scholarly journals Influence of molecular transport on burning rate and conditioned species concentrations in highly turbulent premixed flames

2021 ◽  
Vol 928 ◽  
Author(s):  
H.C. Lee ◽  
P. Dai ◽  
M. Wan ◽  
A.N. Lipatnikov
Keyword(s):  
Burning Rate ◽  
Burning Velocity ◽  
Molecular Transport ◽  
Leading Edge ◽  
Lewis Number ◽  
Single Species ◽  
Turbulent Flames ◽  
Computational Domain ◽  
Differential Diffusion ◽  
Preferential Diffusion

Apparent inconsistency between (i) experimental and direct numerical simulation (DNS) data that show the significant influence of differential diffusion on the turbulent burning rate and (ii) recent complex-chemistry DNS data that indicate mitigation of the influence of differential diffusion on conditioned profiles of various local flame characteristics at high Karlovitz numbers, is explored by analysing new DNS data obtained from lean hydrogen–air turbulent flames. Both aforementioned effects are observed by analysing the same DNS data provided that the conditioned profiles are sampled from the entire computational domain. On the contrary, the conditioned profiles sampled at the leading edge of the mean flame brush do not indicate the mitigation, but are significantly affected by differential diffusion phenomena, e.g. because reaction zones are highly curved at the leading edge. This observation is consistent with a significant increase in the computed turbulent burning velocity with decreasing Lewis number, with all the results considered jointly being consonant with the leading point concept of premixed turbulent combustion. The concept is further supported by comparing DNS data obtained by allowing for preferential diffusion solely for a single species, either atomic or molecular hydrogen.

Influence of Lewis Number on Heat Release Rate in Premixed Syngas Flames

2019 ◽  
Author(s):  
Kedar G. Bhide ◽  
Sheshadri Sreedhara
Keyword(s):  
Heat Release ◽  
Fuel Consumption ◽  
Heat Release Rate ◽  
Release Rate ◽  
Consumption Rate ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Attractive Alternative ◽  
Differential Diffusion ◽  
Fuel Consumption Rate

Abstract Syngas is an attractive alternative to currently popular hydrocarbon fuels due to its ability to be synthesized from multiple sources and lower carbon content. Direct Numerical Simulation (DNS) studies on premixed and non-premixed syngas flames have recently received attention. In this light, DNS of turbulent premixed syngas has been performed. Influence of turbulence and differential diffusion effects on chemical pathways of fuels like Hydrogen and methane has been studied in the past. Similar study on syngas flame has not been reported in the literature. Two cases with variation in the intensity of turbulence have been reported in this study. Effect of differential diffusion and turbulence on heat release rate and fuel consumption rate has been discussed. The behavior of heat release rate and fuel consumption rate was largely similar between laminar and turbulent flames considered in this study. Influence of species Lewis number was found to be more pronounced than that of turbulence.

An Experimental Study on Properties of Local Burning Velocity for Hydrogen Added Hydrocarbon Premixed Turbulent Flames

2011 ◽  
Author(s):  
Masaya Nakahara ◽  
Koichi Murakami ◽  
Jun Hashimoto ◽  
Atsushi Ishihara
Keyword(s):  
Burning Velocity ◽  
Quantitative Relationship ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Laminar Flames ◽  
Mean Values ◽  
Laser Tomography ◽  
Turbulence Condition ◽  
Turbulent Burning Velocity ◽  
Local Burning

This study is performed to investigate directly the local flame properties of turbulent propagating flames at the same weak turbulence condition (u′/SL0 = 1.4), in order to clarify basically the influence of the addition of hydrogen to methane or propane mixtures on its local burning velocity. The mixtures having nearly the same laminar burning velocity with different rates of addition of hydrogen δH are prepared. A two-dimensional sequential laser tomography technique is used to obtain the relationship between the flame shape and the flame displacement. The local flame displacement velocity SF is quantitatively obtained as the key parameters of the turbulent combustion. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effects of positive stretch and curvature on burning velocity. It was found that the trends of the mean values of measured SF with respect to δH, the total equivalence ratio Φ and fuel types corresponded well its turbulent burning velocity. The trend of the obtained Ma could explain the local burning velocity of turbulent flames only qualitatively. Based on the Ma, the local burning velocity at the part of turbulent flames with positive stretch and curvature, SLt, is estimated quantitatively. As a result, a quantitative relationship between the estimated SLt and the SF at positive stretch and curvature of turbulent flames could be observed for mixtures with increasing the Lewis number.

scholarly journals Isolating strain and curvature effects in premixed flame/vortex interactions

2017 ◽  
Vol 831 ◽  
pp. 618-654 ◽  
Author(s):  
F. Thiesset ◽  
F. Halter ◽  
C. Bariki ◽  
C. Lapeyre ◽  
C. Chauveau ◽  
...  
Keyword(s):  
Leading Edge ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Reliable Estimate ◽  
Vortex Interactions ◽  
Curvature Effects ◽  
Stretch Rate ◽  
Fuel Consumption Rate ◽  
Flame Response ◽  
Displacement Speed

