Soot Absorption Coefficient Research Articles

Flame soot absorption coefficient and temperature reconstruction through line-of-sight attenuation and light field imaging

The accuracy and efficiency of 3-D flame temperature field reconstruction using the light field imaging technique heavily depend on soot radiation characteristics. In this study, we employ the line-of-sight attenuation method to reconstruct the soot absorption coefficient distribution in a pure absorbing flame. Utilizing these soot absorption coefficients, the radiative transfer equation is effectively transformed within the framework of the light field imaging technique into a linear inverse problem and also outlines the flame boundary. This proposed strategy reduces the unwanted detection rays significantly, thus eliminating the extensive computational processing. Consequently, the proposed approach substantially enhances the accuracy and efficiency of flame temperature reconstruction. Numerical simulations were carried out on a bimodal asymmetric flame to validate the noise tolerance capabilities, investigate the effects of varying voxel numbers on flame division and carry out a comparative study. Experimental studies were also conducted to reconstruct flame temperature and soot absorption coefficient distributions under different combustion operating conditions. Thermocouple measurements were performed and compared with the reconstructed temperatures. Outcomes achieved from both numerical simulations and experimental studies demonstrate the feasibility, accuracy and robustness of the proposed method.

2D 2-Colour time-Resolved laser induced incandescence sizing of ultra-fine soot particles in a methane diffusion flame

Planar, two-dimensional, two-colour, time-resolved laser-induced incandescence (2D-2C-TiRe-LII) is employed to investigate the formation of soot in a standard methane laminar diffusion flame for the first time. The work builds on previous developments in 2D LII by time-resolving the signal decay by shifting the gating time of ICCD cameras. The two-colour configuration allows application of the technique to extract the initial temperature of soot particles immediately after the laser pulse, instead of relying on estimates of the soot absorption coefficient, thus allowing higher accuracy in the determination of very small particle sizes produced in methane flames, using a minimum error approach. The technique is combined with a previous work on Continuous Wave Laser Cavity Extinction (CW-LCE) to allow absolute measurements of sub-ppm low soot volume fractions with very small particle sizes.

Influence of soot on radiative heat transfer in bubbling fluidized bed combustors

The effect of soot particles and aggregates on radiative heat transfer in bubbling fluidized bed combustors is investigated. For this purpose, a soot radiative property model based on Rayleigh scattering theory is coupled with in-house developed 1-D gray and spectral radiation codes and a 3-D spectral radiation code based on method of lines solution of discrete ordinates method for the application of (i) a 1-D slab problem involving combustion gases and soot and (ii) freeboard of a 0.3 MWt atmospheric bubbling fluidized bed combustion test rig including combustion gases, fly ash particles and soot. The predictions of the 1-D radiation code reveal that (i) Planck-mean soot radiative properties provide reasonably accurate radiative heat transfer predictions (ii) soot particle diameter and number of soot particles in an aggregate do not considerably influence the net wall heat fluxes and source terms as absorption coefficient of soot is not significantly affected by those parameters and is orders of magnitude higher than scattering coefficient. Application of 3-D radiation model to the test rig shows that soot significantly contributes to radiative heat transfer in the freeboards of bubbling fluidized bed combustors. Soot has a noticeable impact on radiation even at very low volume fractions.

Soot Pyrometry by Emission Measurements at Different Wavelengths in Laminar Axisymmetric Flames

ABSTRACT In this work, a multi-wavelength pyrometry method is proposed in order to evaluate soot temperature for laminar axisymmetric non-premixed flames. A single radiative model that combines three detection wavelengths is developed to increase the spectral information used in the soot temperature determination. A simple set-up is used in order to capture the soot emission at two or three wavelengths. The robustness of the methodology is assessed by a radiative model, which includes a sensitivity and relative error analysis for the soot temperature, and accounts for self-absorption effects. The model and procedure are verified by employing numerical temperature and soot volume fraction fields to recreate the convoluted soot emissions. Soot emission measurements involve different combinations of wavelengths that are employed to evaluate soot temperature without requiring additional measurements of the soot absorption coefficient when soot self-absorption is neglected. The error on the determined soot temperature was estimated to be lower than 60 K under this assumption for soot volume fractions under 10 ppm. In addition, an analysis of the signal noise effect on the temperature shows that the modulated absorption/emission (MAE) technique is more sensitive than the emission (EMI) technique. A sensitivity analysis showed that the presented three-wavelength EMI model is less sensitive to small deviations on the measured soot emission ratio than the classical two-wavelength approach. This feature makes the model suitable to improve the accuracy on the determination of soot temperature when the noise level of the signal is significant.

