A hybrid transport-diffusion model for radiative transfer in absorbing and scattering media

International audienceA new multi-scale hybrid transport-diffusion model for radiative transfer calculations is proposed. In this model, the radiative intensity is decomposed into a macroscopic component calculated by the diffusion equation, and a mesoscopic component. The transport equation for the mesoscopic component allows to correct the estimation of the diffusion equation, and then to obtain the solution of the linear radiative transfer equation. In this work, results are presented for stationary and transient radiative transfer cases, in examples which concern solar concentrated and optical tomography applications. The Monte Carlo and the discrete-ordinate methods are used to solve the mesoscopic equation. It is shown that the multi-scale model allows to improve the efficiency of the calculations when the medium is close to the diffusive regime. Moreover, the development of methods for coupling the radiative transfer equation with the diffusion equation becomes easier with this model than with the usual domain decomposition methods

Pressurized Carbon Dioxide as Heat Transfer Fluid: Influence of Radiation on Turbulent Flow Characteristics in Pipe

International audienceThe influence of radiative heat transfer in a CO2 pipe flow is numerically investigated at different pressures. Coupled heat and mass transfer, including radiation transport, are modeled. The physical models and the high temperature and high pressure radiative properties method of computation are presented. Simulations are conducted for pure CO2 flows in a high temperature pipe at 1100 K (with radius 2 cm) with a fixed velocity (1 m.s −1) and for different operating pressures, 0.1, 1, 5 and 20 MPa (supercritical CO2). The coupling between the temperature and velocity fields is discussed and it is found that the influence of radiation absorption is important at low pressure and as the operating pressure increases above 5 MPa the influence of radiation becomes weaker due to an increase of CO2 optical thickness

Remote Measurement of Heliostat Reflectivity with the Backward Gazing Procedure

Concentrated solar power is a promising technique enabling renewable energy production with large scale solar power plants in the near future. Estimating quantitatively the reflectivity of a solar concentrator is a major issue, since it has a significant impact on the flux distribution formed at the solar receiver. Moreover, it is desirable that the mirrors can be measured during operation in order to evaluate environmental factors such as day/night thermal cycles or soiling and ageing effects at the reflective surfaces. For that purpose, a backward gazing method that was originally developed to measure mirror shape and misalignment error was developed. The method operates in quasi real-time without disturbing the heat production process. It was successfully tested at the Themis solar tower power plant in Targasonne, France. Its basic principle consists in acquiring four simultaneous images of a Sun-tracking heliostat, captured from different observation points located near the thermal receiver. The images are then processed with a minimization algorithm allowing the determination of the reflectance and slopes errors of the mirrors. In this communication, it is shown that the algorithm allows one to get quantitative reflectivity maps at the surface of the heliostat. The measurement is fully remote and is used to evaluate surface reflectivity that depends on optical coatings quality and soiling effects. Preliminary results obtained with a Themis heliostat are presented. They show that reflectivity measurements can be carried out within repeatability about ± 5% Peak-to-Valley (PTV) and 1% RMS. Ways to improving these numbers are discussed in the paper

Computation of canting errors in heliostats by flux map fitting: experimental assessment

In solar tower plants, thousands of heliostats reflect sunlight into a central receiver. Heliostats consist of a subset of mirrors called facets that must be perfectly oriented (i.e., canted) to concentrate as much solar radiation as possible. This study presents and validates the so-called flux map fitting technique to detect and correct canting errors. The computed distributions were matched to a series of images through an optimization algorithm. According to the sensitivity analysis, three images spread along a single day provide sufficient information for the algorithm to succeed. Using this methodology, four heliostats at the THEMIS research facility were recanted, thereby substantially increasing the optical quality in three of them. The procedure to infer the heliostat aimpoint was assessed.Universidad Carlos III de Madrid (2020/00051); European Commission (823802); Horizon 2020 Framework Programme (727762). We thank the CNRS-PROMES laboratory, UPR 8521, belonging to the French National Center for Scientific Research (CNRS), for providing access to its facilities, the support of its scientific and technical staff, and the financial support of the SFERA-III project (Grant Agreement No. 823802). We thank the technical help provided by Yann Volut, William Baltus, and Antoine Pérez. The research project VISHELIO-CM-UC3M has been funded by the call "Programa de apoyo a la realización de proyectos interdisciplinares de I+D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020" under the framework of the "Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid". This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 727762, project acronym NEXT-CSP.Publicad

Fast and Accurate Computation of Flux Density Formed by Solar Concentrators and Heliostats

Computing the flux densities provided by solar concentrators or focusing heliostats can be done in two different ways: A grid ray-tracing (GRT) procedure that makes use of a large number of ray bundles, starting from the solar disk and finally impinging the focal plane of the concentrator. The method is reliable and accurate, but requires extensive computing times. Alternatively, the flux densities can be estimated by using convolution algorithms. This latter method requires much less computing time, but is known to be less accurate when the incidence angle of the sunrays on the reflector increases. The objective of this contribution is to define an algorithm based on convolution products and fast Fourier transforms having high accuracy. The results show that RMS error differences between both models are typically lower then 1%

