Soot Radiation Research Articles

Interactive transient flamelet modeling for soot formation and oxidation processes in laminar non-premixed jet flames

The Interactive Transient Flamelet (ITF) model has been devised to realistically simulate slow processes such as soot formation and radiation in a laminar non-premixed flame with a relatively long residence time. In context with the ITF model, soot formation in the laminar non-premixed jet flame is modeled by the two-equation soot model to simulate physical processes such as nucleation, surface growth, oxidation and agglomeration. In the present ITF approach, gaseous chemistry is effectively coupled with soot chemistry. The present ITF model accounts for radiative cooling induced by gaseous species and soot particles in the same mixture fraction space simultaneously while it treats the unphysical diffusion of soot to be practically zero. Moreover, the present ITF procedure can maintain consistency to extend the present transient flamelet model to the simulation of turbulent non-premixed sooting flames. To validate the present ITF approach together with the two-equation soot model, in terms of soot volume fraction, number density, temperature, OH and C2H2 mass fractions, as well as reaction rates for nucleation, surface growth, and oxidation, numerical results are compared with those obtained by the full transport equation approach. Furthermore, to assess the applicability of the present ITF approach and to evaluate the effects of gaseous differential diffusion on the soot formation processes, numerical results obtained by the present ITF model with equal and differential diffusion are also compared with experimental data in terms of temperature; CH4, OH, H2O, CO2 and C2H2 mole fractions; and soot volume fraction. Based on these numerical results, the detailed discussion has been made for the capability and limitations of the present ITF approach to predict the precise flame structure and soot formation characteristics in the laminar non-premixed methane/air jet flames.

Dependence of sooting characteristics and temperature field of co-flow laminar pure and nitrogen-diluted ethylene–air diffusion flames on pressure

Pressure dependence of sooting characteristics and the flame temperature field of pure ethylene and ethylene diluted with nitrogen in co-flow laminar diffusion flames was investigated experimentally. The pressure range for ethylene was from atmospheric to 7atm and to 20atm for nitrogen-diluted ethylene flames. Spectrally-resolved line-of-sight soot radiation emission measurements were used to obtain radially resolved temperatures and soot volume fractions by using an Abel type inversion algorithm. A constant mass flow rate of ethylene was maintained at 0.48mg/s at all pressures to match the carbon flow rates of gaseous alkane fuels experiments reported previously. Visible flame heights, as marked by the luminous soot radiation, initially increased with pressure, but changed little above 5atm. Maximum local soot volume fraction of ethylene flames seems to scale with pressure raised to the third power (about 2.8). This is argued to be a relatively stronger pressure dependence of maximum soot volume fraction as compared to other gaseous fuels. A similarly higher pressure dependence was observed when the maximum soot yields of ethylene and other gaseous fuels were compared. It was shown that the soot yield dependence of ethylene flames does not conform to the unified dependence on pressure which was demonstrated for gaseous alkane fuels recently. The sooting propensity of nitrogen-diluted ethylene flames was shown to be less than that of n-heptane flames diluted with similar amount of nitrogen. Flame temperature profiles and averaged temperatures of ethylene flames showed similar characteristics as the other gaseous fuels, however radial temperature gradients in ethylene flames were much higher than those in gaseous alkane fuel flames.

Gasoline effects on spray characteristics, mixing and auto-ignition processes in a CI engine under Partially Premixed Combustion conditions

