Turbulence Radiation Interactions Research Articles

Analysis of the turbulence–radiation interactions for large eddy simulations of turbulent flows

In large eddy simulations (LES) of radiative heat transfer, the subgrid-scale (SGS) radiative emission and the SGS radiative absorption are two unclosed terms that arise after applying the filtering operation to the radiative transfer equation and which represent the effect of the SGS motions on the evolution of the resolved radiative heat flux. In the present work, a-priori tests based on direct numerical simulation of homogeneous isotropic turbulence have been carried out. It was found that the effect of the SGS radiative absorption may be neglected in LES, whereas the SGS radiative emission has to be modelled, particularly for engineering applications where the grids are generally coarse, and in flows with high turbulence intensities. Future works should be devoted to the development of SGS models for radiative transfer.

Approximate solutions of the filtered radiative transfer equation in large eddy simulations of turbulent reactive flows

An analysis of the relevance of turbulence–radiation interaction in the numerical simulation of turbulent reactive flows is presented. A semi-causal stochastic model was used to generate a time-series of turbulent scalar fluctuations along optical paths of Sandia flame D, a widely studied piloted turbulent jet nonpremixed flame. The radiative transfer equation was integrated along these paths for every realization using a grid resolution typical of a direct numerical simulation. The correlated k-distribution method was employed to compute the radiative properties of the medium. The results were used to determine the ensemble average, as well as the extreme values, of quantities that indicate the importance of the turbulence–radiation interaction. Several approximate methods are then proposed to solve the filtered radiative transfer equation in the framework of large eddy simulations. The proposed methods are applicable along with combustion models that either assume the filtered probability density function of a conserved scalar or solve a transport equation for a joint scalar or joint scalar/velocity filtered density function. It is concluded that the errors resulting from neglecting the turbulence–radiation interaction in large eddy simulations are much lower than those found in Reynolds-averaged Navier–Stokes calculations. The optically thin fluctuation approximation may be extended to large eddy simulations yielding predictions in excellent agreement with the reference solution. If the turbulence–radiation interaction is accounted for using this approximation, the average relative error of the filtered total radiation intensity is generally below 0.3% for the studied flame.

Direct numerical simulation of turbulence–radiation interactions in a statistically one-dimensional nonpremixed system

An important issue in chemically reacting turbulent flows is the interaction between turbulence and radiation (TRI), which arises from highly nonlinear coupling between fluctuations in temperature and species composition of the flow field with the fluctuations of radiative intensity. Here direct numerical simulation (DNS) has been employed to investigate TRI in canonical nonpremixed systems in three-dimensional geometries. A photon Monte Carlo method has been used to solve the radiative transfer equation (RTE), which has been coupled with the flow solver. Radiation properties employed here correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient, which mimics that of typical hydrocarbon–air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient–Planck function correlation, and the absorption coefficient–intensity correlation have been examined for intermediate-to-large values of the optical thickness, and contributions from all three correlations were found significant but the relative importance of their contribution varies with optical thickness.

Monte Carlo simulation of radiative heat transfer and turbulence interactions in methane/air jet flames

A Photon Monte Carlo method combined with a composition PDF method is employed to model radiative heat transfer in combustion applications. Turbulence–radiation interactions (TRIs) can be fully taken into account using the proposed method. Sandia's Flame D and artificial flames derived from it are simulated and good agreement with experimental data is found. The effects of different TRI components are investigated. It is shown that, to predict the radiation field accurately, emission TRI must be taken into account, while, as expected, absorption TRI is negligible in the considered nonsooting methane/air jet flames if the total radiation quantities are concerned, but non-negligible for evaluation of local quantities. The influence of radiation on the turbulent flow field is also discussed.

