Unified behaviour of maximum soot yields of methane, ethane and propane laminar diffusion flames at high pressures

Soot concentration and temperature distributions within the flame envelope of laminar diffusion flames of methane and ethane at elevated pressures were measured in a high-pressure combustion chamber. Methane measurements were made with two different fuel flow rates: 0.43 mg/s (0.32 mg/s carbon flow rate) for the pressure range of 15–60 atm, and 0.83 mg/s for the pressure range of 5–20 atm (0.62 mg/s carbon flow rate). For the ethane flames, the flow rate was 0.78 mg/s (0.62 mg/s carbon flow rate) and the pressure range was 2–15 atm. From the soot concentration distribution, soot yields were calculated as a function of flame height and pressure. Maximum soot yields from the current study and the previous measurements in similar flames with methane, ethane, and propane flames were shown to display a unified behaviour. Maximum soot yields, when scaled properly, were represented by an empirical exponential function in terms of the reduced pressure, actual pressure divided by the critical pressure of the fuel. The maximum soot yield seems to reach a plateau asymptotically as the pressure exceeds the critical pressure of the fuel.     

Soot formation and temperature field structure in laminar propane-air diffusion flames at elevated pressures

The effect of pressure on soot formation and the structure of the temperature field was studied in coflow propane-air laminar diffusion flames over the pressure range of 0.1 to 0.73 MPa in a high-pressure combustion chamber. The fuel flow rate was selected so that the soot was completely oxidized within the visible flame and the flame was stable at all pressures. Spectral soot emission was used to measure radially resolved soot volume fraction and soot temperature as a function of pressure. Additional soot volume fraction measurements were made at selected heights using line-of-sight light attenuation. Soot concentration values from these two techniques agreed to within 30% and both methods exhibited similar trends in the spatial distribution of soot concentration. Maximum line-of-sight soot concentration along the flame centerline scaled with pressure; the pressure exponent was about 1.4 for pressures between 0.2 and 0.73 MPa. Peak carbon conversion to soot, defined as the percentage of fuel carbon content converted to soot, also followed a power-law dependence on pressure, where the pressure exponent was near to unity for pressures between 0.2 and 0.73 MPa. Soot temperature measurements indicated that the overall temperatures decreased with increasing pressure; however, the temperature gradients increased with increasing pressure.     

Comparison of structures of laminar methane–oxygen and methane–air diffusion flames from atmospheric to 60 atm

A combined experimental and numerical study was conducted to examine the structure of laminar methane–oxygen diffusion flames in comparison with methane–air flames. Soot measurements made in these flames indicated that the maximum soot yields of methane–air flames are consistently higher than methane–oxygen flames at all pressures. The maximum soot yield of the methane–oxygen flames reaches a peak near 40 atm and then starts decreasing as the pressure further increased. The maximum soot yield of the methane–air flames plateaus at about 40 atm and does not change much with further increases in pressure. Methane–oxygen flames display a distinct two-zone structure which is visible from atmospheric pressure up to 60 atm. The inner zone, similar to hydrocarbon-air diffusion flames, has a yellow/orange colour and is surrounded by an outer blue zone. This outer zone was shown to have a stratified structure with a very steep equivalence ratio gradient. The main reactions in this zone were shown to be the oxidation of hydrogen and carbon monoxide produced within the inner zone. The methane–air diffusion flames had a thin layer of blue outer zone at atmospheric pressure; however, this zone completely disappeared when the pressure was increased above atmospheric. The presence of the two-zone structure in the methane–oxygen flames was attributed to the intensified penetration of oxygen into the core flow. The higher diffusivities, steeper oxygen concentration gradients, and enhanced entrainment increase the transport of oxygen to the flame. As such, there is sufficient oxygen present near the base of the flame to support the diffusion flame in the inner zone of the methane–oxygen flames. The abundance of oxygen near the centerline, even in the lower portion of the flame, also promotes the oxidation of soot.     

