Diagnosis of turbulence radiation interaction in turbulent flames and implications for modeling in large Eddy simulation

International audienceAn a priori study of the turbulence radiation interaction (TRI) is performed on numerical data from Direct Numerical Simulation (DNS) of a turbulent flame. The influence of the various correlations that appear in the radiative emission is investigated and their impact is evaluated in the context of Large Eddy Simulation (LES). In LES, only filtered quantities are computed, where the filter is the grid. The radiative emission is reconstructed first from the exact, then filtered solution variables and the sensitivity to the filter size is evaluated. Three approaches are used to take into account the subgrid scale correlations: the no-TRI, partial TRI and full TRI approaches. Results show that the full TRI is exact compared to the reference emission and that the partial TRI performs worse than the no-TRI for the studied configuration. This indicates that in the studied case, the TRI must be considered in LES in a full formulation

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

International audienceRadiation 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 100 K to 150 K 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. (C) 2011 Published by Elsevier Inc. on behalf of The Combustion Institute

Assessment of the single-mixture gas assumption for the correlated K-distribution fictitious gas method in H2O-CO2-CO mixture at high temperature

International audienceThis 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 (10(5) Pa) and at high altitude (16.6 km, 10(4) Pa). 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: 2000-2500 cm(-1), 3450-3850 cm(-1) and 3850-4150 cm(-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

A theoretical and experimental study of the time-dependent radiative properties of a solar bubbling fluidized bed receiver

International audienceIn order to evaluate the potential of solar bubbling fluidized bed receivers compared to other methods for the solar heating of gases at high temperature, a thorough knowledge of the heat transfer of the receiver is necessary. Since the external energy source of the system is radiative and because of high working temperatures, it is particularly important to model the radiative heat transfer to later predict the temperature field in the solar receiver. The aim of this study is to model the radiative flux distribution in a fluidized bed by taking into account the time-dependent absorption and scattering of light in the particulate medium. For this purpose, we propose a model using the Monte Carlo Method as well as a time-dependent field of optical properties that was predicted using a Computational Fluid Dynamics tool implemented with an Eulerian model. A statistical treatment using the k-distribution method was later applied to the time-dependency of the radiative properties of the solar fluidized bed receiver. This method has proven to be useful to reduce computational time while keeping a good accuracy. An experimental set-up was designed to validate the numerical predictions of the particle volume fraction and the penetration of radiation into the fluidized bed. The good agreement of the current model with the experimental data confirms its suitability. (C) 2014 Elsevier Ltd. All rights reserved

Accurate solutions for radiative heat transfer in two-dimensional axisymmetric enclosures with gas radiation and reflective surfaces

Il y a une erreur d'impression à l'intérieur de la revue, qui indique pour référence le volume 46. Il s'agit bien du volume 47, n°1.International audienceAccurate solutions for benchmarking purposes in two-dimensional axisymmetric enclosures with reflective surfaces have been obtained using the Monte Carlo method (MCM) based on the net exchange formulation (NEF). Previous applications of the MCM-NEF have been restricted to multidimensional problems with black boundaries or one-dimensional problems with gray boundaries. Here, the extension to multidimensional enclosures with gray boundaries is presented. The medium is a mixture of H 2 O, CO 2 , N 2 , and soot at atmospheric pressure, and its radiative properties are computed using the correlated k-distribution method. Predictions obtained using the discrete ordinates method are included, showing good agreement with the benchmark MCM=NEF solutions

Unsteady coupling of Navier-Stokes and radiative heat transfer solvers applied to an anisothermal multicomponent turbulent channel flow

Eurotherm Seminar 83 on Computational Thermal Radiation in Participating Media III, Lisbon, PORTUGAL, APR 15-17, 2009International audienceDirect numerical simulations (DNS) of an anisothermal reacting turbulent channel flow with and without radiative source terms have been performed to study the influence of the radiative heat transfer on the optically non-homogeneous boundary layer structure. A methodology for the study of the emitting/absorbing turbulent boundary layer (TBL) is presented. Details on the coupling strategy and the parallelization techniques are exposed. An analysis of the first order statistics is then carried out. It is shown that, in the studied configuration, the global structure of the thermal boundary layer is not significantly modified by radiation. However, the radiative transfer mechanism is not negligible and contributes to the heat losses at the walls. The classical law-of-the-wall for temperature can thus be improved for RANS/LES simulations taking into account the radiative contribution

Méthode de Monte Carlo et synthèse d'images : application à des milieux diffusants en transfert radiatif

International audienceL'objectif principal de cette étude est la prise en compte de la diffusion multiple pour le calcul de transferts radiatifs dans des géométries complexes. Pour traiter la diffusion, un algorithme basé sur la méthode de Monte Carlo a été développé et implémenté dans un environnement issu d'un code de synthèse d'images existant. A l'aide de propriétés d'invariance de la statistique des chemins de diffusion multiple, nous avons validé cette approche dans le cas de trois géométries tridimensionnelles usuelles : sphère, cylindre et pyramide

Effects of radiative heat transfer on the structure of turbulent supersonic channel flow

International audienceThe interaction between turbulence in a minimal supersonic channel and radiative heat transfer is studied using large-eddy simulation. The working fluid is pure water vapour with temperature-dependent specific heats and molecular transport coefficients. Its line spectra properties are represented with a statistical narrow-band correlated-k model. A grey gas model is also tested. The parallel no-slip channel walls are treated as black surfaces concerning thermal radiation and are kept at a constant temperature of 1000 K. Simulations have been performed for different optical thicknesses (based on the Planck mean absorption coefficient) and different Mach numbers. Results for the mean flow variables, Reynolds stresses and certain terms of their transport equations indicate that thermal radiation effects counteract compressibility (Mach number) effects. An analysis of the total energy balance reveals the importance of radiative heat transfer, compared to the turbulent and mean molecular heat transport

COMBINED CONDUCTIVE-RADIATIVE HEAT TRANSFER ANALYSIS IN COMPLEX GEOMETRY USING THE MONTE CARLO METHOD

Deterministic methods are commonly used to solve the heat balance equation in three-dimensional (3D) geometries. This article presents a preliminary study to the use of a stochastic method for the computation of the temperature in complex 3D geometries where the combined conductive and radiative heat transfers are coupled in the porous solid phase. The Monte Carlo algorithm and its results are validated by a comparison with the results obtained with a conventional finite-volume method

Radiative transfer and spectroscopic databases: A line-sampling Monte Carlo approach

Issu de : Eurotherm conference n° 105 - Computational thermal radiation in participating media V, Albi, FRANCE, 1-3 April 2015International audienceDealing with molecular-state transitions for radiative transfer purposes involves two successive steps that both reach the complexity level at which physicists start thinking about statistical approaches: (1) constructing line-shaped absorption spectra as the result of very numerous state-transitions, (2) integrating over optical-path domains. For the first time, we show here how these steps can be addressed simultaneously using the null-collision concept. This opens the door to the design of Monte Carlo codes directly estimating radiative transfer observables from spectroscopic databases. The intermediate step of producing accurate high-resolution absorption spectra is no longer required. A Monte Carlo algorithm is proposed and applied to six one-dimensional test cases. It allows the computation of spectrally integrated intensities (over 25 cm−1 bands or the full IR range) in a few seconds, regardless of the retained database and line model. But free parameters need to be selected and they impact the convergence. A first possible selection is provided in full detail. We observe that this selection is highly satisfactory for quite distinct atmospheric and combustion configurations, but a more systematic exploration is still in progress