1,721,018 research outputs found
Assessment of an algebraic equilibrium wall-function for supercritical flows
No full textIn this work the application of an algebraic equilibrium wall-function to real-gas flows is presented and analyzed. The aim is to assess capabilities of existing algebraic wall-functions in supercritical conditions. In particular a systematic analysis on the coupled wall-function of Cabrit and Nicoud is carried out a-priori on a wall-resolved Large Eddy Simulations (WR-LES) database, featuring cryogenic para-hydrogen flow in a heated pipe at supercritical pressure. The model is shown to overestimate the wall-temperature for increasing values of the imposed heat flux and to slightly underestimate the skin friction velocity. The causes of the failure are investigated. In particular the focus is on the equilibrium boundary layer hypothesis, on the validity of the Van Driest transformation for supercritical, stratified flows and on the ideal-gas assumption employed in the original derivation of the model. For the latter, a consistent thermodynamic correction is proposed in order to extend the applicability of the mentioned wall-function to any arbitrary Equation of State (EOS). The proposed extension is tested a-priori and is shown to provide improved temperature and skin friction velocity predictions at wall, although still presenting relevant deviations from the reference database solution. The discrepancies seem to be addressed to the equilibrium assumption and to the Van Driest transformation. The former in particular accounts for 5-20%
of the reference value for the skin friction velocity, among all cases and y^+
examined, and for 1-20%
in terms of wall-temperature depending on the considered heat flux. The latter is shown to fail on the mentioned database for increasing stratifications, with errors between
1-40% depending on the considered case. An additional analysis of more recently proposed transformations for variable property flows reveals the same limitations
Direct numerical simulation of transcritical jets at moderate Reynolds number
No full textThis paper presents a direct numerical simulation dataset comprising four transcritical nitrogen jets characterized by the same Reynolds number and different pressures, ranging from near critical up to largely supercritical. Simulations are carried out using a well-established high-order numerical method in conjunction with high-fidelity real fluid equation of state and transport properties. Although being characterized by the same Reynolds number, jets under transcritical conditions at different pressures behave in a significantly different way at both large- and small-scale. At near-critical pressure conditions, jets are more influenced by the solid wall effect caused by the steep stratification induced by the pseudo-boiling region. Globally, such gradients delay the complete mixing of the jet, whereas locally they promote baroclinic vorticity generation
Subgrid modeling of intrinsic instabilities in premixed flame propagation
Full text linkThis work is devoted to the investigation and subgrid-scale modeling of intrinsic flame instabilities occurring in the propagation of a deflagration wave. Such instabilities, of hydrodynamic and thermodiffusive origin, are expected to be of particular relevance in recent technological trends such as in the use of hydrogen as a clean energy carrier or as a secondary fuel in hydrogen enriched combustion. A dedicated set of direct numerical simulations is presented and used, in conjunction with coherent literature results, in order to develop scaling arguments for the propagation speed of self-wrinkled flames which are also supported by the outcomes of a weakly non-linear model, namely the Sivashinsky equation. The observed scaling is based on the definition of the number of unstable wavelengths in a reference hydrodynamic lengthscale, in other words the ratio between the neutral or cutoff lengthscale of intrinsic instabilities and the lateral domain of a planar flame. The scalings are then employed to develop an algebraic model for the wrinkling factor in the context of a flame surface density closure approach. An a-priori analysis shows that the model correctly captures the flame wrinkling caused by intrinsic instability at sub grid level. A strategy to include the developed self-wrinkling model in the context of a turbulent combustion model is finally discussed on the basis of the turbulence induced cut-off concept
Workshop on Fundamental Understanding and Modelling of High Pressure Turbulent Premixed Combustion
No full textSelf-wrinkling induced by Darrieus-Landau instability in turbulent premixed Bunsen flames from low to moderately high Reynolds numbers
No full textExperimental data obtained via particle image velocimetry are used to investigate the self-wrinkling of premixed flame fronts induced by Darrieus-Landau (DL) instability and its interaction with turbulence from low to moderately high Reynolds numbers in a Bunsen configuration. At low Reynolds, hence in quasilaminar conditions, the DL instability is experimentally triggered by varying the mixture ratio. Conversely, in turbulent cases, the DL instability is triggered by varying the Bunsen nozzle diameter so that flames can be compared at the same equivalence ratio and jet Reynolds numbers. The differences between stable and unstable Bunsen flames at low Reynolds number are discussed in terms of vorticity generation downstream in the flame as well as total flame strain and its normal and tangential components. The straining pattern of a single DL cusp is calculated along the flame front to create a reference case for higher turbulence intensity cases and assess the influence of self-wrinkling of the flame front on the reactant flow field, i.e., the channeling effect. The results obtained are consistent with recent direct numerical simulations of single DL-cusp propagating in a quiescent environment. In addition, inspection of strain-curvature joint probability density function, curvature correlation coefficient and crossing length statistics will show that, even from the intermediate Reynolds numbers explored, unstable flames under the influence of self-wrinkling effects possess the statistical characteristics that stable flames gain only at high turbulence levels, where a unified and turbulence dominated regime is likely to be reached, although with the persistence of some residual differences
Strain rates, flow patterns and flame surface densities in hydrodynamically unstable, weakly turbulent premixed flames