Recent research has shown that one of the paths to reduce pollutant emissions in diesel engines is to bring the operating conditions towards those of a gasoline engine, through homogeneous combustion and high octane fuels. Reduced soot, NOx, and fuel consumption are some of the benefits of using gasoline fuel combined with EGR in compression ignition engines in partially premixed combustion modes. Thus, a comparative study using diesel fuel and gasoline has been conducted focusing on the spray characteristics, mixing and autoignition process in these new combustion modes using a conventional diesel injection system.In this work, an experimental study has been carried out comparing the effects of penetration and cone angle under non-evaporating conditions for both fuels in a constant volume test rig. Besides, liquid length measurements and combustion process under partially premixed combustion (PPC) conditions in a single cylinder transparent engine have been done. The analysis of the results for both fuels shows no major differences on penetration and spray cone angle between them. Under vaporizing conditions, diesel spray exhibited a significantly longer liquid length than gasoline, due to the higher volatility of gasoline. In quantitative terms, liquid length was found to be 1.8 to 2.4 times longer for the diesel fuel. 1D spray model calculations confirmed the experimental results.The combustion process evaluation under PPC conditions shows differences in ignition delay and RoHR even under same thermodynamic conditions. Besides, Combustion images indicate differences in the autoignition and combustion process for diesel and gasoline fuels. Gasoline shows lower soot radiation and better combustion phasing.Differences in the mixing process evaluated with the 1D model show the mixing process is governed mainly by the fuel reactivity, the ignition delay and, to a lower extent, by the spray characteristics. The overall equivalence ratio obtained through the 1D model at the start of combustion is leaner for the gasoline case. This results agrees with a longer ignition delay for this fuel, due to lower fuel reactivity, and with the lower light intensity detected in the combustion images, where the leaner the equivalence ratio the lower the intensity.

Evaluation of the gray gas model to compute radiative transfer in non-isothermal, non-homogeneous participating medium containing CO2, H2O and soot

Thermal radiation is usually the dominant heat transfer mode in combustion processes due to the presence of participating gases and soot at high temperatures. However, gases and soot present quite distinct spectral behavior in participating gases, the spectrum can be formed by millions of spectral lines, while in soot, the absorption coefficient varies linearly with the wavenumber. Therefore, when soot radiation is dominant, one can attempt to use simpler approaches, such as the gray gas (GG) model, instead of the sophisticate gas models that are required in gas-dominant radiation. This paper analyses the conditions in which the GG model can be used to provide reasonably accurate computations of radiation in soot–gaseous mixtures. Results are obtained for a one-dimensional slabs formed by a mixture of CO2, H2O and soot, and then compared with the benchmark line-by-line integration. This study shows that, even for relatively moderate concentrations of soot, the GG model can provide sufficiently accurate estimations. In addition, this paper presents newly obtained GG correlations for soot, based on empirical equations, and for CO2 and H2O, based on HITEMP2010 database.

The influence of soot radiation on NO emission in practical biodiesel combustion

This study is motivated by so-called “Biodiesel NOx Penalty”, which is the often reported increase in nitrogen oxides emissions with biodiesel, relative to petroleum diesel. Biodiesel also reportedly decreases exhaust soot concentrations via a reduction of soot formation during combustion. Soot is a known source of radiation heat transfer during diesel combustion, and can cause a reduction in flame temperature relative to non-sooting conditions. As NO formation is strongly dominated by high combustion temperature, the heat transfer from soot to surroundings can reduce NO formation. The primary objective of this study is to assess if a decrease in soot radiation with biodiesel fuel leads to the increase of NO emission. In the current study, a metallic fuel additive (barium), which is known to effectively reduce soot formation during combustion, is used to control the soot formation of both petroleum diesel and 100% palm olein biodiesel. This study shows that the soot radiation effect might not be a dominant factor on NO formation, thus likely contributes little to biodiesel’s increase of NOx emissions.

Measurement and Modeling of Particle Radiation in Coal Flames

This work aims at developing a methodology that can provide information of in-flame particle radiation in industrial-scale flames. The method is based on a combination of experimental and modeling work. The experiments have been performed in the high-temperature zone of a 77 kWth swirling lignite flame. Spectral radiation, total radiative intensity, gas temperature, and gas composition were measured, and the radiative intensity in the furnace was modeled with an axisymmetric cylindrical radiation model using Mie theory for the particle properties and a statistical narrow-band model for the gas properties. The in-flame particle radiation was measured with a Fourier transform infrared (FTIR) spectrometer connected to a water-cooled probe via fiber optics. In the cross-section of the flame investigated, the particles were found to be the dominating source of radiation. Apart from giving information about particle radiation and temperature, the methodology can also provide estimates of the amount of soot radiation and the maximum contribution from soot radiation compared to the total particle radiation. In the center position in the flame, the maximum contribution from soot radiation was estimated to be less than 40% of the particle radiation. As a validation of the methodology, the modeled total radiative intensity was compared to the total intensity measured with a narrow angle radiometer and the agreement in the results was good, supporting the validity of the used approach.

