Richard L. Axelbaum

The Effect of Flame Structure on Soot-Particle Inception in Diffusion Flames

Abstract The effect of flame structure on soot particle inception is studied by varying the mixture fraction at stoichiometry Z st and, consequently, flame location. Z st is varied by reassigning the nitrogen from the oxidizer to the fuel such that the flame temperature is not changed. Strain rates in the counterflow flame and flame heights in the coflow flame are measured in ethylene flames to identify the sooting limits based on the appearance of luminous soot. Numerical experiments with a counterflow diffusion-flame code employing C2 kinetics are also performed to understand the effects of Z st on flame structure and to interpret the experimental results. The results show that as Z st is increased and the flame shifts towards the fuel, soot inception is suppressed. In the counterflow flame, no luminous soot is detected at a strain rate greater than 60 s −1 for Z st ≥ 0.16, as compared to 175 s −1 for the ethylene/air flame, Z st = 0.064. The laminar ethylene coflow flame is soot free at Z st ≥ 0.72, regardless of flame height. For small Z st , where changes in Z st are primarily due to changes in fuel concentration, the effect on soot inception is primarily fuel dilution. For large Z st , where the oxygen concentration is appreciably increased, the subsequent shift in the OH profile towards the fuel side of the flame can have a dramatic influence on inception. The shift in OH essentially narrows the soot zone suggesting that it may be possible to obtain soot-free conditions for many fuels if the structure of the flame can be adjusted to the extent that significant OH exists on the fuel side of the flame at a temperature of ca. 1300 K. The experimental and numerical results demonstrate that this requirement can be satisfied for ethylene flames.

The influence of carbon dioxide and oxygen as additives on soot formation in diffusion flames

A study of carbon dioxide and oxygen addition on soot formation has been performed such that the effects of dilution, temperature and direct chemical participation have been isolated for the additives on both the fuel and oxidizer sides. By measuring soot inception limits in the counterflow flame and integrated soot volume fractions in the coflow flame, the influence of the additives on soot inception, growth and burnout has also been ascertained. Results demonstrate that carbon dioxide, whether added to the fuel or oxidizer side, can suppress soot formation chemically. The effect of oxygen addition is more complex. When added to the fuel side of an ethylene flame, the addition leads to an abrupt increase in the inception limit, indicating that the inception chemistry has been accelerated. The addition to propane, however, is initially suppressive and results in a significant reduction in the soot inception limit which is more than can be accounted for by dilution. The addition becomes promoting as the oxygen mole fraction approaches 40%. Finally, the effect of oxygen concentration on the oxidizer side, for both ethylene and propane flames, is almost totally thermal.

Thermophoretic Effects on Seeding Particles in LDV Measurements of Flames

Abstract The motion of LDV seeding particles under the influence of viscous and thermophoretic forces in the rapidly-accelerating, high-temperature-gradient flame environment was studied via the counterfiow premixed twin-flame configuration. Results demonstrate that thermophoretic force can induce significant lag between the fluid and,particle velocities in the active preheat zone of a flame, and suggest that caution should be exercised when interpreting LDV data obtained in this region. A thermophoretic velocity correction to the LDV-determined velocity, with known experimental or computational temperature profile, is proposed. Additional considerations of LDV diagnoslics and its determination of laminar flame speed are also presented.

On the structural sensitivity of purely strained planar premixed flames to strain rate variations

Abstract The effects of aerodynamic straining on the structure and response of adiabatic, unrestrained, equidiffusive, planar premixed flames were experimentally and computationally studied via the counterflow, twin-flame configuration formed by oppositely directed identical jets of nitrogen-diluted, near-stoichiometric methane/air mixtures. Experimentally, the velocity, temperature and major species concentration profiles were determined as functions of the applied strain rate by using LDV and spontaneous Raman scattering. Computationally, the experimental situation was simulated with detailed reaction mechanisms and transport properties. Both the experimental and computational results show that the temperature and species structure of the flame in the direction normal to the flame surface remains largely similar in response to variations in strain rate as long as the flame is sufficiently far away from the stagnation surface so that incomplete reaction is minimal. These results substantiate the concepts that the scalar structure of the flame, and thereby the flame thickness, are insensitive to strain rate variations for these purely strained flames, and that these flames cannot be extinguished by straining alone. The computed results are further shown to agree quantitatively with the experimental data, hence supporting the usefulness of the computational model for the simulation of strained flames. Implications of present findings on the concept of the local flow time, the extinction of strained flames, the modelling of turbulent flames through the concept of laminar flamelets, and flame stabilization and blowoff, are discussed.

Soot formation in strained diffusion flames with gaseous additives

The effects of various gaseous additives on soot formation in strained diffusion flames are reported. The additives N2, Ar, He, H2, and CO were introduced with fuels C2H4, C3H8, and C4H10, and were selected to isolate the effects of dilution, temperature, preferential diffusion, and active chemical participation resulting from the additive. Special emphasis was placed on understanding the mechanisms by which CO and H2 addition influence soot inception. Measurements were made of the limiting strain rate for complete suppression of soot, i.e., the soot-particle inception limit, Kp, in the counterflow diffusion flame. Some laser-extinction measurements of soot volume fraction were also made in the coflow flame to determine the applicability of the results to this geometry. The addition of inerts to the fuel decreases the sooting limit due to the reduction in fuel concentration and temperature. Concentration modification due to preferential diffusion enhances the suppressive effect of He, causing it to be the most effective additive considered. The behavior of the reactive additives is more complex. The addition of H2 increases flame temperature but decreases Kp for the fuels considered. Preferential diffusion is partially responsible for this behavior, however direct chemical suppression may also play a role in the strongly suppressive effects of this additive. The chemical role of H2 is discussed in the context of Frenklachs H abstraction/C2H2 addition model for PAH formation. Carbon monoxide addition to C2H4 results in a monotonic decrease in Kp that is primarily a consequence of dilution. For CO addition to the alkanes there is initially an increase in Kp followed by a decrease for XCO > 0.5, suggesting a small chemical enhancement. Coflow results tend to support these findings: For C2H4 the results are consistent with dilution while for C3H8 a small chemical enhancement combined with suppression due to dilution nets a weak suppression of soot formation. This finding, that CO can enhance inception chemistry in alkanes, requires further study.

Effects of Structure and Hydrodynamics on the Sooting Behavior of Spherical Microgravity Diffusion Flames

Abstract This study is an examination of the sooting behavior of spherical microgravity diffusion flames burning ethylene at atmospheric pressure in a 2.2-s drop tower. In a novel application of microgravity, spherical flames were employed to allow convection across the flame to be either from fuel to oxidizer or from oxidizer to fuel. Thus, spherical microgravity flames are capable of allowing stoichiometric mixture fraction, Z st , and direction of convection across the flame to be controlled independently. This allowed for a study of the phenomenon of permanently blue diffusion flames—flames that remain blue as strain rate approaches zero. Z st was varied by changing inert concentrations such that adiabatic flame temperature did not change. At low Z st , nitrogen was supplied with the oxidizer, and at high Z st , it was provided with the fuel. Flame structure, quantified by Z st , was found to have a profound effect on soot production. Soot-free conditions were observed at high Z st and sooting conditions were observed at low Z st regardless of convection direction. Convection direction was found to have a smaller impact on soot inception, suppressing formation when convection at the flame sheet was directed towards the oxidizer. A numerical analysis was developed to simulate steady state conditions and aided the interpretation of the results. The analysis revealed that steady state was not achieved for any of the flames, but particularly for those with pure ethylene or oxygen flowing from the porous burner. Furthermore, despite the fact that all flames had the same adiabatic flame temperature, the actual peak temperatures differed considerably. While transient burner heating and burner radiation reduced flame temperature, gas-phase radiative heat loss was the dominant mechanism accounting for these differences.

