Acoustic Reynolds Stress: The Source of Coherent Structures During Combustion Instability - Novel Combustion Concepts for Sustainable Energy Development - page 357

usual approach seems to be adopted globally, particularly during the years of

economic downturn.
The present chapter is on combustion instability, which has been a problem
plaguing engine design, development and operation for several decades now. It is a
phenomenon wherein the interaction between the

'

ow processes such as
unsteady heat release, shedding of vortical structures, etc. and acoustics leads to
production, growth and sustenance of high amplitude sound. Some of the signifi-

fl uid fl

-
cant mechanisms of combustion instability involve vortex shedding (Schadow and
Gutmark 1992 ) and equivalence ratio

fl fluctuations (Lieuwen and Zinn 1998 ), both

leading to heat release rate fl fluctuations that act as a source to acoustic field.

Combustion instability has been a problem of the proverbial ' six blind men and

an elephant . The acousticians look at the combustion process merely as a source of

unsteady heat release. The combustion scientists investigate unsteady

'

fl ames

without regard to the excitation of acoustics in the engine subject to certain

boundary conditions. The unsteady fluid mechanics that is coupled with combustion

is usually left uncovered in this process.
The vortex shedding mechanism, in particular, for instance, is prevalent because
combustors usually adopt geometric constructs such as sudden expansions, bluff-
bodies or a strong swirl

fl

fl flow to create regions of fl flow recirculation that would

facilitate flame holding. However, such recirculation zones involve inherently

unsteady shear layers that oscillate and in turn, lead to fluctuating heat release. The
important aspect to consider here is the dynamics of combusting shear layers which
are likely to be very different from shear layer dynamics observed usually under
cold

fl

fl flow conditions. For instance, it is well known that in the wake fl flow of a bluff-

body flame holder, a Karman vortex street of alternating pairs of vortices is usually

observed under isothermal

fl

ow
structures in an otherwise turbulent wake. However, under statistically steady
combustion conditions, the Karman vortex street is replaced by extended shear
layers from the separation points due to the dilatation caused by the heat release in
the

fl flow conditions. These are the dominant large-scale fl

fl flame held by the bluff body. These extended shear layers exhibit Kelvin

-
Helmholtz instability waves that result in relatively small-scale vortical roll-ups as
the most dominant

flow conditions. On the
contrary, large-scale coherent structures (of the order of the Karman vortex street,
but symmetrical, in the case of bluff-body

fl flow structures under such reacting fl

flame holders) have been reported during
the advent of combustion instability in burners. The present work attempts to focus
on the origin or reappearance of such

fl

flow structures under combustion conditions
when intense acoustic excitation and feedback as during instability ensues.
The acoustic-

fl

ow interaction involved in combustion instability is gov-
erned by the compressible Navier

fl ame- fl

Stokes (NS) equations. Many investigators have
solved the compressible NS equations directly to study combustion instability in
practical combustors, e.g., (Poinsot and Candel 1988 ; Menon and Jou 1991 ).
Further, approaches such as URANS (Brookes et al. 2001 )/LES (Huang and Yang
2004 ) are also performed to compute

-

flows that are prone to combustion instability.
A hybrid approach involving compressible LES and a Helmholtz acoustic solver is
also reported (Roux et al. 2005 ; Selle et al. 2006 ).

fl