Full-Spectrum Correlated-k Research Articles

Improving the modeling of spectral radiation penetration into the condensed materials with the separated full spectrum correlated-k method

The large temperature difference between the radiation source and the condensed materials in fire scenarios, makes the general form of full spectrum correlated-k method unable to accurately model both absorption and emission within the condensed phase. In this paper, a new solution form for the FSCK method is presented which accurately accounts for both. This so-called “separated” form of FSCK method solves the contributions of medium emission and boundary's incident intensity separately by implementing two different reference temperatures. The advantages of the separated FSCK method is exhibited through three case studies using the transmissivity and radiative heat source calculated by high resolution line by line calculations as the benchmark. The test cases represent a layer of six different liquid and solid hydrocarbon fuels for which the various levels of irradiation from a radiating source is introduced by the temperature of its upper wall and an effective emissivity. The case studies provide a sensitivity analysis for the magnitude and spectral form of the irradiation at the boundary. The separated form of FSCK exhibits its best performance when the incident intensity and medium emission are in the same order of magnitude. Moreover, when the peak region of the boundary's spectral irradiation is closer to the peaks of the absorption coefficient spectrum of the condensed phase, the accuracy of both separated and classical FSCK solutions decreases, though, the separated solution still provides better accuracy.

Effects of oxygen depletion on soot production, emission and radiative heat transfer in opposed-flow flame spreading over insulated wire in microgravity

The main objective of this article is to investigate experimentally and numerically the effects of reduced-oxygen contents on the soot production and emission from solid fuels in microgravity, which constitute an important issue in terms of fire safety for manned space missions. Due to its convenience for the implementation of soot-related optical diagnostics, the configuration of a flame spreading in an opposed flow over thin nickel chromium (NiCr) wires coated by low density PolyEthylene (LDPE) is considered. Experiments are conducted at a pressure of 101.3 kPa and an oxidizer velocity of 150 mm/s. The oxygen mole fraction in the oxidizer, XO2, is varied from 18% to 21% by nitrogen dilution of air. The modeling strategy lies on a surrogate fuel to mimic the combustion of LDPE by preserving its stoichiometry and the laminar smoke-point (LSP) flame height. The numerical model considers a detailed chemistry, a two-equation soot production model involving laminar smoke point (LSP)-based soot formation rates and oxidation by OH and O2, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method, and a simple degradation model for LDPE. Based on experimental evidence, the soot formation rate is scaled by the adiabatic flame temperature to account for thermal effects due to variation in XO2. The model reproduces quantitatively the increase in flame size, residence time, and soot volume fraction observed experimentally as XO2 is enhanced as well as the transition from a non-smoking to a smoking flame, which occurs for XO2 between 19% and 20%. The increase in soot volume fraction results of a combined enhancement in both residence time, owing to an increase in the fuel mass flow rate, and soot formation rate due to higher temperature in the soot formation region. The radiant fraction increases significantly with XO2 from about 17% for XO2=18% to about 36% for XO2= 21%. This increase in radiative losses is accompanied by a reduction of the temperature in the soot oxidation region. Therefore, for increasing XO2, the soot oxidation process is governed by a competition between oxygen-enhanced conditions that promote the formation of soot oxidizing species and the increase in radiative losses that dampens their formation. For the present flames, the first mechanism prevails as XO2 increases from 18% to 19% whereas the second dominates as XO2 is further increased, leading to smoking flames as actually observed for XO2=20 and 21%. The radiant fraction at the smoke-point transition and the soot oxidation freezing temperature are in line with those reported at normal gravity. Finally, model results show that, whatever XO2, the contribution of radiation to the heating process is negligible ahead of the pyrolysis front and is largely overcome by surface radiative losses along the pyrolysis region.

