Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.     

Effects of structure and hydrodynamics on the sooting behavior of spherical microgravity diffusion flames

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.     

A model for sooting in diffusion flames

For laminar diffusion flames the onset of sooting can be predicted by a simple extension of the Bauke-Schumann theory of such flames. Deviations from these predictions may be caused by effectively finite soot-oxidation rates and by enhanced mixing due to turbulence. Both effects have been enhanced. It is shown that the fraction of carbon in the fuel liberated as soot is determined largely by the physical characteristics of the flame rather than by the detailed chemical nature of the fuel. The effect on soot production of the addition of oxygen to the fuel is studied under conditions for which the physical parameters are carefully controlled.     

Measurements of sooting tendency in laminar diffusion flames of n-heptane at elevated pressure

This research focuses on the effects of an increasing pressure on the soot formation during combustion of vaporized liquid fuel. Therefore soot formation is measured in a laminar diffusion flame, with n-heptane as fuel, over a range of pressures from 1.0 to 3.0 bar. The soot volume fraction in the diffusion flames has been measured using Laser-Induced Incandescence (LII) calibrated by means of the Line Of Sight Attenuation (LOSA) technique. The values of the calibration factors between LII intensities and soot volume fraction from LOSA are slightly varied for different pressure. The integral soot volume fractions show power law dependence on pressures, being proportional to pⁿ, with n being 3.4 ± 0.3 in the pressure range of 1.0–3.0 bar.     

Effect of strain rate on sooting limits in counterflow diffusion flames of gaseous hydrocarbon fuels: Sooting temperature index and sooting sensitivity index

The effect of the strain rate on the sooting limits in counterflow diffusion flames was investigated in various gaseous hydrocarbon fuels by varying the nitrogen dilution in the fuel and oxidizer streams. The sooting limit was defined as the critical fuel and oxygen mole fraction at which soot started to appear in the elastic light scattering signal. The sooting region for normal alkane fuels at a specified strain rate, in terms of the fuel and oxygen mole fraction, expanded as the number of carbon atoms increased. The alkene fuels (ethylene, propene) tested had a higher propensity for sooting as compared with alkane fuels with the same carbon numbers (ethane, propane). Branched iso-butane had a higher propensity for sooting than did n-butane. An increase in the strain rate reduced the tendency for sooting in all the fuels tested. The sensitivity of the sooting limit to the strain rate was more pronounced for less sooting fuels. When plotted in terms of calculated flame temperature, the critical oxygen mole fraction exhibited an Arrhenius form under sooting limit conditions, which can be utilized to significantly reduce the effort required to determine sooting limits at different strain rates. We found that the limiting temperatures of soot formation flames are viable sooting metrics for quantitatively rating the sooting tendency of various fuels, based on comparisons with threshold soot index and normalized smoke point data. We also introduce a sooting temperature index and a sooting sensitivity index, two quantitative measures to describe sooting propensity and its dependence on strain rate.     

Parametric effects on sooting in turbulent acetylene diffusion flames

Optical extinction and temperature measurements have been made in free turbulent diffusion acetylene flames over a wide range of velocities and nozzle sizes. The measurements show that the soot volume profiles scale as the flame time constant. Soot formation and oxidation rates are controlled by mixing rates at low flow rates and by soot kinetics at high flow rates. The temperature in the downstream oxidizing region is the most important determinant of the flame's smoking propensity. Inert diluents greatly reduce soot formation and burnout rates and O₂ addition increases formation rates.     

Effect of ferrocene addition on sooting limits in laminar premixed ethylene-oxygen-argon flames

The critical sooting C/O ratio was measured for a series of atmospheric-pressure, laminar premixed ethylene-oxygen-argon flames doped with a small amount of ferrocene. Comparisons of the doped and undoped flames show marked increases in sooting tendency for flames doped with ferrocene. Under the same flame conditions, the critical C/O ratios in doped flames were uniformly lower than those of undoped flames. A five-step reaction mechanism of ferrocene decomposition, leading to the formation of the cyclopentadienyl, was proposed. Detailed kinetic modeling of the experimental flames showed little changes in the flame temperature and the aromatic concentration upon ferrocene doping. The experimental and computational results support the early suggestion that in premixed flames the increase in sooting tendency due to ferrocene addition is the result of induced nucleation by iron oxide nanoparticles. These particles provide a surface to initiate soot surface growth.     

Cool sooting flames of hydrocarbons

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Investigation of the effect of molecular structure on sooting tendency in laminar diffusion flames at elevated pressure

A specially designed High Pressure Vessel and Burner and fueling system (called “doped flame”) are presented in this paper. This setup allows for soot measurements in laminar diffusion flames of liquid fuel blends at elevated pressure. Fuels with two typical molecular structures, namely linear and saturated cyclic hydrocarbons, are examined in both non-oxygenated (n-hexane (C₆H₁₄) and cyclohexane (C₆H₁₂)) and oxygenated form (1-hexanol (C₆H₁₄O) and cyclohexanol (C₆H₁₂O)). All compounds are blended into n-heptane. Focus of the research is on soot volume fraction at elevated pressure in the range of 1.5–2.0 bar. Sooting tendency is evaluated by means of Laser Induced Incandescence (LII) with Line of Sight Attenuation calibration (LOSA), and the data suggests that soot is more prevalent for cyclic structures relative to their linear counterparts.     