Modélisation et simulation de l'émission énergétique et spectrale d'un jet réactif composé de gaz et de particules à haute température issus de la combustion d'un objet pyrotechnique

Les travaux réalisés durant la thèse s'inscrivent dans une problématique scientifique liée à l'étude des transferts radiatifs. Plus particulièrement, l'application de cette étude est la télédétection infrarouge d'un écoulement diphasique réactif et turbulent à haute température. Cette étude a pour objectif la modélisation et la simulation du rayonnement infrarouge émis par cet écoulement et re¸cu par un détecteur. Pour développer un outil de simulation numérique de la signature infrarouge d'un jet de gaz et de particules à haute température, les espèces majoritaires qui sont responsables de l'émission du rayonnement ont été identifiées lors d'expérimentations. Les campagnes expérimentales ont permis la construction de bases de données concernant les gaz (CO2-CO-H2O) et les particules (oxydes métalliques) présents dans le jet. Connaissant la nature des gaz et des particules, le calcul de leurs propriétés radiatives doit être réalisé. Cette étape est nécessaire puisque ces propriétés caractérisent l'émission de rayonnement par le jet et elles doivent être connues pour résoudre l'équation de transfert radiatif. Pour les gaz, un code de calcul raie par raie de spectres synthétiques a été développé. De plus, pour diminuer le temps de calcul d'une signature infrarouge, il est préférable d'utiliser des modèles spectraux de bandes étroites. Le modèle de télédétection infrarouge est un modèle spectral utilisant des k(coefficient d'absorption)-distributions sous l'hypothèse des k-corrélés avec l'approximation d'un gaz unique pour le mélange associée à l'hypothèse des gaz fictifs. Les paramètres de ce modèle (CKFG-SMG), ont été tabulés et validés dans l'étude. En ce qui concerne les propriétés radiatives des nuages de particules sphériques, le modèle de Mie est utilisé car il est valable pour les gammes de fractions volumiques rencontrées. Pour tester l'influence de la diffusion, une étude de sensibilité à la diffusion a été réalisée. En effet, nous avons quantifié l'erreur commise sur le flux émis par différentes couches si les processus de diffusion du rayonnement sont négligés. Cette étude a montré que l'influence de la diffusion peut être négligée dans le cadre de notre étude. La modélisation de la signature infrarouge du jet diphasique réactif issu de la combustion du matériau pyrotechnique, nécessite la connaissance des températures et des concentrations en gaz et particules, en tous les points du jet. Ce jet diphasique réactif a été simulé à l'aide du logiciel Fluent. De plus, une interface graphique a été développée qui recrée la scène optronique en se servant des profils aérothermochimiques du jet diphasique et des données concernant la position du détecteur. De cette fa¸con, un outil de simulation numérique de la signature infrarouge du jet (SIRJET) a été développé qui inclue un modèle de transfert radiatif (lancer de rayon) ainsi que les paramètres tabulés (gaz et particules) du modèle spectral de télédétection infrarouge (CK, CKFG, CK-SMG, CKFG-SMG). Enfin, une confrontation est présentée entre une mesure et le résultat d'une simulation de la signature infrarouge d'un jet diphasique à haute température. ABSTRACT : The works realized during the thesis join a scientific problem connected to the study of radiative transfer. More particularly, the application of this study is the infrared remote sensing of a two-phase reactive turbulent flow at high temperature. This study has for objective the modelling and the simulation of the infrared intensity emitted by this jet and received by a detector. To develop a software dedicated to the simulation of the infrared signature of a jet containing gases and particles at high temperature, the major species involved were identified during experiments. Knowing the nature of gases (H2O-CO2-CO) and particles (oxidised metal), the calculation of their radiative properties must be realized. This stage is necessary because these properties characterize the infrared emission by the jet and they have to be known to resolve the radiative transfer equation. For gases, a line-by-line code was developed to compute synthetic spectra. Furthermore, to decrease the time of calculation of an infrared signature, it's better to use spectral models of narrow bands. The model of infrared remote sensing is the correlated-k model with the single mixture gas assumption used with the fictitious gases idea. The parameters of this model (CKFG-SMG) were tabulated and validated in the study. As regards, the radiative properties of the spherical particles clouds were computed with the Mie theory that is valid for the range of volume fraction considered in our study. To test the influence of scattering, a sensitivity study to the volume fraction and the refractive index was done which allows us to neglect scattering processes in the development of radiative transfer software designed to compute infrared signature within the framework of our study. Infrared signature modelling of the hot jet requires the knowledge of temperatures and concentrations of gases and particles, at each position in the jet. This two-phase reactive and turbulent flow was simulated with the commercial Fluent software. Furthermore, a graphic interface was developed which simulate the optronic scene by means of aerothermochemical profiles of the jet and datas concerning the position of the detector. In this way, a software of infrared signature computation ( SIRJET) was developed which includes a model of radiative transfer (ray tracing) as well as tabulated parameters (gases and particles) of various spectral model used in infrared remote sensing (CK, CKFG, CK-SMG, CKFG-SMG). Finally, a confrontation is presented between a measure and a simulated infrared signature of a hot jet

