Ignition, Flame Growth and Extinction in Premixed Combustion
Dominated by a Strongly Oscillating Flow Field
Jimmy Olofsson1, Sven-Inge Möller2, Annika Lindholm2,3, Johan Hult2,4 and Marcus Aldén2
1Dantec Dynamics A/S, Skovlunde, Denmark; 2Lund University, Faculty of Engineering LTH, Lund, Sweden; 3Current adress: Siemens

Industrial Turbomachinery AB, Finspång, Sweden; 4Current adress: MAN Diesel & Turbo SE, Copenhagen, Denmark

Introduction

Helmholtz type pulse combustor test cases

In the search for environmentally friendlier combustion systems, the numberof applications operated in lean premixed mode has increased. As theperformance of these systems are pushed towards the operational limits, theprobability for combustion instabilities or combustion induced oscillations
increases significantly. Oscillations may severely impair the performance ofthe combustion system and can lead to mechanical failure due to vibrations
or increased heat transfer. These oscillations are utilized in applications
such as dump combustors for waste incineration and pulse combustors.

The combustor consists of a flapper valve, a combustion
chamber and a tail pipe. The pressure actuated flapper
valve positioned inside the inlet decoupler controls injection
and mixing of fresh air/fuel. Adjusting the position of thestagnation plate changes the operating characteristics interms of burner input, air flow, operating frequency andair/fuel ratio.

This study concerns experimental studies of the flame behaviour in a
Helmholtz type pulse combustor. The experimental data is used to verify
possible ignition delay due to fluid dynamic strain as well as to identify
possible reignition sources. Time-resolved Planar Laser-Induced
Fluorescence (PLIF) of OH and line-of-sight integrated chemiluminescence
images, supplemented by flow fields by means of Laser Doppler
Anemometry (LDA) are employed.

High-speed laser diagnostic set-up
OH PLIF and chemiluminescence images were recorded simultaneously.
OH was excited at 283 nm and fluorescence was detected at 309 nm. A
cluster of four frequency doubled Nd:YAG lasers aligned onto a single path,
were used to pump a tuneable dye laser to generate the excitation
wavelength. The time separation between the laser pulses was set to be 100– 325 µs, in order to fully capture the motion of the turbulent reacting flow. A
high-speed framing camera, consisting of eight intensified CCD detectors
was used for the image acquisition. Four CCDs were used to image OH
PLIF and the other four were used to image OH chemiluminescence. The
combustion chamber top view indicates the three planes where OH PLIF
was collected.

Case

12

Power Input (kW)

14.8

12.4

Operating freq. (Hz)6299

Stagnation plate position (mm)2513Equivalence ratio φ0.55

0.85

Pg (kPa) (gas supply)

1.25

1.05

Pid (kPa) (inlet decoupler)

1.07

1.23

Air inlet with fan
Inlet decoupler
with mixing valve
Volume: 25 l.

Inlet tube
Length: 210 mm.
Inner diam.: 20 mm.

Flue gas

Tail pipe
Length: 1340 mm. Exhaust decoupler
Inner diam.: 36 mm. Volume: 35 l.
Combustion chamber
Volume: 1.5 l.

Quartz windows

8080(mm)

Stagnation plate Ø 25 mm

150

Flow velocity fields superposed on OH LIF image data
Ensemble-averaged velocity fields from LDA superposed on measured instantaneous OH LIF at the centre plane, Case 1,
are shown for four different instants of the operating cycle (upper row). Also ensemble-averaged velocity fields aresuperposed on ensemble-averaged axial velocity fluctuations, v’’ax (lower row).
Case 1:t/τ = 0.025t/τ = 0.275

t/τ = 0.475

t/τ = 0.675

OH

10 m/sRelative OH concentration

Relative OH concentration

Relative OH concentration

Relative OH concentration

Axial velocity fluctuations

Axial velocity fluctuations

Axial velocity fluctuations

Axial velocity fluctuations

Peak of injection: Twovortices causing recirculation in the centre
back towards the stagnation plate. High velocity
fluctuations localized to theinlet jet and the downstream side of the vortex.

Ignition: The inlet jet has
slowed down but thecounter flow velocity in thecentre is close to its maximum. Homogenous velocity
fluctuations in the centre
indicates efficient mixing.

Peak of combustion:
Velocities are low and small
homogenous velocity fluctuations are observed inmost parts of the chamber.

v’’ax

A = centre plane
B = centre + 15 mm
C = centre + 30 mm

60 mm

Start of injection: The inlet
jet can be observed inareas with slightly higher
velocity fluctuations.

Time-resolved OH PLIF and Chemiluminescence imaging
The chemiluminescence images are line-of-sight integrated over the entire
depth of the combustion chamber. The OH images are planar andpresented at their actual position and size during the measurements.
Case 1:t/τ = 0t/τ = 0.3

Chem.

Sheet AOH

Sheet BOH
Sheet COHt/τ = 0.5t/τ = 0.7

Heat release distribution depends on inlet jet
velocity which significantly affects theinjection, mixing and ignition. At high inlet jetvelocities (Case 1) delayed ignition due to fluid
dynamic strain is seen. Pre-ignition occursthat is later extinguished. OH emanating frompre-ignition may play an important role in thesubsequent ignition. The ignition starts insmall islands with no distinguishable flame
front. After main ignition, combustion isobserved in a region extending over almost
the entire combustion chamber, except for theregion close to the inlet. A large amount of theheat release takes place further downstream
in the combustion chamber, no heat releaseoccurs in the inlet region.

At low inlet jet velocities (Case 2) the flame is located to a smaller part of thecombustion chamber near the inlet. The reaction zone is located along the edgesof the entraining air/fuel jet. The data also verifies the presence of a radical pool assuggested in earlier work as a possible ignition source. The flame front is almost
laminar, wrinkled but distinguishable. Later in the cycle the transport of air/fuel has
reached the centre and combustion takes place in an almost volumetric way.
Case 2:t/τ = 0Chem.

Sheet BOH
Sheet COHAcknowledgement

SWEDISH FOUNDATION for
STRATEGIC RESEARCH

t/τ = 0.3t/τ = 0.5t/τ = 0.7