scholarly journals STRUCTURE OF A FLAME FRONT PROPAGATING AGAINST THE FLOW NEAR A COLD WALL

2002 ◽  
Vol 12 (11) ◽  
pp. 2547-2555 ◽  
Author(s):  
VADIM N. KURDYUMOV ◽  
AMABLE LIÑÁN
Keyword(s):  
Flame Front ◽  
Velocity Gradient ◽  
Front Propagation ◽  
Lewis Number ◽  
Critical Value ◽  
Constant Density ◽  
Density Approximation ◽  
Cold Wall ◽  
Low Velocity ◽  
Velocity Region

The flashback or propagation of premixed flames against the flow of a reacting mixture, along the low velocity region near a cold wall, is investigated numerically. The analysis, carried out using the constant density approximation for an Arrhenius overall reaction, accounts for the effects of the Lewis number of the limiting reactant. Flame front propagation and flashback are only possible for values of the near wall velocity gradient below a critical value. The flame propagation becomes chaotic for small values of the Lewis number.

A Numerical Simulation to the Influence of Unsteady Strain Flow on Twin Premixed Flames

2019 ◽  
Vol 11 (3) ◽  
Author(s):  
Faisal Al-Malki
Keyword(s):  
Numerical Simulation ◽  
Finite Elements ◽  
Strain Rate ◽  
Premixed Flames ◽  
Lewis Number ◽  
Critical Value ◽  
The Other ◽  
Constant Density ◽  
Nonmonotonic Dependence

The aim of this paper was to examine the response of twin premixed flames formed in a counterflow configuration to the presence of an unsteady straining flow. We began by describing the problem mathematically using the thermodiffusive model with constant density and then adopted a finite elements approach to solve the problem numerically. The study has shown that the role of flow on flame propagation is determined by three main parameters, namely, flow amplitude A, strain rate ε, and fuel Lewis number LeF. For LeF ≥ 1, the flow is seen to promote flame extinction, while LeF < 1 the flow clearly enhances the flame reactivity. Qualitatively, it has been shown that for LeF = 1, there exists a critical value of A (that varies with ε) below which the reactivity decreases monotonically with A. For small LeF < 1, on the other hand, the reactivity was seen to increase with A. For LeF > 1, however, a nonmonotonic dependence, especially for small ε, is predicted.

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.

Metastability for a generalized Burgers equation with applications to propagating flame fronts

1999 ◽  
Vol 10 (1) ◽  
pp. 27-53 ◽  
Author(s):  
X. SUN ◽  
M. J. WARD
Keyword(s):  
Flame Front ◽  
Principal Eigenvalue ◽  
Vertical Channel ◽  
Front Propagation ◽  
Slow Motion ◽  
Diffusion Limit ◽  
Asymptotic Results ◽  
Small Diffusion ◽  
Generalized Burgers Equation ◽  
Exponentially Small

In the small diffusion limit ε→0, metastable dynamics is studied for the generalized Burgers problemformula hereHere u=u(x, t) and f(u) is smooth, convex, and satisfies f(0)=f′(0)=0. The choice f(u)=u2/2 has been shown previously to arise in connection with the physical problem of upward flame-front propagation in a vertical channel in a particular parameter regime. In this context, the shape y=y(x, t) of the flame-front interface satisfies u=−yx. For this problem, it is shown that the principal eigenvalue associated with the linearization around an equilibrium solution corresponding to a parabolic-shaped flame-front interface is exponentially small. This exponentially small eigenvalue then leads to a metastable behaviour for the time- dependent problem. This behaviour is studied quantitatively by deriving an asymptotic ordinary differential equation characterizing the slow motion of the tip location of a parabolic-shaped interface. Similar metastability results are obtained for more general f(u). These asymptotic results are shown to compare very favourably with full numerical computations.

scholarly journals Structure of accretion flows in the nova-like cataclysmic variable RW Tri

2020 ◽  
Vol 497 (2) ◽  
pp. 1475-1487
Author(s):  
G Subebekova ◽  
S Zharikov ◽  
G Tovmassian ◽  
V Neustroev ◽  
M Wolf ◽  
...  
Keyword(s):  
Binary Systems ◽  
Cataclysmic Variable ◽  
Low Velocity ◽  
Time Resolved ◽  
Narrow Component ◽  
Accretion Flows ◽  
Photometric Observations ◽  
Hα Emission ◽  
Usual Interpretation ◽  
Velocity Region

ABSTRACT We obtained photometric observations of the nova-like (NL) cataclysmic variable RW Tri and gathered all available AAVSO and other data from the literature. We determined the system parameters and found their uncertainties using the code developed by us to model the light curves of binary systems. New time-resolved optical spectroscopic observations of RW Tri were also obtained to study the properties of emission features produced by the system. The usual interpretation of the single-peaked emission lines in NL systems is related to the bi-conical wind from the accretion disc’s inner part. However, we found that the Hα emission profile is comprised of two components with different widths. We argue that the narrow component originates from the irradiated surface of the secondary, while the broader component’s source is an extended, low-velocity region in the outskirts of the accretion disc, located opposite to the collision point of the accretion stream and the disc. It appears to be a common feature for long-period NL systems – a point we discuss.

