Spectral Soot Emission Research Articles

Effects of intramolecular oxygen and partial premixing on soot formation in ethane/ethanol flames

Detailed soot concentration and temperature fields were measured in coflow laminar diffusion flames of neat ethane and ethane/ethanol mixtures, and partially premixed flames of ethane/ethanol mixtures. Two-dimensional diffuse-light line-of-sight attenuation and spectral soot emission techniques were employed for soot volume fraction and temperature measurements, respectively. The choice of flow rates prioritized tractability and comparability between flames. To this end, the total carbon mass flow rate and the C/H ratio were kept constant for all ethane/ethanol mixture flames, i.e., oxygen was introduced into the fuel stream either as intramolecular oxygen in ethanol or both as intermolecular and molecular oxygen. Soot concentration was observed to decrease monotonically with an increasing ethanol content in ethane/ethanol mixtures without synergistic effects. Soot concentration was also measured in neat ethane flames with flow rates matching those of ethane in ethane/ethanol mixtures. A comparison of these flames with ethane/ethanol flames revealed a striking soot suppression effect by ethanol addition. Although the carbon flow rate was less in neat ethane flames than the total carbon flow rate of their corresponding ethane/ethanol flames, the peak soot volume fractions were significantly higher in neat ethane flames. This points to a chemical soot suppression effect beyond that can be explained by the lower sooting propensity of ethanol fuel. These results were discussed in light of the synergistic effect studies in the literature. Lastly, measurements from partially-premixed ethane/ethanol mixtures are reported. The increased oxygen content significantly increased the peak soot volume fraction in these flames. These results support that intermolecular oxygen in ethanol and molecular oxygen in the fuel tube behave drastically differently in an ethane base flame. Ethanol addition does not exhibit synergistic effects on soot formation, whereas molecular oxygen addition increases maximum soot concentration, indicating the activation of unique soot production pathways.

Infrared spectral soot emission for robust and high-fidelity flame thermometry.

Spectral soot emission (SSE) in the visible spectrum is a popular technique for non-intrusive thermometry in sooting flames. However, its accuracy is restricted by uncertainties in the wavelength dependence of soot optical properties. We propose a novel infrared spectral soot emission method that successfully addresses this issue. Comprehensive light extinction experiments were firstly conducted to explore the spectral variation of soot optical property. The results indicated a wavelength independence of the soot absorption function provided the wavelength of the incident light is larger than 1000 nm, thereby indicating through the Kirchhoff law the potential of a robust thermometry using infrared (>1000 nm) spectral soot emissions. Proof-of-concept experiments were performed for sooting premixed flames of ethylene with different equivalence ratios. The results demonstrated that the new method provided more accurate temperature results compared with its visible-NIR counterpart, particularly at flame positions where nascent soot particles are present. The proposed method is, to our knowledge, the first infrared spectral soot emission-based thermometry, and is believed to offer a solution to improving the fidelity of SSE with a cost-effective optical setup.

Development and validation of a hybrid constraint spectral thermometry for laminar sooting flames.

A hybrid constraint spectral thermometry (HCST) method based on combined light extinction and spectral soot emission was developed for temperature measurements in sooting flames. Light extinction measurements were performed in the 635-1064 nm spectral range to constrain the fitting of the soot optical property E(m) as a function of the wavelength for separately measured spectral emission data. This hybrid constraint methodology helps to avoid complete reliance on either measurement, which significantly increases the immunity of the temperature determination to measurement noises. After numerically validating the performance of HCST, measurements were performed for a series of representative sooting premixed flames. Additional measurements were conducted with the proven tunable diode laser absorption technique to provide a reference temperature, and satisfactory agreements demonstrate the accuracy and robustness of the proposed HCST method.

Probing sooting limits in counterflow diffusion flames via multiple optical diagnostic techniques

Sooting limit representing the critical flame condition where soot starts to appear is a useful quantitative index to assess the sooting tendency of a particular fuel. Experimental determination of sooting limit is closely related to the establishment of a well-defined “nucleation” flame which facilitates further scrutinization of the soot formation process. The present study was devoted to probing the earliest soot appearance in typical counterflow diffusion flames. The sooting limit was defined as the critical fuel (or oxygen) mole fraction in the nitrogen-balanced fuel (or oxidizer) stream. Multiple optical diagnostic techniques including flame luminosity, light extinction and scattering, spectral soot emission (SSE) as well as laser-induced incandescence (LII) were used to determine the sooting limits. The results showed that the value of sooting limit can be quantitatively different among the used detection techniques. Flame luminosity and SSE were found to be most sensitive to the presence of nascent soot, followed by LII and laser light scattering, while light extinction is the least sensitive technique. The effects of the wavelength of probing light on sooting limits were also assessed for the laser extinction and scattering technique. The contribution of the present work can be summarized into the following two aspects: (1) a quantitative database of sooting limits as determined from different soot detection measurements is provided to facilitate the choice of nascent soot-probing technique; (2) the first appearance of soot particles in nucleation flames are fully characterized and interpreted through a cross-comparison among multiple diagnostics techniques. It is also the authors’ expectation that the present results may provide new insights into the understanding of soot nucleation process and the interaction between light and nascent soot.