Experimental study of soot and temperature field structure of laminar co-flow ethylene–air diffusion flames with nitrogen dilution at elevated pressures

An experimental study has been conducted to investigate soot formation in laminar co-flow ethylene–air diffusion flames with nitrogen dilution from a co-flow circular nozzle at pressures from 10 to 35 atm. Spectral soot emission (SSE) diagnostic technique was used to determine the radially resolved soot and temperature field structure. Constancy of ethylene and nitrogen flow rates were maintained and the flow rates of ethylene and nitrogen were selected such that no smoke was emitted even at the highest soot loadings. The flame height, marked by visible flame radiation, remained constant at about 5.5 mm and the cross-sectional area of the flame decreased with increasing pressure. At 10 atm, the peak soot concentration of less than 8 ppm, was measured near the flame tip. At 35 atm, the peak soot concentration of about 62 ppm, was measured near the mid-height of the flame. The conversion of carbon in the fuel to soot was strongly dependent on pressure particularly in the lower pressure range. At higher pressure this dependence was weaker. The peak carbon conversion to soot, 6.5%, was observed at 30 atm and remained constant to 35 atm. Temperatures increased along the flame axis and the peak temperature was observed near the flame tip to indicate complete soot oxidation.     

Effects of gravity and pressure on laminar coflow methane–air diffusion flames at pressures from 1 to 60 atmospheres

The effects of pressure and gravity on the sooting characteristics and flame structure of coflow methane–air laminar diffusion flames between 1 and 60 atm were studied numerically. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows which include complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which was verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed under both normal- and zero-gravity conditions to help separate and identify the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the sooting behavior and structure of the flames through their effects on buoyancy and temperature. Zero-gravity flames generally have lower temperatures, broader soot-containing zones, and higher soot volume fractions than normal-gravity flames at the same pressure. Buoyancy forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures caused the zero-gravity flames to lengthen. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. In the zero-gravity flames, increasing pressure beyond 20 atm caused the maximum carbon conversion factor to decrease.     

Flame length in the wake of a burning hydrocarbon drop

The flame in the wake of a hydrocarbon droplet is described by a model which combines recent experimental evidence on a large simulated droplet and the early concepts of Wohl et al. [10] on laminar diffusion flames. The model accounts for simultaneous diffusive combustion, premixed combustion, and pyrolysis in the near wake and for burning and coagulation of soot in the far wake. Predicted flame heights are in reasonable agreement with experiment for envelope and wake flames at 1 atm. The predicted pressure dependence for the heights of the near-wake flame zone and of the far-wake flame zone in an envelope flame of n-heptane is in good agreement with experiment up to 30 atm. The model suggests that for small droplets vapor pyrolysis becomes a significant effect only at elevated pressures.     

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.     

Soot concentration and temperature measurements in co-annular, nonpremixed CH4/air laminar flames at pressures up to 4 MPa

Laminar nonpremixed methane-air flames were studied over the pressure range of 0.5 to 4 MPa using a new high-pressure combustion chamber. Flame characterization showed very good flame stability over the range of pressures, with a flame tip rms flicker of less than 1% in flame height. At all pressures, soot was completely oxidized within the visible flame. Spectral soot emission (SSE) and line-of-sight attenuation (LOSA) measurements provided radially resolved measurements of soot volume fraction and soot temperature at pressures from 0.5 to 4.0 MPa. Such measurements provide an improved understanding of the influence of pressure on soot formation and have not been reported previously in laminar nonpremixed flames for pressures above 0.4 MPa. SSE and LOSA soot concentration values typically agree to within 30% and both methods exhibit similar trends in the spatial distribution of soot concentration. Maximum soot concentration depended on pressure according to a power law, where the exponent on pressure is about 2 for the range of pressures between 0.5 and 2.0 MPa, and about 1.2 for 2.0 to 4.0 MPa. Peak carbon conversion to soot also followed a power-law dependence on pressure, where the pressure exponent is unity for pressures between 0.5 and 2.0 MPa and 0.1 for 2.0 to 4.0 MPa. The pressure dependence of sooting propensity diminished at pressures above 2.0 MPa. Soot concentrations measured in this work, when transformed to line-integrated values, are consistent with the measurements of Flower and Bowman for pressures up to 1.0 MPa [Proc. Combust Inst. 21 (1986) 1115-1124] and Lee and Na for pressures up to 0.4 MPa [JSME Int. J. Ser. B 43 (2000) 550-555]. Soot temperature measurements indicate that the overall temperatures decrease with increasing pressure; however, the differences diminish with increasing height in the flame. Low down in the flame, temperatures are about 150 K lower at pressures of 4.0 MPa than those at 0.5 MPa. In the upper half of the flame the differences reduce to 50 K.     