This study focuses on the response of premixed flames to a transient hydrodynamic perturbation in an intermediate situation between laminar stretched flames and turbulent flames: an axisymmetric vortex interacting with a flame. The reasons motivating this choice are discussed in the framework of turbulent combustion models and flame response to the stretch rate. We experimentally quantify the dependence of the flame kinematic properties (displacement and consumption speeds) to geometrical scalars (stretch rate and curvature) in flames characterized by different effective Lewis numbers. Whilst the displacement speed can be readily measured using particle image velocimetry and tomographic diagnostics, providing a reliable estimate of the consumption speed from experiments remains particularly challenging. In the present work, a method based on a budget of fuel on a well chosen domain is proposed and validated both experimentally and numerically using two-dimensional direct numerical simulations of flame/vortex interactions. It is demonstrated that the Lewis number impact neither the geometrical nor the kinematic features of the flames, these quantities being much more influenced by the vortex intensity. While interacting with the vortex, the flame displacement (at an isotherm close to the leading edge) and consumption speeds are found to increase almost independently of the type of fuel. We show that the total stretch rate is not the only scalar quantity impacting the flame displacement and consumption speeds and that curvature has a significant influence. Experimental data are interpreted in the light of asymptotic theories revealing the existence of two distinct Markstein numbers, one characterizing the dependence of flame speed to curvature, the other to the total stretch rate. This theory appears to be well suited for representing the evolution of the displacement speed with respect to either the total stretch rate, curvature or strain rate. It also explains the limited dependence of the flame displacement speed to Lewis number and the strong correlation with curvature observed in the experiments. An explicit relationship between displacement and consumption speeds is also given, indicating that the fuel consumption rate is likely to be altered by both the total stretch rate and curvature.

Flame stretch rate as a determinant of turbulent burning velocity

1992 ◽  
Vol 338 (1650) ◽  
pp. 359-387 ◽  
Keyword(s):  
Burning Velocity ◽  
Turbulent Flame ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Stretch Rate ◽  
Dimensionless Groups ◽  
Flame Stretch ◽  
Experimental Values ◽  
Turbulent Burning Velocity

A rational basis for correlating turbulent burning velocities is shown to involve the product of the Karlovitz stretch factor and the Lewis number. A generalized expression is derived to show how flame stretch is related to the velocity field. A new dimensionless correlation of experimental values of turbulent burning velocities is presented. Dimensionless groups also are used in correlations of laminar and turbulent flame extinction stretch rates. A distribution function of stretch rates in turbulent flames, based on an earlier one of Yeung et al ., is proposed and the experimental data are well predicted by a theory based on flamelet extinction by flame stretch with this distribution. Uncertainties arise concerning the role of negative stretch rate. Laminar flamelet modelling of complex combustion appears to have a broader validity than might be expected and some explanation for this is offered.

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.

scholarly journals Two-Phase Dusty Fluid Flow Along a Rotating Axisymmetric Round-Nosed Body

2017 ◽  
Vol 139 (8) ◽  
Author(s):  
Sadia Siddiqa ◽  
Naheed Begum ◽  
M. A. Hossain ◽  
Rama Subba Reddy Gorla
Keyword(s):  
Boundary Layer ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Solid Particles ◽  
Dust Particles ◽  
Computational Domain ◽  
Two Phase ◽  
Carrier Fluid ◽  
Boundary Layer Equations ◽  
Implicit Finite Difference

This article is concerned with the class of solutions of gas boundary layer containing uniform, spherical solid particles over the surface of rotating axisymmetric round-nosed body. By using the method of transformed coordinates, the boundary layer equations for two-phase flow are mapped into a regular and stationary computational domain and then solved numerically by using implicit finite difference method. In this study, a rotating hemisphere is used as a particular example to elucidate the heat transfer mechanism near the surface of round-nosed bodies. We will investigate whether the presence of dust particles in carrier fluid disturbs the flow characteristics associated with rotating hemisphere or not. A comprehensive parametric analysis is presented to show the influence of the particle loading, the buoyancy ratio parameter, and the surface of rotating hemisphere on the numerical findings. In the absence of dust particles, the results are graphically compared with existing data in the open literature, and an excellent agreement has been found. It is noted that the concentration of dust particles’ parameter, Dρ, strongly influences the heat transport rate near the leading edge.

scholarly journals Grid Dependency Study for the NASA Rotor 37 Compressor Blade

1997 ◽  
Author(s):  
Andrea Arnone ◽  
Ennio Carnevale ◽  
Michele Marconcini
Keyword(s):  
Total Pressure ◽  
Turbulence Modeling ◽  
Leading Edge ◽  
Substantial Improvement ◽  
Compressor Blade ◽  
Computational Domain ◽  
Temperature Ratio ◽  
Turbulent Transition ◽  
Total Temperature ◽  
The Impact

