Correlated-k Research Articles

Performance of the Atmospheric Radiative Transfer Simulator (ARTS) in the 600–1650 cm−1 Region

The Atmospheric Radiative Transfer Simulator (ARTS) has been widely used in the radiation transfer simulation from microwave to terahertz. Due to the same physical principles, ARTS can also be used for simulations of thermal infrared (TIR). However, thorough evaluations of ARTS in the TIR region are still lacking. Here, we evaluated the performance of ARTS in 600–1650 cm−1 taking the Line-By-Line Radiative Transfer Model (LBLRTM) as a reference model. Additionally, the moderate resolution atmospheric transmission (MODTRAN) band model (BM) and correlated-k (CK) methods were also used for comparison. The comparison results on the 0.001 cm−1 spectral grid showed a high agreement (sub-0.1 K) between ARTS and LBLRTM, while the mean bias difference (MBD) and root mean square difference (RMSD) were less than 0.05 K and 0.3 K, respectively. After convolving with the spectral response functions of the Atmospheric Infra-Red Sounder (AIRS) and the Moderate Resolution Imaging Spectroradiometer (MODIS), the brightness temperature (BT) differences between ARTS and LBLRTM became smaller with RMSDs of <0.1 K. The comparison results for Jacobians showed that the Jacobians calculated by ARTS and LBLRTM were close for temperature (can be used for Numerical Weather Prediction application) and O3 (excellent Jacobian fit). For the water vapor Jacobian, the Jacobian difference increased with an increasing water vapor content. However, at extremely low water vapor values (0.016 ppmv in this study), LBLRTM exhibited non-physical mutations, while ARTS was smooth. This study aims to help users understand the simulation accuracy of ARTS in the TIR region and the improvement of ARTS via the community.

Analysis of spectral radiative entropy generation in a non-gray planar participating medium at radiative equilibrium with two different boundary conditions

In the present study, spectral radiative entropy generation analysis is presented for radiative heat transfer between two parallel plates containing non-gray absorbing and emitting medium at radiative equilibrium. In order to evaluate the effect of boundary condition (BC) on the entropy generation, two configurations of BC are considered. In the first configuration, two bounding walls are assumed to be at prescribed temperatures. In the second configuration, one wall is assumed to be at a prescribed temperature and the other wall is assumed to have prescribed heat flux BC. The discrete ordinates method and the finite volume method are respectively used for the angular and spatial discretization of the radiative transfer equation and radiative entropy generation equations. While the correlated-k (CK) non-gray model is used to evaluate spectral radiative properties. Five cases are considered, including two homogeneous cases and two non-homogeneous cases with a single participating gas (i.e. H2O or CO2) and one homogeneous case with a mixture of H2O, CO2 and soot particles. Spectral, volumetric and total radiative entropy generations are calculated for each BC configuration and case. Also, the effects of the concentration of participating gas, wall emissivity and total gas pressure on radiative entropy generation are attentively analyzed. The results show that irreversible radiative transfer at the wall with lower temperature is the main source of radiative entropy generation in the system. Soot particles increase radiative entropy generation due to absorption and emission process but decrease radiative entropy generation due to walls radiation. The radiative entropy generation is approximately constant when the concentration of participating gas varies for both BC's. Total radiative entropy generation for the system with the prescribed temperature at walls significantly increases with the increase of wall emissivity; conversely, for the system with mixed BC, total radiative entropy generation linearly decreases. Furthermore, for the system with the prescribed temperature at the walls, radiative entropy generation slightly decreases when medium total pressure increases; while for the problems with mixed BC, increasing medium total pressure does not significantly effect on radiative entropy generation. Finally, results demonstrate that the wall emissivity has a considerable effect on the radiative entropy generation for radiative heat transfer in the non-gray medium in radiative equilibrium.

Spectral radiative entropy generation in a non-gray planar participating medium including H2O and CO2

Spectral radiative entropy generation through a non-gray absorbing and emitting planar medium is studied. Spectral radiative properties are calculated using the correlated-k (CK) non-gray model with updated parameters based on the HITEMP2010 high-resolution database. The angular and spatial discretization of the radiative transfer equation and radiative entropy generation equation are performed using the discrete ordinates method and the finite volume method, respectively. First, results obtained using the CK model, are compared with the results of the gray model. The calculations reveal that there is a significant difference between results obtained by the gray model and the non-gray model, and the gray assumption generates a remarkable error in the calculations of radiative entropy generation. Then, for comprehensive analysis, five cases have been considered including two homogeneous and isothermal cases, two non-homogeneous and non-isothermal cases filled with H2O or CO2 and one homogeneous and non-isothermal case with a mixture of H2O and CO2. Spectral, local and total radiative entropy generation are calculated for each case and the effect of temperature of the medium, the concentration of participating gas and wall emissivity on radiative entropy generation is attentively investigated. Results show that the radiative entropy generation in the system increases as three aforementioned parameters increase but the effect of gas temperature variations is more than others. Further, the sensitivity of radiative entropy generation due to wall radiation to wall emissivity is decreased when εw rises.

