Dynamics and stability of lean-premixed swirl-stabilized combustion

A review of active control of combustion instabilities

This article contains a review of combustion instabilities coupled by pressure waves and of the related pulse combustor and active control principles and applications. The basic mechanisms leading to combustion oscillations are first identified from experimental evidence gathered in recent years. It is shown that in the presence of pressure waves, the flow features large scale motions which drive the instability. The dynamics of the flame is then dominated by processes of hydrodynamic instability, vortex roll-up, vortex interactions, front and reacting stream pulsations, periodic extinctions and reignitions, self acceleration. Knowledge accumulated on these various aspects has provided guidelines in combustor design. Progress accomplished in the special domain of pulse combustors is surveyed. These devices which operate under sustained pressure oscillations have been the subject of intensive research with as objective the understanding of the dynamics of the flow in various geometrical situations in order to improve the heat transfer efficiency and diminish the level of pollutant formation. While pulse combustors use pressure oscillations in normal operation, in most circumstances the presence of pressure waves causes problems of vibration, enhanced heat fluxes to the walls, cyclic fatigue, and in some cases important damage to the combustor. One generally wishes to eliminate or attenuate these waves. This is usually achieved by changes in the geometrical design and in the operation of the system. New possibilities based on active control have been investigated in recent years. The basic principles and the initial demonstrations of active instability control are first reviewed. It is then shown that such methods constitute a promising technology and that they are also a unique tool in the experimental investigation of combustion instability and pressure related phenomena.

Combustion instabilities coupled by pressure waves and their active control

Instantaneous Planar Measurements of Velocity and Large-Scale Vorticity and Strain Rate in an Engine Using Particle-Image Velocimetry

Chemiluminescence from species such as CH*, C2*, OH* and CO2* often are used as a quantitative diagnostic in experimental studies of premixed combustion. This paper reports results from a numerical investigation of CO2* chemiluminescence as a quantitative diagnostic in laminar and turbulent premixed flames. Calculations are carried out using a complex reaction mechanism for methane and propane and a model for CO2* chemiluminescence. Relationships between chemiluminescent intensity and both H-atom concentration and heat release rate are quantified as functions of dilution, equivalence ratio, steady and unsteady strain-rate. These relationships are monotonic, but not unique; they depend on which flame parameter is varied. However, the effect of unsteadiness on the relationship for a strained flame is negligible, and this allows the use of chemiluminescence-based diagnostics for measurement of relative H-atom concentration and relative heat release rates in unsteady laminar and turbulent premixed fl...

CO2* Chemiluminescence in Premixed Flames

Combustor performance enhancement through direct shear layer excitation

Although organized vortical motions have been observed in a variety of turbulent flames, there have been only a few experimental studies which have directly examined the structure of a flame imbedded in an unsteady vortical flow. In the present work, this problem is investigated using a laminar co-flowing jet flame in which a periodic vortical motion is induced by acoustic excitation of the fuel stream. A variety of optical planar imaging techniques are employed to define the instantaneous flame structure. Particle tracking is used to obtain the instantaneous two-dimensional velocity field. These measurements are combined to follow the evolution of the flame in the unsteady velocity field over one cycle of the excitation. The flame is observed to break up into a series of axisymmetric flamelets which convect and distort under the influence of buoyancy and the vortical flow. When the velocity field is viewed in a frame of reference moving with the flamelets the flow pattern is seen to consist of a pair of saddle points on either side of the flow centerline near the flame tip and a single saddle point on the centerline near the base of the flame. This latter saddle point is also evident in images of light scattered from TiO 2 particles added to the fuel stream. Entrained fluid is fed into a large axisymmetric vortex surrounding the base of the flame. Because of heat release, the flame acts as an effective volume source in the flow. A high strain rate exists on the centerline at the base of the flame, and planar laser-induced fluorescence measurements of the concentration of OH show local extinction of the flame.

U. Vandsburger

Papers

Combustor performance enhancement through direct shear layer excitation

An investigation of the structure of a laminar non-premixed flame in an unsteady vortical flow

Effect of fuel spray vaporization on the stability characteristics of a dump combustor