The NASA Rotor 37 has been computed by several authors in the last few years with relative success. The aim of this work is to present a systematic grid dependency study in order to quantify the amount of uncertainty that comes from the grid density. The computational domain is divided onto several regions (i.e. leading edge, trailing edge, shear layer …) and for each of them, the impact of the grid density is investigated. By means of this analysis, substantial improvement has been obtained in the prediction of efficiency and exit angle. On the contrary, the improvement achieved in total pressure and total temperature ratio is less remarkable. It is believed that only after a systematic grid dependency study can the contribution of turbulence modeling, laminar-turbulent transition, and boundary conditions be analyzed with success.

scholarly journals Observations on Jet-Flame Blowout

2008 ◽  
Vol 2008 ◽  
pp. 1-7 ◽  
Author(s):  
N. J. Moore ◽  
J. L. McCraw ◽  
K. M. Lyons
Keyword(s):  
Reaction Zone ◽  
Mixture Fraction ◽  
Flame Structure ◽  
Leading Edge ◽  
Transient Behavior ◽  
Turbulent Flames ◽  
Ignition Source ◽  
Jet Flame ◽  
Physically Based ◽  
Air Diffusion

The mechanisms that cause jet-flame blowout, particularly in the presence of air coflow, are not completely understood. This work examines the role of fuel velocity and air coflow in the blowout phenomenon by examining the transient behavior of the reaction zoneat blowout. The results of video imaging of a lifted methane-air diffusion flame at near blowout conditions are presented. Two types of experiments are described. In the first investigation, a flame is established and stabilized at a known, predetermined downstream location with a constant coflow velocity, and then the fuel velocity is subsequently increased to cause blowout. In the other, an ignition source is used to maintain flame burning near blowout and the subsequent transient behavior to blowout upon removal of the ignition source is characterized. Data from both types of experiments are collected at various coflow and jet velocities. Images are used to ascertain the changes in the leading edge of the reaction zone prior to flame extinction that help to develop a physically-based model to describe jet-flame blowout. The data report that a consistent predictor of blowout is the prior disappearance of the axially oriented flame branch. This is witnessed despite a turbulent flames' inherent variable behavior. Interpretations are also made in the light of analytical mixture fraction expressions from the literature that support the notion that flame blowout occurs when the leading edge reaches the vicinity of the lean-limit contour, which coincides approximately with the conditions for loss of the axially oriented flame structure.

Differential Diffusion Modeling in LES/FDF Simulations of Turbulent Flames

AIAA Journal ◽  
2019 ◽  
Vol 57 (8) ◽  
pp. 3206-3212 ◽  
Author(s):  
Hua Zhou ◽  
Tianwei Yang ◽  
Zhuyin Ren
Keyword(s):  
Turbulent Flames ◽  
Diffusion Modeling ◽  
Differential Diffusion

scholarly journals Towards the distributed burning regime in turbulent premixed flames

2019 ◽  
Vol 871 ◽  
pp. 1-21 ◽  
Author(s):  
A. J. Aspden ◽  
M. S. Day ◽  
J. B. Bell
Keyword(s):  
Laminar Flame ◽  
Three Dimensional ◽  
Lewis Number ◽  
Reaction Rates ◽  
Turbulent Flames ◽  
Fuel Temperature ◽  
Transport Models ◽  
Eddy Viscosity Model ◽  
Order Of Magnitude ◽  
Number Species

Three-dimensional numerical simulations of canonical statistically steady, statistically planar turbulent flames have been used in an attempt to produce distributed burning in lean methane and hydrogen flames. Dilatation across the flame means that extremely large Karlovitz numbers are required; even at the extreme levels of turbulence studied (up to a Karlovitz number of 8767) distributed burning was only achieved in the hydrogen case. In this case, turbulence was found to broaden the reaction zone visually by around an order of magnitude, and thermodiffusive effects (typically present for lean hydrogen flames) were not observed. In the preheat zone, the species compositions differ considerably from those of one-dimensional flames based a number of different transport models (mixture averaged, unity Lewis number and a turbulent eddy viscosity model). The behaviour is a characteristic of turbulence dominating non-unity Lewis number species transport, and the distinct limit is again attributed to dilatation and its effect on the turbulence. Peak local reaction rates are found to be lower in the distributed case than in the lower Karlovitz cases but higher than in the laminar flame, which is attributed to effects that arise from the modified fuel-temperature distribution that results from turbulent mixing dominating low Lewis number thermodiffusive effects. Finally, approaches to achieve distributed burning at realisable conditions are discussed; factors that increase the likelihood of realising distributed burning are higher pressure, lower equivalence ratio, higher Lewis number and lower reactant temperature.

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