Turbulence Radiation Interactions Research Articles

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.

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.

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.

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.

Predicting the mean and RMS fields in subsonic hydrogen jet fires

In this paper two modelling approaches for predicting the mean and root mean squared (RMS) fields of turbulent hydrogen jet fires are evaluated. This is of interest if the turbulence–radiation interaction in such jets is to be modelled rigorously. One of the modelling strategies is based on calculating the variance of the fields of interest from a prescribed probability density function (PDF) and the second is to solve a modelled transport equation for field variances. In addition three different PDFs are considered: a β PDF, a clipped Gaussian PDF and a truncated Gaussian PDF. The different modelling strategies are evaluated using a number of experimental jet fires taken from the open literature. The conclusions of the study are the PDF approach for predicting the RMS fields are superior to solving a transport equation for the variance of the field variables of interest. Of the PDFs considered the β PDF and truncated Gaussian PDF give similar predictions of the jet fire structure being more accurate than the clipped Gaussian PDF.

Analysis of the interaction between turbulent combustion and thermal radiation using unsteady coupled LES/DOM simulations

Radiation exchanges must be taken into account to improve the prediction of heat fluxes in turbulent combustion. The strong interaction with turbulence and its role on the formation of polluting species require the study of unsteady coupled calculations using Large Eddy Simulations (LESs) of the turbulent combustion process. Radiation is solved using the Discrete Ordinate Method (DOM) and a global spectral model. A detailed study of the coupling between radiative heat transfer and LES simulation involving a real laboratory flame configuration is presented. First the impact of radiation on the flame structure is discussed: when radiation is taken into account, temperature levels increase in the fresh gas and decrease in the burnt gas, with variations ranging from 100K to 150K thus impacting the density of the gas. Coupling DOM and LES allows to analyze radiation effects on flame stability: temperature fluctuations are increased, and a wavelet frequency analysis shows changes in the flow characteristic frequencies. The second part of the study focuses on the Turbulence Radiation Interaction (TRI) using the instantaneous radiative fields on the whole computational domain. TRI correlations are calculated and are discussed along four levels of approximation. The LES study shows that all the TRI correlations are significant and must be taken into account. These correlations are also useful to calculate the TRI correlations in the Reynolds Averaged Navier–Stokes (RANS) approach.

Turbulence–Radiation Interaction: From Theory to Application in Numerical Simulations

Theoretical analysis and experimental investigations have shown that the mean heat fluxes in turbulent gaseous flows are influenced not only by the mean scalar fields (temperature and molar fraction of the species) but also by the scalar fluctuations. It is widely recognized that the increase of radiative fluxes in comparison with laminar flows may exceed 100%. This interaction between turbulence and radiation is mainly due to the nonlinearity between radiative emission and temperature. It is particularly important in reactive flows, since temperature fluctuations are typically higher in these flows than in nonreactive ones. In this paper, a survey of the theory concerning turbulence–radiation interaction (TRI) is presented, along with applications in numerical simulations. We first present experimental and theoretical fundamentals on TRI. Then, direct numerical simulation and stochastic methods are addressed. Although they provide reliable information on TRI, they are too computationally demanding for practical applications. We will then focus on methods based on the solution of the time-averaged form of the conservation equations. Although many different approaches are available, we will concentrate on two methods. One is based on the solution of the time-averaged form of the radiative transfer equation using the optically thin fluctuation approximation and a combustion model based on a prescribed probability density function (pdf) approach. The second one is based on the photon Monte Carlo method for radiative transfer calculations in media represented by discrete particle fields and a combustion model based on the Monte Carlo solution of the transport equation for the joint pdf of scalars. Finally, the role of TRI in large eddy simulation is discussed, and the main consequences of TRI in combustion systems are summarized.

Assessment of a presumed joint pdf for the simulation of turbulence–radiation interaction in turbulent reactive flows

Turbulent reactive flows are an important problem in engineering, with a major impact on combustion efficiency, pollutant emissions and climate change. The numerical simulation of these flows is difficult, not only due to the need to address different physical phenomena, but also due to the interaction between them. The interaction between turbulence and radiation, due to the strong non-linearity between temperature and radiative emission, is the subject of the present work. To account for this interaction, the time-averaged form of the radiative transfer equation may be solved. The correlation between the absorption coefficient of the medium and the radiation intensity is often neglected in simulations, because the radiation intensity depends on the temperature and chemical composition along an optical path, and therefore it may be argued that it is relatively independent of local turbulent fluctuations. However, this approximation may introduce errors that are not negligible. A model has recently been proposed to determine that correlation, which relies on the assumption that the joint probability density function of mixture fraction and radiation intensity is a two-dimensional clipped Gaussian. This assumption is investigated here using experimental data for the mean and variance of mixture fraction for a turbulent free jet flame. It is shown that the presumed joint pdf accurately allows the evaluation of the mean radiation emission and absorption, but the prediction of the correlation between fluctuations of the absorption coefficient and fluctuations of the radiation intensity is not so good near the end of radial optical paths.

