The Effect of Mean Turbulent Strain Rate on the Flame Speed of Premixed, Growing Flames

2000 ◽  
Vol 123 (1) ◽  
pp. 175-181 ◽  
Author(s):  
D. S.-K. Ting ◽  
M. D. Checkel
Keyword(s):  
Strain Rate ◽  
Growth Model ◽  
Linear Correlation ◽  
Turbulent Flame ◽  
Lewis Number ◽  
Flame Speed ◽  
Experimental Measurements ◽  
Turbulent Flame Speed ◽  
Markstein Number ◽  
The Mean

This paper presents a flame growth model based on experimental measurements of flame speed and mean turbulent strain rate. Methane/air mixtures of 0.7 and 0.9 equivalence ratios were centrally spark-ignited in a 125 mm cubical chamber. Based on schlieren images and combustion pressure traces, a linear correlation was found between the turbulent flame speed and the turbulent strain rate. For these unity-Lewis-number and near-zero-Markstein-number flames, the effectiveness of turbulent strain in enhancing the flame speed was found to increase linearly with the mean flame radius over the range of conditions tested.

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.

Investigation of Turbulence Models Applied to Premixed Combustion Using a Level-Set Flamelet Library Approach

2004 ◽  
Vol 126 (4) ◽  
pp. 701-707 ◽  
Author(s):  
Ulf Engdar ◽  
Per Nilsson ◽  
Jens Klingmann
Keyword(s):  
Level Set ◽  
Turbulence Intensity ◽  
Turbulent Flame ◽  
Turbulence Models ◽  
Flame Speed ◽  
Premixed Combustion ◽  
Chemical Mechanism ◽  
Turbulent Flame Speed ◽  
The Mean ◽  
The Impact

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by an external code, which employs a detailed chemical mechanism. However, a model for the turbulent flame speed is required, which, among other considerations, depends on the turbulence intensity, i.e., these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-Cd, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-ε model, a cubic nonlinear k-ε model, the standard k-ω model and the shear stress transport (SST) k-ω model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the nonlinear k-ε model offers no advantage over the standard model, regardless of flame speed model. The k-ω based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Mu¨ller flame speed model shows the least sensitivity to the choice of turbulence model.  

Investigation of Turbulence Models Applied to Premixed Combustion Using a Level-Set Flamelet Library Approach

2003 ◽  
Author(s):  
Ulf Engdar ◽  
Per Nilsson ◽  
Jens Klingmann
Keyword(s):  
Level Set ◽  
Turbulence Intensity ◽  
Turbulent Flame ◽  
Turbulence Models ◽  
Flame Speed ◽  
Premixed Combustion ◽  
Chemical Mechanism ◽  
Turbulent Flame Speed ◽  
Non Linear ◽  
The Mean

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by an external code, which employs a detailed chemical mechanism. However, a model for the turbulent flame speed is required, which, amongst other considerations, depends on the turbulence intensity, i.e. these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-CD, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-ε model, a cubic non-linear k-ε model, the standard k-ω model and the Shear Stress Transport (SST) k-ω model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the non-linear k-ε model offers no advantage over the standard model, regardless of flame speed model. The k-ω based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Mu¨ller flame speed model shows the least sensitivity to the choice of turbulence model.

scholarly journals Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process

Fluids ◽  
2019 ◽  
Vol 4 (3) ◽  
pp. 146 ◽  
Author(s):  
Aaron Endres ◽  
Thomas Sattelmayer
Keyword(s):  
Boundary Layer ◽  
Gas Turbines ◽  
Separation Zone ◽  
Turbulent Flame ◽  
Pressure Rise ◽  
Flame Speed ◽  
Zone Size ◽  
Detailed Chemical Kinetics ◽  
Turbulent Flame Speed ◽  
Quenching Distance

Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. This was done by means of large eddy simulations with finite rate chemistry as well as detailed chemical kinetics and diffusion models at pressures between 0 . 5 and 3 . It was found that the flashback propensity increases with increasing pressure. The separation zone size and the turbulent flame speed at flashback conditions decrease with increasing pressure, which decreases flashback propensity. At the same time the quenching distance decreases with increasing pressure, which increases flashback propensity. It is not possible to predict the occurrence of boundary layer flashback based on the turbulent flame speed or the ratio of separation zone size to quenching distance alone. Instead the interaction of all effects has to be accounted for when modelling boundary layer flashback. It was further found that the pressure rise ahead of the flame cannot be approximated by one-dimensional analyses and that the assumptions of the boundary layer theory are not satisfied during confined boundary layer flashback.

scholarly journals Extension of the turbulent flame speed closure model to ignition in multiphase flows

2013 ◽  
Vol 160 (2) ◽  
pp. 351-365 ◽  
Author(s):  
Jan M. Boyde ◽  
Patrick C. Le Clercq ◽  
Massimiliano Di Domenico ◽  
Manfred Aigner
Keyword(s):  
Multiphase Flows ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Closure Model ◽  
Turbulent Flame Speed