Pressure effects on the soot production and radiative heat transfer of non-buoyant laminar diffusion flames spreading in opposed flow over insulated wires

Abstract This paper investigates experimentally and numerically pressure effects on soot production and radiative heat transfer in non-buoyant opposed-flow flames spreading over wires coated by Low Density PolyEthylene (LDPE). Experiments, conducted in parabolic flights, consider pressure levels ranging from 50.7 kPa to 121.6 kPa and an oxidizer flowing parallel to the wire's axis at a velocity of 150 mm/s and composed of 20% O2/80% N2 in volume. The numerical model includes a detailed chemistry, a two-equation smoke-point based soot production model, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method and a simple degradation model for LDPE. An analysis of the experimental data shows that the spread rate, the pyrolysis mass flow rate, and the residence time for soot formation are independent of pressure whereas the soot formation rate is third-order in pressure. The model reproduces quantitatively the effects of pressure on soot production and captures the transition from non-smoking to smoking flames. The radiant fraction increases with pressure because of an enhancement in soot radiation whereas the contribution of radiating gases remains approximately constant over the range of pressures considered. In addition, gas radiation dominates at pressure lower than 75 kPa whereas soot radiation prevails at higher-pressure levels. Consistently with the data obtained at normal gravity, the smoke-point transition is found to occur for a radiant fraction of about 0.3 and the soot oxidation freezing temperature is estimated in the range 1350–1450 K. Eventually, whatever the pressure considered, the surface re-radiation from the wire is higher than the incident radiative flux from the flame to the surface along the entire wire. This shows that radiative heat transfer contributes negatively to the heating of the unburnt LDPE and to the heat balance along the pyrolysing surface.

Assessment of engineering gas radiative property models in high pressure turbulent jet diffusion flames

This article aims to determine the most relevant engineering global method for gas radiative property modeling to be applied in the simulations of combustion problems. Two versions of the full-spectrum correlated k (FSCK) model, the Rank-Correlated full-spectrum k-distribution/ Spectral-Line-Weighted-sum-of-gray-gases (RC-FSK/RC-SLW) and a new version of the Weighted-Sum-of-Grey-Gases (WSGG) model are compared with the Narrow-Band CK (NBCK) model in four turbulent axisymmetric jet diffusion flames fueled either by hydrogen or methane at atmospheric and higher pressures. These comparisons are performed in decoupled radiative heat transfer calculations with the thermal fields being prescribed. The databases and coefficients associated to these different models are determined from a unique Line-By-Line database in order to allow a relevant comparison. Model results suggest that the SLW/FSCK methods coupled to the so-called improved scheme proposed by Cai and Modest or the Rank-Correlated SLW/FSK model, along with k-g distributions generated from accurate high-resolution high-temperature databases are the most mature gas radiative property models to be implemented in CFD codes dealing with combustion problems involving gas-soot mixtures.

Locally correlated SLW model for prediction of gas radiation in non-uniform media and its relationship to other global methods

Following previous theoretical development based on the assumption of a rank correlated/comonotonic spectrum, the Locally Correlated SLW (LC-SLW) method is outlined. The relationship between the LC-SLW method and the Reference Approach SLW (RA-SLW) method and the Rank Correlated SLW method (RC-SLW) is established. Further, the relationship between these SLW model variants and other global models including the Full Spectrum Correlated-k (FSCK) and Absorption Distribution Function (ADF) models is defined. The predictive accuracy of the various SLW models is compared. It is found that, despite the fact that the RC-SLW model demonstrates better overall accuracy, the LC-SLW model may generally prove more accurate in regions of higher gas temperature. While the LC-SLW model yields improved accuracy over the RC-SLW model in regions of high temperature, it is shown that the LC-SLW model cannot claim universal improvement in predictive accuracy at all conditions.

Investigation of gas and particle radiation modelling in wet oxy-coal combustion atmospheres