An experimental analysis on the evolution of the transient tip penetration in reacting Diesel sprays

Schlieren imaging has helped deeply characterize the behavior of Diesel spray when injected into an oxygen-free ambient. However, when considering the transient penetration of the reacting spray after autoignition, i.e. the Diesel flame, few studies have been found in literature. Differences among optical setups as well as among experimental conditions have not allowed clear conclusions to be drawn on this issue. Furthermore, soot radiation may have a strong effect on the image quality, which cannot be neglected.The present paper reports an investigation on the transient evolution of Diesel flame based upon schlieren imaging. Experimental conditions have spanned values of injection pressure, ambient temperature and density for typical Diesel engine conditions. An optimized optical setup has been used, which makes it possible to obtain results without soot interference. Based on observations for a long injection event (4ms Energizing Time), the analysis has resorted to extensive comparison of inert and reacting sprays parameters, which have made it possible to define different phases after autoignition.Shortly after autoignition, axial and radial expansion of the spray have been observed in terms of tip penetration and radial cone angle. After that, during a stabilization phase, the reacting spray penetrates at a similar rate as the inert one. Later, the reacting spray undergoes an acceleration period, where it penetrates at a faster rate than the inert one. Finally, the flame enters a quasi-steady penetration phase, where the ratio of reacting and inert penetration stabilizes at a nearly constant value. The duration of the reacting spray penetration stages shows modifications when varying engine parameters such as air temperature, air density, injection pressure, and nozzle diameter. However, the proportionality between reacting and inert penetration has been observed to depend mainly on temperature, in agreement with observed reductions in entrainment when shifting from inert to reacting conditions.

An assessment of radiation modeling strategies in simulations of laminar to transitional, oxy-methane, diffusion flames

Twenty four, laboratory scale, laminar to transitional, diffusion oxy-methane flames were simulated employing different radiation modeling options and their predictions compared against experimental measurements of: temperature, flame length and radiant fraction. The models employed were: gray and non-gray formulations of a recently proposed weighted-sum-of-gray gas model, non-adiabatic extension of the equilibrium based mixture fraction model and investigations into the effects of: the thermal boundary conditions, soot and turbulence radiation interactions (TRI).Predictions of gas, wall temperatures and flame lengths were in good agreement with experimental measurements. Flame lengths determined through the axial profiles of OH confirmed with the experimental trends by increasing with increase in fuel-inlet Reynolds numbers and decreasing with the increase in O2 composition in oxidizer. The temperature and flame length predictions were not sensitive to the radiative property model employed.There were significant variations between the gray and non-gray model radiant fraction predictions with the variations in general increasing with decrease in Reynolds numbers possibly attributed to shorter flames and steeper temperature gradients. The inclusion of soot model and TRI model did not affect our predictions as a result of low soot volume fractions and the radiation emission enhancement to the temperature fluctuations being localized to the flame sheet.

Soot Formation Modeling of n-Heptane Sprays Under Diesel Engine Conditions Using the Conditional Moment Closure Approach

Numerical simulations of soot formation of n-heptane autoigniting spray in a constant-volume vessel under diesel engine conditions with different ambient densities (14.8 and 30 kg/m3) and ambient oxygen concentrations (8–21% O2 mol fraction) were performed using two-dimensional, first-order conditional moment closure (CMC). Soot formation was modeled with a semiempirical two-equation model that considers simultaneous soot particle inception, surface growth, coagulation, and oxidation by O2 and OH. Soot radiation was accounted for with an optical-thin formulation. Results are compared to experimental data by means of ignition delay time, lift-off length, and soot volume fraction distribution. Good predictions of ignition delay and lift-off for all nine cases are achieved. High volume fraction soot location and semi-quantitative distribution have been well described even with this comparatively simple soot model. The findings suggest that the conditional moment closure approach is a promising framework for soot modeling under diesel engine conditions.