Turbulence radiation interaction in Reynolds-averaged Navier–Stokes simulations of nonpremixed piloted turbulent laboratory-scale flames

Numerical simulation results are presented for two axisymmetric, nonluminous turbulent piloted jet diffusion flames: Sandia Flame D (SFD) and Delft Flame III (DFIII). Turbulence is represented by a Reynolds stress transport model, while chemistry is modeled by means of steady laminar flamelets. We use the preassumed PDF approach for turbulence–chemistry interaction. A weighted sum of gray gases model is used for the gas radiative properties. The radiative transfer equation is solved using the discrete ordinates method in the conservative finite-volume formulation. The radiative loss leads to a decrease in mean temperature, but does not significantly influence the flow and mixing fields, in terms either of mean values or of rms values of fluctuations. A systematic analysis of turbulence–radiation interaction (TRI) is carried out. By considering five different TRI formulations, and comparing also with a simple optically thin model, individual TRI contributions are isolated and quantified. For both flames, effects are demonstrated of (1) influence of temperature fluctuations on the mean Planck function, (2) temperature and composition fluctuations on the mean absorption coefficient, and (3) correlation between absorption coefficient and Planck function. The strength of the last effect is stronger in DFIII than in SFD, because of stronger turbulence–chemistry interaction and lower mean temperature in DFIII. The impact of the choice of TRI model on the prediction of the temperature-sensitive minor species NO is determined in a postprocessing step with fixed flow and mixing fields. Best agreement for NO is obtained using the most complete representation of TRI.

Stochastic time series analysis of pulsating buoyant pool fires

A novel time series model that captures the puffing effect was developed to simulate the fluctuating scalar (temperature and species) fields of buoyant pool fires. In an ethylene fueled pool fire ( Fr f = 0.109, D = 7.1 cm), stochastic time series of temperature ( T), carbon dioxide (CO 2) concentration and soot volume fraction ( f v) were simulated and time series of line-of-sight spectral radiation intensity ( I λ ) were calculated. The model inputs, such as the puffing frequency ( f 0) and mean and root-mean-square (rms) of the fluctuating scalars, were obtained by analyzing I λ time series measurements of the same pool fire. The autocorrelation coefficients of the measured I λ exhibit a periodically oscillating behavior with significant negative values. The oscillation frequency is very close to the empirically determined characteristic puffing frequency in pulsating buoyant pool fires ( f 0 = 1.68 D −0.5). The power spectral density (PSD) profiles of the measured I λ also exhibit significant peaks at the characteristic puffing frequency. Distributions of mean and rms T, CO 2 concentration and f v in the present pool fire were estimated using a non-intrusive tomographic technique considering turbulence-radiation interactions. Mean T and CO 2 concentration distributions were also compared to those obtained using gas chromatography (GC). Statistical properties (autocorrelation coefficients, PSDs and probability density functions) of the calculated I λ time series of the present pool fire matched those of the measured I λ very well. The present time series model can be extended to other pulsating flows, such as buoyant diffusion flames and buoyant plumes.

The effect of turbulence–radiation interaction on radiative entropy generation and heat transfer

The analysis under the second law of thermodynamics is the gateway for optimisation in thermal equipments and systems. Through entropy minimisation techniques it is possible to increase the efficiency and overall performance of all kinds of thermal systems. Radiation, being the dominant mechanism of heat transfer in high-temperature systems, plays a determinant role in entropy generation within such equipments. Turbulence is also known to be a major player in the phenomenon of entropy generation. Therefore, turbulence–radiation interaction is expected to have a determinant effect on entropy generation. However, this is a subject that has not been dealt with so far, at least to the extent of the authors’ knowledge. The present work attempts to fill that void, by studying the effect of turbulence–radiation interaction on entropy generation. All calculations are approached in such a way as to make them totally compatible with standard engineering methods for radiative heat transfer, namely the discrete ordinates method. It was found that turbulence–radiation interaction does not significantly change the spatial pattern of entropy generation, or heat transfer, but does change significantly their magnitude, in a way approximately proportional to the square of the intensity of turbulence.