In this study, the impact of various treatments of gas and particle radiative properties on modelling radiative heat transfer in a 2D-squared enclosure with conditions representative to the post-burner region of an oxy-coal furnace with wet flue-gas recirculation is systematically investigated. The benchmark solutions for the radiative source terms and the net wall heat fluxes are computed with a narrow-band correlated-k (NBCK) model to account for the non-gray gas radiation in conjuction with the Mie-theory to compute the particle radiative properties. Comparisons with this benchmark solution show that the use of gray Henyey-Greenstein functions obtained with the Planck-mean values for the asymmetry factors of the coal/char and ash particles is sufficient to model accurately the scattering phase functions. In addition, the use of Planck-mean values for all the particle radiative properties provides predictions within 20% of the reference solutions. On the other hand, model results show that the application of a constant refractive index for ash particles induce large discrepancies, whereas such approximation can be made for coal particles without loss of accuracy. Finally, a significant amount of computational cost can be saved with no significant loss of accuracy by considering a wide-band correlated-k model instead of the NBCK model. The full-spectrum correlated-k (FSCK) scheme proposed by Modest and Riazzi (2005) based on gray scattering coefficients and phase functions improves further the computational saving with higher mean relative errors of about 5% for the radiative source terms and net wall heat fluxes. Thus, it is concluded that the FSCK model is appropriate to model the radiative heat transfer including gas and particle radiation for engineering applications.

Prediction of the radiative heat transfer in small and large scale oxy-coal furnaces

Predicting thermal radiation for oxy-coal combustion highlights the importance of the radiation models for the spectral properties of gases and particles. This study numerically investigates radiation behaviours in small and large scale furnaces through refined radiative property models, using the full-spectrum correlated k (FSCK) model and Mie theory based data, compared with the conventional use of the weighted sum of grey gases (WSGG) model and the constant values of the particle radiation properties. Both oxy-coal combustion and air-fired combustion have been investigated numerically and compared with combustion plant experimental data. Reasonable agreements are obtained between the predicted results and the measured data. Employing the refined radiative property models achieves closer predicted heat transfer properties to the measured data from both furnaces. The gas-phase component of the radiation energy source term obtained from the FSCK property model is higher within the flame region than the values obtained by using the conventional methods. The impact of using non-grey radiation behaviour of gases through the FSCK is enhanced in the large scale furnace as the predicted gas radiation source term is approximately 2–3 times that obtained when using the WSGG, while the same term is in much closer agreement between the FSCK and the WSGG for the pilot-scale furnace. The predicted total radiation source term (from both gases and particles) is lower in the flame region after using the refined models, which results in a hotter flame (approximately 50–150 K higher in this study) compared with results obtained from conventional methods. In addition, the predicted surface incident radiation reduces by using the refined radiative property models for both furnaces, in which the difference is relevant with the difference in the predicted radiation properties between the two modelling techniques. Numerical uncertainties resulting from the influences of combustion model, turbulent particle dispersion and turbulence modelling on the radiation behaviours are discussed.

A calibrated soot production model for ethylene inverse diffusion flames at different Oxygen Indexes

A numerical investigation was carried out in order to gain insights into the soot production mechanisms by studying the effect of the oxygen concentration of the oxidant stream, known as Oxygen Index (OI), on soot production and flame radiation in laminar Inverse Diffusion Flames (IDF). Several laminar axisymmetric IDF were simulated, varying the OI from 17% to 35%. All flames were fueled with pure ethylene. Comparisons to experimental data were intended to assess and improve the capabilities of a two-equation acetylene/benzene-based semi-empirical soot production model and a Full-Spectrum correlated-k (FSCK) radiative property model. Higher OI were found to generate shorter flames, presenting higher temperatures and an increase in the soot production and energy irradiated. Results show that simulations predict correctly the experimental behavior observed by changing the OI, but reliable predictions are limited to values under 25%. In order to improve the generality of the results, the soot production model was modified, incorporating the soot surface aging effect in an approximated way on the surface growth mechanism, and then it was calibrated. A square root dependence was considered in the specific soot surface area rather than a linear dependence. A sensitivity analysis shows that the surface growth process is the most sensitive in terms of predicting the soot content produced. Simulations after calibration were in satisfactory agreement with the experimental measurements, presenting accurate predictions from both local and overall points of view. Results demonstrate that IDF is a relevant configuration to obtain insights concerning soot modeling, providing a decoupled oxidation process, similar residence times at different OI, and thermal age controlled by temperature alone. Results also demonstrate that considering an aging effect on the soot surface reactivity is necessary in order to properly model the variations induced by changing the OI.

