Influence of flame geometry on turbulent premixed flame propagation: a DNS investigation

2012 ◽  
Vol 709 ◽  
pp. 191-222 ◽  
Author(s):  
T. D. Dunstan ◽  
N. Swaminathan ◽  
K. N. C. Bray
Keyword(s):  
Turbulent Flame ◽  
Leading Edge ◽  
Propagation Speed ◽  
Algebraic Model ◽  
Mean Flow ◽  
Continuous Growth ◽  
Turbulent Premixed Flame ◽  
Turbulent Flame Speed ◽  
Flame Brush ◽  
Turbulent Premixed

AbstractThe sensitivity of the turbulent flame speed to the geometry of the flame is investigated using direct numerical simulations of turbulent premixed flames in three canonical configurations: freely propagating statistically planar flames, planar flames stabilized in stagnating flows, and rod-stabilized V-flames. We consider both the consumption speed, which measures the integrated rate of burning, and the propagation speed, which measures the speed of an isosurface within the flame brush. An algebraic model for the propagation speed of the leading edge of the flame brush, which is blind to flame geometry, is also applied to the data for the purposes of establishing its range of validity and the causes of its failure. The turbulent consumption speed is found to be strongly geometry dependent, primarily due to the continuous growth of the flame brush thickness. Changes in the structure and consumption speed of instantaneous flame fronts are found to be only weakly sensitive to flame geometry. The turbulent propagation speed is analysed in terms of its reactive, diffusive and turbulent flux components. All three terms are shown to be significant, both through the flame brush and along the leading edge. The leading-edge propagation speed is found to be sensitive to flame geometry only in the V-flames under certain conditions. It is suggested that this apparent geometry dependence, which the model cannot capture, results from the relation between the turbulence and mean flow time scales in these particular cases, and is not intrinsic to the flame geometry itself.

Modeling CO With Flamelet-Generated Manifolds: Part 2 — Application

2012 ◽  
Author(s):  
Graham Goldin ◽  
Zhuyin Ren ◽  
Hendrik Forkel ◽  
Liuyan Lu ◽  
Venkat Tangirala ◽  
...  
Keyword(s):  
Reaction Rate ◽  
Source Term ◽  
Turbulent Flame ◽  
Leading Edge ◽  
Flame Speed ◽  
Reaction Progress ◽  
Turbulent Flame Speed ◽  
Flamelet Generated Manifold ◽  
The Mean ◽  
Flame Brush

Conventional Flamelet Generated Manifold (FGM) closure of the mean progress variable reaction rate assumes PDF shapes to account for turbulent fluctuations. The FGM parameters are commonly assumed to be statistically independent, and the marginal PDFs invariably require second moments, which are difficult to model accurately and have limited coefficients that can be adjusted to calibrate the simulation. A new model is presented which locates the flame brush with a turbulent flame speed model, and applies the FGM kinetic rate to model kinetically limited processes, such as CO quenching, behind the flame-front. The model is applied to 3D RANS simulations of an equivalence ratio sweep in the GE Entitlement Rig perfectly premixed combustor experiment. Calculating the mean FGM reaction progress source term with standard assumed shape PDFs leads to a narrow flame brush and equilibrium CO outlet emissions. By limiting the mean FGM reaction progress source term by the turbulent flame speed model, the flame brush is broadened and super-equilibrium CO is predicted at the outlet. Good agreement with measurement is obtained with default model coefficients. Since the majority of the mean reaction progress source term is limited by the turbulent flame speed reaction rate, it is demonstrated that the model is relatively insensitive to assumed shape PDFs for the FGM rate, as well as the parameter used to determine the turbulent flame leading edge.

scholarly journals Statistics of Conditional Fluid Velocity in the Corrugated Flamelets Regime of Turbulent Premixed Combustion: A Direct Numerical Simulation Study

2011 ◽  
Vol 2011 ◽  
pp. 1-13 ◽  
Author(s):  
Nilanjan Chakraborty ◽  
Andrei N. Lipatnikov
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Fluid Velocity ◽  
Turbulent Flame ◽  
Leading Edge ◽  
Lewis Number ◽  
Navier Stokes ◽  
Mean Velocity ◽  
Turbulent Premixed Flame ◽  
Turbulent Premixed