Comparison of CK model and line by line method using old and updated parameters/databases

Thermal radiation transfer in a 2D enclosure filled with non-gray gases, namely H2O, CO2, or their mixture, is calculated using the line-by-line approach and the correlated-k (CK) model. This study shows the effect of spectral databases on the accuracy of LBL and CK model calculations. Line-by-line calculations were carried out using two databases for high temperature, i.e. the HITEMP1995 and the HITEMP2010 high-resolution databases. Calculations of the CK model were conducted using two sets of parameters, i.e. the old (earlier) parameters {A. Soufiani, J. Taine, High temperature gas radiative property parameters of statistical narrow-band model for H2O, CO2and CO and correlated-k (CK) model for H2O and CO2, Int. J. Heat Mass Transfer 40 (1997) (987–991)} and the new (updated) parameters {(Rivière, P. and A. Soufiani, Updated band model parameters for H2O, CO2, CH4and CO radiation at high temperature, International Journal of Heat and Mass Transfer, 2012. 55(13), 3349–3358)}. The angular and spatial discretization of the radiative transfer equation is performed using the discrete ordinates method and finite volume method, respectively. Seven test cases with different medium temperature (both isothermal and non-isothermal) and homogeneous or non-homogeneous concentrations of participating gases are considered. This study intends to provide a comprehensive evaluation of the CK model with two sets of parameters and LBL method using the HITEMP95 database in a wide range of medium temperatures (from 1000 to 2500 K) and concentration of participating gases (from 10% to 100% mole fraction). The line-by-line results based on the HITEMP2010 high-resolution spectral database are used as benchmarks to evaluate the accuracy of other approximate models. Very good agreement for both the wall heat flux and the radiative source term was observed between the LBL results using the HITEMP2010 database and the results of the CK model in all the test cases. It is concluded that for all considered test cases, the CK method with the new parameters provides better accuracy than with the old ones.

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.

Updated band model parameters for H2O, CO2, CH4 and CO radiation at high temperature

Statistical narrow-band (SNB) model parameters for H2O, CO2, CH4 and CO, and correlated-k (CK) parameters for H2O and CO2 are generated from line by line calculations and recently improved spectroscopic databases in wide temperature and spectral ranges. Results from the new parameters are compared to direct line by line calculations and to results from earlier model parameters [A. Soufiani, J. Taine, High temperature gas radiative property parameters of statistical narrow-band model for H2O, CO2 and CO and correlated-k (ck) model for H2O and CO2, Int. J. Heat Mass Transfer 40 (1997) 987–991] in terms of band averaged spectral transmissivities, Planck mean absorption coefficients, and total emissivities. The comparisons show first a good agreement between updated SNB, CK and LBL results. Significant improvements on earlier parameters are observed for H2O and CO2, especially at very high temperatures and path lengths. Model parameters and computer programs illustrating their implementation are provided as Supplementary data.

Linear regression models and neural networks for the fast emulation of a molecular absorption code

The background scene generator MATISSE, whose main functionality is to generate natural background radiance images, makes use of the so-called Correlated K (CK) model. It necessitates either loading or computing thousands of CK coefficients for each atmospheric profile. When the CK coefficients cannot be loaded, the computation time becomes prohibitive. The idea developed in this paper is to substitute fast approximate models for the exact CK generator; using the latter, a representative set of numerical examples is built and used to train linear or nonlinear regression models. The resulting models enable an accurate CK coefficient computation for all the profiles of an image in a reasonable time.

Assessment of the Single-Mixture Gas Assumption for the Correlated K-Distribution Fictitious Gas Method in H2O–CO2–CO Mixture at High Temperature

This paper deals with the comparison of spectral narrow band models based on the correlated-K (CK) approach in the specific area of remote sensing of plume signatures. The CK models chosen may or may not include the fictitious gas (FG) idea and the single-mixture-gas assumption (SMG). The accuracy of the CK and the CK-SMG as well as the CKFG and CKFG-SMG models are compared, and the influence of the SMG assumption is inferred. The errors induced by each model are compared in a sensitivity study involving the plume thickness and the atmospheric path length as parameters. This study is conducted in two remote-sensing situations with different absolute pressures at sea level (105Pa) and at high altitude (16.6km, 104Pa). The comparisons are done on the basis of the error obtained for the integrated intensity while leaving a line of sight that is computed in three common spectral bands: 2000–2500cm−1, 3450–3850cm−1, and 3850–4150cm−1. In most situations, the SMG assumption induces negligible differences. Furthermore, compared to the CKFG model, the CKFG-SMG model results in a reduction of the computational time by a factor of 2.

