Full-spectrum Correlated-k Model Research Articles

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.

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.

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.

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 radiative property models on soot production in laminar coflow ethylene diffusion flames

Two axisymmetric laminar coflow non-smoking and smoking ethylene diffusion flames are studied numerically in order to assess the influence of different radiative property models on the soot formation and oxidation processes. Simulations are carried out by considering the Steady Laminar Flamelet (SLF) concept and a modified two-equation acetylene-based model to describe the soot nucleation, surface growth and oxidation processes. Several radiative property models are considered: the simple Optically Thin Approximation (OTA), the Weighted-Sum of Grey-Gases (WSGG), the Grey-Wide-Band model (GWB), the Statistical Narrow Band Correlated-k model (SNBCK) and the Full Spectrum Correlated-k model (FSCK). Comparisons between calculations carried out with the SNBCK model and experimental data show a reasonable agreement. Model results show that the choice of the radiative property models influence largely the soot prediction, especially in the upper part of the flame where oxidation occurs. Simulations show that the reabsorption of spectral gas and soot is an important feature and thus the commonly used OTA or grey models introduce large discrepancies. The GWB model leads to improved solutions, but it should be avoided if accurate predictions are desired. The FSCK provides equivalent results as compared to the SNBCK model with a substantial gain in CPU time.

Multigroup Correlated-k Distribution Method for Nonequilibrium Atomic Radiation

the shock layer were overcome by splitting the full spectrum into a number of nonoverlapping part spectra. Results for one-dimensional nonhomogeneous gas slabs are presented, comparing line-by-line benchmarks with the fullspectrum correlated-k model, showing very good accuracy with a maximum error of 2–3% for the stagnation-line flowfield of the Stardust vehicle and more severe nonhomogeneous two-cell problems.

A multi-scale full-spectrum correlated- k distribution for radiative heat transfer in inhomogeneous gas mixtures

A multi-scale full-spectrum correlated- k distribution model has been developed and tested for radiative transfer calculations in absorbing/emitting molecular gases. The gas or gas mixture is broken up into different groups by separating different absorbing species and, for each specie, by collecting them into spectral groups according to the lower level energy of their spectral lines. Like all k-distribution methods as well as the full-spectrum correlated- k (FSCK) model, the new model may be used with any arbitrary radiative transfer equation solver. Results for one- and two-dimensional inhomogeneous gas mixtures with varying temperature and mole fraction fields are presented and compared with line-by-line benchmarks and the FSCK model, showing very good accuracy in situations with severe temperature gradients and/or sharp mole fraction ratio changes.