Influence of thermal radiation on soot production in Laminar axisymmetric diffusion flames

The aim of this paper is to study the effect of radiative heat transfer on soot production in laminar axisymmetric diffusion flames. Twenty-four C1–C3 hydrocarbon–air flames, consisting of normal (NDF) and inverse (IDF) diffusion flames at both normal gravity (1g) and microgravity (0g), and covering a wide range of conditions affecting radiative heat transfer, were simulated. The numerical model is based on the Steady Laminar Flamelet (SLF) model, a semi-empirical two-equation acetylene/benzene based soot model and the Statistical Narrow Band Correlated K (SNBCK) model coupled to the Finite Volume Method (FVM) to compute thermal radiation. Predictions relative to velocity, temperature, soot volume fraction and radiative losses are on the whole in good agreement with the available experimental data. Model results show that, for all the flames considered, thermal radiation is a crucial process with a view to providing accurate predictions for temperatures and soot concentrations. It becomes increasingly significant from IDFs to NDFs and its influence is much greater as gravity is reduced. The radiative contribution of gas prevails in the weakly-sooting IDFs and in the methane and ethane NDFs, whereas soot radiation dominates in the other flames. However, both contributions are significant in all cases, with the exception of the 1g IDFs investigated where soot radiation can be ignored. The optically-thin approximation (OTA) was also tested and found to be applicable as long as the optical thickness, based on flame radius and Planck mean absorption coefficient, is less than 0.05. The OTA is reasonable for the IDFs and for most of the 1g NDFs, but it fails to predict the radiative heat transfer for the 0g NDFs. The accuracy of radiative-property models was then assessed in the latter cases. Simulations show that the gray approximation can be applied to soot but not to combustion gases. Both the non-gray and gray soot versions of the Full Spectrum Correlated k (FSCK) model can be then substituted for the SNBCK with a reduction in CPU time by a factor of about 20 in the latter case.

Computations of soot formation in ethylene/air counterflow diffusion flames and its interaction with radiation

A methodology is presented which allows to predict soot levels produced in simple, one-dimensional laminar flames. The method is applied to the calculation of a set of well documented ethylene/air counterflow diffusion flames, using a detailed chemical mechanism (Davis et al., 1999 [1]) and a semi-empirical, two-equation soot model from Leung and Lindstedt (1991) [2]. Modifications of the original soot model are made in order to retrieve the experimental measurements of Hwang and Chung (2001) [3]. To account for radiative heat losses, a second series of fully coupled gas/soot/radiation simulations of the counterflow flames is performed. This allows to assess the effect of soot and gas radiation on soot formation and on the flame structure.

EXPERIMENTAL STUDY OF TURBULENT LPG TURBULENT INVERSE DIFFUSION FLAME IN COAXIAL BURNER

The present experimental study investigates the turbulent LPG Inverse Diffusion Flame (IDF) stabilized in a coaxial burner in terms of flame appearance, visible flame length, centerline temperature distribution and oxygen concentration and NOX emission characteristics. The effect of air-fuel jet velocities on visible flame length is interpreted using global strain rate and a new devised parameter called Modified Momentum Ratio. The centerline temperature exhibits a steeper increase in the lower premixed zone of the IDF due to the enhanced premixing. Subsequently it declines gradually in the upper luminous portion owing to soot radiation and heat losses to the ambient. Further, the centerline oxygen depletes rapidly in the lower blue zone but found to increase gradually in the upper luminous portion of IDF. The centerline temperature and oxygen distribution along the flame length revealed the dual flame structure of IDF. The EINOX values exhibited a bell shaped profile and reached a maximum value around stoichiometric overall equivalence ratio.

Evaluation of the influence of OEC on soot formation and thermal radiation in confined acetylene diffusion flames

The effect of OEC on soot formation and thermal radiation was studied in confined acetylene diffusion flames. Confined flames are widely used in industrial settings. The flames were produced in a combustion chamber with a burner operating with a parallel annular coaxial flow of the oxidizer. The soot concentration was calculated by the laser-induced light extinction method. The thermal radiation was measured with a radiometer in the narrow band of influence of soot radiation. The oxygen content in the combustion air was less than 30% – 21 to 25% – which does not require significant retrofitting of existing equipment when combustion conditions are varied. The results suggest that the use of OEC enables soot formation and thermal radiation in confined acetylene flames to be managed and controlled.

