Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

However, there remains no consensus regarding the stabilization mechanism. Proposed models differ primarily on the role of premixing
at the flame front. Given a jet of undiluted fuel issuing into air, it is undisputed that some degree of fuel-air premixing
occurs upstream of the lifted flame. The stabilization theory of Vanquickenbourne and van Tiggelen assumed full premixedness
of fuel and air at the stabilization point, with stabilization occurring through a balance of the local jet axial velocity and turbulent
flame speed. Subsequently, Peters and Williams argued that, in axisymmetric turbulent jets, insufficient molecular mixing occurs
upstream of the flame front to support the notion of premixed flame propagation. Instead, stabilization was proposed to be governed
by the quenching of thin, laminar diffusion flamelets. Muller et al. later extended this work to consider partially premixed
flamelets, concluding again that flamelet quenching was important to flame stabilization. Recently, triple flame theories have been
applied to the stabilization problem. Essential to the formation of the triple flame structure is a gradient in the fuel mixture fraction
profile normal to the flow direction, ranging from fuel-rich to fuel-lean conditions across the profile. On either side of the stoichiometric
point, a premixed flame branch forms. The excess fuel and oxidizer from the rich and lean branches, respectively, then burn
as a downstream diffusion flame. These three structures - the rich and lean premixed branches and the diffusion tail motivate the
’triple flame’ nomenclature. In the idealized case of a uniform incoming velocity profile, the premixed branches present a convex
surface to the flow, with the fuel-rich and fuel-lean sides receding owing to the reduced flame speed with departure from stoichiometry.
The addition of the diffusion tail downstream of this convex surface results in the characteristic triple flame shape, which
resembles a boat anchor. In a lifted turbulent jet flame, however, the incoming velocity profile can be expected to be highly nonuniform.
Veynante et al. computed triple flames with vortices superposed on the incoming velocity profiles. Under these conditions
the flame branches are highly distorted from the idealized shape. For sufficiently high strain rates, one of the premixed
branches may be extinguished while the other branches continue to burn. Because of these departures from the idealized triple
flame structure, the term ’leading-edge flame’or ’edge flame’ is preferred when describing flame stabilization involving upstream
partial premixing with a trailing diffusion flame branch.
Author
Jet Flow; Diffusion Flames; Flame Stability; Premixed Flames
20010022639 Stanford Univ., Center for Turbulence Research, Stanford, CA USA
Stochastic Modeling of Scalar Dissipation Rate Fluctuations in Non-Premixed Turbulent Combustion
Pitsch, Heinz, Stanford Univ., USA; Fedotov, Sergei, Manchester Univ., UK; Annual Research Briefs - 2000: Center for Turbulence
Research; December 2000, pp. 91-103; In English; See also 20010022631; No Copyright; Avail: CASI; A03, Hardcopy;
A03, Microfiche
In non-premixed combustion chemical reactions take place when fuel and oxidizer mix on a molecular level. The rate of
molecular mixing can be expressed by the scalar dissipation rate, which is for the mixture fraction Z defined as chi = 2D(sub z)(del
Z)(exp 2), where D(sub Z) is the diffusion coefficient of the mixture fraction. The scalar dissipation rate appears in many models
for turbulent non-premixed combustion as, for instance, the flamelet model, the Conditional Moment Closure (CMC) model, or
the compositional pdf model. In common technical applications, it has been found that if the scalar dissipation rates are much lower
than the extinction limit, fluctuations of this quantity caused by the turbulence do not influence the combustion process. However,
it has been concluded from many experimental and theoretical studies that there is a strong influence of these fluctuations if conditions
close to extinction or auto-ignition are considered. For instance, in a system where the scalar dissipation rate is high enough
to prohibit ignition, random fluctuations might lead to rare events with scalar dissipation rates lower than the ignition limit, which
could cause the transition of the whole system to a burning state. In this study, we investigate the influence of random scalar dissipation
rate fluctuations in non-premixed combustion problems using the unsteady flamelet equations. These equations include
the influence of the scalar dissipation rate and have also been shown to provide very reasonable predictions for non-premixed
turbulent combustion in a variety of technical applications. However, it is clear that these equations are actually not capable of
describing all of the features which might occur in turbulent non-premixed flames. For instance, in jet diffusion flames, local
extinction events might occur close to the nozzle because of high scalar dissipation rates. These extinguished spots might reignite
downstream, not by auto-ignition, but by heat conduction and diffusive mass exchange with the still burning surroundings. It
should be kept in mind that the motivation in this work is not to predict actual turbulent reacting flows, but to study the dynamical
system defined by the equations described in the following section. The advantage of the present simplified approach allows a
study of the extinction process isolated from auto-ignition and re-ignition events.
Author
Turbulent Combustion; Dissipation; Mathematical Models; Stochastic Processes; Flame Stability; Turbulent Flames; Fuel Combustion
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