Direct numerical simulation of turbulence–radiation interactions in homogeneous nonpremixed combustion systems

An important issue in chemically reacting turbulent flows is the interaction between turbulence and radiation (turbulence–radiation interactions—TRI). TRI arises from highly nonlinear coupling between fluctuations in temperature and species composition with the fluctuations of radiative intensity. Here direct numerical simulation (DNS) has been employed to investigate TRI in a statistically homogeneous nonpremixed system. A photon Monte Carlo method has been used to solve the radiative transfer equation (RTE). Radiation properties correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient that mimics that of typical hydrocarbon-air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient-Planck function correlation, and the absorption coefficient-intensity correlation have been examined for intermediate-to-large values of the optical thickness, and contributions from all three correlations have been found to be significant. Emission TRI has also been estimated analytically using a β PDF model. The β PDF results are in good agreement with DNS results. This suggests that relatively simple models for emission TRI in nonpremixed systems might be developed based on a modeled equation for mixture fraction variance.

Detailed numerical simulation of thermal radiation influence in Sandia flame D

In order to investigate the influence of thermal radiation in turbulent combustion processes, Sandia flame D is numerically simulated, with multiple-time scale (MTS) k– ε turbulence model for turbulence, the combination of probability density function (PDF) transportation method, Lagrangian flamelet model (LFM) and the detailed chemical reaction mechanism GRI 3.0 (consisting of 53 species and 325 elemental reactions) for combustion and finite volume/correlated- k (FV/CK) method for radiation heat transfer. To account for turbulence’s influence on radiation, the effects of turbulence–radiation interactions (TRI) are investigated in radiation calculations and it is recommended that for detailed numerical simulation TRI should be considered. Numerical results with and without radiation influence being taken into accounted are compared with experimental data. Different from reports by other researchers, our simulation results show that although the magnitude of thermal radiation is relatively small, its influence on combustion process is significant. It is suggested that turbulence and chemical reactions may magnify the influence of thermal radiation.

Multiscale Modeling of Turbulence, Radiation, and Combustion Interactions in Turbulent Flames

Traditional modeling of radiative transfer in reacting flows has ignored turbulence–radiation interactions (TRI), due to difficulties caused by their inherent nonlinearities and their vast range of length scales and time scales. The state-of-the-art of modeling TRI is reviewed, and some results are presented, in which TRI are calculated from basic principles from the composition PDF method and from DNS calculations. The results show that, in turbulent jet flames, TRI are always of great importance, and that they are dominated by the correlation between the absorption coefficient and the radiative Planck function. Introduction Most fires as well as commercial combustion processes, such as boilers, internal combustion engines, gas turbines, etc., involve high temperatures and, therefore, thermal radiation usually contributes a significant fraction to the overall heat transfer rates. At the same time, the vast majority of these combustion applications occur under turbulent conditions, which is known to enhance heat transfer. Radiation, chemical kinetics and turbulence individually are among the most challenging fundamental and practical problems of computational science and engineering, due to their inherent nonlinearities and their vast range of length scales and time scales. In turbulent combustion, these phenomena are coupled in interesting and highly nonlinear ways, leading to entirely new classes of interactions. In much the same way as convection is aided by turbulence, so is radiation, which in the presence of chemical reactions may increase several fold due to turbulence interactions. While coupling between turbulence and chemistry has received a great deal of attention over the years [1, 2], turbulence-radiation interaction (hereafter TRI) has until recently been ignored by virtually all investigations due to its extreme complexity, even though its importance has been widely recognized [3–7]. Preliminary and state-of-the-art calculations have shown that TRI always increases the heat loss from a flame, and that this additional heat loss can reach 60% of the total and more, leading to a reduction in the local gas temperature of 200 C or more. Therefore,

Direct numerical simulation of turbulence/radiation interaction in premixed combustion systems

An important fundamental issue in chemically reacting turbulent flows is turbulence/radiation interaction (TRI); TRI arises from highly nonlinear coupling between temperature and composition fluctuations. Here, a photon Monte Carlo method for the solution of the radiative transfer equation has been integrated into a turbulent combustion direct numerical simulation (DNS) code. DNS has been used to investigate TRI in a canonical configuration with systematic variations in optical thickness. The formulation allows for nongray gas properties, scattering, and general boundary treatments, although in this study, attention has been limited to gray radiation properties, no scattering, and black boundaries. Individual contributions to emission and absorption TRI have been isolated and quantified. Of particular interest are intermediate values of optical thickness where, for example, the smallest hydrodynamic and chemical scales are optically thin while the largest turbulence scales approach an optically thick behavior. In the configuration investigated, the temperature self-correlation contribution (emission) is primarily a function of the ratio of burned-gas temperature to unburned-gas temperature, and is the dominant contribution to TRI only in the optically thin limit. Even in the most optically thin case considered, the absorption coefficient–Planck function correlation and absorption coefficient–intensity correlation are not negligible. At intermediate values of optical thickness, contributions from all three correlations are significant.