Numerically resolved line by line radiation spectrum of large kerosene pool fires

A numerical model is presented for spectral characteristics of radiation coming from a pool fire flame. The case studies are 1.75 m × 1.75 m and 2.5 m × 2.5 m Kerosene pool fires. Transient heat and mass transfer of the system was solved using a CFD model of a 4 m × 4 m × 5 m rectangular domain built in Fire Dynamic Simulator (FDS) with LES of turbulence and a two-step combustion reaction. Transient profiles of gas compositions, soot concentration and temperature along a line of sight of an imaginary sensor were collected from the CFD simulations, and instantaneous solutions of the thermal radiation along the line were calculated using high-resolution LBL spectral absorption profiles of combustion gases together with a model for soot absorption coefficient, based on spectrally dependant complex index of refraction. The transient spectra, consisting of numerous instantaneous intensity solutions, were then averaged and compared against the similar experimentally measured data. The line of sight and other settings of the model were carefully checked to be consistent with the experiments performed for the same system. The modelling results revealed the strong absorption effect of cold atmospheric gases while the emission peak of hot CO2 at ~2200 cm−1 in fire is still quite distinguishable from the spectral profile of hot blackbody even at 23 m away from the centre of the flame. This emission peak can be therefore used for detection of the fire. The spectral changes of the spectrum are explained and a sensitivity analysis is performed to study the effects of the sensor's distance from the pool, pool size, and modelling and operational conditions, such as relative humidity and radiative fraction.

Assessments on SGG and WSGG spectral models in high-ash coal combustion

In radiation process, a variety of models can be used to represent the spectral radiative properties of combustion gases. In coal combustion, the high level of soot concentration brings higher complexity to the formulation. Several pieces of research are found, comparing different radiation models usually applied to idealized laboratory situations. Only a few works explore the behavior of such models in real scale applications, such as coal-fired furnaces. Comparisons among different arrangements of the SGG and WSGG models were carried out by means of CFD simulations, applied to a boiler of a real 160 MW coal-fired power plant. Five different cases were simulated. The most simple case was set with a fixed absorption coefficient SGG model, while the most detailed case was achieved by employing a WSGG model with gas and soot absorption coefficients overlay. Temperature fields were well represented by all cases, with maximum deviations of 6%. Predictions on radiative heat flux for the SGG models were significantly lower than the WSGG simulations, with relative deviations up to 16.46%. Even though deviations on radiative heat flux are significant, results suggest the suitability of simple gray gas models in fast assessments of industrial applications such as coal-fired furnaces.

Effect of soot self-absorption on color-ratio pyrometry in laminar coflow diffusion flames.

This Letter reports on the effect of self-absorption on measured temperature for color-ratio soot pyrometry with a color camera. A series of increasingly nitrogen diluted atmospheric pressure ethylene/air laminar coflow diffusion flames are studied, providing flames with different optical path lengths, soot loading, and soot optical properties. Numerical calculations are used to simulate the change in collected flame emission signal with and without light attenuation using experimentally measured maps of the soot absorption coefficient. This parameter implicitly contains information about soot volume fraction and soot optical properties. The ratio of these calculations is used to correct the raw color-channel signals, resulting in temperature maps with improved accuracy. The change in calculated temperature varies spatially within each flame, with the maximum correction quantified to be 22K for a flame with a maximum optical depth of 0.31. This correction is as much as 42 and 75K for simulated flames with the same optical properties, structure, and a factor of two and five increase in soot volume fraction, respectively.