A calibration-free two-dimensional spectral soot emission platform for temperature and soot measurements

A simple and cost-effective temperature and soot volume fraction measurement technique that does not rely on spectral or absolute irradiance calibration is developed. The technique is based on a three-sensor CMOS camera, where the broadband sensitivity of RGB sensors were narrowed down to representative wavelengths using a tri-bandpass filter. A telecentric imaging lens was used to improve the accuracy of the tomographic deconvolution of the axisymmetrical ethylene laminar diffusion flames stabilized on a co-flow burner. Temperature and soot concentration measurements have been compared with results previously reported from the same burner. Good agreement for temperature measurements and excellent agreement for soot concentration measurements have been shown. Additionally, to demonstrate the dynamic range of the system, two-dimensional temperature and soot volume fraction fields were measured for four cases of dilution corresponding to various soot loadings. As the dilution rate was increased, the peak soot volume fraction was drastically reduced and temperatures were increased in accordance with the reduced dissipative heat losses from broadband soot radiation.

A machine learning approach assisting soot radiation-based thermometry to recover complete flame temperature field in a laminar flame

Soot radiation-based thermometry is a popular approach for flame temperature diagnostics. However, it usually suffers from flame temperature information lost in the regions of soot absent or less significant area. The originality of the present work lies in the development of a two-step Multi-Layer Perceptron (MLP) neural network method to assist soot radiation-based thermometry for flame temperature field retrieval. It completes the whole temperature field in a steady axis-symmetric Santoro laminar flame, which was originally measured by the Modulated Absorption Emission technique (MAE). Using temperature fields provided by numerical simulation of this standard Santoro flame, the two-step Multi-Layer Perceptron neural network model is trained and tested, and the temperature recovery sensitivity to the noises is theoretically investigated. Moreover, this two-step MLP approach for flame temperature completion is further proofed by experimental temperature results. And the two-step MLP prediction uncertainty is estimated as ± 85.5 K experimentally. The developed approach could assist other soot radiation-based thermometry, i.e., spectral soot emission, to provide the complete flame temperature fields.

Soot maturity studies in methane-air diffusion flames at elevated pressures using laser-induced incandescence

Two-dimensional laser-induced incandescence (LII) measurements were used for quantitative soot volume fraction (fv) measurements in methane-air diffusion flames at pressures ranging from 0.1 to 0.5 MPa. Additionally, laser-induced fluorescence (LIF) was used for visualization of polycyclic aromatic hydrocarbons (PAHs) considered as important soot precursors. A heat and mass transfer-based LII model was used for the analysis of experimental LII fluence curves to evaluate soot absorption functions, E(m), at various spatial positions in the flames. Based on previous work, variations in the evaluated E(m) was assumed to be related to soot maturity. Generally, the most mature soot was found at the spatial position of maximum fv along the central axis and at the flame edges. Also, the soot maturity at the position of maximum fv at each pressure was found to increase from the flame at 0.1 MPa to 0.5 MPa. A critical examination of the assumptions and uncertainties in the E(m) analysis was made. The present study emphasizes the limitation of assigning soot a constant E(m) in the overall flame for temperature and soot concentration evaluation using the optical methods spectral soot emission (SSE) and line-of-sight attenuation (LOSA), which are commonly used non-intrusive optical diagnostic techniques in sooting high-pressure flames. The study also demonstrates the critical choice of fluence in quantitative imaging LII measurements of fv when the E(m) of soot spans over large range of values.

