962), James and Purdy (1962), Girard et al. (1979), Karim and Lam (1986) and Karim et al. (1987). This investigation considers some aspects of flame propagation and mass transfer within confined, stratified gaseous environment. The diluents nitrogen and helium were used in turn to overlay initially combustible methane-air or hydrogen-air mixtures. Mixture stratification was produced by permitting the two initially homogeneous gaseous systems to interdiffuse at constant temperature within a long, vertical, smooth, closed, circular tube. Only upward flame propagation was examined, as this mode is expected to involve the widest flammability limits and the fastest propagation for both homogeneous and stratified methane-air mixtures when confined in tubes (Liebman et al., 1971).

Effect of low temperature on the rich flammability limits in air of hydrogen and some fuel mixtures containing hydrogen

The rich flammability limits in air of hydrogen at atmospheric pressure and for upward flame propagation were established consistently under isothermal initial conditions at low temperatures extending down to −60°C. Moreover, the lean limits were determined down to −100°C. The flammability limits of a range of binary combinations of hydrogen with methane, ethylene and propane were also established. It is shown that the rich flammability limits of hydrogen-ethylene mixtures deviate very significantly from the commonly used predictive technique of Le Chatelier's rule in a wide range of fuel composition. Considerable deviation was also observed for hydrogen-propane and hydrogen-methane mixtures containing relatively small concentrations of propane or methane. The extent of the relative deviation was found to be virtually independent of the initial temperature of the mixtures for the conditions considered.

Effect of low temperature on the rich flammability limits of some gaseous fuels and their mixtures

The rich flammability limits in air of methane, ethane, propane, butane, ethylene and propylene at atmospheric pressure and for upward flame propagation were established consistently under isothermal initial conditions at low temperatures extending down to −60°C. In addition, the flammability limits of a range of binary combinations of their mixtures were also established. On the basis of the experimental data obtained, some guidelines for predicting the rich flammability limits are presented and discussed.

Effect of gravity on laminar premixed gas combustion II: Ignition and extinction phenomena

Minimum ignition energies and flame radii as a function of time were measured for near-limit, limit, and sublimit fuel-lean methaneair mixtures burning at one-g and zero-g. Minimum ignition energy values were the same at one-g and zero-g except for mixtures very near the zero-g flammability limit and leaner, where the zero-g values were much higher than the one-g values. For sublimit mixtures at zero-g a previously unreported mode of unstable flame propagation was observed; this mode was characterized by a flame radius increasing in proportion to the square root of the time lapse from ignition, an energy release often orders of magnitude greater than the spark energy input, and sudden extinction. This mode of flame propagation was observed at all gas pressures tested but was more pronounced at higher pressures. All zero-g propagation was spherically symmetric except for a few unusual flame extinguishments at high pressures. The principal conclusions are that flame extinguishment at zero-g is caused by a flame-front instability and that gravitational forces have a stabilizing effect on upward flame propagation. The cause of the instability could not be determined; further experiments which might aid in determining the cause are suggested.

Flame Propagation Through Atmospheres Involving Concentration Gradients Formed by Mass Transfer Phenomena

The purpose of the present work is to establish experimentally the nature of the phenomena and the rate of fire spread through explosive media involving concentration gradients in a fuel-air mixture. Most of the work reported has been limited to upward flame propagation using methane as a fuel. Wide deviations from the quasi-steady approach were observed when a flame propagates along a negative concentration gradient in lean methane-air mixtures. It appears that the observed rate of fire spread in stratified lean mixtures can be correlated directly in terms of local concentration gradients and the corresponding propagation rates under homogeneous conditions.

Some observations on near-limit flames

Lean mixtures of hydrogen-air and rich mixtures of n -butane-air were spark ignited within a 12-ft-diam sphere. At hydrogen concentrations below 8.0%, and at butane concentrations above 6.1%, the pressure rises were very small since little of the combustible mixture was consumed by upward flame propagation. With the onset of downward propagation at 8.5% hydrogen or 6.0% butane, combustion was complete. The limits of downward flame propagtion are compared with available data on the detonable limits of gas mixtures when initiated by solid explosives. The interpretation follows Spalding's treatment of the optimum gas concentrations for spark ignition, using the concept of preferential diffusion.

The concentration limits for coal dust-air mixtures for upward propagation of flame in a vertical tube

The upper and lower limits of concentration for upward propagation of flame in a vertical tube are given on a probability basis for coal dust-air mixtures. The 0·5 probability value of the lower limit varied from 0·23 to 0·31 g/l. and the upper limit varied from 2·07 to 1·02 g/l.; the actual value depending on the definition of propagation used. The standard errors are given and are all less than ten per cent of the limit values. The merits of this particular method of defining limits are discussed and compared with those of other methods. The apparatus consists of a vertical Perspex tube of 14·2 cm internal diameter and 4·68 m in length, with a vibrating hopper at the upper end which feeds dust into the tube at predetermined uniform rates. The dust, a silkstone coal dust ground to give a nominal 85 per cent through B.S.S.240, is ignited at the open lower end of the tube by means of a small methane flame and the coal dust-air flame propagates upwards towards the closed end.

Explosive Limits of Hydrogen‐Chlorine Mixtures

The lower explosive limit concentration of hydrogen (with upward flame propagation) in mixtures of chlorine containing oxygen, nitrogen, and carbon dioxide in proportions typical of electrolytic chlorine were determined at pressures from 0 to 135 psig. The explosive limit was found to be quite sensitive to variations of pressure and composition. It varied from about 3.1 to about 8.1% hydrogen depending on pressure and chlorine concentration.

The influence of incombustible vapours on the limits of inflammability of gases and vapours in air

The influence of some incombustible vapours upon the upward propagation of flame through mixtures of combustible gases and vapours with air contained in a 1 7/8 in. glass tube has been examined and some comparisons made with results in a 4 in. tube. From the data obtained with the smaller tube it has been possible to plot diagrams for each of the combustibles with inert vapour and air showing completely the compositions of inflammable and non-inflammable mixtures. From these, the limiting ‘safe’ composition of mixtures of inert vapour with air and with the combustible respectively have been derived. The combinations studied were hydrogen, methane, ethylene, carbon monoxide, n -hexane, cyclohexane and benzene with methyl bromide; hydrogen and n -hexane with difluorodichloromethane, and methane with carbon tetrachloride. A sample of coal gas, the composition of which was determined, has also been examined with methyl bromide as the inert vapour, and it is shown that the limit curve agrees fairly well with that calculated by Le Chatelier’s rule from the curves for its constituents.

Seminar on chemical induction of genetic changes in Aspergilli

The rich flammability limits in air of hydrogen at atmospheric pressure and for upward flame propagation were established consistently under isothermal initial conditions at low temperatures extending down to −60°C. Moreover, the lean limits were determined down to −100°C. The flammability limits of a range of binary combinations of hydrogen with methane, ethylene and propane were also established. It is shown that the rich flammability limits of hydrogen-ethylene mixtures deviate very significantly from the commonly used predictive technique of Le Chatelier's rule in a wide range of fuel composition. Considerable deviation was also observed for hydrogen-propane and hydrogen-methane mixtures containing relatively small concentrations of propane or methane. The extent of the relative deviation was found to be virtually independent of the initial temperature of the mixtures for the conditions considered.