1,720,997 research outputs found
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
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
Influence of the Darrieus-Landau instability on the propagation of planar turbulent flames
No full textThe propagation of premixed flames in weak two-dimensional homogeneous turbulent flows is studied numerically via a hybrid Navier-Stokes/front capturing methodology within the context of a hydrodynamic model, which treats the flame as a surface of density discontinuity separating the burnt and unburnt gases. The focus is the influence of the Darrieus-Landau instability on the turbulent flame, which has been recognized recently to have a dramatic effect on its structure and the turbulent flame speed. Such instability, controlled by a parameter inversely proportional to the Markstein length, can be triggered in a laboratory setting by variations in system pressure or in fuel type and composition. Particular attention in this study is devoted to the influence of the Darrieus-Landau instability on a turbulent, statistically planar flame. Results are therefore limited to positive Markstein length corresponding to lean hydrocarbon-air or rich hydrogen-air mixtures. We show that, although the planar flame under similar but laminar conditions is stable, it is nonetheless affected by the instability in the presence of a turbulent incident flowfield. The turbulent flame speed is observed to exhibit, in addition to the effect of thermal expansion, a nontrivial dependence on the instability parameter and on the turbulence integral scale both effects modulating, in the weak turbulence regime, the well established quadratic dependence of turbulent flame speed on turbulence intensity. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved
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
Turbulent propagation of premixed flames in the presence of Darrieus-Landau instability.
No full textWe investigate the role played by hydrodynamic instability in the wrinkled flamelet regime of turbulent combustion, where the intensity of turbulence is small compared to the laminar flame speed and the scale large compared to the flame thickness. To this end the Michelson-Sivashinsky (MS) equation for flame front propagation in one and two spatial dimensions is studied in the presence of uncorrelated and correlated noise representing a turbulent flow field. The combined effect of turbulence intensity, integral scale, and an instability parameter related to the Markstein length are examined and turbulent propagation speed monitored for both stable planar flames and corrugated flames for which the planar conformation is unstable. For planar flames a particularly simple scaling law emerges, involving quadratic dependence on intensity and a linear dependence on the degree of instability. For corrugated flames we find the dependence on intensity to be substantially weaker than quadratic, revealing that corrugated flames are more resilient to turbulence than planar flames. The existence of a threshold turbulence intensity is also observed, below which the corrugated flame in the presence of turbulence behaves like a laminar flame. We also analyze the conformation of the flame surface in the presence of turbulence, revealing primary, large-scale wrinkles of a size comparable to the main corrugation. When the integral scale is much smaller than the characteristic corrugation length we observe, in addition to primary wrinkles, secondary small-scale wrinkles contaminating the surface. The flame then acquires a multi-scale, self-similar conformation, with a fractal dimension, for one-dimensional flames, plateauing at 1.23 for large intensities. The existence of an intermediate integral scale is also found at which the turbulent speed is maximized. When two-dimensional flames are subject to turbulence, the primary wrinkling patterns give rise to polyhedralcellular structures which bear a very close resemblance to those observed in experiments on hydrodynamically unstable expanding spherical flames
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
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