Large eddy simulation of upward flame spread on PMMA walls with a fully coupled fluid–solid approach

A fully coupled fluid–solid approach has been developed within FireFOAM 2.2.x, a large eddy simulation (LES) based fire simulation solver within the OpenFOAM® toolbox. Due consideration has been given to couple the radiative heat transfer and soot treatment with pyrolysis calculations. Combustion is modeled using the newly extended eddy dissipation concept (EDC) for the LES published by the authors’ group. Soot formation and oxidation are handled by the published extension of the laminar smoke point concept to turbulent fires using the partially stirred reactor (PaSR) concept also from the authors’ group. The gases radiation properties are evaluated using the established weighted sum of grey gas model while soot absorption coefficient is calculated using a single Planck-mean absorption coefficient. The effect of in-depth radiation is treated with the relatively simple Beer's law and the solid surface regression length is calculated from the local pyrolysis rate. Systematic validation studies have been conducted with several published experiments including simple pyrolysis test without the gaseous region, small scale wall fires and large scale flame spread. The predictions are in very good agreement with the relevant experimental data, demonstrating that the present modeling approach can be used to predict upward flame spread over PMMA with reasonable accuracy. Further parametric studies have also been conducted to demonstrate the effectiveness of the present modifications to capture the underlying physics. The detailed field predictions for vortex structures and flame volume including laminar–turbulent transition have also been analysed to uncover further insight of the unsteady flame spread phenomena. Potentially, the model can be used to aid further fundamental studies of the flame spread phenomena such as investigating the effects of width, inclination angles and side walls on flame spread as well as the predictions of flame spread in practical applications.

Modelling soot production and thermal radiation for turbulent diffusion flames

This paper presents a systematic investigation on the strategies required for modelling soot when performing CFD simulations of turbulent flames. The first test case consists in a 3D enclosure containing a mixture of N2, CO2, H2O and soot (Coelho, 2003). Results obtained with the gray implementation of the Discrete Ordinate Method in ANSYS Fluent are compared against literature data, assessing different formulations for the absorption coefficient of soot. These considerations are exploited through reactive RANS simulations accounting for radiative heat transfer, with the purpose to test different models for soot formation process on a turbulent non-premixed kerosene-air flame (Young et al., 1994).

Absorption turbulence-radiation interactions in sooting turbulent jet flames

The effects of absorption Turbulence-Radiation Interactions (TRI) on radiative heat transfer are investigated along characteristic diametric paths of three ethylene/air flames using a stochastic space and time series model. The three flames cover a wide range of scales and optical thicknesses. Model results show that the contribution of absorption TRI to the total absorption increases with the optical thickness but remains lower than 10%. Absorption TRI is largely dominated by soot radiation. The cross-correlation between the soot absorption coefficient and the radiative intensity is found to be mainly negative, leading to an overall negative absorption TRI. Finally, the Optically Thin Fluctuation Approximation can be applied to the modeling of turbulent jet flames despite noticeable discrepancies on mean spectrally-integrated radiative intensity for the large-scale optically-thick flame.

Numerical modeling of soot formation in a turbulent C2H4/air diffusion flame

Soot formation in a lifted C 2 H 4 -Air turbulent diffusion flame is studied using two different paths for soot nucleation and oxidation; by a 2D axisymmetric RANS simulation using ANSYS FLUENT 15.0. The turbulence-chemistry interactions are modeled using two different approaches: steady laminar flamelet approach and flamelet-generated manifold. Chemical mechanism is represented by POLIMI to study the effect of species concentration on soot formation. P1 approximation is employed to approximate the radiative transfer equation into truncated series expansion in spherical harmonics while the weighted sum of gray gases is invoked to model the absorption coefficient while the soot model accounts for nucleation, coagulation, surface growth, and oxidation. The first route for nucleation considers acetylene concentration as a linear function of soot nucleation rate, whereas the second route considers two and three ring aromatic species as function of nucleation rate. Equilibrium-based and instantaneous approach has been used to estimate the OH concentration for soot oxidation. Lee and Fenimore-Jones soot oxidation models are studied to shed light on the effect of OH on soot oxidation. Moreover, the soot-radiation interactions are also included in terms of absorption coefficient of soot. Furthermore, the soot-turbulence interactions have been invoked using a temperature/mixture fraction-based single variable PDF. Both the turbulence-chemistry interaction models are able to accurately predict the flame liftoff height, and for accurate prediction of flame length, radiative heat loss should be accounted in an accurate way. The soot-turbulence interactions are found sensitive to the PDF used in present study.