2D soot concentration and burning rate of a vertical PMMA slab using Laser-Induced Incandescence

In the field of fire studies, it is interesting to provide useful data for the validation of soot production and radiation models. 2D soot concentration in the flame and burning rate of the solid surface have been determined in the case of the combustion of a vertical PMMA slab. The local soot concentration has been measured with the Laser-Induced Incandescence method. This one has been calibrated with in situ extinction measurements performed simultaneously (at 1064nm). The interference signals of LII caused by laser scattering and Laser-Induced fluorescence have been considered and eliminated by a well suited detection. The flat field effect caused by the ICCD camera has also been corrected. The trapping effect on the LII signal has also been considered. The flame grows on the slab after the ignition, and after 1500s a steady state of combustion appears. During this period, the soot profiles in the boundary layer have been measured at two heights in the flame and their main features will be discussed. It has been possible to determine the burning rate of the PMMA slab from the observation of the displacement of soot profiles in the camera field. The values at both heights are respectively 5.55 and 6.95g/s/m2. These values will be compared with results obtained in other studies.

Turbulence-radiation interactions in large-eddy simulations of luminous and nonluminous nonpremixed flames

Turbulence-radiation interactions (TRI) are explored in large-eddy simulations (LES) of luminous and nonluminous nonpremixed jet flames. The simulations feature a transported filtered density function (FDF) method for subfilter-scale fluctuations in composition and temperature, and a fully coupled photon Monte Carlo (PMC) method for radiative transfer with line-by-line (LBL) spectral resolution. The model is exercised to isolate and quantify individual contributions to TRI for conditions that range from small optically thin flames to relatively large optically thick flames, including spectral molecular gas radiation and broadband soot radiation. The results provide new physical insight into TRI and guidance for modeling. In all cases, emission TRI are responsible for a significant fraction of the radiative emission, and that fraction increases with increasing optical thickness. For simulations where 84% of the turbulence kinetic energy is resolved, contributions of subfilter-scale fluctuations to emission TRI exceed those of resolved-scale fluctuations. The largest contributions to emission TRI are the absorption coefficient–temperature correlation and the temperature self-correlation. Absorption TRI are evident only for relatively high optical thicknesses. In all cases, the contributions of subfilter-scale fluctuations to absorption TRI are negligible.

Laminar Smoke Point Based Subgrid Soot Radiation Modeling Applied to LES of Buoyant Turbulent Diffusion Flames

Large eddy simulations (LES) of gaseous buoyant turbulent flames have been conducted with the application of a flamelet based soot-radiation model. The subgrid model applies a turbulent eddy description of soot formation, oxidation and radiation and is based on the laminar smoke point concept. Two parameters, a local turbulent strain rate and prior enthalpy loss/gain fraction influence the soot formation and radiation. Radiation heat transfer is simulated by solving the finite volume discretized form of the radiative transfer equation (RTE) with the subgrid soot-radiation model implemented. The radiant heating of surfaces in close proximity of the flames is computed and predicted heat fluxes and surface temperatures are compared against experimental data. Fire growth in a rack storage arrangement is simulated with the application of a pyrolysis model. Computed heat release rate (HRR) is compared against experimental data.

High speed imaging of OH* chemiluminescence and natural luminosity of low temperature diesel spray combustion

This study focused on spray combustion of ultra low sulfur diesel (ULSD) fuel under low oxygen conditions with low temperature combustion (LTC) mode in an optically accessible constant volume combustion chamber. The ambient oxygen concentration was configured as 10% and 15% to achieve low flame temperature. The ambient gas temperature varied from 800K to 1200K. High speed imaging of OH* chemiluminescence and natural luminosity (NL) was used to visualize the instantaneous spray combustion process. The heat release rate was analyzed using the transient combustion pressure and the flame structure was studied based on the combustion images. Results show that a higher oxygen concentration case features a shorter ignition delay and higher heat release rate. The LTC mode can be realized by decreasing the oxygen concentration and ambient temperature simultaneously and it features a longer ignition delay, a slower reaction rate, and apparently lower soot radiation heat loss. The visualization results of NL and OH* show that the high temperature reaction occurs mainly in the mid-stream and downstream of the spray combustion, but not in the region very close to the chamber wall. This study validates the LTC process by showing very weak OH* chemiluminescence signal. The results also indicate that in order to realize LTC mode, it is important to control the ambient oxygen and ambient temperature at the same time. By only reducing the ambient oxygen concentration it may not be effective to suppress soot generation.

