Static and Dynamic Combustion stability

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3.1.1
Static and Dynamic
Combustion stability
Timothy C. Lieuwen
Associate Professor
School of Aerospace Engineering
Georgia Institute of Technology
Atlanta, GA 30332-0150
email: tim.lieuwen@aerospacegatech.edu
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3.1.1-1 Introduction
The objective of this article is to provide the reader
with some background on blowoff and combustion instability,
often referred to as a combustor’s “static stability” and “dynamic
stability”. In particular, this chapter will focus upon this
phenomenon in lean, premixed combustion systems operating
with any of a variety of fuels, such as natural gas or synthetic-
gas.
Blowoff refers to the fl ame physically leaving the
combustor and “blowing out” of the combustor. This issue is
often referred to as “static stability”. Blowoff occurs when
the fl ame cannot be anchored in the combustor. Combustion
instability, or “dynamic instabilities” refer to damaging
oscillations driven by fl uctuations in the combustion heat release
rate. These oscillations cause wear and damage to combustor
components and, in extreme cases, can cause liberation of pieces
into the hot gas path and resulting damaging to downstream
turbine components.
3.1.1-2 Static Stability
As the propagation speed of essentially all fl ames is
substantially lower than fl ow velocities in realistic systems,
special fl ame stabilization systems are necessary to anchor the
fl ame. These include rapid expansions or bluff bodies in the fl ow,
so that there is a re-circulating fl ow fi eld that recirculates hot
products back to the incoming reactants. Swirling combustors
introduce this recirculation with purely aerodynamic means - the
fl ow actually reverses direction and forms a recirculation bubble
when the fl uid has a suffi cient swirl number, a phenomenon
referred to as “vortex breakdown”.
Whatever the stabilization method, a fl ame can only be
stabilized in a combustor over a certain range of conditions, even
if those conditions lie within its fl ammability limits. For example,
at a fi xed stoichiometry, as the fl ow velocity is increased, at some
point the fl ame will not be able to remain anchored but will blow
off. Alternatively, at a fi xed fl ow velocity, as the equivalence
ratio is decreased, at some point the fl ame blows off.
Predicting blowout behavior is complicated by a lack
of understanding of the fl ame characteristics at the stabilization
point. Nonetheless, empirically anchored phenomenological
methods for correlating blowout behavior have been reasonably
successful. Most approaches consider the ratio of two time
scales: a chemical kinetic time and residence time, τchem/τ
.
res
The chemical time characterizes how much time is required
for the reaction while the residence time characterizes the time
which the reactants reside in the reaction zone1. This ratio is
often referred to as a combustor loading parameter. Simply
put, if this residence time is shorter than the chemical time, the
fl ame will blow off. It must be emphasized that the detailed
fl ow and chemical processes are much more complex than this
simple picture might suggest; nonetheless, more sophisticated
approaches generally reduce to a correlation of this form.
When applied to blowoff limits of premixed fl ames,
this chemical time can be estimated as: