389 research outputs found
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
44th Meeting of the Italian section of the combustion institute. Combustion for sustainability
No full textCarbon-free energy storage is a key enabler of renewable resources, given its role
in overcoming the structural intermittency of such resources. Grid-scale storage
can be flexibly provided by chemical energy via hydrogen or alternative carbon-neutral vectors, ammonia being a promising candidate. Ammonia possesses some
crucial advantages over hydrogen, namely a lower cost per unit of stored energy,
higher volumetric energy density, and a widespread production and distribution
capacity [1]. Ammonia can indeed be utilized via direct combustion in gas turbines
and internal combustion engines although it poses challenges due to its low
reactivity and high NOx emissions. This calls for an intense research effort in
ammonia combustion both at the experimental and numerical levels. Computational
fluid dynamics research on ammonia combustion is still in its infancy and new
numerical tools are therefore needed. In this study, we utilize an in-house, high
order, low-Mach number reactive flow solver, especially suited for DNS, based on
the massively parallel spectral element code nek5000. We utilize the CFD
infrastructure on problems specifically targeting ammonia combustion. Given the
high hydrogen content and the resulting low effective Lewis numbers of typical
reactive ammonia mixtures, the focus is directed towards the analysis of
thermodiffusively unstable ammonia flames, exhibiting a characteristic cellular
conformation. Such flames are known to greatly affect the global consumption
speed due to their wrinkling tendency which may also affect NOx emissions. Twodimensional simulations are performed to analyse the main features of
thermodiffusively unstable laminar ammonia flames. We also recently proposed
novel data-driven models [2,3] for the sub-grid modelling of such intrinsic
instabilities which can prove of great value in LES codes. In this context, we seek
DNS datasets that are minimal in size and yet still fully representative of the
morphological and propagative features of larger ammonia flames.
[1] Valera-Medina, A., et al., Prog. In En. And Comb. Sci. (2018), 69, pp.63-102.
[2] Lapenna, P.E., et al., Comb. Th. And Modelling (2021), 25 (6), pp.1064-1085.
[3] Lapenna, P.E., et al., Proc. Comb. Inst. (2021), 38(2), pp.2001-2011
Subgrid modeling of intrinsic instabilities in premixed flame propagation
No full textThis 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
Direct numerical simulation of thermodiffusively unstable lean NH3/H2-air flame
No full textCarbon-neutral fuels and energy carriers are crucial for decarbonization, and
ammonia has recently gained attention as a promising candidate due to its potential
as a hydrogen carrier and carbon-free fuel. However, ammonia combustion poses
challenges due to its low reactivity and high nitrogen oxide emissions, which require
additional research efforts. In this contribution, we use a high-order, low-Mach
number reactive flow solver to perform a direct numerical simulation of ammonia
premixed combustion and shed light on intrinsic instabilities of the ensuing flame.
The thermodiffusive stability limits of mixtures of technical interest are investigated
and a numerical reconstruction of the dispersion relation of a target mixture is carried
out. Then a direct simulation is performed on a medium-scale domain in order to
investigate the impact of intrinsic instability on flame propagation and pollutants
formation. The onset of cellular structures typical of intrinsically unstable flames is
observed resulting in regions of super-adiabatic temperatures leading to enhanced
pollutant formation
Low-mach number simulations of transcritical flows
No full textA numerical framework for the direct simulation, in the low-Mach number limit, of reacting and non-reacting transcritical flows is presented. The key feature are an efficient and detailed representation of the real fluid properties and an high-order spatial discretization. The latter is of fundamental importance to correctly resolve the largely non-linear behavior of the fluid in the proximity of the pseudo-boiling. The validity of the low-Mach number assumptions is assessed for a previously developed non-reacting DNS database of transcritical and supercritical mixing. Fully resolved DNS data employing high-fidelity thermodynamical models are also used to investigate the spectral characteristic as well as the differences between transcritical and supercritical jets
Myoblast transplantation for heart failure: where are we heading? Alfieri O, Livi U, Martinelli L, Arpesella G, Valfrè C, Lapenna E, Desnos M, Hagège AA, Menasché P
No full textDirect numerical simulation of thermodiffusively unstable lean NH3/H2-air flames
No full textCarbon-neutral fuels and energy carriers are crucial for decarbonization, and ammonia has recently gained attention
as a promising candidate due to its potential as a hydrogen carrier and carbon-free fuel. However, ammonia
combustion poses challenges due to its low reactivity and high nitrogen oxide emissions, which require additional
research efforts. In this contribution, we use a high-order, low-Mach number reactive flow solver to perform a direct
numerical simulation of ammonia premixed combustion and shed light on intrinsic instabilities of the ensuing flame.
The thermodiffusive stability limits of mixtures of technical interest are investigated and a numerical reconstruction
of the dispersion relation of a target mixture is carried out. Then a direct simulation is performed on a medium-scale
domain in order to investigate the impact of intrinsic instability on flame propagation and pollutants formation. The
onset of cellular structures typical of intrinsically unstable flames is observed resulting in regions of super-adiabatic
temperatures leading to enhanced pollutant formation. The data obtained can be further employed in the context of
data-driven models for sub-grid modeling for large eddy simulation
Interplay of Darrieus-Landau instability and weak turbulence in premixed flame propagation
No full textIn this study we investigate, both numerically and experimentally, the interplay between the intrinsic Darrieus-Landau (DL) or hydrodynamic instability of a premixed flame and the moderately turbulent flow field in which the flame propagates. The objective is threefold: to establish, unambiguously, through a suitably defined marker, the presence or absence of DL-induced effects on the turbulent flame, to quantify the DL effects on the flame propagation and morphology and, finally, to asses whether such effects are mitigated or suppressed as the turbulence intensity is increased. The numerical simulations are based on a deficient reactant model which lends itself to a wealth of results from asymptotic theory, such as the determination of stability limits. The skewness of the flame curvature probability density function is identified as an unambiguous morphological marker for the presence or absence of DL effects in a turbulent environment. In addition, the turbulent propagation speed is shown to exhibit a distinct dual behavior whereby it is noticeably enhanced in the presence of DL instability while it is unchanged otherwise. Furthermore, increasing the turbulence intensity is found to be mitigating with respect to DL-induced effects such as the mentioned dual behavior which disappears at higher intensities. Experimental propane and/or air Bunsen flames are also investigated, utilizing two distinct diameters, respectively, above and below the estimated DL cutoff wavelength. Curvature skewness is still clearly observed to act as a marker for DL instability while the turbulent propagation speed is concurrently enhanced in the presence of the instability
Interplay of hydrodynamic instabilities and turbulence in premixed flames
No full textThis thesis is devoted to the numerical investigation of premixed Flames subject to intrinsic hydrodynamic instabilities as well as turbulence. Laminar as well as turbulent premixed combustion can be largely influenced by the onset of hydrodynamic instabilities which can cause a significant increase in flame corrugation and, as a result, in the turbulent flame speed, especially when low turbulence intensity level are present. Indeed, such instability is responsible for the formation of sharp folds and creases in the flame front and for the wrinkling observed, undergo certain conditions, over the surface of expanding flames. Hydrodynamic instability is a result of thermal expansion across the flame and its role is particularly dominant in large-scale flames, when flames are constrained by domains larger than several hundred times the flame thickness. The understanding of these phenomena and their interaction with turbulence can play a potentially significant role in practical combustion systems such as gas turbines and, more generally, in industrial and domestic burners. The aim of this thesis is to develop a numerical tool capable of simulating the propagation of turbulent premixed flames under the influence hydrodynamic instabilities and gather qualitative and quantitative data on flame properties such as morphology and global propagation
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
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