Effects of benzene, cyclo-hexane and n-hexane addition to methane on soot yields in high-pressure laminar diffusion flames

Effects of doping high pressure methane diffusion flames with benzene, cyclo-hexane and n-hexane were investigated to assess the sooting propensity of three hydrocarbons with six carbons at elevated pressures. Amount of liquid hydrocarbons added to methane constituted 7.5% of the total carbon content of the fuel stream. The pressure range investigated extended up to 10 bar and the experiments were carried out in a high pressure combustion chamber capable of establishing stable laminar diffusion flames with various fuels at elevated pressures and was used in similar experiments previously. Temperatures and soot volume fractions were measured using the spectral soot emission technique capturing spectrally-resolved line-of-sight intensities which were subsequently inverted using an Abel type algorithm to obtain radial distributions assuming that the flames are axisymmetric. The total mass carbon flow of the fuel stream was kept constant at 0.524 mg/s in neat methane, benzene-doped methane, cyclo-hexane-doped methane, and n-hexane-doped methane flames to have tractable measurements at all pressures. Measured maximum soot volume fractions and evaluated maximum soot yields showed that benzene-doped methane flame had the higher values than cyclo-hexane doped methane flames which in turn had higher values than n-hexane doped methane flames at all pressures. Sooting propensity dependence of the three hydrocarbons on pressure can be ranked as, in descending order, n-hexane, cyclo-hexane, and benzene; however, the difference between pressure dependencies of n-hexane and cyclo-hexane was within the measurement error margins. Ratio of soot yields of benzene to n-hexane doped flames changed from about 2 at 2 bar to 1.2 at 10 bar; the ratio of benzene to cyclo-hexane doped flames showed similar trends.

The chemical structure effects of alkylbenzenes on soot formation in a laminar co-flow flame

Alkylbenzene isomers are often used in transportation fuel surrogates but have different sooting tendencies. There is a need to understand how their chemical structures affect the soot formation mechanism. In this study, the chemical structure effects of alkylbenzenes (1,2,4-trimethylbenzene (124TMB) and n-propylbenzene (PBZ)) on soot formation in a laminar diffusion flame were experimentally and numerically investigated. In the experiment, the optical time-resolved laser-induced incandescence (LII) and spectral soot emission (SSE) diagnostics were used to measure radial soot volume fraction, primary particle diameter and flame temperature profiles. The radial number density was also experimentally derived. The results are consistent with the literature as 124TMB exhibits higher soot concentration than PBZ. From the analyses of the primary particle diameters and the derived number density, this is caused by the higher soot nucleation rate for 124TMB. This conclusion is supported by numerical modeling, which utilized the detailed CoFlame code with a moderately reduced CRECK mechanism. The simulation results show that 124TMB has earlier soot inception and Polycyclic Aromatic Hydrocarbon (PAH) addition than PBZ. Consistent with both earlier soot inception and PAH addition, the 124TMB model predicts earlier pyrene (A4) formation, suggesting 124TMB has alternative reaction pathways for pyrene formation. The reaction pathway analysis suggests that pyrene is formed via the aromatic radical recombination route, as opposed to the conventional HACA mechanism. Bypassing the formation of the second ring could be the reason for 124TMB having earlier soot nucleation. In contrast, the PBZ model predicts that the formation of pyrene follows the slower HACA pathways, leading to later soot nucleation.

Soot formation in diluted laminar ethene, propene and 1-butene diffusion flames at elevated pressures

Soot formation characteristics of ethene, propene, and 1-butene, the most abundant unsaturated intermediates in thermal decomposition of paraffinic hydrocarbons, were investigated in laminar diffusion flames stabilized on a co-flow burner installed in a high-pressure combustion chamber with optical access. All three olefins were diluted with nitrogen to produce sooting but non-smoking diffusion flames at desired pressures. Pressure range was 1–2.5 bar with 1-butene, and 1–8 bar with propene and ethene. Upper pressure limits of 1-butene and propene were established by their respective vapour pressure characteristics. The spectral soot emission technique, in which radiation emitted by the soot within the flame was collected as line-of-sight intensity and spectrally resolved over the range 690–945 nm, was used to measure radially-resolved temperature and soot volume fraction. The carbon mass flow rates of the three fuels were kept constant at 0.505 mg/s to facilitate direct comparison among the fuels at elevated pressures. With the same dilution level, the sooting propensity increased from ethene to 1-butene as expected; however, the pressure sensitivity of propene and 1-butene differed significantly from that of ethene. Soot yields in both propene and 1-butene flames showed a much weaker dependence on pressure than the soot in ethene flames. In the decomposition of propene and 1-butene, allyl radical and 1,3-butadiene are known to form in critical quantities leading to formation of higher molecular growth species specifically six-membered ring aromatics, and presence of these simple aromatics is argued to play a role in lowering the pressure sensitivity of the soot in C3 and C4 olefin flames.