On the shapes of the presumed probability density function for the modeling of turbulence–radiation interactions

The laminar flamelet equations in combination with the joint probability density function (PDF) transport equation of mixture fraction and turbulence frequency have been used to simulate turbulent jet diffusion flames. To check the suitability of the presumed shapes of the PDF for the modeling of turbulence–radiation interactions (TRI), two types of presumed joint PDFs are constructed by using the second-order moments of temperature and the species concentrations, which are derived by the laminar flamelet model. The time-averaged radiative source terms and the time-averaged absorption coefficients are calculated by the presumed joint PDF approaches, and compared with those obtained by the laminar flamelet model. By comparison, it is shown that there are obvious differences between the results of the independent PDF approach and the laminar flamelet model. Generally, the results of the dependent PDF approach agree better with those of the flamelet model. For the modeling of TRI, the dependent PDF approach is superior to the independent PDF approach.

Statistical modeling of thermal radiation transfer in buoyant turbulent diffusion flames

A statistical (Monte Carlo) method for radiative heat transfer has been incorporated in CFD modeling of buoyant turbulent diffusion flames in stagnant air and in a cross-wind. The model and the computational tool have been developed and applied to simulate both burner flames with controlled fuel supply rate and in self-sustained pool fires with burning rates coupled with flame radiation. The gas–soot mixture was treated either as gray (using the effective absorption coefficient derived from total emissivity data or the Planck mean absorption coefficient) or as non-gray (using the weighed sum of gray gases model). The comparison of predicted radiative heat fluxes indicates applicability of the gray media assumption in modeling of thermal radiation in case of high soot content. The effect of turbulence-radiation interaction is approximately taken into account in calculation of radiation emission, which is corrected to allow for temperature self-correlation and absorption-temperature correlation. In modeling buoyant propane flames in still air above 0.3 m diameter burner, extensive comparison is presented of the predictions with the measurements of gas species concentrations, temperature, velocity and their turbulent fluctuations, and radiative heat fluxes obtained in flames with different heat release rates. Similar to previously published experimental data, the predicted burning rate of flames above the acetone pools exposed to flame radiation increases with the pool diameter and approaches a constant level for large pool sizes. The magnitude of predicted burning rates is shown to be in agreement with the reported measurements. Augmentation of burning rate of the pool fire in a cross-wind because of increased net radiative heat flux received by the fuel surface and non-monotonic dependence of burning rate on cross-wind velocity, subject to the pool diameter, is predicted. The statistical treatment of thermal radiation transfer has been found to be robust and computationally efficient.

Monte Carlo modeling of radiative transfer in a turbulent sooty flame

The calculation of radiative transfer within a sooty turbulent ethylene–air diffusion jet flame has been carried out by using a Monte Carlo method and an accurate CK model for the gases. The influence of the turbulence–radiation interaction (TRI) has been studied. In the TRI modeling, the radiative properties of the assumed homogeneous turbulent structures are randomly obtained from a multidimensional probability density function (PDF) of the reaction progress variable, of the mixture ratio and of the soot volume fraction. This joint PDF is obtained from an Eulerian–Lagrangian turbulent combustion model and the sizes of the turbulent structures are directly derived from a k– ϵ model. In the considered flame, the TRI effect is an increase of the radiative heat loss by about 30%. The radiative heat loss becomes almost equal to one-third of the chemical heat release. Soot particles play the most important role in the global radiative heat loss but the influence of gaseous species like CO 2 and H 2O can be important in the local energy balance.