Assessment of several gas radiation models for radiative heat transfer calculations in a three-dimensional oxy-fuel furnace under coal-fired conditions

Oxy-fuel coal combustion is a promising Carbon Capture and Storage technology that allows retro-fitting of existing power plants. Due to high concentrations of CO2 and H2O and the strong and irregular spectral variation of their absorption coefficients, prediction of radiative heat transfer in oxy-fuel combustion systems requires a non-grey gas radiation model. Several models, including narrow-band correlated-k (NBCK), wide-band correlated-k (WBCK), full-spectrum correlated-k (FSCK), and weighted-sum-of-grey-gases (WSGG) models, were used to predict radiative heat transfer by CO2 and H2O in a three-dimensional real-size virtual coal-fired furnace under air- and oxy-fuel (dry and wet flue-gas recycle) conditions. The best agreement with the benchmark solution calculated by the NBCK model with the updated EM2C parameters is achieved by the FSCK model. The WBCK model is less accurate. The WSGG parameter sets of Kangwanpongpan et al. (2012) and Bordbar et al. (2014) are most suited for oxy-fuel combustion among the six sets considered. The FSCK model offers the best performance in terms of both accuracy and computational efficiency among the considered gas radiation models and is recommended for CFD simulations of real-sized oxy-coal systems.

Calculations of radiative heat transfer in an axisymmetric jet diffusion flame at elevated pressures using different gas radiation models

Radiation heat transfer in axisymmetric jet diffusion flames under conditions relevant to oxygen-enriched combustion at total pressures of 1, 10, 20, and 30atm was calculated using several gas radiation models: line-by-line (LBL), narrow-band correlated-k (NBCK), wide-band correlated-k (WBCK), full-spectrum correlated-k (FSCK), spectral-line based weight-sum-of-gray-gases (SLW), and weight-sum-of-gray-gases (WSGG). An optimized NBCK model, an optimized FSCK model, and a WBCK model were proposed and evaluated. The LBL results are used as the benchmark solution in the evaluation of other gas radiation models. The optimized NBCK model and the optimized FSCK model are much more computationally efficient than the standard implementation of these models with very little loss in accuracy. The NBCK, WBCK, and FSCK models are accurate and their normalized errors in both the radiative source term and radiative flux remain less than about 7% and display essentially no dependence on the total pressure. Whatever the pressure considered, the FSCK is found to provide accurate predictions by considering only 10 Gauss points. For the same number of gray gases, the SLW is less accurate than the FSCK, especially at pressures higher than the atmospheric pressure. However, its accuracy can be significantly improved to reach that of the FSCKby increasing the number of gray gases. The accuracy of WSGG models deteriorates somewhat with increasing the total pressure in the prediction of radiative heat flux, though it displays no significant dependence on the total pressure in the calculation of the radiative source term. The spectral line broadening has a non-negligible influence on radiative heat transfer in the jet diffusion flame. The somewhat increased inaccuracy of the WSGG model with increasing the total pressure is at least partially due to the application of the model parameters derived at 1atm to high pressures. The normalized errors of WSGG are about 10 to 20%. The optimized FSCK model is found much more accurate than the popular WSGG model with a comparable computational efficiency and is therefore recommended for large-scale CFD applications.

A comprehensive evaluation of different radiation models in a gas turbine combustor under conditions of oxy-fuel combustion with dry recycle