Computational Fluid Dynamics Modeling of Oxy-Fuel Flames: The Role of Soot and Gas Radiation

This work applies a computational fluid dynamics (CFD) approach to examine gas and soot-related radiation mechanisms in air and oxy-fuel flames operated with propane as fuel. In oxy-fuel combustion, CO2 and H2O replace the N-2 in air combustion. As a result, the radiative heat transfer characteristics differ between the combustion atmospheres. Moreover, changes in soot formation have been observed in oxy-fuel compared to air-fired flames. Both gas- and soot-related radiation can be essential for the design of oxy-fuel furnaces and need to be accounted for when temperature and heat transfer conditions are modeled. The aim of the work is to determine the respective impact of combustion gases and soot in heat transfer modeling of the flames. Both gray and nongray approaches are used to account for the gas radiation and the results are compared to measured data from a 100 kW oxy-fuel unit to investigate if a gray model is sufficient to generate a reliable solution when applied in CFD simulations of oxy-fuel combustion. In addition, calculations of the radiative source term are performed for a domain between two infinite plates, with temperature and concentration profiles from the CFD simulations of the present work. It is shown that the nongray approach accurately predicts the source term in both combustion environments, whereas the gray model fails in predicting the source term. The source term has a direct influence on the temperature field in CFD calculations. However, this work also shows that the inclusion of soot radiation is more critical in sooty air and oxy-fuel flames than the use of a more rigorous description of the radiative properties of the gas.

Modelling thermal radiation in buoyant turbulent diffusion flames

This work focuses on the numerical modelling of radiative heat transfer in laboratory-scale buoyant turbulent diffusion flames. Spectral gas and soot radiation is modelled by using the Full-Spectrum Correlated-k (FSCK) method. Turbulence-Radiation Interactions (TRI) are taken into account by considering the Optically-Thin Fluctuation Approximation (OTFA), the resulting time-averaged Radiative Transfer Equation (RTE) being solved by the Finite Volume Method (FVM). Emission TRIs and the mean absorption coefficient are then closed by using a presumed probability density function (pdf) of the mixture fraction. The mean gas flow field is modelled by the Favre-averaged Navier–Stokes (FANS) equation set closed by a buoyancy-modified k-ϵ model with algebraic stress/flux models (ASM/AFM), the Steady Laminar Flamelet (SLF) model coupled with a presumed pdf approach to account for Turbulence-Chemistry Interactions, and an acetylene-based semi-empirical two-equation soot model. Two sets of experimental pool fire data are used for validation: propane pool fires 0.3 m in diameter with Heat Release Rates (HRR) of 15, 22 and 37 kW and methane pool fires 0.38 m in diameter with HRRs of 34 and 176 kW. Predicted flame structures, radiant fractions, and radiative heat fluxes on surrounding surfaces are found in satisfactory agreement with available experimental data across all the flames. In addition further computations indicate that, for the present flames, the gray approximation can be applied for soot with a minor influence on the results, resulting in a substantial gain in Computer Processing Unit (CPU) time when the FSCK is used to treat gas radiation.

Radiation Properties of Oxygen-Enhanced Normal and Inverse Diffusion Flames

Radiative heat transfer in oxygen-enhanced inverse flame configurations is an important area of study for fundamental combustion research and for terrestrial and spacecraft fire safety. Motivated by this, heat flux distributions, total radiative heat loss and spectral radiation intensities were investigated experimentally for oxygen-enhanced normal and inverse laminar ethane diffusion flames with increasing heat release rates. The oxygen mole fraction in the oxidizer was varied as 21%, 30%, 50%, and 100% with coflowing normal and inverse flame burners used to stabilize the flames. The inverse diffusion flames were essentially nonluminous while the normal diffusion flames with identical heat release rates were highly luminous. Oxygen enhancement led to reduced flame lengths, increased luminosities and increased total radiative heat loss and spectral radiation intensities for both normal and inverse diffusion flames. Using flame length as the characteristic length parameter, the normalized radiative heat flux distributions for flames approximately collapsed together, further establishing the effectiveness of the single point radiant output measurement technique. Radiative heat loss fractions of normal and inverse diffusion flames with varying oxygen concentrations in the oxidizer are compared. The radiation spectra of all flames included significant contributions from gas radiation from carbon dioxide and water vapor and the radiation spectra of the high oxygen concentration flames included contributions from soot radiation.