The Effect of Lewis Number on Instantaneous Flamelet Speed and Position Statistics in Counter-Flow Flames With Increasing Turbulence

2017 ◽  
Author(s):  
Sean D. Salusbury ◽  
Ehsan Abbasi-Atibeh ◽  
Jeffrey M. Bergthorson
Keyword(s):  
Flame Front ◽  
Turbulence Intensity ◽  
High Speed ◽  
Rayleigh Scattering ◽  
Turbulent Flame ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Laminar Flames ◽  
Counter Flow

Differential diffusion effects in premixed combustion are studied in a counter-flow flame experiment for fuel-lean flames of three fuels with different Lewis numbers: methane, propane, and hydrogen. Previous studies of stretched laminar flames show that a maximum reference flame speed is observed for mixtures with Le ≳ 1 at lower flame-stretch values than at extinction, while the reference flame speed for Le ≪ 1 increases until extinction occurs when the flame is constrained by the stagnation point. In this work, counter-flow flame experiments are performed for these same mixtures, building upon the laminar results by using variable high-blockage turbulence-generating plates to generate turbulence intensities from the near-laminar u′/SLo=1 to the maximum u′/SLo achievable for each mixture, on the order of u′/SLo=10. Local, instantaneous reference flamelet speeds within the turbulent flame are extracted from high-speed PIV measurements. Instantaneous flame front positions are measured by Rayleigh scattering. The probability-density functions (PDFs) of instantaneous reference flamelet speeds for the Le ≳ 1 mixtures illustrate that the flamelet speeds are increasing with increasing turbulence intensity. However, at the highest turbulence intensities measured in these experiments, the probability seems to drop off at a velocity that matches experimentally-measured maximum reference flame speeds in previous work. In contrast, in the Le ≪ 1 turbulent flames, the most-probable instantaneous reference flamelet speed increases with increasing turbulence intensity and can, significantly, exceed the maximum reference flame speed measured in counter-flow laminar flames at extinction, with the PDF remaining near symmetric for the highest turbulence intensities. These results are reinforced by instantaneous flame position measurements. Flame-front location PDFs show the most probable flame location is linked both to the bulk flow velocity and to the instantaneous velocity PDFs. Furthermore, hydrogen flame-location PDFs are recognizably skewed upstream as u′/SLo increases, indicating a tendency for the Le ≪ 1 flame brush to propagate farther into the unburned reactants against a steepening average velocity gradient.

Flame Characteristics and Turbulent Flame Speeds of Turbulent, High-Pressure, Lean Premixed Methane/Air Flames

2005 ◽  
Author(s):  
P. Griebel ◽  
R. Bombach ◽  
A. Inauen ◽  
R. Scha¨ren ◽  
S. Schenker ◽  
...  
Keyword(s):  
Flame Front ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Operating Conditions ◽  
Turbulent Flame Speed ◽  
Front Position ◽  
Preheating Temperature ◽  
Lean Premixed

The present experimental study focuses on flame characteristics and turbulent flame speeds of lean premixed flames typical for stationary gas turbines. Measurements were performed in a generic combustor at a preheating temperature of 673 K, pressures up to 14.4 bars (absolute), a bulk velocity of 40 m/s, and an equivalence ratio in the range of 0.43–0.56. Turbulence intensities and integral length scales were measured in an isothermal flow field with Particle Image Velocimetry (PIV). The turbulence intensity (u′) and the integral length scale (LT) at the combustor inlet were varied using turbulence grids with different blockage ratios and different hole diameters. The position, shape, and fluctuation of the flame front were characterized by a statistical analysis of Planar Laser Induced Fluorescence images of the OH radical (OH-PLIF). Turbulent flame speeds were calculated and their dependence on operating conditions (p, φ) and turbulence quantities (u′, LT) are discussed and compared to correlations from literature. No influence of pressure on the most probable flame front position or on the turbulent flame speed was observed. As expected, the equivalence ratio had a strong influence on the most probable flame front position, the spatial flame front fluctuation, and the turbulent flame speed. Decreasing the equivalence ratio results in a shift of the flame front position farther downstream due to the lower fuel concentration and the lower adiabatic flame temperature and subsequently lower turbulent flame speed. Flames operated at leaner equivalence ratios show a broader spatial fluctuation as the lean blow-out limit is approached and therefore are more susceptible to flow disturbances. In addition, because of a lower turbulent flame speed these flames stabilize farther downstream in a region with higher velocity fluctuations. This increases the fluctuation of the flame front. Flames with higher turbulence quantities (u′, LT) in the vicinity of the combustor inlet exhibited a shorter length and a higher calculated flame speed. An enhanced turbulent heat and mass transport from the recirculation zone to the flame root location due to an intensified mixing which might increase the preheating temperature or the radical concentration is believed to be the reason for that.

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