The oxy-fuel combustion is a promising CO2 capture technology from combustion systems. This process is characterized by much higher CO2 concentrations in the combustion system compared to that of the conventional air-fuel combustion. To accurately predict the enhanced thermal radiation in oxy-fuel combustion, it is essential to take into account the non-gray nature of gas radiation. In this study, radiation heat transfer in a 3D model gas turbine combustor under two test cases at 20atm total pressure was calculated by various non-gray gas radiation models, including the statistical narrow-band (SNB) model, the statistical narrow-band correlated-k (SNBCK) model, the wide-band correlated-k (WBCK) model, the full spectrum correlated-k (FSCK) model, and several weighted sum of gray gases (WSGG) models. Calculations of SNB, SNBCK, and FSCK were conducted using the updated EM2C SNB model parameters. Results of the SNB model are considered as the benchmark solution to evaluate the accuracy of the other models considered. Results of SNBCK and FSCK are in good agreement with the benchmark solution. The WBCK model is less accurate than SNBCK or FSCK. Considering the three formulations of the WBCK model, the multiple gases formulation is the best choice regarding the accuracy and computational cost. The WSGG model with the parameters of Bordbar et al. (2014) [20] is the most accurate of the three investigated WSGG models. Use of the gray WSSG formulation leads to significant deviations from the benchmark data and should not be applied to predict radiation heat transfer in oxy-fuel combustion systems. A best practice to incorporate the state-of-the-art gas radiation models for high accuracy of radiation heat transfer calculations at minimal increase in computational cost in CFD simulation of oxy-fuel combustion systems for pressure path lengths up to about 10barm is suggested.

Effects of total pressure on non-grey gas radiation transfer in oxy-fuel combustion using the LBL, SNB, SNBCK, WSGG, and FSCK methods

The effects of total pressure on gas radiation heat transfer are investigated in 1D parallel plate geometry containing isothermal and homogeneous media and an inhomogeneous and non-isothermal CO2–H2O mixture under conditions relevant to oxy-fuel combustion using the line-by-line (LBL), statistical narrow-band (SNB), statistical narrow-band correlated-k (SNBCK), weighted-sum-of-grey-gases (WSGG), and full-spectrum correlated-k (FSCK) models. The LBL calculations were conducted using the HITEMP2010 and CDSD-1000 databases and the LBL results serve as the benchmark solution to evaluate the accuracy of the other models. Calculations of the SNB, SNBCK, and FSCK were conducted using both the 1997 EM2C SNB parameters and their recently updated 2012 parameters to investigate how the SNB model parameters affect the results under oxy-fuel combustion conditions at high pressures. The WSGG model considered is the recently developed one by Bordbar et al. [19] for oxy-fuel combustion based on LBL calculations using HITEMP2010. The total pressure considered ranges from 1 up to 30atm. The total pressure significantly affects gas radiation transfer primarily through the increase in molecule number density and only slightly through spectral line broadening. Using the 1997 EM2C SNB model parameters the accuracy of SNB and SNBCK is very good and remains essentially independent of the total pressure. When using the 2012 EM2C SNB model parameters the SNB and SNBCK results are less accurate and their error increases with increasing the total pressure. The WSGG model has the lowest accuracy and the best computational efficiency among the models investigated. The errors of both WSGG and FSCK using the 2012 EM2C SNB model parameters increase when the total pressure is increased from 1 to 10atm, but remain nearly independent of the total pressure beyond 10atm. When using the 1997 EM2C SNB model parameters the accuracy of FSCK only slightly decreases with increasing the total pressure.

LES and RANS of air and oxy-coal combustion in a pilot-scale facility: Predictions of radiative heat transfer

This study evaluates the use of large eddy simulation (LES) and Reynolds-averaged Navier Stokes (RANS) models for the prediction of turbulent coal combustion under air and oxyfuel environments in a pilot-scale 250kWth furnace. The furnace is part of the UKCCSRC Pilot-scale Advanced Capture Technology (PACT) facilities and was designed for detailed analysis of the combustion process. The prediction of thermal radiation is validated against experimental measurements under both air- and oxy-firing regimes. Two radiation models were evaluated during the RANS calculations, the widely used weighted sum of grey gases (WSGG) and the full-spectrum correlated k (FSCK) model, while the LES case was calculated using the FSCK radiation model. The results show that the choice in gas radiation model demonstrates only a small change in the temperature and heat flux predictions in the RANS calculations, while the LES solutions are able to achieve better agreement with measured values than the RANS predictions for both air-fired and oxyfuel coal combustion.