The statistics of mean fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes (RANS) simulations have been studied using a Direct Numerical Simulation database of statistically planar turbulent premixed flame representing the corrugated flamelets regime combustion. Expressions for conditional mean velocity and conditional velocity correlations which are derived based on a presumed bimodal probability density function of reaction progress variable for unity Lewis number flames are assessed in this study with respect to the corresponding quantities extracted from DNS data. In particular, conditional surface averaged velocities(ui)¯Rsand the velocity correlations(uiu)j¯Rsin the unburned reactants are demonstrated to be effectively modelled by the unconditional velocities(ui)¯Rand velocity correlations(uiuj)¯R, respectively, for the major part of turbulent flame brush with the exception of the leading edge. By contrast, conditional surface averaged velocities(ui)¯Psand the velocity correlations(uiu)j¯Psin fully burned products are shown to be markedly different from the unconditional velocities(ui)¯Pand velocity correlations(uiuj)¯P, respectively.

scholarly journals Effects of pressure gradients on turbulent premixed flames

1997 ◽  
Vol 353 ◽  
pp. 83-114 ◽  
Author(s):  
DENIS VEYNANTE ◽  
THIERRY POINSOT
Keyword(s):  
Pressure Gradient ◽  
Turbulent Transport ◽  
Turbulent Flame ◽  
Premixed Flames ◽  
Flame Speed ◽  
Pressure Gradients ◽  
Turbulent Premixed Flames ◽  
Turbulent Flame Speed ◽  
Flame Brush ◽  
Turbulent Premixed

In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations.A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale lt.A simple model for the turbulent flux u″c″ is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.

Analysis of Mean Orientation Vector and Mean Curvature in Statistically Steady Turbulent Premixed Combustion

2015 ◽  
Author(s):  
Jaesung Kwon ◽  
Dohyun Kim ◽  
Kang Y. Huh
Keyword(s):  
Mean Curvature ◽  
Leading Edge ◽  
Premixed Combustion ◽  
Relative Importance ◽  
Governing Equations ◽  
Turbulent Premixed Combustion ◽  
Orientation Vector ◽  
The Mean ◽  
Flame Brush ◽  
Turbulent Premixed

Formal governing equations are derived for the mean orientation vector, 〈n〉f, and the mean curvature, 〈∇ · n〉f, in turbulent premixed combustion. Balance is checked to evaluate all component terms and to understand their physical implications in DNS of two test flames. The terms involving ∇T(vn + Sd) and n″(vn + Sd)″ are dominant to determine 〈n〉f through a flame brush and at the leading edge. All listed terms are relevant to determine 〈∇·n〉f, while those involving ∇T2νn+SdΣ′f and (∇·n)″ (vn + Sd)″ become important at the edges. Different trends are observed on the dominant terms for thicker flamelets with the Karlovitz number greater than unity. Further investigation may be required to clarify relative importance of the component terms in different regimes of realistic flame conditions.

Boundary Layer Flashback Prediction for Turbulent Premixed Jet Flames: Comparison of Two Models

2019 ◽  
Author(s):  
Alireza Kalantari ◽  
Nicolas Auwaijan ◽  
Vincent McDonell
Keyword(s):  
Boundary Layer ◽  
Gas Turbine ◽  
Gas Turbines ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Operating Conditions ◽  
Pollutant Emissions ◽  
Jet Flames ◽  
Turbulent Flame Speed ◽  
Turbulent Premixed

Abstract Lean-premixed combustion is commonly used in gas turbines to achieve low pollutant emissions, in particular nitrogen oxides. But use of hydrogen-rich fuels in premixed systems can potentially lead to flashback. Adding significant amounts of hydrogen to fuel mixtures substantially impacts the operating range of the combustor. Hence, to incorporate high hydrogen content fuels into gas turbine power generation systems, flashback limits need to be determined at relevant conditions. The present work compares two boundary layer flashback prediction methods developed for turbulent premixed jet flames. The Damköhler model was developed at University of California Irvine (UCI) and evaluated against flashback data from literature including actual engines. The second model was developed at Paul Scherrer Institut (PSI) using data obtained at gas turbine premixer conditions and is based on turbulent flame speed. Despite different overall approaches used, both models characterize flashback in terms of similar parameters. The Damköhler model takes into account the effect of thermal coupling and predicts flashback limits within a reasonable range. But the turbulent flame speed model provides a good agreement for a cooled burner, but shows less agreement for uncooled burner conditions. The impact of hydrogen addition (0 to 100% by volume) to methane or carbon monoxide is also investigated at different operating conditions and flashback prediction trends are consistent with the existing data at atmospheric pressure.

