Abstract:
A swirl stabilised dump combustor capable of imposing flow perturbations
creating combustion instabilities has been designed and commissioned. The
capability of supplying different fuel mixtures (methane hydrogen blends) has
been incorporated. Additional capability is the facility to preheat the combustion
air prior to chamber entry and to be able to introduce dilution air into the
chamber. The chamber itself is of fused silica quartz to allow non-intrusive
optical diagnostics.
High speed CH* Chemiluminescence has been performed to qualitative
characterise the unstable heat release rate of pure methane and methane
hydrogen blended flames to allow analysis of the mean deconvoluted flame
structure. High speed Stereoscopic Particle Imaging Velocimetry (SPIV) has been
used to acquire the flow field throughout the chamber and focusing upon the
Annulus entry. These diagnostics have been phase locked to the imposed
perturbation.
A selection of conditions is presented with three different perturbation
frequencies within the low frequency range. These reveal vastly different
reacting and flow field structures. The difference of structures is attributed to
behaviour of the IRZ (Internal Recirculation Zone) and CRZ (Corner
Recirculation Zone) in altering the flame shape. All conditions exhibited the
axisymmetric/bubble vortex breakdown mechanism responsible for stabilisation.
Both single cell and double cell structures were observed in the mean flow field
vector maps. The mechanism of oscillating heat release rate is attributed to
oscillations of flame surface area. Profiles of integrated heat release rate and
flame exhibit the same profile shape and behaviour correlating very well. The
inclusion of hydrogen had no quantifiable impact upon the mean reacting or flow
field structures using the current diagnostics. Investigation into the nature of the
turbulence of the shear layers close to the annulus is presented for three
perturbation frequencies. This highlighted periodic structures within the
turbulence corresponding to the imposed perturbation frequency. It was found
that excitation of both shear layers for all turbulent components was not always
true and depended upon the perturbation frequency and flow structure close to
the annulus. Two oppositely rotating vorticity structures were revealed attached
to the outer and inner circumference of the annulus. These structures protruded
into the chamber and spread radially. Frequency analysis of these two structures
revealed both were oscillating at the perturbation frequency indicating vorticity
shedding. The mean vorticity structures are shown to be influenced also by the
behaviour of the recirculation zones.