Numerical investigation of soot formation and oxidation processes under large two-stroke marine diesel engine-like conditions using integrated CFD-chemical kinetics

In this reported work, multi-dimensional computational fluid dynamics studies of diesel combustion and soot formation processes in a constant volume combustion chamber and a marine diesel engine are carried out. The key interest here is firstly to validate the coupling of a newly developed skeletal n-heptane mechanism and a revised multi-step soot model using laser extinction measurements of diesel soot obtained at different ambient pressure levels in an optical accessible, constant volume chamber experiment. It is revealed that ignition delay times and liftoff lengths generated using the new skeletal model are close to those produced by the larger and more comprehensive chemical mechanisms, apart from those at the low pressure condition. The current study also demonstrates that the variation of averaged soot volume fraction with respect to the change of combustion chamber pressure captured using the revised soot model agrees reasonably well with the measurements in terms of peak values. The numerical model is subsequently applied to investigate the flame development, soot/nitrogen monoxide formation and heat transfer in a two-stroke, low-speed uniflow-scavenged marine diesel engine operating at full load condition, where optical measurements are not available. Comparisons to the experimental data show that the simulated pressure rise starts 1.0 crank angle degree in advance and the calculated peak pressure is 1.7% lower. The associated flame liftoff length is negligible, yielding higher local equivalence ratio and soot volume fraction values as compared to those under similar test condition in the constant volume chamber. With the use of the revised model, the total heat transfer to the walls calculated when soot radiative heat loss is taken into account is approximately 30% higher compared to that when only convective heat loss is considered. The averaged nitrogen monoxide concentration is 7.7% lower when both convective and soot radiative heat losses are accounted for but the net soot mass production is less sensitive to soot radiation. A sensitivity study reveals that neither increasing nor decreasing the soot absorption coefficient by 30% from the baseline setup is influential to nitrogen monoxide formation, soot mass production and heat transfer. The findings here aid to gain insights and provide a better understanding of the combustion and soot processes in large, uniflow-scavenged marine engines. The numerical model developed in this work can also be applied to explore different phenomena in this combustion system.

Measurement of distributions of temperature and wavelength-dependent emissivity of a laminar diffusion flame using hyper-spectral imaging technique

A generalized method to estimate a two-dimensional (2D) distribution of temperature and wavelength-dependent emissivity in a sooty flame with spectroscopic radiation intensities is proposed in this paper. The method adopts a Newton-type iterative method to solve the unknown coefficients in the polynomial relationship between the emissivity and the wavelength, as well as the unknown temperature. Polynomial functions with increasing order are examined, and final results are determined as the result converges. Numerical simulation on a fictitious flame with wavelength-dependent absorption coefficients shows a good performance with relative errors less than 0.5% in the average temperature. What’s more, a hyper-spectral imaging device is introduced to measure an ethylene/air laminar diffusion flame with the proposed method. The proper order for the polynomial function is selected to be 2, because every one order increase in the polynomial function will only bring in a temperature variation smaller than 20 K. For the ethylene laminar diffusion flame with 194 ml min−1 C2H4 and 284 L min−1 air studied in this paper, the 2D distribution of average temperature estimated along the line of sight is similar to, but smoother than that of the local temperature given in references, and the 2D distribution of emissivity shows a cumulative effect of the absorption coefficient along the line of sight. It also shows that emissivity of the flame decreases as the wavelength increases. The emissivity under wavelength 400 nm is about 2.5 times as much as that under wavelength 1000 nm for a typical line-of-sight in the flame, with the same trend for the absorption coefficient of soot varied with the wavelength.

Effects of soot absorption coefficient–Planck function correlation on radiative heat transfer in oxygen-enriched propane turbulent diffusion flame

The main objective of this article is to quantify the influence of the soot absorption coefficient–Planck function correlation on radiative loss and flame structure in an oxygen-enhanced propane turbulent diffusion flame. Calculations were run with and without accounting for this correlation by using a standard k–ε model and the steady laminar flamelet model (SLF) coupled to a joint Probability Density Function (PDF) of mixture fraction, enthalpy defect, scalar dissipation rate, and soot quantities. The PDF transport equation is solved by using a Stochastic Eulerian Field (SEF) method. The modeling of soot production is carried out by using a flamelet-based semi-empirical acetylene/benzene soot model. Radiative heat transfer is modeled by using a wide band correlated-k model and turbulent radiation interactions (TRI) are accounted for by using the Optically-Thin Fluctuation Approximation (OTFA). Predicted soot volume fraction, radiant wall heat flux distribution and radiant fraction are in good agreement with the available experimental data. Model results show that soot absorption coefficient and Planck function are negatively correlated in the region of intense soot emission. Neglecting this correlation is found to increase significantly the radiative loss leading to a substantial impact on flame structure in terms of mean and rms values of temperature. In addition mean and rms values of soot volume fraction are found to be less sensitive to the correlation than temperature since soot formation occurs mainly in a region where its influence is low.