Transferts radiatifs dans les récepteurs-réacteurs solaires à haute température

Les projets de recherche, sur les nouveaux procédés de conversion de l'énergie solaire concentrée, doivent relever le défi de la simulation des récepteurs solaires. En effet, des mécanismes complexes de couplage écoulement/rayonnement/réaction chimique/croissance de particules doivent être simulés pour développer les connaissances et les applications. Dans ce mémoire, l'influence du rayonnement a été étudiée dans un réacteur solaire de craquage du méthane pour la production d'hydrogène et de particules de noirs de carbone. L'influence des transferts radiatifs a aussi été étudiée dans des récepteurs et réacteurs volumiques poreux à particules ou en mousse ainsi que des récepteurs surfaciques plans et à cavité. De plus, des campagnes expérimentales ont été menées pour valider les modèles construits notamment ceux sur les récepteurs volumiques. La nécessité d'augmenter les rendements de ces récepteurs requiert de limiter les pertes par émission thermique. C'est pourquoi, des travaux ont été réalisés dans l'objectif d'étudier le gain d'efficacité que procure l'utilisation de matériaux spectralement sélectifs pour des récepteurs volumiques et surfaciques à haute température. Pour l'ensemble de ces travaux, des modèles numériques de transferts radiatifs (Monte Carlo, Ordonnées Discrètes, 2-Flux) et de transferts couplés ont été développés et utilisés. De plus, des modèles de propriétés radiatives spectrales de parois opaques rugueuses, de milieux semi-transparents composés d'un mélange gaz-particules ou d'un milieu macroporeux ou de gaz à haute température ont été proposés et leurs domaines de validité étudiés

GacA is essential for Group A <i>Streptococcus </i>and defines a new class of monomeric dTDP-4-dehydrorhamnose reductases (RmlD)

The sugar nucleotide dTDP-L-rhamnose is critical for the biosynthesis of the Group A Carbohydrate, the molecular signature and virulence determinant of the human pathogen Group A Streptococcus (GAS). The final step of the four-step dTDP-L-rhamnose biosynthesis pathway is catalyzed by dTDP-4-dehydrorhamnose reductases (RmlD). RmlD from the Gram-negative bacterium Salmonella is the only structurally characterized family member and requires metal-dependent homo-dimerization for enzymatic activity. Using a biochemical and structural biology approach, we demonstrate that the only RmlD homologue from GAS, previously renamed GacA, functions in a novel monomeric manner. Sequence analysis of 213 Gram-negative and Gram-positive RmlD homologues predicts that enzymes from all Gram-positive species lack a dimerization motif and function as monomers. The enzymatic function of GacA was confirmed through heterologous expression of gacA in a S. mutans rmlD knockout, which restored attenuated growth and aberrant cell division. Finally, analysis of a saturated mutant GAS library using Tn-sequencing and generation of a conditional-expression mutant identified gacA as an essential gene for GAS. In conclusion, GacA is an essential monomeric enzyme in GAS and representative of monomeric RmlD enzymes in Gram-positive bacteria and a subset of Gram-negative bacteria. These results will help future screens for novel inhibitors of dTDP-L-rhamnose biosynthesis

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

An in silico model for identification of small RNAs in whole bacterial genomes: characterization of antisense RNAs in pathogenic Escherichia coli and Streptococcus agalactiae strains

Characterization of small non-coding ribonucleic acids (sRNA) among the large volume of data generated by high-throughput RNA-seq or tiling microarray analyses remains a challenge. Thus, there is still a need for accurate in silico prediction methods to identify sRNAs within a given bacterial species. After years of effort, dedicated software were developed based on comparative genomic analyses or mathematical/statistical models. Although these genomic analyses enabled sRNAs in intergenic regions to be efficiently identified, they all failed to predict antisense sRNA genes (asRNA), i.e. RNA genes located on the DNA strand complementary to that which encodes the protein. The statistical models enabled any genomic region to be analyzed theorically but not efficiently. We present a new model for in silico identification of sRNA and asRNA candidates within an entire bacterial genome. This model was successfully used to analyze the Gram-negative Escherichia coli and Gram-positive Streptococcus agalactiae. In both bacteria, numerous asRNAs are transcribed from the complementary strand of genes located in pathogenicity islands, strongly suggesting that these asRNAs are regulators of the virulence expression. In particular, we characterized an asRNA that acted as an enhancer-like regulator of the type 1 fimbriae production involved in the virulence of extra-intestinal pathogenic E. coli