Anisotropic enhancement of turbulence in large-scale, low-intensity turbulent premixed propane–air flames

2002 ◽  
Vol 462 ◽  
pp. 209-243 ◽  
Author(s):  
JUNICHI FURUKAWA ◽  
YOSHIKI NOGUCHI ◽  
TOSHISUKE HIRANO ◽  
FORMAN A. WILLIAMS
Keyword(s):  
Turbulent Combustion ◽  
Large Scale ◽  
Isotropic Turbulence ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Vertical Plane ◽  
Turbulent Pipe Flow ◽  
Laser Doppler Velocimeter ◽  
Arrival Times ◽  
Flame Brush

The density change across premixed flames propagating in turbulent flows modifies the turbulence. The nature of that modification depends on the regime of turbulent combustion, the burner design, the orientation of the turbulent flame and the position within the flame. The present study addresses statistically stationary turbulent combustion in the flame-sheet regime, in which the laminar-flame thickness is less than the Kolmogorov scale, for flames stabilized on a vertically oriented cylindrical burner having fully developed upward turbulent pipe flow upstream from the exit. Under these conditions, rapidly moving wrinkled laminar flamelets form the axisymmetric turbulent flame brush that is attached to the burner exit. Predictions have been made of changes in turbulence properties across laminar flamelets in such situations, but very few measurements have been performed to test the predictions. The present work measures individual velocity changes and changes in turbulence across flamelets at different positions in the turbulent flame brush for three different equivalence ratios, for comparison with theory.The measurements employ a three-element electrostatic probe (EP) and a two-component laser-Doppler velocimeter (LDV). The LDV measures axial and radial components of the local gas velocity, while the EP, whose three sensors are located in a vertical plane that passes through the burner axis, containing the plane of the LDV velocity components, measures arrival times of flamelets at three points in that plane. From the arrival times, the projection of flamelet orientation and velocity on the plane are obtained. All of the EP and LDV sensors are located within a fixed volume element of about 1 mm diameter to provide local, time-resolved information. The technique has the EP advantages of rapid response and good sensitivity and the EP disadvantages of intrusiveness and complexity of interpretation, but it is well suited to the type of data sought here.Theory predicts that the component of velocity tangent to the surface of a locally planar flamelet remains constant in passing through the flamelet. The data are consistent with this prediction, within the accuracy of the measurement. The data also indicate that the component of velocity normal to the flamelet, measured with respect to the flamelet, tends to increase in passing through the flamelet, as expected. The flamelets thereby can generate anisotropy in initially isotropic turbulence. They also produce differences in turbulent spectra conditioned on unburnt or burnt gas. Local modifications of turbulence by flamelets thus are demonstrated experimentally. The modifications are quantitatively different at different locations in the turbulent flame brush but qualitatively similar in that the turbulence is enhanced more strongly in the radial direction than in the axial direction at all positions in these flames.

Pipe Flow Measurements of a Transparent Non-Newtonian Slurry

1989 ◽  
Vol 111 (3) ◽  
pp. 331-336 ◽  
Author(s):  
J. T. Park ◽  
R. J. Mannheimer ◽  
T. A. Grimley ◽  
T. B. Morrow
Keyword(s):  
Velocity Profile ◽  
Newtonian Fluid ◽  
Power Law ◽  
Pipe Flow ◽  
Large Scale ◽  
Longitudinal Velocity ◽  
Tangential Velocity ◽  
Turbulent Pipe Flow ◽  
Laser Doppler Velocimeter ◽  
Flow Measurements

An experimental description of the flow structure of non-Newtonian slurries in the laminar, transitional, and full turbulent pipe flow regimes is the primary objective of this research. Experiments were conducted in a large-scale pipe slurry flow facility with an inside pipe diameter of 51 mm. The transparent slurry formulated for these experiments from silica, mineral oil, and Stoddard solvent exhibited a yield-power-law behavior from concentric-cylinder viscometer measurements. The velocity profile for laminar flow from laser Doppler velocimeter (LDV) measurements had a central plug flow region, and it was in agreement with theory. The range of the transition region was narrower than that for a Newtonian fluid. The mean velocity profile for turbulent flow was close to a 1/7 power-law velocity profile. The rms longitudinal velocity profile was also similar to a classical turbulent pipe flow experiment for a Newtonian fluid; however, the rms tangential velocity profile was significantly different.