Non-gray modeling of radiative heat transfer in hydrogen combustion scenarios

SUMMARY New weighted-sum-of-gray-gases model (WSGGM) parameters for H2O vapor are derived from emissivity correlations in the open literature and presented for use in hydrogen combustion simulations. Predictions employing the new WSGGM are seen to compare favorably against the spectral-line-based WSGGM on benchmark problems as well as in media conditions representative of turbulent hydrogen diffusion flames (Sandia Flame A and a model hydrogen gas-turbine combustor). The Sandia Flame A calculations were performed in a decoupled manner employing experimental measurements of temperature and gas compositions as inputs. The measured temperature variance data were employed to model the turbulence–radiation interactions. A coupled computational fluid dynamic (CFD) calculation was performed to obtain the conditions within the gas-turbine combustor geometry. The observed accuracies of the proposed set of WSGGM parameters in conditions encompassing a wide range of H2O vapor concentrations and temperature non-homogeneities encountered in combustion media make them amenable to implementation in CFD codes. Copyright © 2011 John Wiley & Sons, Ltd.

Study of Emission Turbulence-Radiation Interaction in Hypersonic Boundary layers

Direct numerical simulations are conducted to study the effects of emission turbulence-radiation interaction in hypersonic turbulent boundary layers, representative of the Orion Crew Exploration Vehicle at peak-heating condition during reentry. A nondimensional governing parameter to measure the significance of emission turbulence-radiation interaction is proposed, and the direct numerical simulation fields with and without emission coupling are used to assess emission turbulence-radiation interaction. Both the uncoupled and coupled results show that there is no sizable interaction between turbulence and emission at the hypersonic environment under investigation. An explanation of why the intensity of emission turbulence-radiation interaction in the hypersonic boundary layer is smaller than that in many combustion flows is provided.

Relevance of the subgrid-scales for large eddy simulations of turbulence–radiation interactions in a turbulent plane jet

An a priori study based on direct numerical simulation (DNS) of a non-isothermal turbulent plane jet has been carried out in order to analyse the role of the small-scales of turbulence on thermal radiation. Filtered DNS and large eddy simulation (LES) without subgrid-scale (SGS) model have been estimated for the radiative heat transfer. The comparison of the results highlights the subgrid-scale influence over the filtered radiation quantities, such as the radiative intensity and the radiative emission. The influence of the optical thickness is also studied. It is shown that the subgrid-scales are not significant near the centerline of the jet, where the radiative heat transfer is more important, and therefore that the SGS can be neglected in this configuration. However, when the optical thickness increases, the SGS become relevant and SGS modeling may be needed.

A new weighted-sum-of-gray-gases model for CO 2–H 2O gas mixtures

Decoupled radiative transfer calculations of Sandia Flame D are performed employing experimental measurements of temperature and gas compositions and the discrete ordinates method. Turbulence radiation interactions are accounted for employing temperature variance measurements. The spatial variations in the mean H 2O/CO 2 ratios within the flame are seen to vary widely and sometimes fall outside the range of the weighted-sum-of-gray-gases model (WSGGM) parameters reported in the literature. Therefore, parameters for a new WSGGM are computed from total emissivity correlations encompassing the range of the H 2O/CO 2 ratios encountered within the flame. Predictions from the new model compare favorably against the spectral-line-based WSGGM and existing benchmarks.

A comparison of gray and non-gray modeling approaches to radiative transfer in pool fire simulations

Decoupled radiative heat transfer calculations of 30 cm-diameter toluene and heptane pool fires are performed employing the discrete ordinates method. The composition and temperature fields within the fires are created from detailed experimental measurements of soot volume fractions based on absorption and emission, temperature statistics and correlations found in the literature. The measured temperature variance data is utilized to compute the temperature self-correlation term for modeling turbulence–radiation interactions. In the toluene pool fire, the presence of cold soot near the fuel surface is found to suppress the average radiation feedback to the pool surface by 27%. The performances of four gray and three non-gray radiative property models for the gases are also compared. The average variations in radiative transfer predictions due to differences in the spectroscopic and experimental databases employed in the property model formulations are found to be between 10% and 20%. Clear differences between the gray and non-gray modeling strategies are seen when the mean beam length is computed based on traditionally employed geometric relations. Therefore, a correction to the mean beam length is proposed to improve the agreement between gray and non-gray modeling in simulations of open pool fires.