scholarly journals Flame Front Propagation in an Optical GDI Engine under Stoichiometric and Lean Burn Conditions

Energies ◽  
2017 ◽  
Vol 10 (9) ◽  
pp. 1337 ◽  
Author(s):  
Santiago Martinez ◽  
Adrian Irimescu ◽  
Simona Merola ◽  
Pedro Lacava ◽  
Pedro Curto-Riso
Keyword(s):  
Flame Front ◽  
Front Propagation ◽  
Lean Burn ◽  
Gdi Engine

Effect of Thermal Boundary Conditions on Propagation of Non-Equidiffusive Flames

2020 ◽  
Author(s):  
Vyacheslav (Slava) Akkerman
Keyword(s):  
Boundary Conditions ◽  
Flame Front ◽  
Surface Area ◽  
Thermal Conditions ◽  
Lewis Number ◽  
Wall Friction ◽  
Adiabatic Wall ◽  
Narrow Channels ◽  
Flame Instability ◽  
Aspect Ratios

Abstract Boundary conditions constitute one of the key factors influencing combustion in chambers with large aspect ratios such as narrow channels or pipes. Specifically, the flame shape and propagation velocity are impacted by wall friction and wall heat transfer. Both factors continuously influence the shape of the flame front, thereby resulting in its larger surface area as compared to a planar flame front. Such a corrugated flame consumes more fuel per unit time and thereby propagates faster than the planar flame at the same thermal-chemical conditions. Consequently, a flame accelerates due to the boundary conditions. In the recent years, there have been many studies scrutinizing the role of boundary conditions in flame acceleration scenario by means of analytical formulations, numerical studies or experimental measurements. However, the majority of these works was limited to equidiffusive flames, where the thermal-to-mass diffusivity ratio (the Lewis number; Le) is unity. In this respect, the present work removes this limitation by analyzing non-equidiffusive (Le &lt; 1 or Le &gt; 1) flames propagating in pipes of various widths. Specifically, a parametric study has been conducted by means of simulations of the basic hydrodynamic and combustion equations. In this particular study, two-dimensional channels with smooth walls and different thermal conditions such as isothermal and adiabatic walls, have been employed for various Lewis numbers in the range 0.2 ≤ Le ≤ 2.0, and for various Reynolds number associated with the flame propagation in the range 5 ≤ Re ≤ 30. As a result, a strong coupling between the wall conditions and the variations of the Lewis and Reynolds numbers is demonstrated. Specifically, it is observed that the increase in the Lewis number results in moderation of flame tip acceleration. It is also found that there is a change in the burning rate and surface area of the flame front at the lower Lewis numbers, where flames appear unstable against the diffusional-thermal flame instability. Moreover, a substantial difference between the cases of isothermal and adiabatic wall conditions is demonstrated.

scholarly journals Thermal theory of a laminar flame front near a cold wall

1953 ◽  
Vol 4 (1) ◽  
pp. 173-177 ◽  
Author(s):  
Theodore von Kármán ◽  
Gregorio Millán
Keyword(s):  
Flame Front ◽  
Laminar Flame ◽  
Cold Wall ◽  
Thermal Theory ◽  
Laminar Flame Front

A simultaneous inversion of seismic traveltimes and amplitudes for velocity and attenuation

Geophysics ◽  
1996 ◽  
Vol 61 (6) ◽  
pp. 1738-1757 ◽  
Author(s):  
Don W. Vasco ◽  
John E. Peterson ◽  
Ernest L. Majer
Keyword(s):  
Perturbation Approach ◽  
Well Log ◽  
Ray Theory ◽  
Fracture Zones ◽  
Low Velocity ◽  
Simultaneous Inversion ◽  
Attenuation Structure ◽  
Paraxial Ray ◽  
The Relationship ◽  
Velocity Region

It is possible to efficiently use traveltime and amplitude information to infer variations in velocity and Q. With little additional computation, terms accounting for source radiation pattern and receiver coupling may be included in the inversion. The methodology is based upon a perturbation approach to paraxial ray theory. The perturbation approach linearizes the relationship between velocity deviations and traveltime and amplitude anomalies. Using the technique, we infer the velocity and attenuation structure at a fractured granitic site near Raymond, California. A set of four well pairs are examined and each is found to contain two zones of strong attenuation. The velocity variations contain an upper low velocity region corresponding to the uppermost attenuating zone. The location of these zones agrees with independent well‐log and geophysical data. The velocity and attenuation anomalies appear to coincide with extensively fractured sections of the borehole and may indicate fracture zones rather than individual fractures.

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