I. Glassman

A Multiple-step Overall Kinetic Mechanism for the Oxidation of Hydrocarbons

Abstract —Extensive experimental results were obtained on the oxidation of many aliphatic hydrocarbons in a high temperature, turbulent flow reactor developed for kinetic studies. These results indicated the viability of presenting this complex kinetic situation in the format of a simplified, overall kinetic scheme which could accurately predict the major species formed and the temperature-time history (rate of heat release) of the system. The proposed overall mechanism follows the general form:

Sooting Behavior of Gaseous Hydrocarbon Diffusion Flames and the Influence of Additives

Abstract The sooting behavior of laminar diffusion flames were altered by addition of diluents to the fuel flow. Fuel additives which decrease the sooting tendency had an effectiveness in order of their molar specific heat capacity; helium exhibited some exaggerated trends, probably due to its high thermal diffusivity. The effect of water vapor, carbon dioxide and sulphur dioxide in reducing the tendency to soot was purely thermal with no observable chemical interaction. Trace amounts of oxidizers and halogenated compounds increased the sooting tendency substantially. Results from tests with blended fuels and fuel-hydrogen nitrogen mixtures in which the C/H ratio and temperature were controlled revealed that C/H ratio was not a dominant parameter in the sooting tendency. These results and temperature measurements provided evidence that the sooting behavior of a diffusion flame is controlled by the initial fuel pyrolysis which is dominated by the flame temperature and the field it causes. Since accurate ca...

Flame spread in an opposed forced flow: the effect of ambient oxygen concentration

The velocity of flame propagation over the surface of thick PMMA and thin paper sheets has been measured as a function of the velocity and oxygen concentration of a forced gas flow opposing the direction of flame propagation. It is shown that although for thin fuels the flame spread rate always decreases as the opposed flow velocity increases, for thick fuels the dependence of the spread rate on the gas velocity is also a function of the ambient oxygen concentration. For low oxygen concentrations the flame spread rate decreases as the velocity of the gas flow increases. For high oxygen concentrations, however, the spread rate increases with the flow velocity, reaches a maximum and then decreases as the gas velocity increases. The velocity of the opposed flow at which the maximum occurs is a function of the oxygen concentration, decreasing as the concentration decreases. Following phenomenological considerations and simplified descriptions of the primary mechanisms occurring during the flame spread process, the experimental results are correlated by two non-dimensional parameters, one describing the gas phase kinetic effects and the other describing the process of heat transfer from the flame to the fuel. Such a correlation provides a powerful means of predicting the flame spread prcess as well as physical insight into the mechanisms controlling the propagation of the flame.

Sooting Correlations for Premixed Flames

Abstract —Using a Bunsen-type burner in which the name temperature was controlled by nitrogen addition, the tendency of hydrocarbon fuels to form soot has been determined systematically. The effective sooting equivalence ratio reported is based on the stoichiometric relation in which the carbon and hydrogen in the fuel are converted to carbon monoxide and water. respectively, rather than the conventional equivalence ratio or the C/O ratio. The temperature effect is clearly shown by plotting the log of the effective equivalence ratio versus the reciprocal of the (adiabatic) flame temperature, and this dependency is similar for all fuels examined and is relatively weak. At a given flame temperature, an increase in either the number of carbon atoms or the C/H ratio in the fuel molecule increases the tendency to soot. These effects are attributed to the increased pyrolysis rate and the decreased oxidative attack, respectively. No significant effects are found with regard to the structure of the fuel molecule....

Influence of Laboratory Parameters on Flame Spread Across Liquid Fuels

Abstract The rate of flame spread across the surface of liquid fuels (mainly n-decane, but also n-nonane, n-undecane, dipentene, kerosene, and n-butanol) has been examined in detail for a wide range of laboratory variables. Many hundreds of carefully controlled tests have been performed aimed at elucidating the dependence of the flame spreading velocity on (1) the purity of the fuel, (2) the mode of ignition, (3) the temperature of the fuel. (4) the dimensions and material of the fuel container, and (5) the depth of the fuel layer. The effects of environmental parameters (atmospheric pressure, air temperature and relative humidity) have also been investigated, but non-systematically. The results show that the flame spreading velocity is susceptible to changes in most of these parameters. This work constitutes the first systematic variation of all the laboratory variables with the view to obtaining flame spreading data which are meaningfully applicable to any apparatus and also to ‘real’ liquid fuel fires....