Radiation characteristics and turbulence–radiation interactions in sooting turbulent jet flames

A comprehensive modeling strategy including detailed chemistry, soot and radiation models coupled with state-of-the-art closures for turbulence–chemistry interactions and turbulence–radiation interactions is applied to various luminous turbulent jet flames. Six turbulent jet flames are simulated with Reynolds numbers varying from 6700 to 15,000, two fuel types (pure ethylene, 90% methane–10% ethylene blend) and different oxygen concentrations in the oxidizer stream (from 21% O2 to 55% O2). All simulations are carried out with a single set of physical and numerical parameters (model constants). A Lagrangian particle Monte Carlo method is used to solve a modeled joint probability density function (PDF) transport equation, which allows accurate closure for turbulence–chemistry interactions including nonlinear soot subprocesses. Radiation is calculated using a particle-based photon Monte Carlo method that is coupled with the PDF method to accurately account for both emission and absorption turbulence–radiation interactions (TRI). Line-by-line databases are used for accurate spectral radiative properties of CO2 and H2O; soot radiative properties also are modeled as nongray. For the flames that have been investigated, soot emission can be almost 45% of the total emission, even when the peak soot volume fraction is of the order of a few parts-per-million (ppm) and up to 99% of soot emission can escape the domain without re-absorption. Turbulence–radiation interactions have a strong effect on the net radiative heat loss from these sooting flames. For a given temperature, species and soot distribution, TRI increases emission from the flames by 30–60%, and the net heat loss from the flame increases by 45–90% when accounting for TRI. This is higher than the corresponding increase in radiative heat loss due to TRI in nonsooting flames. Absorption TRI was found to be negligible in these laboratory-scale sooting flames with soot levels on the order of a few ppm, but may be important in larger industrial-scale flames and in higher-pressure systems.

The influence of the non-resolved scales of thermal radiation in large eddy simulation of turbulent flows: A fundamental study

The filtered radiative transfer equation has been solved without subgrid-scale model and compared to a-priori estimations from direct numerical simulation of statistically stationary homogeneous isotropic turbulence. The importance of the large eddy simulation non-resolved scales on the thermal radiation is analysed in this academic case. The influence of the mean and variance of temperature of the system on the subgrid-scale correlations is studied. It is shown that the turbulence–radiation interaction (TRI) is greater for low-temperature flows, where the radiative transfer is lower. Moreover, the strongest subgrid-scale correlations, which are the temperature self-correlation and the absorption coefficient-temperature correlation, have opposite effects, which suggests that it is better to neglect both correlations, instead of modelling only one. Therefore, the assumption of neglecting the TRI in LES leads to good predictions. Three-dimensional filtering effects, as well as the grid influence on non-local quantities, such as the radiation intensity, are also analysed. These effects have a local influence, but are negligible in the tested cases.

Composition PDF/photon Monte Carlo modeling of moderately sooting turbulent jet flames

A comprehensive model for luminous turbulent flames is presented. The model features detailed chemistry, radiation and soot models and state-of-the-art closures for turbulence–chemistry interactions and turbulence–radiation interactions. A transported probability density function (PDF) method is used to capture the effects of turbulent fluctuations in composition and temperature. The PDF method is extended to include soot formation. Spectral gas and soot radiation is modeled using a (particle-based) photon Monte Carlo method coupled with the PDF method, thereby capturing both emission and absorption turbulence–radiation interactions. An important element of this work is that the gas-phase chemistry and soot models that have been thoroughly validated across a wide range of laminar flames are used in turbulent flame simulations without modification. Six turbulent jet flames are simulated with Reynolds numbers varying from 6700 to 15,000, two fuel types (pure ethylene, 90 % methane– 10 % ethylene blend) and different oxygen concentrations in the oxidizer stream (from 21 % O 2 to 55 % O 2 ). All simulations are carried out with a single set of physical and numerical parameters (model constants). Uniformly good agreement between measured and computed mean temperatures, mean soot volume fractions and (where available) radiative fluxes is found across all flames. This demonstrates that with the combination of a systematic approach and state-of-the-art physical models and numerical algorithms, it is possible to simulate a broad range of luminous turbulent flames with a single model.

