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

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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
LIF images to be recorded at any desired crank angles of the engine cycle. The integration time for the ICCD camera was set to 50 ns in order to minimize the flame chemiluminescence background. 200 300 400 500 600 700 800 900 1000 1100 1200 TDC 5 TDC 10 TDC 20 TDC 30 TDC Figure 9. CO TPLIF images at different crank angles. Single-shot CO PLIF images from the engine were recorded at subsequent crank angles, examples are shown in Figure 9. At TDC just before the ignition, a faint CO LIF image was recorded. This small amount of CO originates from the exhaust gas of the previous cycle, remembering this is a side valve engine with high amounts of residuals. At 5 and 10 CAD after TDC, the increased intensity and spread out CO LIF distribution reveal the flame location. At 20 and 30 CAD after TDC, the CO PLIF images show a more intense signal in the left part of the combustion chamber (furthest away from the spark plug) and only a weak signal in the right part indicating the fading away of the reaction zone of the flame. Compared with the flame chemiluminescence images shown in Figure 8, one can find that although a strong flame chemiluminescence was observed at 30 CAD after TDC the CO LIF had already decreased. It should be kept in mind that the recording of chemiluminescence is a line-of-sight technique integrating across the entire thickness of the combustion chamber whereas the PLIF technique monitors a thin slice in the middle of the combusting volume. The results indicate that the CO concentration reaches the highest value in the flame reaction zone in this high pressure engine combustion environment. This is in agreement with the expected scenario where CO is formed as an intermediate species when the fuel is decomposed and then consumed as the temperature increases. At high 5 temperature and pressure, the interference from photolysis of hot CO2 molecule [24] can be a problem in the CO LIF detection. From the fact that only weak CO LIF was detected at 30 TDC, where high temperature are revealed from the chemiluminescence measurement, one can judge that the hot CO2 photo-fragmentation are of little concern in this experiment. This might be due to the high nascent CO concentration in the engine flame. Intensity (arb. Units) Intensity (arb. units) Intensity (arb. units) Intentisty (arb. units) 6 x104 5 4 3 2 1 0 7 x104 6 5 4 3 2 1 0 6 x104 5 4 3 2 1 0 14 x104 12 10 8 6 4 2 scattered 460 nm laser light emission from C2 5 degree ATDC 15 degree ATDC scattered 460 nm laser light 25 degree ATDC scattered 460 nm laser light a b c 20 degree BTDC exhaust cycle scattered 460 nm laser light d 0 450 500 550 600 650 700 Wavelength (nm) Figure 10. Spectra of TPLIF of CO from engine chamber. In order to clarify any other possible interference in the recorded CO LIF images, spectral
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Investigation of Two-Photon Laser-Induced Fluorescence Detection ...

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