Importance of Turbulence-Radiation Interactions in Turbulent Diffusion Jet Flames

Traditional modeling of radiative transfer in reacting flows has ignored turbulence-radiation interactions (TRI). Radiative fluxes, flux divergences and radiative properties have been based on mean temperature and concentration fields. However, both experimental and theoretical work have suggested that mean radiative quantities may differ significantly from those predictions based on the mean parameters because of their strongly nonlinear dependence on the temperature and concentration fields. The composition PDF method is able to consider many nonlinear interactions rigorously, and the method is used here to study turbulence-radiation interactions. This paper tries to answer two basic questions: (1) whether turbulence-radiation interactions are important in turbulent flames or not; and (2) if they are important, then what correlations need to be considered in the simulation to capture them. After conducting many flame simulations, it was observed that, on average, TRI effects account for about 1/3 of the total drop in flame peak temperature caused by radiative heat losses. In addition, this study shows that consideration of the temperature self correlation alone is not sufficient to capture TRI, but that the complete absorption coefficient-Planck function correlation must be considered.

Measurements and Calculations of Spectral Radiation Intensities for Turbulent Non-Premixed and Partially Premixed Flames

Spectral radiation intensities leaving diametric and chord-like paths for six non-sooting flames were measured using an infrared array spectrometer. The spectral radiation intensities were also computed using the mean property approach and a time and space series simulation approach. Turbulence/radiation interactions (TRI) in these flames were investigated by comparing the two sets of computations to the experimental data. The effects of TRI are significant for regions away from the flame axis. The new data and findings are of value in the evaluation of radiation models, which are increasingly used in turbulent combustion calculations.

An averaged formulation of the M1 radiation model with mean absorption coefficients and presumed probability density functions for turbulent flows

After the presentation of the M 1 macroscopic radiative model with mean absorption coefficients, an averaged formulation of this model is derived in order to account for turbulence–radiation interactions. This averaged model is closed assuming the shape of the probability density functions and computing the variances of the radiative quantities. Further on, macroscopic models for the variances of both radiative energy and flux are derived and discussed.

Calculation of Radiative Heat Transfer in Combustion Systems

Most practical combustion systems involve complex geometry configurations and CFD techniques used for the calculation of flow and combustion in such geometries use body-fitted non-orthogonal mesh systems. This paper reviews some of the currently available radiative heat transfer calculation techniques suitable for such CFD applications. The Monte Carlo method, the discrete transfer method, the YIX method, the discrete ordinates method and the finite volume method are discussed and some notable applications related to combustion problems are reviewed. Comparative results using all the methods outlined are presented for bench mark problems and their applicability to complex geometry situations are discussed. Radiative heat flux predictions for an S.I. engine simulation are presented to demonstrate the capability of the discrete transfer method in a pent-roof complex geometry combustion chamber. The paper also describes a ray based technique for the handling of turbulence-radiation interactions in combustion and its application is demonstrated in the prediction of a methane diffusion flame.

Application of composition PDF methods in the investigation of turbulence–radiation interactions

The composition probability density function (PDF) method is used to study radiating reactive flows. The method is able to treat turbulence–radiation interactions (TRI) in a rigorous way: many unclosed terms due to TRI in the traditional Reynolds-averaging process can be calculated exactly and all others can be accurately modeled by using the optically thin eddy approximation. The application of the method is demonstrated by considering a simplified methane/air diffusion flame, which shows enhancement of the radiative fluxes as a result of TRI. The importance of considering different TRI terms is investigated, indicating that the absorption coefficient–Plank function correlation is the most important.

Turbulence-Radiation Interactions in Nonreactive Flow of Combustion Gases

A nonreactive hot mixture of radiatively participating species, typically carbon dioxide and water vapor, may be found in the exhaust sections of almost all combustors. Since the scalar fluctuations in such nonreactive flows are substantially smaller than in flames, it is commonly believed that the effects of turbulence-radiation interactions (TRI) on altering wall heat fluxes in nonreactive flows are negligible. Such belief, however, has not been substantiated by evidence to date. The purpose of this note is to investigate the conditions under which turbulence-radiation interactions may be important in nonreactive flows. The final outcome was found to be largely dependent on how the scalar fluctuations correlate, rather than the magnitude of the fluctuations themselves. It was found that for most situations of practical interest, TRI effects are indeed negligible.