Investigation of Two-Photon Laser-Induced Fluorescence Detection ...

C 2 interferences
(c 1 Π g – b 1 Π u)
(d 3 Π g – a 3 Π u)
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
Distance (mm)
400
450
500
550
600
650
Wavelength (nm)
Figure 6. A CO fluorescence spectrum from a
methane/air (φ=1.5) burner is shown above. The C 2
lines around 474 nm and 520 nm are visible.
Engine measurements
The measurements in the cell and the premixed
flame were followed by measurements in a small
single-cylinder four-stroke Briggs & Stratton engine
[23]. The engine featured a side-valve design with the
spark plug located above the exhaust valve. The
original crankcase, cylinder liner and piston remained
unaltered, while the cylinder head was modified to
allow for optical access to the combustion chamber.
Three windows were mounted in the cylinder head
for this purpose. Two vertical windows, one on the
side of the combustion chamber, made it possible to
have a horizontal laser sheet passing through it. The
induced fluorescence was imaged through a third
window mounted horizontally above the two valves.
A picture taken from above, through the top window,
is shown in Figure 7. In this image the intake valve
can be seen to the left and the exhaust valve to the
right. The spark plug electrodes are also visible in the
lower right corner. The field of view for the ICCD
camera is also shown in this figure, where the outer
white frame illustrates the area covered during the
flame chemiluminescence imaging and the inner
frame illustrates the area covered during the CO LIF
imaging.
Isooctane was used as the fuel for the engine
measurements. Shown in Figure 8 are single-shot
images of the flame chemiluminescence. These
images are recorded at subsequent crank angle
positions to cover the different parts of the engine
cycle. The exposure time for the ICCD camera was
set to 5 µs, which corresponded to a crank shaft
rotation of less than 0.04 crank angle degrees (CAD).
At Top Dead Center (TDC), a developing flame
kernel originating from the vicinity of the spark plug
can be seen; 5 CAD after TDC, the flame is more
spread out from the spark plug; the flame then
continues to expand as expected and as shown by the
images recorded at 15 and 30 CAD after TDC.
4
Figure 7. Vision of the engine chamber from the top
window. Outer write frame indicate the chemiluminescence
collection area; inner write frame
indicates the LIF imaging area.
300 600 900 400 1000 1800 2400
TDC
15 TDC
5 TDC 30 TDC
Figure 8. Flame chemiluminescence emission taken
with 5 microsecond gate time.
During the engine LIF experiments a singlemode
Nd:YAG laser (Spectra Physics, PRO 290-10)
pumping a OPO laser system (Spectra Physics,
MOPO 730-10) was utilized to produce the required
230 nm laser radiation. When pumped with 550 mJ
per pulse at 355 nm, the OPO delivered 75 mJ per
pulse at 460 nm. After frequency doubling in a BBO
crystal, laser radiation at 230 nm with a pulse energy
of 15 mJ was obtained. For this setup the 230 nm
laser has an estimated linewidth of ~ 0.3 cm -1 . The
230 nm beam was formed into a 12 mm wide laser
sheet with a thickness of approximately 300 µm. The
laser sheet was sent through the combustion chamber
right above the valves as indicated in Figure 9. The
ICCD camera was used for detection of the LIF
through the top window perpendicular to the laser
sheet. A long-pass filter was used for suppressing the
scattered laser radiation while transmitting the redshifted
fluorescence. The engine was run at 1200 rpm
corresponding to a firing frequency of 10 Hz
matching the repetition rate of the laser system.
Setting the engine as master, a trigger signal for each
engine cycle, locked at a selected crank angle
position, was sent to a pulse-delay generator
(Stanford, DG535), which was used to synchronize
the laser system and the ICCD camera. This enabled