Pressure dependence of primary soot particle size determined using thermophoretic sampling in laminar methane-air diffusion flames

Optical techniques, such as the light extinction and scattering as well as the laser-induced incandescence and the spectral soot emission, have been used routinely to measure soot concentrations in atmospheric flames. Laser-induced incandescence and light scattering have been proven to measure the primary soot particle size as well in atmospheric flames; however, using these two techniques for the purpose of primary soot particle sizing in diffusion flames at elevated pressures has been found to be problematic. One of the popular methods of studying soot morphology and primary size in atmospheric flames is thermophoretic sampling followed by transmission electron microscopy analysis. A high pressure thermophoretic sampling system was built and used successfully to measure the size of primary soot particles in laminar diffusion flames of methane at pressures above atmospheric. The multi-probe sampling system was fitted inside the high-pressure combustion chamber that had been used previously for high-pressure soot formation studies. Soot samples taken at various pressures were analyzed subsequently by transmission electron microscopy to estimate the primary soot particle sizes. The soot volume fractions and soot temperatures were measured by spectral soot emission technique at the same height above the burner rim as the thermophoretic sampling. The mean primary soot particle size, measured at a constant height from the burner exit at all pressures, decreased about 35% from 2 to 10 atm whereas the soot volume fraction increased by a factor of more than 50. Experimental results of mean primary soot sizes and the corresponding soot volume fractions imply that the number of soot nuclei in soot inception region of the laminar diffusion flames must have a strong sensitivity to pressure. The higher amounts of soot are mainly determined by the increasing nucleation leading to higher primary soot particle number densities as the pressure is increased.

Numerical and experimental study of the influence of CO2 and N2 dilution on soot formation in laminar coflow C2H4/air diffusion flames at pressures between 5 and 20 atm

The effects of fuel dilution by CO2 and N2 on soot formation and the flame structure in laminar coflow C2H4/air diffusion flames at pressures between 5 and 20atm were investigated both experimentally and numerically. Experimentally a constant ethylene flow rate and a constant dilution rate of 1:2 (fuel:diluent by mass) were maintained throughout the experiments. The flames were stable and non-smoking over the pressure range investigated. The radially-resolved soot volume fraction and temperature distributions were measured by the spectral soot emission (SSE) technique. Numerical calculations were conducted using two C2 chemistry models with formation of PAHs up to pyrene and a soot model incorporating pyrene collision as the soot inception step and hydrogen-abstraction acetylene addition mechanism and PAH condensation as the surface growth processes. The two C2 chemistry models were the ABF mechanism [Appel et al. (2000)] and the DLR mechanism [Slavinskaya and Frank (2009)]. The DLR mechanism predicted little or no chemical effect of CO2 dilution, depending on the pressure, in the present context. Numerical results are in qualitative agreement with experimental measurements. Soot volume fractions and carbon conversion are lower in the CO2-diluted flames due to the additional chemical effect of CO2. CO2 is still more effective than N2 as a diluent to suppress soot formation at elevated pressures. The primary pathway for the chemical effect of CO2 dilution is through the reverse reaction of CO+OH↔CO2+H. The chemical effect of CO2 lowers the rates of soot inception, C2H2 addition, and PAH condensation. The effectiveness of the CO2 chemical effect on soot formation suppression diminishes with increasing pressure. The diminishing effectiveness of the chemical effect of CO2 dilution with increasing pressure is due to the significant decrease in the H radical mole fraction.

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 20atm. 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 0.55mg/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 1atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO2 concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO2 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.

Sooting behaviour of n-heptane laminar diffusion flames at high pressures

The effect of pressure on sooting behaviour of n-heptane is studied in co-flow n-heptane/air laminar diffusion flames at pressures above atmospheric in a high pressure combustion chamber. The fuel is diluted with either nitrogen or helium to keep a non-smoking flame at elevated pressures, and the selected fuel mass flow rate of n-heptane provided diffusion flames in which the soot was completely oxidized within the visible flame envelope. The flame stability proved to be a challenge and stable flames were possible only at certain pressures for a sufficiently long duration to permit measurements. The soot volume fractions and temperatures were measured by spectral soot emission as a function of pressure for nitrogen-diluted n-heptane flames at 2, 5 and 7atm. For helium-diluted n-heptane flames, line of sight soot emission data at 3, 4, and 5atm are presented at two heights above the burner exit. Comparison of limited nitrogen-diluted n-heptane data to previous measurements of soot yields indicate that soot formation in diffusion flames of n-heptane seems to be slightly more sensitive to pressure than that in aliphatic gaseous fuel diffusion flames within the pressure range considered in this work.

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.