Influence of Hydrogen Addition on Lean Premixed Methane-Air Flame Statistics

2010 ◽  
Author(s):  
Baris Yilmaz ◽  
Sibel O¨zdogan ◽  
Iskender Go¨kalp
Keyword(s):  
High Pressure ◽  
Software Package ◽  
Turbulence Modeling ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Premixed Combustion ◽  
High Pressure Chamber ◽  
Hydrogen Addition ◽  
Flame Brush ◽  
Lean Conditions

Hydrogenated premixed methane/air flames under lean conditions are simulated in this study. The computational model of the high pressure chamber setup of Orleans - ICARE (France) has been developed. The k-ε turbulence model with Pope-correction is used for turbulence modeling. The laminar flame properties are computed using GRI-Mech 3.0 mechanism with Chemkin software package. The turbulent flame front statistics are investigated with three premixed combustion models, namely Zimont, Coherent Flame Model (CFM) and modified version of CFM model (MCFM) models. It has been observed that increasing the volumetric percentage of hydrogen in the mixture results in reducing the flame-end position. The flame brush thickness becomes thinner as well. Satisfactory results have been obtained compared to experiments.

Implications of laminar flame finite thickness on the structure of turbulent premixed flames

2015 ◽  
Vol 787 ◽  
pp. 116-147 ◽  
Author(s):  
Kim Q. N. Kha ◽  
Vincent Robin ◽  
Arnaud Mura ◽  
Michel Champion
Keyword(s):  
Turbulent Combustion ◽  
Laminar Flame ◽  
Finite Thickness ◽  
Turbulent Flame ◽  
Leading Edge ◽  
Special Focus ◽  
Fast Chemistry ◽  
Wrinkled Flame ◽  
Turbulent Premixed

A layered description of the structure of turbulent flame brushes is provided for situations featuring large but finite values of the Damköhler number, which correspond to the wrinkled flame regime of turbulent premixed combustion. One special focus of this study is placed on the description of the leading edge of the turbulent flame brush, the role of which is known to be essential with respect to propagation, transport and stabilization issues. On the basis of rather simple and well-identified working hypotheses, the influence of slight increases in the Karlovitz number values is revealed. The phenomenology and associated statistics are also investigated analytically, which leads to a mathematical description of the leading edge internal structure. With respect to the progress variable statistics, i.e. probability density function, this leading edge can indeed be thought of as the inner part of a boundary layer where the influence of the finite thickness of laminar flamelets can no longer be neglected. From the proposed description, standard fast-chemistry closures, which are currently used to perform the numerical simulation of turbulent combustion, may easily be generalized to account for the finite-rate chemistry effects occurring in this sublayer, thus emphasizing the interest of the present analysis for turbulent combustion theory and modelling.

Flashback in Lean Prevaporized Premixed Combustion: Nonswirling Turbulent Pipe Flow Study

2003 ◽  
Vol 125 (3) ◽  
pp. 670-676 ◽  
Author(s):  
O. Scha¨fer ◽  
R. Koch ◽  
S. Wittig
Keyword(s):  
Flame Propagation ◽  
Pipe Flow ◽  
Laminar Flame ◽  
Fluid Dynamic ◽  
Turbulent Flame ◽  
Side Wall ◽  
Turbulent Pipe Flow ◽  
Flame Velocity ◽  
Fundamental Study ◽  
Wall Boundary Layer

A fundamental study has been performed on the upstream flame propagation of a turbulent kerosene flame, stabilized in a confined stagnation flow at atmospheric pressure. Besides temperature and equivalence ratio, mixture properties and fluid dynamic parameters have been varied. The flashback phenomenon is discussed in terms of critical mean velocities and additionally based on detailed LDV data at the outlet of the premixing duct. The largest critical velocities uc for flashback are found for the “perfectly” premixed case and equivalence ratios close to stoichiometric, which is in accordance with the theory on laminar flame propagation. In the case of a homogeneous mixture, flashback is determined by the velocity distribution at the outlet of the premixing section. In the undisturbed pipe flow the flame propagates through the wall boundary layer. The data for this case are compared with the theory of side-wall quenching in terms of a critical Peclet number and critical velocity gradients at the wall. Both are deduced from the experimental data. Reducing the velocity on the axis forces the flame to propagate through the center at a velocity predicted by correlations on turbulent flame velocity.