Assessments of Spectral Narrow Band and Weighted-Sum-of-Gray-Gases Models for Computational Fluid Dynamics Simulations of Pool Fires

The statistical narrow band (SNB), correlated-k (CK), and weighted-sum-of-gray-gases (WSGG) models have been incorporated into a computational fluid dynamics (CFD) code. Their accuracy and computing times are evaluated for three pool fires scenarios. CFD_CK and CFD_SNB yield similar predictions, and the former is three times more CPU demanding than the latter. CFD_CK is unrealistic for practical fire applications. Temperature and velocity predictions with CFD_WSGG and CFD_SNB agree better in the persistent and plume regions, but significant discrepancies are found in the intermittent region. The overall predictions of the two approaches show reasonable agreement. The WSGG model should not be discarded for CFD fire simulations.

Determination of the gas temperature profile in a large-scale furnace using a fast/efficient inversion scheme for the SRS technique

Measurements of gas temperature profiles in a large-scale test furnace are made by inverting the measured spectral data at 4.3 μm band with inversion algorithms. A 3.4 m-long stainless steel tube is employed as the test furnace through which flue gas flows. Three inversion algorithms are tested: modified constrained inversion method (MCIM), base function-based inversion method (BFIM) and BFIM-based MCIM (BCIM). The correlated-k (CK)-based WNB model is employed for calculation of spectral intensities during the inversion process. The overall performances of algorithms are scrutinized with the inversion results. To verify the accuracy, the inverted temperatures are compared with measured ones using thermocouples. BFIM is good for initial guess and MCIM is good for fine touch, while BCIM gives fairly good results for all cases considered. Thus, BCIM is considered to be applicable, with good accuracy and capability, to inverse problems in large-scale furnaces.

Evaluation Of The Correlated-K And Other Gas Radiation Models For Combustion Applications

The correlated-k (CK) distribution gas model is evaluated for some typical fires and combustion applications. The method based on the cumulative distribution function of the absorption coefficient has been widely used for atmospheric applications, but little tested for combustion systems. The CK approach, suitable for mixtures of gases and scattering particles is applied in the current work to PMMA and natural gas fires. For comparison purposes, results obtained with the narrow band statistical (NBS) and weighted sum of gray gases (WSGG) models are also presented for the same scenarios. The results clearly show that the CK approach provides a good level of accuracy with acceptable CPU times for the fire configurations tested, in the prediction of radiative quantities.

MODTRAN Cloud and Multiple Scattering Upgrades with Application to AVIRIS

Recent upgrades to the MODTRAN atmospheric radiation code improve the accuracy of its radiance predictions, especially in the presence of clouds and thick aerosols, and for multiple scattering in regions of strong molecular line absorption. The current public-released version of MODTRAN (MODTRAN3.7) features a generalized specification of cloud properties, while the current research version of MODTRAN (MODTRAN4) implements a correlated- k (CK) approach for more accurate calculation of multiply scattered radiance. Comparisons to cloud measurements demonstrate the viability of the CK approach. The impact of these upgrades on predictions for AVIRIS viewing scenarios is discussed for both clear and clouded skies; the CK approach provides refined predictions for AVIRIS nadir and near-nadir viewing.

The onset of Rayleigh–Bénard instability in molecular radiating gases

Radiation effect on the onset of Rayleigh–Bénard instability is studied theoretically in the case of real molecular emitting and absorbing gases. Radiative transfer is treated using the full integro-differential formulation without recourse to the differential approximation or to the asymptotic optically thin or optically thick limiting cases. The marginal stability is determined using a combined Galerkin-collocation method based on Chebyshev polynomials. A modified correlated-k (CK) model is developed for NH3, H2O and CO2 in order to take into account the fine spectral structure of the absorption coefficient and of its derivative with respect to temperature. No restrictions are made concerning the nature of the bounding surfaces and their radiative properties. The study of gray media allows us to discuss first the limitations of the differential approximation and the validity range of the optically thin and thick limits. The principle of exchange of stability is found to hold for all the studied cases. For real radiating gases, a reasonable agreement with the few available experimental data which are related to NH3 is first shown and then a parametric study for H2O and CO2 is reported. It is shown that neither radiation effect on the static temperature profile nor the radiative damping of thermal disturbances can be neglected. Radiation effects on the delay of instability onset increase with temperature, layer depth, and decrease with boundary emissivities. Pressure effects depend on the level of saturation of gas active absorption bands.