Dehydrogenation and growth of soot in premixed flames

Dehydrogenation of soot formed in premixed sooting flames burning methane and ethylene in similar equivalence ratio and temperature conditions was studied in detail by measuring the type of C–H bonds, as derived from a newly developed FT-IR quantitative method, and the soot absorption coefficient. The measurement of the aliphatic and aromatic hydrogen content of soot has shown to give useful insights in the different dehydrogenation and soot mass growth processes observed for the methane and ethylene flame. In the ethylene flame, aliphatic hydrogen is preferentially and almost completely removed in the early soot formation region and the reduction of soot formation rate was observed after the aliphatic hydrogen decrease. This can be attributed to the decrease of radical sites formation and the consequent loss of surface reactivity toward further carbon addition. Downstream of the soot formation region, coagulation and thermal annealing of the ethylene soot particles, accompanied by a negligible dehydrogenation, become the predominant phenomena. The corresponding steep rise of the absorption coefficient is a signature of the increase of aromatic structures interconnected by conjugated sp2 bonds, marking the end of soot formation and growth process. In the methane flame, the decrease of aliphatic hydrogen was found to occur to a lower extent and delayed in respect to the maximum soot formation rate. The reduced soot dehydrogenation observed in the methane flame was attributed to the lower [H]/[H2] ratio, as testified by the larger presence of molecular hydrogen, which reduces the number of radical sites necessary for soot dehydrogenation and aromatic growth. In this condition, the later phases of coagulation and internal structural carbon rearrangements did not significantly occur, as demonstrated by the low value and steadiness of the methane soot absorption coefficient.

Light Absorption Coefficient and Hydrogen Content as Key Properties for Inferring Structural Features of Soot

The relationship between the light absorption coefficient of flame-formed soot and its hydrogen content was investigated by coupling UV-Visible spectroscopy with elemental analysis and FT-IR quantification of hydrogen tethered to soot sampled along the axis of a premixed ethylene flame. Just after soot inception, the preferential depletion of aliphatic hydrogen was observed moving from 0.2 to 0.1 hydrogen mole fraction and in correspondence of a relatively slight increase of the absorption coefficient. Soot formation rate decreased just after the disappearance of aliphatic hydrogen due to the consequent decrease of active sites on soot particles necessary for further carbon addition and growth. Crossing the threshold hydrogen mole fraction value of about 0.1, aromatic hydrogen resulted to survive to dehydrogenation producing a highly light-absorbing carbon. Thus, the high absorption coefficient appeared to be due to coagulation and thermal annealing marking the end of the soot formation process.

Extension of weighted sum of gray gas data to mathematical simulation of radiative heat transfer in a boiler with gas-soot media.

In this study an expression for soot absorption coefficient is introduced to extend the weighted-sum-of-gray gases data to the furnace medium containing gas-soot mixture in a utility boiler 150 MWe. Heat transfer and temperature distribution of walls and within the furnace space are predicted by zone method technique. Analyses have been done considering both cases of presence and absence of soot particles at 100% load. To validate the proposed soot absorption coefficient, the expression is coupled with the Taylor and Foster's data as well as Truelove's data for CO2-H2O mixture and the total emissivities are calculated and compared with the Truelove's parameters for 3-term and 4-term gray gases plus two soot absorption coefficients. In addition, some experiments were conducted at 100% and 75% loads to measure furnace exit gas temperature as well as the rate of steam production. The predicted results show good agreement with the measured data at the power plant site.

