Theory of attached and lifted diffusion flames

Wichman, Indrek S.; Ramadan, Bassem H.

doi:10.1063/1.869829

Cited by 22 publications

(8 citation statements)

References 23 publications

(51 reference statements)

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“…To elucidate flame stability mechanisms, the structure of the flame-stabilizing region must be known. Numerous studies of the structure and stability of laminar diffusion flames have been conducted in both normal earth gravity (1g) and microgravity (lg) [1][2][3][4][5][6][7][8][9][10][11][12]. Although several attempts [1][2][3] were made to reveal the internal structure of the flame-stabilizing region experimentally, most previous works were limited to overall flame characteristics.…”

Section: Introductionmentioning

confidence: 99%

Further studies of the reaction kernel structure and stabilization of jet diffusion flames

Takahashi

Katta

2005

Proceedings of the Combustion Institute

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Section: Introductionmentioning

confidence: 99%

Further studies of the reaction kernel structure and stabilization of jet diffusion flames

Takahashi

Katta

2005

Proceedings of the Combustion Institute

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“…Since this is a difficult quantity to calculate, even for a quiescent gas in an idealized geometry , here the influences of streamwise convection are neglected and the quenching characteristics of the flame tip are calculated in the absence of these real effects. Such calculations have been performed previously by Wichman & Ramadan (1998), Wichman et al (1999) and Wichman (1999). For the flame leading edge (tip) characteristic length scale (figure 1), we have (4.13) where the dimensionless multiplicative constant is of order unity.…”

Section: (B) Microstructure Of a Spreading Diffusion Flamementioning

confidence: 99%

“…The latter varies most near ξ = −Δ (figures 5 and 6). In particular, the mixing of the reactants near the flame leading edge and in the quench layer should resemble the distribution described in Wichman & Ramadan (1998), where an onion-shaped intermeshing set of isochors indicated diffusive mixing. Without this mixing, the flame-tip structure, consisting of a 'terminated' gas-phase flame with its point of extremely high reactivity 5 located almost exactly at its termination point, would not be possible.…”

Section: (B) Microstructure Of a Spreading Diffusion Flamementioning

confidence: 99%

Theoretical and numerical analysis of a spreading opposed-flow diffusion flame

Yang

Wichman

2009

Proc. R. Soc. A.

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This article describes the macroscopic and microscopic features of flames spreading over solid-fuel surfaces by examining and comparing three models. The first model examines ignition and flame spread over a solid-fuel surface using a two-dimensional numerical simulation code. This model employs variable density, variable thermophysical properties and one-step global finite-rate chemistry. The second model, a macroscopic 'field' model, is solved in terms of the mixture fraction (Z ) and total enthalpy (H ) functions. Comparisons are made with numerical predictions for primitive quantities: temperature, species distributions and velocity fields; and derived quantities: heat flux, mass flux, mixture fraction, enthalpy function and flame stretch rate. The third model yields a 'localized' flame structure description near the flame attachment point. Theoretical formulas are produced for the quenching distance, the leading edge heat flux, and the flame structure, as characterized by reactivity, temperature field and species distributions. The analytical predictions are compared with numerical simulations to derive flame microstructure scaling parameters.

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“…This radical flux is believed to increase the chemical reaction rates for important reaction steps near a stabilization point that is located in a small premixing zone. Wichman and Ramadan [6] state that, on the contrary, upstream radical transport is not a necessary condition, and that flame stabilization can result just due to the existence of a premixing zone of sufficient reactivity. Chung and Lee [7] offer two examples of nonpremixed flame stabilization.…”

Section: Introductionmentioning

confidence: 99%

On the similitude between lifted and burner-stabilized triple flames - A numerical and experimental investigation

Azzoni

Ratti

Puri

2000

38th Aerospace Sciences Meeting and Exhibit

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We have investigated lifted triple flames and addressed issues related to flame stabilization. The stabilization of nonpremixed flames has been argued to result due to the existence of a premixing zone of sufficient reactivity, which causes propagating premixed reaction zones to anchor a nonpremixed zone. We first validate our simulations with detailed measurements in more tractable methane-air burner-stabilized flames. Thereafter, we simulate lifted flames without significantly modifying the boundary conditions used for investigating the burner-stabilized flames. The similarities and differences between the structures of lifted and burner-stabilized flames are elucidated, and the role of the laminar flame speed in the stabilization of lifted triple flames is characterized. The reaction zone topography in the flame is as follows. The flame consists of an outer lean premixed reaction zone, an inner rich premixed reaction zone, and a nonpremixed reaction zone where partially oxidized fuel and oxidizer (from the rich and lean premixed reaction zones, respectively) mix in stoichiometric proportion and thereafter burn. The region with the highest temperatures lies between the inner premixed and the central nonpremixed reaction zone. The heat released in the reaction zones is transported both upstream (by diffusion) and downstream to other portions of the flame. Measured and simulated species concentration profiles of reactant (O 2 , CH 4 ) consumption, intermediate (CO, H 2 ) formation followed by intermediate consumption and product (CO 2 , H 2 O) formation are presented. A lifted flame is simulated by conceptualizing a splitter wall of infinitesimal thickness. The flame liftoff increases the height of the inner premixed reaction zone due to the modification of the upstream flow field. However, both the lifted and burner-stabilized flames exhibit remarkable similarity with respect to the shapes and separation distances regarding the three reaction zones. The heat-release distribution and the scalar profiles are also virtually identical for the lifted and burner-stabilized flames in mixture fraction space and attest to the similitude between the burner-stabilized and lifted flames. In the lifted flame, the velocity field diverges upstream of the flame, causing the velocity to reach a minimum value at the triple point. The streamwise velocity at the triple point is Ϸ0.45 m s Ϫ1 (in accord with the propagation speed for stoichiometric methane-air flame), whereas the velocity upstream of the triple point equals 0.7 m s Ϫ1 , which is in excess of the unstretched flame propagation speed. This is in agreement with measurements reported by other investigators. In future work we will address the behavior of this velocity as the equivalence ratio, the inlet velocity profile, and inlet mixture fraction are changed.

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Theory of attached and lifted diffusion flames

Cited by 22 publications

References 23 publications

Further studies of the reaction kernel structure and stabilization of jet diffusion flames

Further studies of the reaction kernel structure and stabilization of jet diffusion flames

Theoretical and numerical analysis of a spreading opposed-flow diffusion flame

On the similitude between lifted and burner-stabilized triple flames - A numerical and experimental investigation

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