Soot formation in laminar ethane diffusion flames at pressures from 0.2 to 3.3 MPa

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow ethane-air laminar diffusion flames over the pressure range of 0.1–3.34MPa in a high pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 1.52 to 3.34MPa, whereas it increased considerably from atmospheric to 1.52MPa. Flame cross-sectional area, measured at the flame height of 5mm either bounded by maximum flame temperature or maximum soot volume fraction contours, showed an inverse dependence on pressure. Peak carbon conversion to soot, defined as the percentage of fuel’s carbon content converted to soot, showed a strong dependence on pressure at lower pressures; but this dependence grew weaker as the pressure was increased. This dependence can be expressed as a pressure scaling in the form of a power law. However, the exponent of pressure was not constant: it was about 2.2 for pressures between 0.2 and 0.51MPa, about 1.1 for pressures between 0.51 and 1.52MPa, and about 0.4 for pressures between 1.52 and 3.34MPa. Averaged flame temperatures decreased with increasing pressure as a result of enhanced heat loss from the flame by soot radiation. The maximum temperature gradients increased with pressure at lower flame heights; at higher locations in the flame, after an initial increase at the lower pressure range, gradients reached a plateau at about 1.5–2.0MPa.

Soot formation and temperature structure in small methane–oxygen diffusion flames at subcritical and supercritical pressures

An experimental study was conducted to examine the characteristics of laminar methane–oxygen diffusion flames up to 100 atmospheres. The influence of pressure on soot formation and on the structure of the temperature field was investigated over the pressure range of 10–90 atmospheres in a high-pressure combustion chamber using a non-intrusive, line-of-sight spectral soot emission diagnostic technique. Two distinct zones characterized the appearance of a methane and pure oxygen diffusion flame: an inner luminous zone similar to the methane–air diffusion flames, and an outer diffusion flame zone which is mostly blue. The flame height, marked by the visible soot radiation emission, was reduced by over 50% over the pressure range of 10–100 atmospheres. Between 10 and 40 atmospheres, the soot levels increased with increasing pressure; however, above 40 atmospheres the soot concentrations decreased with increasing pressure.

Soot formation and temperature field structure in co-flow laminar methane–air diffusion flames at pressures from 10 to 60 atm

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane–air laminar diffusion flames over a wide pressure range, from 10 to 60atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.

Numerical and experimental study of an axisymmetric coflow laminar methane–air diffusion flame at pressures between 5 and 40 atmospheres

A numerical and experimental study of an axisymmetric coflow laminar methane–air diffusion flame at pressures between 5 and 40 atm was conducted to investigate the effect of pressure on the flame structure and soot formation characteristics. Experimental work was carried out in a new high-pressure combustion chamber described in a recent study [K.A. Thomson, Ö.L. Gülder, E.J. Weckman, R.A. Fraser, G.J. Smallwood, D.R. Snelling, Combust. Flame 140 (2005) 222–232]. Radially resolved soot volume fraction was experimentally measured using both spectral soot emission and line-of-sight attenuation techniques. Numerically, the elliptic governing equations were solved in axisymmetric cylindrical coordinates using the finite volume method. Detailed gas-phase chemistry and complex thermal and transport properties were employed in the numerical calculations. The soot model employed in this study accounts for soot nucleation and surface growth using a semiempirical acetylene-based global soot model with oxidation of soot by O 2, OH, and O taken into account. Radiative heat transfer was calculated using the discrete-ordinates method and a nine-band nongray radiative property model. Two soot surface growth submodels were investigated and the predicted pressure dependence of soot yield was compared with available experimental data. The experiment, the numerical model, and a simplified theoretical analysis found that the visible flame diameter decreases with pressure as P a −0.5 . The flame-diameter-integrated soot volume fraction increases with pressure as P a 1.3 between 5 and 20 atm. The assumption of a square root dependence of the soot surface growth rate on the soot particle surface area predicts the pressure dependence of soot yield in good agreement with the experimental observation. On the other hand, the assumption of linear dependence of the soot surface growth rate on the soot surface area predicts a much faster increase in the soot yield with pressure than that observed experimentally. Although pressure affects the gas-phase chemistry, the increased soot production with increasing pressure seems primarily due to enhanced mixture density and species concentrations in the pressure range investigated. The increased pressure causes enhanced air entrainment into the fuel stream around the burner rim, leading to accelerated fuel pyrolysis. In the pressure range of 20 to 40 atm both the model and experiment show a diminishing sensitivity of sooting propensity to pressure with a greater decrease in the predicted sensitivity of soot propensity to pressure than the experimental results.

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.