Evaluation of FSK models for radiative heat transfer under oxyfuel conditions

Oxyfuel is a promising technology for carbon capture and storage (CCS) applied to combustion processes. It would be highly advantageous in the deployment of CCS to be able to model and optimise oxyfuel combustion, however the increased concentrations of CO2 and H2O under oxyfuel conditions modify several fundamental processes of combustion, including radiative heat transfer. This study uses benchmark narrow band radiation models to evaluate the influence of assumptions in global full-spectrum k-distribution (FSK) models, and whether they are suitable for modelling radiation in computational fluid dynamics (CFD) calculations of oxyfuel combustion. The statistical narrow band (SNB) and correlated-k (CK) models are used to calculate benchmark data for the radiative source term and heat flux, which are then compared to the results calculated from FSK models. Both the full-spectrum correlated k (FSCK) and the full-spectrum scaled k (FSSK) models are applied using up-to-date spectral data. The results show that the FSCK and FSSK methods achieve good agreement in the test cases. The FSCK method using a five-point Gauss quadrature scheme is recommended for CFD calculations in oxyfuel conditions, however there are still potential inaccuracies in cases with very wide variations in the ratio between CO2 and H2O concentrations.

Radiative heat transfer in the core of axisymmetric pool fires – I: Evaluation of approximate radiative property models

Radiative heat transfer calculations are conducted in two laboratory-scale axisymmetric methane pool fires generated on a burner of 0.38 m diameter with heat release rates (HRR) of 34 and 176 kW by using the ‘exact’ Line-By-Line (LBL) method, the narrow band correlated-k (NBCK) model, the full-spectrum correlated-k (FSCK) model, the multi-scale full-spectrum k-distribution (MSFSK) model, and a grey wide-band model (WBM). For each radiative model the corresponding absorption coefficients for carbon dioxide, water vapor, carbon monoxide and methane are generated from the same high-resolution spectroscopic database. Model results show that the contribution of carbon monoxide can be neglected whereas that of methane increases with HRR. In addition, the grey approximation for soot holds for these weakly sooting flames. Comparisons with LBL solutions show that WBM fails to predict accurately the radiative heat transfer through the fuel rich core. The FSCK model presents the best compromise in terms of accuracy and computational efficiency for the 34 kW pool fire. However, significant discrepancies are observed for the 176 kW pool fire where the strong attenuation of radiation by methane invalidates the ‘correlated’ assumption of the absorption coefficient. MSFSK and NBCK models provide very accurate predictions, with the MSFSK model being more efficient when overlap parameters are tabulated as a function of temperature and composition.

Radiative Heat Transfer Through the Fuel-Rich Core of Laboratory-Scale Pool Fires

Radiative heat transfer calculations are conducted along the axis of six axisymmetric pool fires by using the “exact” line-by-line (LBL) method, the narrow band correlated k (NBCK) model, the full-spectrum correlated k (FSCK) model, the multi-scale full-spectrum k-distribution (MSFSK) model, and the wide-band model implemented in the fire dynamic simulator (FDS). The two baseline cases correspond to 34 kW and 176 kW methane pool fires generated on a burner of 0.38 m diameter. For each heat release rate, two other moderately and heavily sooting pool fires were generated by considering higher soot volume fractions while keeping temperature and gaseous species concentrations unaltered. For each radiative model, the corresponding absorption coefficients for carbon dioxide, water vapor, carbon monoxide, and methane were generated from the same high-resolution spectroscopic databases. Model results show that the contribution of carbon monoxide to the radiative intensity can be neglected, whereas that of methane increases with the heat release rate (HRR) and decreases as the soot loading increases. It is also found that the gray approximation for soot holds for the 34 kW pool fires and the weakly and moderately sooting 176 kW pool fires but ceases to be valid for the heavily sooting 176 kW pool fire. Concerning the accuracy of the different approximate radiative models, comparisons with the LBL solutions show that the NBCK model can be used as a reference if LBL solutions are not available. On the other hand, the FDS wide-band model fails in predicting accurately the radiative intensity through the fuel-rich core of pool fires. Finally, the FSCK provide predictions within 10% of LBL solutions with the exception of the heavily sooting 176 kW pool fire where the strong attenuation of radiation by methane invalidates the “correlated” assumption of the absorption coefficient. In this case, the MSFSK model must be considered, improving substantially the predictions of the FSCK.