High-pressure combustion of submillimeter-sized nonane droplets in a low convection environment

An experimental study of droplet combustion of nonane (C₉H₂₀) at elevated pressures burning in air is reported using low gravity and small droplets to promote spherical gas-phase symmetry at pressures up to 30 atm (absolute). The initial droplet diameters range from 0.57 to 0.63 mm and they were ignited by two electrically heated hot wires positioned horizontally on opposite sides of the droplet. The droplet and flame characteristics were recorded by a 16-mm high-speed movie and a high-resolution video camera, respectively. A photodiode is used to measure broadband gray-body emission from the droplet flames and to track its dependence on pressure. Increasing the pressure significantly influences the ability to make quantitative measurements of droplet, soot cloud, and luminous zone diameters. At pressures as low as 2 atm, soot aggregates surrounding the droplet show significant coagulation and agglomeration and at higher pressures the soot cloud completely obscures the droplet, with the result being that the droplet could not be measured. Above 10 atm radiant emissions from hot soot particles are extensive and the resulting flame luminosity further obscures the droplet. Photographs of the luminous zone in subcritical pressures show qualitatively that increasing pressure produces more soot, and the mean photodiode voltage output increases monotonically with pressure. The maximum flame and soot shell diameters shift to later times as pressure increases and the soot shell is located closer to the flame at higher pressure. The soot shell and flame diameter data are correlated by a functional relationship of reduced pressure derived from scaling the drag and thermophoretic forces on aggregates that consolidates all of the data onto a single curve.     

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20 atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55 mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1 atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO₂${\text{CO}}_2$ concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO₂${\text{CO}}_2$ dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.     

A numerical study on the effects of pressure and gravity in laminar ethylene diffusion flames

The effects of pressure and gravity on sooting characteristics and flame structure were studied numerically in coflow ethylene–air laminar diffusion flames between 0.5 and 5 atm. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible, gaseous mixtures which include complex chemistry, detailed radiation heat transfer, and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which has been verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed for both normal- and zero-gravity conditions to help separate the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the flames through their effects on buoyancy and reaction rates. The zero-gravity flames have higher soot concentrations, lower temperatures and broader soot-containing zones than normal-gravity flames at the same pressure. The zero-gravity flames were also found to be longer and wider. Differences were observed between the two levels of gravity when pressure was increased. The zero-gravity flames displayed a stronger dependence of the maximum soot yield on pressure from 0.5 to 2 atm and a weaker dependence from 2 to 5 atm as compared to the normal-gravity flames. In addition, flame diameter decreased with increasing pressure under normal-gravity while it increased with pressure in the zero-gravity cases. Changing the prescribed wall boundary condition from fixed-temperature to adiabatic significantly altered the numerical predictions at 5 atm. When the walls were assumed to be adiabatic, peak soot volume fractions and temperatures increased in both the zero- and normal-gravity flames, emphasizing the importance of heat conduction to the burner rim on flame structure.     