No full textRecent numerical and experimental studies have unveiled a potentially marked difference between the laminar as well as turbulent propagation of premixed flames exhibiting Darrieus-Landau (DL) (or hydrodynamic) instabilities from flames for which instabilities are inhibited. In this study we utilize two-dimensional numerical simulations of slot burner flames as well as experimental Propane-Air Bunsen flames to analyse differences in turbulent propagation, strain rate and induced flow patterns of hydrodynamically stable and unstable flames. We also investigate the effects of hydrodynamic instability on quantities which are directly related to reaction rate closure models, such as flame surface density and stretch factor. A clear enhancement of turbulent flame speed can be observed for unstable flames, generally mitigated at higher turbulence intensity, which is attributed to a flame area increase induced by the characteristic cusp-like DL-induced corrugation, absent in stable flames, which occurs concurrently and in synergy with turbulent wrinkling. Unstable flames also exhibit, both numerically and experimentally, a different correlation between strain rate and flame curvature and are observed to give rise to a channeling of the induced flow in the fresh mixture. Conditionally averaged flame surface density is also observed to attain smaller values in unstable flames, as a result of the thicker turbulent flame brush, indicating that closure models should incorporate instability-related parameters in addition to turbulence-related parameters
Self-wrinkling induced by Darrieus-Landau instability in turbulent premixed Bunsen flames from low to moderately high Reynolds numbers
No full textExperimental data obtained via particle image velocimetry are used to investigate the self-wrinkling of premixed flame fronts induced by Darrieus-Landau (DL) instability and its interaction with turbulence from low to moderately high Reynolds numbers in a Bunsen configuration. At low Reynolds, hence in quasilaminar conditions, the DL instability is experimentally triggered by varying the mixture ratio. Conversely, in turbulent cases, the DL instability is triggered by varying the Bunsen nozzle diameter so that flames can be compared at the same equivalence ratio and jet Reynolds numbers. The differences between stable and unstable Bunsen flames at low Reynolds number are discussed in terms of vorticity generation downstream in the flame as well as total flame strain and its normal and tangential components. The straining pattern of a single DL cusp is calculated along the flame front to create a reference case for higher turbulence intensity cases and assess the influence of self-wrinkling of the flame front on the reactant flow field, i.e., the channeling effect. The results obtained are consistent with recent direct numerical simulations of single DL-cusp propagating in a quiescent environment. In addition, inspection of strain-curvature joint probability density function, curvature correlation coefficient and crossing length statistics will show that, even from the intermediate Reynolds numbers explored, unstable flames under the influence of self-wrinkling effects possess the statistical characteristics that stable flames gain only at high turbulence levels, where a unified and turbulence dominated regime is likely to be reached, although with the persistence of some residual differences
Large scale effects in weakly turbulent premixed flames
No full textIn this study we numerically investigate large scale premixed flames in weakly turbulent flow fields. A large scale flame is classified as such based on a reference hydrodynamic lengthscale being larger than a neutral (cutoff) lengthscale for which the hydrodynamic or Darrieus-Landau (DL) instability is balanced by stabilizing diffusive effects. As a result, DL instability can develop for large scale flames and is inhibited otherwise. Direct numerical simulations of both large scale and small scale three-dimensional, weakly turbulent flames are performed at constant Karlovitz and turbulent Reynolds number, using two paradigmatic configurations, namely a statistically planar flame and a slot Bunsen flame. As expected from linear stability analysis, DL instability induces its characteristic cusp-like corrugation only on large scale flames. We therefore observe significant morphological and topological differences as well as DL-enhanced turbulent flame speeds in large scale flames. Furthermore, we investigate issues related to reaction rate modeling in the context of flame surface density closure. Thicker flame brushes are observed for large scale flames resulting in smaller flame surface densities and overall larger wrinkling factors
Flame Induced Flow Features in the Presence of Darrieus-Landau Instability
No full textThe onset of hydrodynamic or Darrieus-Landau (DL) instability can largely impact on premixed flame morphology, turbulent flame speed and induced flow field. In this work, we focus on the latter induced flow by means of two dimensional direct numerical simulations (DNS) of slot burner flames performed in a parametric fashion. Results from linear stability analysis are used to select the adequate parameter range to be investigated. The presence of DL instability is initially assessed using a recently proposed statistical marker related to flame morphology. The differences between stable and unstable flames are then statistically investigated, utilizing a single, laminar, DL-induced corrugation as a reference state. Such DL-induced effects are investigated at various turbulence intensities, in terms of local propagation, induced strain rate patterns and flow field as well as vorticity production and transformation. Using displacement speed as a measure of local propagation, no noticeable statistical difference is observed between stable and unstable flames while strain rate and vorticity patterns are shown to be largely influenced by the DL induced morphology. From the modeling point view, an enhancement of counter gradient type transport for turbulent scalar fluxes is observed for hydrodynamically unstable flames
Synergistic interplay of thermodiffusive instability and turbulence in premixed flames
No full textIn this work, we experimentally analyze the interplay of thermo-diffusive (TD) intrinsic flame instabilities and turbulence in premixed flame propagation. We utilize methane/hydrogen/air Bunsen flames at atmospheric pressure and variable hydrogen content, and variable turbulence intensity. Experiments are designed to maintain the laminar unstretched premixed flame speed constant by adjusting the equivalence ratio φ for each flame. As the hydrogen content is increased and φ is decreased, thermo-diffusive intrinsic flame instabilities are gradually promoted. We study the effect of thermo-diffusive instability on the global consumption speed by analyzing the contribution of flame surface area increase and flame mean reactivity measured via a stretch factor. We observe that the turbulence-instability interplay mainly occurs through an enhancement of flame reactivity and not flame area. In addition, a power spectral density (PSD) analysis of the flame curvature reveals that the spectra of unstable flames are consistently more energetic due to the wider range of linearly unstable scales interacting with the turbulent integral scale. A forced weakly nonlinear numerical model is also utilized to aid in the understanding of the experimental findings. The model exhibits a characteristic unforced PSD, representing the energy content of the typical spatiotemporal chaotic TD-unstable solution. When forced, the model exhibits PSD that emerge from the interplay of the turbulent spectrum and the characteristic TD-unstable spectrum, and, as a result are consistently more energetic than the TD-stable spectra
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