Simultaneous PIV, OH and Fuel PLIF Measurements in a Low-swirl
Stratified Turbulent Lean Premixed Flame
P. Petersson1, R.Collin2, A. Lantz2, M. Aldén2
1Dantec Dynamics A/S, DK-2740, Skovlunde, Denmark
2Division of Combustion Physics, Lund University, Sweden

Introduction

Burner and operating conditions

This work is part of a ongoing collaborative investigation between
computational and experimental combustion scientists . The focuse is onlaser diagnostics in turbulent flames and state-of-the-art numericalsimulations such as combustion Large Eddy Simulations (LES).

Air and fuel (methane) enters in a lower mixing chamber. Before reaching theswirler, the premixed gas passes through a perforated plate which breaks downlarge flow structures generated upstream. The low swirl is created by outer swirl
vanes, while the turbulent center flow is created by a perforated plate.

 Lean-premixed low swirl flames are suitable benchmarks fortesting numerical simulations such as combustion LES.

Measurements were carriedout for two different operating
conditions.

 There is a lack of comprehensive experimental data including
multi-scalar diagnostics and flow field information for these flows.

Nozzle The selected flame is a relevant premixed target flame, because
the core of the flame can reach the distributed reaction regime
providing a good test of turbulence/flame/chemistry interactions.

Swirler

Re~30000
φ= 0.62
P= 41kW The measurements presented here and in articles have been used
to validate simulations performed at Lund University and at several
other institutes.
 The low swirl burner investigation was originally a joint effort
between TU Darmstad, Lund University Faculty of Engineering,and Lawrence Berkeley Laboratories.

co-flow chamber
The lifted low-swirl flameand laser sheet for the PIV.

Simultaneous velocity and scalar fields

Set-up for laser diagnostics
Two separate laser systems were used for the simultaneous
PIV/PLIF measurements. The counter-propagating laser
beams were formed into overlapping sheets to illuminate aplanar region of the flame, crossing the vertical centreline ofthe burner

532

Particle Image Velocimetry (PIV)For PIV the beams from two frequency doubled Nd:YAGlasers were overlapped to provide successive Miescattering from the seed particles, ~1 µm ZrSiO4. A laser
pulse separation of 30 µs, was required to capture thehigh axial and tangential (out-of-plane) velocities in theouter part of the flame.

Re~20000
φ= 0.62
P= 27kW

Planar Laser-Induced Fluorescence (PLIF)
To achieve simultaneous PLIF from OH andacetone (fuel tracer) a dye laser, pumped witha frequency doubled Nd:YAG laser was used.
Acetone has a broad absorption peak andwas excited using the same excitation
wavelength as used for OH, 283 nm.

The image shows a cross-section through the flame visualizing the fuel-distribution(below and outside the flame), the burnt region and the wrinkeld flame front (OH) andthe superimposed diverging flow-field from PIV.

The resulting fluorescence from the acetone
was emitted in the blue band between 350500 nm. The OH fluorescence was detected
at 309 nm.
Two image-intensified CCD detectors with UV
lenses and were positioned at 90-degree
angle to collect the LIF signals. By using ahigh-reflective band-pass mirror centered at308nm and appropiate filters on the CCDs thetwo LIF signals could be separated.

Flame front location and fuel distribution

The maximum gradient of the OH
signal, giving an approximate
representation of the instantaneous
flame front location.

A combined acetone (fuel tracer) and OH PLIF image inthe low-swirl flame. The fuel distribution is uniform in thelower central part of the fuel stream whereas towards theouter parts the entrainment of air is visible, partly diluting
the mixture and creating the stratified flame conditions.
The acetone concentration was ~0.3% – a compromise
between a sufficiently high LIF signal and a minimum
influence on the flame.

Flame Stabilization Mechanisms
A strong shear is created on both (inner region and co-flow region) sides of the outerhigh velocity swirling flow region. The leading edge flame front is observed to exhibit
large-scale flame front wrinkling resulting in a W-shaped front. This structure isformed due to the interaction of flame front with the large-scale flow motion in theinner low speed zone and the outer high-speed shear-layer of the burner and arebelieved to be an important part of the stabilization of the present flame. This shows
the importance of resolving the dynamics of the flow field correctly to capture theflame stabilization mechanisms. Future work includes combined high-speed
measurements of PIV and PLIF to fully investigate the stabilization mechanism.

Conclusions
The preheat- and reaction layer
approx. visualized by the non-signal
region between acetone and OH.
Acetone is totally oxidized attemperatures above ~1000K.

The combined images from the three techniques gave increased insight in the flame
behaviour, including flow flame interaction, air entrainment and occurrence of “islands”
of reactants in the product region. The fuel distribution was found uniform in the lower
central part of the fuel stream whereas towards the outer parts the entrainment of air isvisible, partly diluting the mixture – creating the stratified flame conditions.
The simultaneous PIV/LIF images indicate that large-scale vortex structures had an
important influence on the flame stabilization mechanism. This was also confirmed byLarge Eddy Simulations performed at Lund University.

Acknowledgement

SWEDISH FOUNDATION for
STRATEGIC RESEARCH