Radiation statistics in homogeneous isotropic turbulence

An analysis of turbulence–radiation interaction (TRI) in statistically stationary (forced) homogeneous and isotropic turbulence is presented. A direct numerical simulation code was used to generate instantaneous turbulent scalar fields, and the radiative transfer equation (RTE) was solved to provide statistical data relevant in TRI. The radiation intensity is non-Gaussian and is not spatially correlated with any of the other turbulence or radiation quantities. Its power spectrum exhibits a power-law region with a slope steeper than the classical −5/3 law. The moments of the radiation intensity, Planck-mean and incident-mean absorption coefficients, and emission and absorption TRI correlations are calculated. The influence of the optical thickness of the medium, mean and variance of the temperature and variance of the molar fraction of the absorbing species is studied. Predictions obtained from the time-averaged RTE are also included. It was found that while turbulence yields an increase of the mean blackbody radiation intensity, it causes a decrease of the time-averaged Planck-mean absorption coefficient. The absorption coefficient self-correlation is small in comparison with the temperature self-correlation, and the role of TRI in radiative emission is more important than in radiative absorption. The absorption coefficient–radiation intensity correlation is small, which supports the optically thin fluctuation approximation, and justifies the good predictions often achieved using the time-averaged RTE.

A NUMERICAL STUDY OF THE INFLUENCE OF TEMPERATURE FLUCTUATIONS IN THE THERMAL RADIATION FIELD

The present paper performs a numerical study of the influence of fluctuations on the temperature field over the thermal radiation field with the purpose to simulate the effect of Turbulence-Radiation Interactions (TRI). To evaluate the behavior of the divergence of the radiant heat flux for a flame in a cylindrical cavity, four temperature profiles are imposed: an average temperature profile and other three with 10%, 20% and 30% of turbulence intensity. The radiative transfer equation is solved using the discrete ordinates method (DOM) and the participating medium is treated as a gray gas. The results demonstrate that the fluctuations of temperature profiles increase significantly the mean divergence of the radiant heat flux in comparison with the average temperature profile, reaching to approximately 20% for the profile with 30% of turbulence intensity.

Large-Eddy Simulation of Turbulence-Radiation Interactions in a Turbulent Planar Channel Flow

Large-eddy simulation (LES) has been performed for planar turbulent channel flow between two infinite, parallel, stationary plates. The capabilities and limitations of the LES code in predicting correct turbulent velocity and passive temperature field statistics have been established through comparison to direct numerical simulation data from the literature for nonreacting cases. Mixing and chemical reaction (infinitely fast) between a fuel stream and an oxidizer stream have been simulated to generate large composition and temperature fluctuations in the flow; here the composition and temperature do not affect the hydrodynamics (one-way coupling). The radiative transfer equation is solved using a spherical harmonics (P1) method, and radiation properties correspond to a fictitious gray gas with a composition- and temperature-dependent Planck-mean absorption coefficient that mimics that of typical hydrocarbon-air combustion products. Simulations have been performed for different optical thicknesses. In the absence of chemical reactions, radiation significantly modifies the mean temperature profiles, but temperature fluctuations and turbulence-radiation interactions (TRI) are small, consistent with earlier findings. Chemical reaction enhances the composition and temperature fluctuations and, hence, the importance of TRI. Contributions to emission and absorption TRI have been isolated and quantified as a function of optical thickness.

A Hybrid Large Eddy Simulation/Filtered Mass Density Function for the Calculation of Strongly Radiating Turbulent Flames

Due to the complex nonlinear coupling of turbulent flow, finite-rate combustion chemistry and thermal radiation from combustion products and soot, modeling, and/or simulation of practical combustors, or even laboratory flames undergoing strong soot formation, remain elusive. Methods based on the determination of the probability density function of the joint thermochemical scalar variables offer a promising approach for handling turbulence-chemistry-radiation interactions in flames. Over the past decade, the development and application of the filtered mass density function (FMDF) approach in the context of large eddy simulations (LES) of turbulent flames have gained considerable ground. The work described here represents the first application of the LES/FMDF approach to flames involving soot formation and luminous radiation. The initial focus here is on the use of a flamelet soot model in an idealized strongly radiating turbulent jet flame, which serves to detail the formulation, highlight the importance of turbulence-radiation interactions, and pave the way for the inclusion of a soot transport and finite-rate kinetics model allowing for quantitative comparisons to laboratory scale sooting flames in the near future.