Radiative extinction of gaseous spherical diffusion flames in microgravity

Radiative extinction of spherical diffusion flames was investigated experimentally and numerically. The experiments involved microgravity spherical diffusion flames burning ethylene and propane at 0.98 bar. Both normal (fuel flowing into oxidizer) and inverse (oxidizer flowing into fuel) flames were studied, with nitrogen supplied to either the fuel or the oxygen. Flame conditions were chosen to ensure that the flames extinguished within the 2.2 s of available test time; thus extinction occurred during unsteady flame conditions. Diagnostics included color video and thin-filament pyrometry. The computations, which simulated flow from a porous sphere into a quiescent environment, included detailed chemistry, transport, and radiation and yielded transient results. Radiative extinction was observed experimentally and simulated numerically. Extinction time, peak temperature, and radiative loss fraction were found to be independent of flow rate except at very low flow rates. Radiative heat loss was dominated by the combustion products downstream of the flame and was found to scale with flame surface area, not volume. For large transient flames the heat release rate also scaled with surface area and thus the radiative loss fraction was largely independent of flow rate. Peak temperatures at extinction onset were about 1100 K, which is significantly lower than for kinetic extinction. An important observation of this work is that while radiative heat losses can drive transient extinction, this is not only because radiative losses are increasing with time but also because the heat release rate is falling off as the flame expands away from the burner and the reactant supply to the flame decreases.     

Kinetic and radiative extinctions of spherical burner-stabilized diffusion flames

Extinction of steady, spherical diffusion flames stabilized by a spherical porous burner was investigated by activation energy asymptotics. An optically-thin radiation model was employed to study the effect of radiation on flame extinction. Four model flames with the same adiabatic flame temperature and fuel consumption rate but different stoichiometric mixture fraction and flow direction, namely the flames with fuel issuing into air, diluted fuel issuing into oxygen, air issuing into fuel, and oxygen issuing into diluted fuel, were adopted to understand the relative importance of residence time and radiation intensity. Results show that for a specified flow rate emerging from the burner, only the kinetic extinction limit at low Damköhler numbers (low residence times) exists. In the presence of radiative heat loss, extinction is promoted so that it occurs at a larger Damköhler number. By keeping the radiation intensity constant while varying the flow rate, both the kinetic and radiative extinction limits, representing the smallest and largest flow rates, between which steady burning is possible, are exhibited. For flames with low radiation intensity, extinction is primarily dominated by residence time such that the high-flow rate flames are easier to be extinguished. The opposite is found for flames suffering strong radiative heat loss. The kinetic extinction limit might occur at mass flow rates lower than what is needed to keep the flame outside of the burner and not observable. An extinction state on the radiative extinction branch can be either kinetic or radiative depending on the process.     

A computational study of oscillatory extinction of spherical diffusion flames

The transient behavior of burner-supported spherical diffusion flames was studied in the transport-induced limit of low mass flow rate and the radiation-induced limit of high mass flow rate which characterize the isola response of flame extinction. Oscillatory instability was observed near both steady-state extinction limits. The oscillation typically grows in amplitude until it becomes large enough to extinguish the flame. The oscillatory behavior was numerically observed using detailed chemistry and transport for methane (50%CH₄/50%He into 21%O₂/79%He) and hydrogen (100% H₂ into 21%O₂/79%He) diffusion flames where the fuel was issued from a point source, and helium was selected as an inert to increase the Lewis number, facilitating the onset of oscillation. In both methane and hydrogen flames, the oscillation always leads to extinction, and no limit cycle behavior was found. The growth rate of the oscillation was found to be slow enough under certain conditions to allow the flame to oscillate for over 450 s, suggesting that such oscillations can possibly be observed experimentally. For the hydrogen flames, however, the frequency of oscillation near the transport-induced limit is much larger, approximately 60 Hz as compared to 0.35 Hz for the methane flame, and the maximum amplitude of temperature oscillations was about 5 K. The distinctively different structures of the hydrogen and methane flames suggest that while both instabilities are thermal-diffusive in origin, oscillations in the hydrogen flames resemble those of premixed flames, while oscillations in the methane flames are non-premixed in character.     

Discrete reaction model for composition of sooting flames

A discrete reaction model of sooting combustion is proposed on the grounds of multi-stage representation of oxidation chemistry. It is demonstrated that the predictions are in fair agreement to the measured data, and show the correct trends with no adjustments to the soot modelling concept. The practical applications of the model are also presented. The algebraic nature of the model relationships makes it easy to bring them into the computational loops of available predictive tools, so that it is believed the present model has the potential to supplant or complement the similar methods in the engineering computational analysis of combustion.     