Sooting Counterflow Diffusion Flames with Varying Oxygen Index

Abstract Varying oxygen concentrations was found to have pronounced effects on the sooting characteristics of diffusion flames. Ethylene and propane flames were stabilized around a porous ceramic cylindrical burner mounted in a vertically oriented, low turbulence wind tunnel. Laser light scattering and extinction, laser Doppler velocimetry and thermocouple temperature measure- ments were obtained along the forward stagnation line. Measurements of soot particle size, number density and volume fraction showed that with an increasing oxygen index the soot volume fractions in both ethylene and propane flames increased at all locations along the stagnation stream- line. For ethylene flames (when the oxygen index increased from 0.18 to 0.28). the maximum soot volume fraction increased by almost an order of magnitude. Propane was found to produce about one half the volume of soot of an ethylene flame at the same oxygen index. The increase in soot volume fraction with increasing oxygen index was attributed to bot...

Flame Spreading Above Liquid Fuels: Surface-Tension-Driven Flows

Abstract Convective heat transfer through the liquid fuel below a spreading flame is considered as a rate controlling mechanism. Thus, a surface-tension-driven liquid flow, induced by the temperature profile ahead of a spreading flame, is analyzed. Velocities, pressures and surface heights are determined for a two-dimensional flame spreading at a steady rate. It is demonstrated that convection can occur near the suface ahead of the flame and in the direction of propagation and, thus, that liquid-phase convective heat transfer can be a plausible rate-controlling mechanism for flame propagation.

High temperature oxidation of aromatic hydrocarbons

High temperature (≈1200 K) oxidationstudies of benzene, toluene and ethyl benzene reveal striking similarities of oxidation pathways in that the overall rate is dominated by the rate of oxidation of the phenyl radical which forms in all three cases. This conclusion is supported by detailed species profiles taken in a turbulent flow reactor. From these profiles a mechanism also has been postulated for phenyl oxidation which follows the route phenoxy, ketocyclohexadienyl, cyclopentaldienyl, butadienyl, vinyl acetylene, butadiene, acetylene and vinyl radicals. CO is expelled from the ring structures to give the next lower order hydrocarbon radical.

Flame Propagation Through Layered Fuel-Air Mixtures

Abstract It is observed that a layered, unconfined combustible mixture supports a flame propagation velocity of 4 to 5 times the laminar flame speed of the stoichiometric mixture of fuel and air. This effect which has been observed by some and more recently in this laboratory, has never been adequately explained. It is in contrast to the classical soap bubble experiment, in which the flame propagation velocity is of the order of 7 to 8 times the laminar flame speed, that is, the order of the unburned to burned gas density ratio To explain this new effect two physical models with different degrees of complexity are explored here. The first is a step-wise premixed fuel-air and pure air case in a gallery of infinite length. The second deals with one of finite length. Both have been mathematically defined to give quantitative results so that the effects of environmental factors can be explicitly determined Solutions of the first model show that the maximum flame propagation speed occurs when the gallery is of...

Fuel Oxygen Effects on Soot Formation in Counterflow Diffusion Flames

Abstract The effect on the soot formation processes of the addition of small quantities of oxygen to the fuel has been studied by making velocity, temperature, soot volume fraction, particle number density, and fluorescence measurements along the frontal stagnation streamline of counterflow diffusion flames established around a porous cylindrical burner. Flames of five fuels with and without oxygen in the fuel stream were studied. In ethene flames, addition of 10 percent oxygen resulted in an increase in the final soot volume fraction (near the stagnation point) of more than 100 percent. For the other fuels, namely propene, propane, isobutane, and n-butane, little change was observed. The difference between ethene and the other fuels is explained on the basis of the H radical concentrations. Oxygen addition decreased the fluorescence signal levels in all the flames, except ethene where an increase was measured.