Effects of low air pressure on radiation-controlled rectangular ethanol and n-heptane pool fires

Experiments on rectangular ethanol and n-heptane pool fires were conducted at different altitudes in Hefei (99.8kPa) and Lhasa (66.5kPa). The burners tested had the same fuel area of 900cm2, but with aspect ratio of long side to short side (n=l/w) varied from 1 and 8. The individual and combined influences of low pressure and aspect ratio on burning rate, temperature, puffing frequency, flame height and radiation for the two fuels were interpreted and formulated. First, burning rate was found to be proportional to ambient air pressure under radiation control, the main reason is that radiative heat flux decreased with pressure due to the pressure affecting the soot absorption coefficient. Flame temperature slightly increased, leading to higher flame puffing frequency at low pressure. Flame height was almost insensitive to pressure as H∝p0. Second, for aspect ratio n, flame temperature was constant and independent of fuel type and burner shape. With increasing n, burner wall temperature increased at the long side, and decreased drastically at the short side, especially n=8. This was attributed to the change of flame tilt and heating of the burner side, caused by variation of entrainment motion. Flame puffing frequency was found to increase with n as a function of f∼(ΔT/T∞)(n+1)/2n. The flame was observed to split into small clusters by enhanced asymmetric entrainment, and H decreased with increasing n as H∼(1/n)2/5Q˙4/15. Considering fuel differences, with increasing n, the burning rate of the ethanol pool fire decreased, and n-heptane showed the opposite trend.

A novel multi−wavelength photoacoustic spectrometer for the measurement of the UV–vis-NIR spectral absorption coefficient of atmospheric aerosols

A multi-wavelength photoacoustic instrument is described, which measures the wavelength dependent optical absorption coefficient (OAC) of soot or soot-containing aerosols in-situ in a range from the ultra-violet to the near-infrared region. The instrument combines a Nd:YAG disc laser (fundamental wavelength 1064 nm, harmonics at 532, 355 and 266 nm) and four photoacoustic detection cells, each purged with the same aerosol sample flow, while being irradiated with one of the four light beams. With the help of a supplementary optical arrangement to illuminate each detection cell with 532 nm light, the system is calibrated against OAC by purging the cells with known concentrations (and hereby known OAC values) of NO 2. This calibration eliminates differences in sensitivity of the detection PA cells and makes the measurement of OAC absolute. The minimum detectable OAC was determined to be 0.2 Mm −1 at 1064 nm and 35.5 Mm −1 at 266 nm, corresponding to a minimum detectable black carbon mass concentration of about 0.1–1 μg/m 3, depending on the wavelength. Comparison measurements with artificially generated soot aerosols showed good agreement of the device with a reference instrument, based on a long path extinction cell (LOPES).

Absorbing aerosols: contribution of biomass burning and implications for radiative forcing

Abstract. Absorbing aerosols supplements the global warming caused by greenhouse gases. However, unlike greenhouse gases, the effect of absorbing aerosol on climate is not known with certainty owing to paucity of data. Also, uncertainty exists in quantifying the contributing factors whether it is biomass or fossil fuel burning. Based on the observations of absorption coefficient at seven wavelengths and aerosol optical depth (AOD) at five wavelengths carried out at Gadanki (13.5° N, 79.2° E), a remote village in peninsular India, from April to November 2008, as part of the "Study of Atmospheric Forcing and Responses (SAFAR)" pilot campaign we discuss seasonal variation of black carbon (BC) concentration and aerosol optical depth. Also, using spectral information we estimate the fraction of fossil-fuel and non-fossil fuel contributions to absorption coefficient and contributions of soot (Black Carbon), non-soot fine mode aerosols and coarse mode aerosols to AOD. BC concentration is found to be around 1000 ng/m3 during monsoon months (JJAS) and around 4000 ng/m3 during pre and post monsoon months. Non-fossil fuel sources contribute nearly 20% to absorption coefficient at 880 nm, which increases to 40% during morning and evening hours. Average AOD is found to be 0.38±0.15, with high values in May and low in September. Soot contributes nearly 10% to the AOD. This information is further used to estimate the clear sky aerosol direct radiative forcing. Top of the atmosphere aerosol radiative forcing varies between −4 to 0 W m−2, except for April when the forcing is positive. Surface level radiative forcing is between −10 to −20 W m−2. The net radiation absorbed within the atmosphere is in the range of 9 to 25 W m−2, of which soot contributes about 80 to 90%.