Finite volume method for non-equilibrium radiative heat transfer

Radiative heat transfer is an important physical phenomenon, especially in the high speed and high temperature flow regimes, making its application important in space exploration vehicles. This paper presents the development and validation of computational tools for modeling the radiative heat transfer that can be either employed by itself or coupled with a computational fluid dynamics solver to analyze aerothermodynamics environments. A generalized mesh is used for discretization of the spatial domain and a finite volume method is used for solving the radiative heat transfer equation. Solution of the radiative heat transfer equation in an angular domain is carried out in a parallel environment. This numerical approach is validated with different benchmark test cases for gray gas radiation in various geometries. To enable the simulation of non-gray gas radiation, the full spectrum correlated-k model is implemented into the radiation solver. A grid sensitivity study is conducted to analyze the dependency of mesh resolution in spatial and angular domains on solution accuracy. The results of the validation studies, grid sensitivity studies, and parallel performance of the implementation are presented in this paper.

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.

Influence of turbulence–radiation interactions in laboratory-scale methane pool fires

The objective of this study is to assess the effects of Turbulence–Radiation Interactions (TRI) on the structure of small-scale pool fires and on the radiative fluxes transferred to surrounding surfaces. Fire-induced flow is modeled by using a buoyancy-modified k-ε model and the Steady Laminar Flamelet (SLF) model coupled with a presumed Probability Density Function (pdf) approach. A 34-kW methane pool fire produced by burner with a diameter of 0.38 m is simulated by neglecting radiation, by considering radiation without TRIs, and by considering radiation with TRIs. Computations carried out with radiation are based on the Full Spectrum Correlated-k (FSCK) method. TRIs are taken into account by considering the Optically-Thin Fluctuation Approximation (OTFA). The mean radiative source term and the mean RTE are then closed by using a presumed pdf of the mixture fraction, scalar dissipation rate, and enthalpy defect. When TRIs are considered predicted flame structure, radiant fraction and radiative fluxes are found in quantitative agreement with the available experimental data. Simulations reveal that TRIs significantly enhance radiative losses and substantially contribute to the drop in temperature due to radiation. TRIs also contribute to reduce turbulence levels and the root mean square (rms) values of temperature fluctuations. In addition radiative heat fluxes on remote targets are found to be considerably higher than those obtained from radiative calculations based on mean properties. Finally different levels of closure for the TRI-related terms are assessed. Model results show that the complete absorption coefficient-Planck function correlation should be considered in order to properly take into account the influence of TRIs on the emission term whereas the effect of absorption coefficient self-correlation on the absorption term is a reduction by about 12% of the radiant fraction.

Modelling thermal radiation from one-meter diameter methane pool fires

The first objective of this article is to implement a comprehensive radiation model in order to predict the radiant fractions and radiative fluxes on remote surfaces in large-scale methane pool fires. The second aim is to quantify the importance of Turbulence-Radiation Interactions (TRIs) in such buoyant flames. The fire-induced flow is modelled by using a buoyancy-modified k-ε model and the Steady Laminar Flamelet (SLF) model coupled with a presumed probability density function (pdf) approach. Spectral radiation is modelled by using the Full-Spectrum Correlated-k (FSCK) method. TRIs are taken into account by considering the Optically-Thin Fluctuation Approximation (OTFA). The emission term and the mean absorption coefficient are closed by using a presumed pdf of the mixture fraction, scalar dissipation rate and enthalpy defect. Two 1m-diameter fires with Heat Release Rates (HRR) of 49 kW and 162 kW were simulated. Predicted radiant fractions and radiative heat fluxes are found in reasonable agreement with experimental data. The importance of TRIs is evidenced, computed radiant fractions and radiative heat fluxes being considerably higher than those obtained from calculations based on mean properties. Finally, model results show that the complete absorption coefficient-Planck function correlation should be considered in order to properly take into account the influence of TRIs on the emission term, whereas the absorption coefficient self-correlation in the absorption term reduces significantly the radiant fractions.