The topic of this study is the numerical simulation of a turbulent non-premixed hydrogen flame with different micromixing models in order to investigate their predictive capability. The two micromixing models are compared. Comparisons with experimental data demonstrate that predictions based on the EMST model are slightly better. The EMST improves largely the precision of the results to the detriment of the RAM and the CPU performances. Overall, profile predictions of mixture fraction, flame temperature and major species are in reasonable agreement with experimental data.
Dans le présent article, les jets turbulents confinés axisymétriques non réactifs sontétudiés en utilisant le modèle de turbulence de second ordre. L'objectif est de mettre enévidence, dans la région initiale du jet, les effets de la variation de densité, provoquée par unécart de température entre le jet d'air chauffé et l'air ambiant, sur les principaux paramètres caractéristiques de telsécoulements. Les résultats numériques obtenus sont alors discutés en comparaison avec ceux issus des expériences de Djeridane [T. Djeridane, Contributionà l'étude expérimentale de jets turbulents axisymétriquesà densité variable, Thèse de doctorat, Univ. Aix-Marseille II, 1994] (relatifs aux jets air/air et He/air) et d'autres trouvées dans la littérature. Les mesures expérimentales de la vitesse longitudinale, de l'écart-type des tensions de Reynolds et du scalaire, présentent qualitativement le même comportement. L'écart entre la simulation numérique et l'expérience n'est pas important et les résultats numériques obtenus sont en général satisfaisants. Mots clés :Turbulence / jets / effets de densité / modélisation au second ordre Abstract -Numerical simulation of an axisymmetric turbulent jet with variable density using a Reynolds stress model. In this paper, the nonreacting axisymmetric confined turbulent jets are studied by using the second order turbulence model. The aim is to present, in the initial region of the jet, the variable density effects, caused by a difference in temperature between the hot air and the ambient air, on the principal characteristic parameters of such flows. The numerical results obtained are then discussed in comparison with experimental data Djeridane [T. Djeridane, Contributionà l'étude expérimentale de jets turbulents axisymétriquesà densité variable, Thèse de doctorat, univ. Aix-Marseille II, 1994] (relating to the air/air and He/air jets) and other found in the literature. Experimental measurements of the axial velocity, the Reynolds stress and the scalar, present qualitatively the same behaviour. The comparison of the numerical simulation to the experiment does not show large difference and the results obtained are in general satisfactory.
The aim of this work is to contribute to the study of chemistry effects on the turbulent scalar field that is, here, the temperature, through a better knowledge of the ratio between the scalar time scale and the mechanical time scale (R t ) in the distributed reaction regime characterized by a Damkö hler number lower than unity. The numerical studies are performed by calculating the order of magnitude of R t in both low exothermic reacting and nonreacting case. Propane has been chosen as a typical hydrocarbon for this study. The molar fractions are computed by the IEM (Interaction Exchange with the Mean) model in which the probability density function is calculated from its transport equation. The models and the numerical simulations are used to describe a jet stirred reactor with subsonic jet injection. The predictions are validated by comparison with experimental data for temperature and concentrations fields. It is pointed that in the non-reacting case, R t parameter is around unity; the temporal scales dynamics and scalar are coupled. In this case, the transport of the passive scalar is ensured by the dynamic field. In the reactive case, however, this coupling is not assured any more. Indeed, the chemical reaction, characterized by a non-linear source term, affects the scalar field by decreasing its characteristic scales, which results in R t lower than unity. The CFD-PDF predictions were within engineering accuracy of experimental data. It can be summarized that the results of exercise are satisfactory, and the CPU-time and RAM memory savings encouraging. INTRODUCTIONTemporal and spatial macroscopic velocity inhomogeneities or fluctuations are ubiquitous in turbulent flows. Large time and length scales (also termed integral scales) are associated to the external (initial=boundary conditions) or internal (flow instabilities) turbulence-triggering mechanisms. At the other end, the Kolmogorov time and length microscales characterize the viscous dissipation of kinetic energy into heat. The transfer of energy between large and small structures or eddies takes place across a wide range of intermediate scales.Heterogeneities in scalar fields can also be induced by injection process (initial and boundary conditions) and=or by the driving fluctuating velocity field. Local chemical source terms can also contribute to generate scalar fluctuations. The ratio of integral length to the Kolmogorov microscale is proportional to the turbulence Reynolds number to the power three-fourths, while the integral time over the Kolmogorov one scales as the Reynolds number to one-half (Dopazo & O'Brien, 1987). In order to resolve the smallest features of fluctuating fields, while keeping integral length scale inside the domain. The required computer storage is thus proportional to the Reynolds number to the nine-fourths power and the computational time goes a power slightly larger than eleven-fourths. This approach to the solutions of turbulent flows is named direct numerical simulation (DNS), as the governing equations are numericall...
In order to respond to the increased demand for clean energy without harming the atmosphere through polluting emissions, Energy production from the hydrogen combustion become largely used. This work presents a numerical study of the injection conditions effect on the structure of the H2-Air diffusion flame. The aim is to reproduce a practical case of non-polluting combustion and resulting in very high temperatures. The configuration is composed of two axisymmetric coaxial jets, as can be found in the diffusion burners. A presumed probability density function (PDF) approach is used to describe the chemistry-turbulence interaction. K-epsilon model of turbulence is used. Particular attention is given to phenomena anchoring or blowout of the flame.
NomenclatureD -nozzle diameter, mm; D a -co-flowing diameter, mm; D eq = D e -equivalent diameter of elliptic nozzle, mm; F c -mass fraction; L p -potential core length from density field; L a -co-flowing length, mm; M -outer to inner specific momentum flux ratio; R v -outer to inner bulk velocity ratio; S -outer to inner density ratio; U -jet exit mean velocity, m/s; U a -co-flowing velocity, m/s; X -distance to nozzle, m; ρ -density: ρ * normalized density = (ρ−ρ e )/(ρ i −ρ e ) or (ρ−ρ He )/(ρ i −ρ He ); (-) i -relative to inner jet; (-) e -relative to external jet; (-) a -relative to ambient fluid.
In this present study, the numerical simulations are performed for an axisymmetric turbulent jet diffusion hydrogen/air flame, by using a hybrid Finite-Volume/Composition PDF-Transport method. This method represents a rational approach for the study of turbulent reacting flows containing signficant turbulencechemistry interactions. The major attraction of Composition PDF method is that the terms associated with chemical reaction appear in closed form, leaving only molecular mixing and turbulent transport terms to be modeled. The accuracy of Composition PDF model calculations depends on the accurate representation of the chemistry and on the mixing model including the value of the mixing-model constant CΦ (mixing-model constant CΦ is the mechanical to scalar time scale ratio (CΦ =τt/τΦ).There has been considerable development in the past three decades in PDF methods, and reviews can be found in Dopazo and O'Brien
The aim of this study is to investigate numerically the effects of four vortices on the dynamic, scalar, and turbulent fields of the hydrogen jet. These vortices, which appear in the vicinities of the nozzle, are created by the vortex generators (VGs), and they are assembled with periodicity or symmetry in order, respectively, to give four vortices of the same or opposite direction. A second-order Reynolds stress model is used to investigate asymmetric turbulent jet.The results indicate that the presence of the vortex near the emission jet section noticeably enhances mixing to ensure a good combustion.
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