Measurements of soot production and thermal radiation from confined turbulent jet diffusion flames of methane

Turbulent methane/air jet diffusion flames at atmospheric and elevated pressure have been studied experimentally to provide data for coupled thermal radiation and soot production model development and validation. Although methane is only lightly sooting at atmospheric pressure, at elevated pressure the soot yield increases greatly. This allows the creation of a highly radiating flame, of moderate optical depth, within a laboratory scale rig. Spatially resolved flame properties needed for model validation have been measured at 1 and 3 atm. These measurements include detailed maps of mean mixture fraction, mean temperature, mean soot volume fraction, and mean and instantaneous spectrally resolved, path integrated radiation intensity.     

Influences of elevated pressures on soot formation in coaxial ethylene–air diffusion flames under different ventilation conditions

The effect of elevated pressures on the soot formation in coaxial ethylene-diffusion flame was investigated under different ventilation conditions, i.e. one with a chamber valve closed and the other with a chamber valve open. Experimental measurements include the soot volume fraction and macroscopic flame behaviors such as visible height and flame shape. The presence of ventilation-assisted flow around the flame was found to further increase the visible flame radius and height at elevated pressures. The maximum soot volume fraction measured within the flame was also increased with the ventilation-assisted flow as well as with the ambient pressure. Experimental results clearly indicate that changes in the physical appearance of the flames altered by ventilation-assisted flow are an important factor to affect the sooting behavior in coaxial diffusion flames.     

Effect of hydrogen addition on NOx formation in high-pressure counter-flow premixed CH4/air flames

A laboratory-scale laminar counterflow burner was used to investigate NO formation in high pressure premixed CH₄/H₂/air flames. New experimental results on NO measurements by LIF were obtained at high pressure in CH₄/H₂/air flames with H₂ content fixed at 20% in the fuel at pressures ranging from 0.1 to 0.7 MPa and an equivalence ratio progressively decreased from 0.74 to 0.6. The effects of hydrogen addition, equivalence ratio and pressure are discussed. These results are satisfactorily compared to the simulations using two detailed mechanisms: GDFkin®3.0_NOmecha2.0 and the mechanism from Klippenstein et al., which are the most recent high-pressure NOx formation mechanisms available in the literature. A kinetic analysis based on Rate of Production/Rate of Consumption and sensitivity analyses of NO is then presented to identify the main pathways that lead to the formation and consumption of NO. In addition, the effect of hydrogen addition on NO formation pathways is described and analysed.     

Soot properties of laminar jet diffusion flames in microgravity

The soot properties of round, non-buoyant, laminar jet diffusion flames are described, based on experiments carried out in microgravity conditions during three flights of the Space Shuttle Columbia (Flights STS-83, 94 and 107). Experimental conditions included ethylene- and propane-fueled flames burning in still air at an ambient temperature of 298 K and ambient pressures of 35-100 kPa. Measurements included soot volume fraction distributions using deconvolved laser extinction imaging and soot temperature distributions using deconvolved multiline emission imaging. Mixture fractions were estimated from the temperature measurements. Flow field modeling based on the work of Spalding is presented. It is shown that most of the volume of these flames is inside the dividing streamline and thus should follow residence time state relationships. Most streamlines from the fuel supply to the surroundings exhibit nearly the same maximum soot volume fraction and maximum temperature. The present work studies whether soot properties of these flames are universal functions of mixture fraction, i.e., whether they satisfy soot state relationships. Soot state relationships were observed, i.e., soot volume fraction was found to correlate reasonably well with estimated mixture fraction for each fuel/pressure selection. These results support the existence of soot property state relationships in steady non-buoyant laminar diffusion flames, and thus in a large class of practical turbulent diffusion flames through the application of the laminar flamelet concept.     

Structure of incipiently sooting ethylene–nitrogen counterflow diffusion flames at high pressures

We report on the structure of C₂H₄/N₂/O₂ counterflow diffusion flames at pressures up to 2.5 MPa. The concentrations of major species, aliphatics up to decane and aromatics up to indene are measured by GC–MS analysis of samples collected along the flame centerline with a capillary probe. The data are compared with results of a computational model with detailed chemistry and transport. A first set of measurements (Series 1) is performed for a fixed stoichiometric mixture fraction Z_st = 0.408, fuel mass fraction Y_F = 0.122, and global strain rate a = 57/s at pressures in the 0.1–0.8 MPa range. The flames are soot-free and permanently blue at all pressures but for the highest value of 0.8 MPa, when visual observation of a faint yellow luminosity reveals incipient sooting. A second set of flames (Series 2) is chosen by a similar criterion, by decreasing the fuel mass fraction to Y_F = 0.0975, lowering the global strain rate to 18.4/s, and covering the 0.855–2.5 MPa pressure range, which also in this case yields incipient sooting only at the highest pressure. In all cases, the flames are immune from buoyancy instabilities. Importantly, the temperature-convective time history is maintained constant in the flames in each series, which allows for a systematic exploration of the influence of only pressure, and indirectly diffusion, on the transition to incipient sooting by monitoring critical soot precursors such as the aromatics. A simple scaling based on Damköhler number suggests that just the increase in concentration of species with pressure suffices to explain the increase in sooting density. Agreement between experiments and computations is very good for major species and C₂H₂, even for the thinnest high-pressure flames. The profiles of these species, methane and temperature collapse for all pressures, once rescaled with a diffusive length δ varying with pressure and strain rate as (p·a)^−1/2. The model properly captures the first steps in fuel consumption by hydrogen abstraction by OH and H. As the pressure rises, the preferential decrease of H mole fraction suggests an increasingly dominant role of OH in an exothermic oxidative process. The observed higher sooting tendency at high pressures correlates with the increase in mole fraction of aromatics, but the model significantly overestimates such an increase. The comprehensive experimental database provides a useful test-bed for further refinements and developments of chemistry models.     

A numerical study of high pressure,laminar, sooting,ethane–air coflow diffusion flames

A numerical study is conducted of ethane–air coflow diffusion flames at pressures from 2 to 15 atm. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a detailed sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with increasing pressure without any tuning of the model for different pressures. The model shows good agreement with the experimental data on both the flame wings and centerline regions. Peak wing and centerline soot volume fractions are found to scale with P^2.49 and P^2.02 respectively. This scaling compares well to that observed experimentally for methane–air and ethylene–air flames. As pressure is increased, the flame cross-sectional area decreases according to P^?1.0 due to a constant mass flux and a thinning of the flame, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation dominates centerline soot formation. Surface growth and PAH condensation increase with increasing pressure primarily due to both of these processes being a function of surface area. This causes increases in soot volume fraction to further accelerate surface growth and PAH condensation, acting in a positive feedback manner. This positive feedback mechanism is initiated by increases in reaction rates caused by increases in gas phase density. Additionally, the significance of surface growth decreases with increasing pressure, while the role of PAH condensation increases.     

Soot surface growth and oxidation at pressures up to 8.0 atm in laminar nonpremixed and partially premixed flames

The flame structure and soot particle surface reaction properties, including growth and oxidation, of laminar jet nonpremixed flames were studied experimentally at pressures of 1.0-8.0 atm. Ethylene-helium mixtures were used in an oxygen/helium coflow at normal temperature (300 K) in order to minimize the effects of buoyancy. The following properties along the axis of flames were measured as a function of distance from the burner exit: soot concentrations by laser extinction, soot temperatures by multiline emission, soot structure by thermophoretic sampling and analysis using transmission electron microscopy (TEM), concentrations of major stable gas species by isokinetic sampling and gas chromatography, concentrations of radical species (H, OH, O) by Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local flame properties in order to find soot surface growth and oxidation rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures involving laminar premixed and diffusion flames fueled with a variety of hydrocarbons. The growth rates from all the available flames were in good agreement with each other and with existing hydrogen-abstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature. Measurements of early soot surface oxidation rates at pressures of 1.0-8.0 atm (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) were found to be consistent with earlier measurements at atmospheric and subatmospheric pressures. The oxidation rates of up to 8 atm in flame environment could be explained by reaction with OH, having a collision efficiency of 0.12.