Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 2
fundamental processes such as ignition, NO, and soot formation, and flame quenching.
Moreover, some turbulent flame models prescribe the turbulent burning velocity as a
function of laminar burning velocity. Thus, detailed information describing the
dependence of the laminar burning velocity, flame thickness, ignition temperature, heat
release rate and flame quenching on various system parameters can be a valuable
diagnostic and design aid.
tel-00623090, version 1 - 13 Sep 2011
There is a significant discrepancy in measuring burning velocities, which gives an
indication of the difficulties and uncertainties associated with experimental
determination of flame properties. In the light of the earlier experimental studies of
Markstein, (1964), the asymptotic analysis of Klimov, (1963), and the computations of
laminar flame structure with detailed chemical kinetics by several researchers, all of
which show the importance of the flame stretch rate (Dixon-Lewis, 1991). There can be
little doubt that this is often neglected key variable (Law, 1989). It follows that any
experimental or computed value of laminar burning velocity should be associated with
a value of the flame stretch rate. Ideally, the stretch-free value of the burning velocity
should be quoted and the influence of stretch rate upon this value should be indicated
by the value of the appropriated Markstein length.
For these reasons this chapter begins with the definition of stretch rate and the
corresponding Karlovitz and Markstein numbers. Following, the theory evolved with the
burning velocity determination are described with emphasis for the constant volume
and constant pressure methods due to be extensively used. This part of the chapter
ends with the flammability limits description as is another important parameter of
premixed laminar flames.
2.5.1 Flame stretch
A flame surface propagating in a uniform flow field is submitted to strain and curvature
effects leading to changes in the frontal area. Karlovitz et al., (1953) and Markstein,
(1964) initiated the study of stretched premixed flames and demonstrated the
importance of the aerodynamic stretching and the preferential diffusion on the flame
response in terms of flame front instability.
The flame stretch factor (κ) is defined as the relative rate of change of flame surface
area (A) (Williams, 1985):
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