An approach to turbulent flame theory

1970 ◽  
Vol 40 (2) ◽  
pp. 401-421 ◽  
Author(s):  
F. A. Williams
Keyword(s):  
Eigenvalue Problem ◽  
Large Scale ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Ensemble Average ◽  
Small Scale ◽  
Grid Turbulence ◽  
Scale Limit

Is it possible to express the problem of calculating turbulent flame speeds as an eigenvalue problem that is analogous to the laminar flame speed problem? It is argued for grid turbulence that the answer is affirmative, and some benefits of pursuing such a calculation are exploited for the limiting case of a first-order reaction with vanishingly small heat release. The streamwise turbulent transport of reactant occupies a central role in the analysis. The equation governing the ensemble average of this quantity assumes different simplified forms in the limits of small-scale and large-scale turbulence. The criterion which is obtained for separating the small-scale and large-scale régimes differs from that of Damköhler and also from that of Kovasznay and Klimov. In the small-scale régime, turbulence produces a spatially varying diffusivity, the form of which can be ascertained only through an investigation of non-linear equations describing the statistical dynamics of production and decay of the velocity–concentration correlation. In the large-scale régime, which is of greater practical importance, the ensemble average of the streamwise turbulent reactant flux satisfies a linear ordinary differential equation whose solution for the growth and decay of the flux contains effects resembling wrinkling of the laminar flame, increasing of the effective diffusivity and augmentation of the effective reaction rate. An exact solution to the linear eigenvalue problem which arises in the large-scale limit reveals that turbulence enhances mean reactant consumption in the upstream portion of the flame and retards reactant consumption downstream. Formulas are given for the increase in flame speed and the increase in flame thickness that are produced by turbulence in the large-scale limit. Since the equations are relatively tractable in the large-scale limit, it is suggested that further study of these equations may yield improved descriptions of realistic turbulent flames.

Flashback in Lean Prevaporized Premixed Combustion: Non-Swirling Turbulent Pipe Flow Study

2001 ◽  
Author(s):  
Olaf Schäfer ◽  
R. Koch ◽  
S. Wittig
Keyword(s):  
Flame Propagation ◽  
Pipe Flow ◽  
Laminar Flame ◽  
Fluid Dynamic ◽  
Turbulent Flame ◽  
Side Wall ◽  
Turbulent Pipe Flow ◽  
Flame Velocity ◽  
Fundamental Study ◽  
Wall Boundary Layer

A fundamental study has been performed on the upstream flame propagation of a turbulent kerosene flame, stabilized in a confined stagnation flow at atmospheric pressure. Besides temperature and equivalence ratio, mixture properties and fluid dynamic parameters have been varied. The flashback phenomenon is discussed in terms of critical mean velocities and additionally based on detailed LDV data at the outlet of the premixing duct. The largest critical velocities uc for flashback are found for the “perfectly” premixed case and equivalence ratios close to stoichiometric, which is in accordance with the theory on laminar flame propagation. In the case of a homogeneous mixture, flashback is determined by the velocity distribution at the outlet of the premixing section. In the undisturbed pipe flow the flame propagates through the wall boundary layer. The data for this case are compared with the theory of side-wall quenching in terms of a critical Peclet number and critical velocity gradients at the wall. Both are deduced from the experimental data. Reducing the velocity on the axis forces the flame to propagate through the center at a velocity predicted by correlations on turbulent flame velocity.

Experiments of Compressible Homogeneous and Isotropic Turbulence Interacting With Expansion Waves

2003 ◽  
Author(s):  
Savvas S. Xanthos ◽  
Yiannis Andreopoulos
Keyword(s):  
Mach Number ◽  
Shock Tube ◽  
Total Pressure ◽  
Large Scale ◽  
Isotropic Turbulence ◽  
Pressure Fluctuations ◽  
Incident Shock ◽  
Expansion Waves ◽  
Curvature Effects ◽  
Homogeneous And Isotropic Turbulence

The interaction of traveling expansion waves with grid-generated turbulence was investigated in a large-scale shock tube research facility. The incident shock and the induced flow behind it passed through a rectangular grid, which generated a nearly homogeneous and nearly isotropic turbulent flow. As the shock wave exited the open end of the shock tube, a system of expansion waves was generated which traveled upstream and interacted with the grid-generated turbulence; a type of interaction free from streamline curvature effects, which cause additional effects on turbulence. In this experiment, wall pressure, total pressure and velocity were measured indicating a clear reduction in fluctuations. The incoming flow at Mach number 0.46 was expanded to a flow with Mach number 0.77 by an applied mean shear of 100 s−1. Although the strength of the generated expansion waves was mild, the effect on damping fluctuations on turbulence was clear. A reduction of in the level of total pressure fluctuations by 20 per cent was detected in the present experiments.

Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method

2013 ◽  
Author(s):  
Scott Martin ◽  
Aleksandar Jemcov ◽  
Björn de Ruijter
Keyword(s):  
Turbulent Combustion ◽  
Large Scale ◽  
Reaction Rates ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Small Scale ◽  
Conditional Moment ◽  
Progress Variable ◽  
Conditional Moment Closure ◽  
Scale Turbulence

Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided. The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled. The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors.

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