Nanoscale unagglomerated nonoxide particles from a sodium coflow flame

Abstract A coflow burner has been developed to react sodium vapor with gaseous halides in a flame configuration similar to the hydrocarbon coflow flame, and produce metals, composites, and nonoxide ceramics. Nanometer-sized particles of elemental titanium and titanium diboride were successfully produced with this burner as indicated by XRD. In a novel approach, the flame is operated under conditions that lead to condensation of the NaCl byproduct onto the particles. The NaCl coating acts to control size and eliminate agglomeration of the particles in the flame because the particles are encapsulated in NaCl. The coating was also found to be highly effective at limiting oxidation during postflame handling of Ti powders. TEM analysis indicates that the as-received products are composed of individual nanoparticles in an NaCl matrix. The NaCl coating has been efficiently removed from the TiB2 particles by both water washing and sublimation at 800°C under vacuum. It is proposed that this method of encapsulation, with a removable coating-material, is a general method of obtaining unagglomerated nanoparticles in a flame. Other applications and coating materials need to be explored.

Experiments on the sooting limits of aerodynamically-strained diffusion flames

An experimental study has been performed with axisymmetric counterflow diffusion flames to investigate the influence of aerodynamic straining on the relevant sooting limits of the lower alkanes. The limits are defined by the critical strain rate at which either soot luminosity, soot particle light-scattering, or fluorescence is negligible compared to the appropriate background signal. The critical strain rates are found to be greatest for the sooting limit based on the fluorescence signal, with those based on luminosity and light-scattering being similar. The fluorescence signal, if attributed to polycyclic aromatic precursors, yields a limit that can be interpreted as the extinction of soot precursors and is suggested to be a possible limit for identifying a completely nonsooting flame condition. The separate effects of flame temperature and fuel concentration on the critical strain rates for soot extinction have also been studied. The results are indicative of how temperature and concentration influence the soot particle inception process and they show that both are potentially important parameters. The critical strain rates display an Arrhenius temperature dependence and this dependence is similar for all alkanes considered.

Gas-phase combustion synthesis of titanium boride (TiB 2 ) nanocrystallites

Two techniques are described for synthesizing nanometer-sized TiB{sub 2} particles by gas-phase combustion reactions of sodium vapor with TiCl{sub 4} and BCl{sub 3}: a low-pressure, low temperature burner and a high-temperature flow reactor. Both methods produce TiB{sub 2} particles that are less than 15 nm in diameter. The combustion by-product, NaCl, is efficiently removed from the TiB{sub 2} by water washing or vacuum sublimation. Material collected from the low-temperature burner and annealed at 1000{degree}C consists of loosely agglomerated particles 20 to 100 nm in size. Washed material from the high-temperature flow reactor consists of necked agglomerates of 3 to 15 nm particles. A thermodynamic analysis of the Ti/B/Cl/Na system indicates that near 100{percent} yields of TiB{sub 2} are possible with appropriate reactant concentrations, pressures, and temperatures. {copyright} {ital 1996 Materials Research Society.}

Soot formation and inert addition in diffusion flames

An experimental study has been conducted in coflow diffusion flames in order to identify the relative importance of fuel concentration dilution and flame temperature reduction on soot formation when inert is added to fuel. Two different methodologies were used to isolate dilution and temperature effects, both involving substitution of inerts with different specific heats. To quantify the extent of soot reduction, laser-light extinction as well as smoke point measurements were made. The results are in agreement with previous studies in counterflow flames and show that soot formation rates in the coflow flame behave nearly linearly with fuel concentration. Furthermore, white temperature exerts a strong influence on soot formation, dilution can also affect formation rates and smoke points when inerts are added to fuels. It is found that the relative importance of dilution and temperature depends on the extent of addition. When moderate amounts of inert are added, the temperature reduction is typically very small so that the effect of dilution can be considerably greater than that of temperature. When large amounts of inert are added, temperature effects may dominate those of dilution although, in an absolute sense, dilution effects could still be important because fuel concentrations are low.