Effects of variable density on response of spherical diffusion flames under rotation

The spherical diffusion flame generated by either a porous burner or a fuel droplet in response to rotational motion was investigated through perturbation analysis, with emphasis on the effects variable density. While it was shown that main feature of the problem was adequately described by the constant-density model, the variable-density formulation revealed two new insights: (1) perturbations due to rotation decrease substantially as compared with the constant-density formulation, suggesting that the perturbing effects of rotation are substantially absorbed and thereby mitigated by the density variation, and (2) magnitude of the perturbations strongly depends on the ratio of the burner/droplet surface temperature to the ambient temperature.     

Soot inception limits in laminar diffusion flames with application to oxy-fuel combustion

For diffusion flames, the combination of oxygen enrichment and fuel dilution results in an increase in the stoichiometric mixture fraction, Zst, and alters the flame structure, i.e., the relationship between the local temperature and the local gas composition. Increasing Zst has been shown to result in the reduction or even elimination of soot. In the present work, the effects of variable Zst on soot inception are investigated in normal and inverse coflow flames, using ethylene as the fuel. Use of the inverse coflow flame underscores the validity of these concepts, since the convective field in the inverse flame directs particles into the fuel-rich region. Sooting limits based on particle luminosity are measured as a function of Zst. The sooting limit is obtained by varying the amount of inert gas until soot appears above a predefined height. For each limit flame, the adiabatic flame temperature is calculated and the flame temperature at the half-height is measured. The flame temperature at the sooting limit is found to increase with Zst for both normal and inverse flames. The effects of residence time are also investigated, and the sooting limit inception temperature is found to be dependent on fuel stream velocity for both the normal and inverse configurations. A simple model applicable to oxy-fuel combustion is presented which describes how increasing Zst results in the reduction and ultimately elimination of soot. This model assumes that soot inception can only occur in a region where critical values for species, temperature, and residence time are met. The soot inception region is shown to be bounded by two isotherms: a low-temperature boundary that is a function of residence time, and a high-temperature boundary that corresponds to the location of a critical local carbon-to-oxygen ratio. The effect of increasing Zst is to move the boundaries of the soot inception zone towards each other, until the zone is infinitely thin and thus the sooting limit is reached. By comparing the model to experimental data, a critical local C/O ratio of 0.53 and a sooting limit inception temperature of 1640 K (for a characteristic residence time of 22 ms) were determined for ethylene.     

Effect of partial premixing on the sooting structure of methane flames

An experimental investigation of the sooting structure of diluted methane–oxygen counterflow flames is reported for partial premixing in the following two nonpremixed flame configurations:

• Case 1:Nonpremixed flame on the oxidizer side of the stagnation plane,
• Case 2:Nonpremixed flame on the fuel side of the stagnation plane.

Effects of both fuel-side and oxidizer-side partial premixing for Cases 1 and 2 were investigated in a low-strain-rate (∼6–8 s⁻¹) counterflow flame. Computations using OPPDIF code were in excellent agreement with the measured concentrations of major species and [OH]. Distribution of measured soot volume fraction and particle sizes are presented along with measured distributions of C₂ hydrocarbon species. Soot loading can increase or decrease depending on (a) the level of partial premixing, (b) the side of partial premixing (fuel side or oxidizer side), and (c) the nonpremixed flame configuration. Of particular interest is the trend for fuel-side partial premixing of Case 1, where the peak soot loading, the peak soot particle diameter, and the thickness of the soot zone initially decrease and then increase with progressive partial premixing. The trends presented are discussed based on chemical, dilution, and flow-field effects of partial premixing on soot growth in counterflow flames. Unlike previous literature, which focused on soot inception, this work emphasizes the role of partial premixing on soot growth by taking into account the changes in the temperature–time history of soot particulates in addition to the previously reported “chemical” and “dilution” effects.     

Numerical and experimental study on silica generating counterflow diffusion flames

Temperatures and concentrations of OH radicals in silica generating counterflow oxy-hydrogen diffusion flames are measured using a broadband coherent anti-Stokes Raman spectroscopy (CARS) and a planar laser induced fluorescence (PLIF) techniques to study thermo-chemical effects of SiCl₄ addition to flames. Numerical analysis considering detailed chemical reactions including silica generating reactions is also conducted. The experimental results demonstrate that temperatures decrease in preheated zone due to the increase in specific heat of the gas mixture while the decrease is mitigated in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl₄. Also, OH concentrations significantly decrease in silica formation flame, which can be attributed to the consumption of oxidative radicals during the silica generating reactions of SiCl₄ and depletion of OH by HCl. The numerical simulation agrees well for flames having relatively low flame temperatures of 1750 K but underestimates the decrease in OH concentration for high temperature flame over 2700 K. The disagreement for the high temperature flames would imply possible OH consumption via direct reactions between OH radicals and silicon chlorides, which is expected to be highly sensitive to temperature.