1,721,056 research outputs found
COMPUTATIONAL METHODS IN CHEMICAL KINETICS REDUCTION AND ANALYSIS FOR COMBUSTION MODELING AND APPLICATIONS TO HYDROCARBON SYSTEMS
No full textMaurizio, Di Giacinto Carlo Massimo,Casciola Roberto, Camuss
Distinguished Paper Award in the Laminar Flames colloquium of the 33rd International Symposium on Combustion
No full textIn this study we numerically implement the hydrodynamic model for a premixed flame as a nonlinear free boundary problem where the flame is tracked via a level set equation and the flow is described by a solution of the variable density Navier-Stokes equations. Unlike an earlier similar study, the present model is enriched by fully accounting for hydrodynamic strain in the flame stretch relation which, in turn, affects the local flame speed. The objective is to comprehensively analyze the effect of strain on the onset of the hydrodynamic instability and on the nonlinear development that takes place beyond its inception. The initial evolution is corroborated with the results of a linear stability analysis for which strain rate effects are fully included. We show that while strain provides an additional stabilizing effect on the short wavelength disturbances, thereby delaying the onset of the hydrodynamic instability, it acts to sharpen the cusps near the troughs of the corrugated flame that develops beyond the stability threshold resulting in a larger flame surface area and a higher propagation speed
Turbulent propagation of premixed flames: the effects of thermal expansion and integral scale
No full textSystematic Investigation of CSP Generated Simplified Kinetics Mechanisms for GRI3.0 Methane/Air Mixtures
No full textCorfù, Greci
Towards an Unsteady/Flamelet Progress Variable method for non-premixed turbulent combustion at supercritical pressures
Full text linkCombustion devices operating at elevated pressures, such as liquid rocket engines (LRE), are usually characterized by supercritical thermodynamic conditions. Propellants injected into the combustion cham- ber experience real fluid effects on both their mixing and combustion. Transition through super-criticality implies abrupt variations in thermochemical properties which, together with chemical reactions and high turbulent levels introduce spatial and temporal scales that make these processes impractical to be simulated directly. Reynolds-Averaged Navier-Stokes (RANS) and Large Eddies Simulation (LES) equipped with suitable turbulent combustion modeling are therefore mandatory to attempt numerical simulation on real- istic length scales. In the present work, the building blocks for extending the unsteady/flamelet progress variable approach for turbulent combustion modeling to supercritical non-premixed turbulent flames are presented. Such approach requires a large number of unsteady supercritical laminar flamelet solutions at supercritical pressures, usually referred as flame structures, to be preliminarily established by solving the flamelet equations with suitable real fluid thermodynamics. Given such unsteady flame structures, flamelet libraries can then be generated for all thermochemical quantities. The explicit dependence on flamelet time is usually eliminated using mixture fraction, reaction progress parameter, and maximum scalar dissipation rate as independent flamelet parameters. Real fluid thermodynamics used for such unsteady supercritical laminar flamelet solutions, is taken into account by means of a computationally efficient cubic equation of state. In order to have a better handling of real gas mixtures, the real gas equation of state is written in a comprehensive three-parameter fashion. A priori analysis at supercritical pressures of transient flame structures is performed in order to study how solutions populate the flamelet state space which is usually characterized by the S-shape diagram representing a collection of steady solutions. High-pressure condi- tions ranging from 60 to 300 bar are chosen as representative of a methane/liquid-oxygen rocket engine operating condition
The "turbulent flame speed" of wrinkled premixed flames
Full text linkThe determination of the turbulent flame speed is a central problem in combustion theory. Early studies by Damkohler and Shelkin resorted to geometrical and scaling arguments to deduce expressions for the turbulent flame speed and its dependence on turbulence intensity. A more rigorous approach was undertaken by Clavin and Williams who, based on a multi-scale asymptotic approach valid for weakly wrinkled flames, derived an expression that apart from a numerical factor recaptures the early result by Damkohler and Shelkin. The common denominator of the phenomenological and the more rigorous propositions is an increase in turbulent flame speed due solely to an increase in flame surface area. Various suggestions based on physical and/or experimental arguments have been also proposed, incorporating other functional parameters into the flame speed relation. The objective of this work is to extend the asymptotic results to a fully nonlinear regime that permits to systematically extract scaling laws for the turbulent flame speed that depend on turbulence intensity and scale, mixture composition and thermal expansion, flow conditions including effects of curvature and strain, and flame instabilities. To this end, we use a hybrid Navier-Stokes/front-capturing methodology, which consistently with the asymptotic model, treats the flame as a surface of density discontinuity separating burned and unburned gases. The present results are limited to positive Markstein length, corresponding to lean hydrocarbon-air or rich hydrogen-air mixtures, and to wrinkled flames of vanishingly small thickness, smaller that the smallest fluid scales. For simplicity we have considered here two-dimensional turbulence, which although lacks some features of real three-dimensional turbulence, is not detrimental when using the hydrodynamic model under consideration, because the turbulent flame retains its laminar structure and its interaction with turbulence is primarily advective/kinematic in nature. (C) 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved
Propagation of wrinkled turbulent flames in the context of hydrodynamic theory
No full textWe study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier-Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon-air or rich hydrogen-air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames
Strain rate effects on the nonlinear development of hydrodynamically unstable flames.
No full textIn this study we numerically implement the hydrodynamic model for a premixed flame as a nonlinear
free boundary problem where the flame is tracked via a level set equation and the flow is described by a
solution of the variable density Navier–Stokes equations. Unlike an earlier similar study, the present model
is enriched by fully accounting for hydrodynamic strain in the flame stretch relation which, in turn, affects
the local flame speed. The objective is to comprehensively analyze the effect of strain on the onset of the
hydrodynamic instability and on the nonlinear development that takes place beyond its inception. The ini-
tial evolution is corroborated with the results of a linear stability analysis for which strain rate effects are
fully included. We show that while strain provides an additional stabilizing effect on the short wavelength
disturbances, thereby delaying the onset of the hydrodynamic instability, it acts to sharpen the cusps near
the troughs of the corrugated flame that develops beyond the stability threshold resulting in a larger flame
surface area and a higher propagation speed
Experimental investigation of Darrieus–Landau instability effects on turbulent premixed flames
No full textThe turbulent propagation speed of a premixed flame can be significantly enhanced by the onset of
Darrieus–Landau (DL) instability within the wrinkled and corrugated flamelet regimes of turbulent com-
bustion. Previous studies have revealed the existence of clearly distinct regimes of turbulent propagation,
depending on the presence of DL instabilities or lack thereof, named here as super- and subcritical respec-
tively, characterized by different scaling laws for the turbulent flame speed.
In this study we present experimental turbulent flame speed measurements for propane/air mixtures at
atmospheric pressure, variable equivalence ratio at Lewis numbers greater than one obtained within a Bun-
sen geometry with particle image velocimetry diagnostics. By varying the equivalence ratio we act on the
cut-off wavelength and can thus control DL instability. A classification of observed flames into sub/super-
critical regimes is achieved through the characterization of their morphology in terms of flame curvature
statistics. Numerical low-Mach number simulations of weakly turbulent two-dimensional methane/air slot
burner flames are also performed both in the presence or absence of DL instability and are observed to
exhibit similar morphological properties.
We show that experimental normalized turbulent propane flame speeds S T =S L are subject to two distinct
scaling laws, as a function of the normalized turbulence intensity U rms =S L , depending on the sub/supercrit-
ical nature of the propagation regime. We also conjecture, based on the experimental results, that at higher
values of turbulence intensity a transition occurs whereby the effects of DL instability become shadowed by
the dominant effect of turbulence
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