Measurements of Stretch Statistics at Flame Leading Points for High Hydrogen Content Fuels

2014 ◽  
Author(s):  
Andrew Marshall ◽  
Julia Lundrigan ◽  
Prabhakar Venkateswaran ◽  
Jerry Seitzman ◽  
Tim Lieuwen
Keyword(s):  
Flame Front ◽  
Hydrogen Content ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Fuel Composition ◽  
Mean Flow ◽  
Tangential Strain ◽  
High Hydrogen ◽  
High Hydrogen Content ◽  
Turbulent Flame Speed

Fuel composition has a strong influence on the turbulent flame speed, even at very high turbulence intensities. An important implication of this result is that the turbulent flame speed cannot be extrapolated from one fuel to the next using only the laminar flame speed and turbulence intensity as scaling variables. This paper presents curvature and tangential strain rate statistics of premixed turbulent flames for high hydrogen content fuels. Global (unconditioned) stretch statistics are presented as well as measurements conditioned on the leading points of the flame front. These measurements are motivated by previous experimental and theoretical work that suggests the turbulent flame speed is controlled by the flame front characteristics at these points. The data were acquired with high speed particle image velocimetry (PIV) in a low swirl burner (LSB). We attained measurements for several H2:CO mixtures over a range of mean flow velocities and turbulence intensities. The results show that fuel composition has a systematic, yet weak effect on curvatures and tangential strain rates at the leading points. Instead, stretch statistics at the leading points are more strongly influenced by mean flow velocity and turbulence level. It has been argued that the increased turbulent flame speeds seen with increasing hydrogen content are the result of increasing flame stretch rates, and therefore SL,max values, at the flame leading points. However, the differences observed with changing fuel compositions are not significant enough to support this hypothesis. Additional analysis is needed to understand the physical mechanisms through which the turbulent flame speed is altered by fuel composition effects.

Spark Ignition Simulations and the Generation of Ignition Maps by Means of a Turbulent Flame Speed Closure Approach

2010 ◽  
Author(s):  
Jan M. Boyde ◽  
Massimiliano Di Domenico ◽  
Berthold Noll ◽  
Manfred Aigner
Keyword(s):  
Turbulent Flame ◽  
Spark Ignition ◽  
Flame Speed ◽  
Test Case ◽  
Ignition Energy ◽  
Mean Flow ◽  
Flame Kernel ◽  
Turbulent Flame Speed ◽  
Flow Parameters ◽  
Partially Premixed

This paper presents a numerical investigation of ignition phenomena in turbulent partially premixed methane/air flames. In this work, a turbulent flame speed closure model (TFC) is employed with an ignition delay module extension. The model is applied to two partially premixed test cases under standard conditions in the configuration of a shearless flame and a counter flow flame, respectively. For both setups, the flame kernel propagation and consequent establishment or extinction of the flame are examined. A shearless configuration represents the first test case under investigation. The study demonstrates the large influence of the mean flow parameters on achieving a successful ignition of the domain. The second test case under examination is a counterflow geometry. A sensitivity analysis with respect to spark ignition position and ignition energy is performed. The simulations show that flame kernel spreading is largely influenced by the magnitude of turbulence occurring in the flow, leading to an enhanced propagation in areas with a moderate turbulence degree, whereas high turbulence can be detrimental for the flame establishment due to extensive heat losses. Another observation is that a successful ignition of the domain can occur, even in cases in which the ignition energy is not placed in an area with flammable mixture. The comparison with experimental data shows a good agreement, both in terms of successful ignition and flame kernel propagation.

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