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

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knowledge no investigation at high pressure and engine applications have been reported. The aim of the present work was to investigate the applicability of CO detection with two-photon LIF in combustion engine. In order to understand the behaviour and achieve a quantitative interpretation of the CO LIF signal from the combustion engine, where both high pressure and high temperature presented, a pressure dependent measurement in a static cell and a flame measurement with different stoichiometries have been performed. Single shot, two dimensional CO LIF imaging in a test engine was finally demonstrated with exclusion of the potential interference from C2 and from photolysis of hot CO2. Figure 1. Schematic energy level diagram for carbon monoxide. Depicted also the two-photon excitation, the fluorescence decay and the collisioninduced triplet state transitions. Cell and flame measurements In order to investigate the potential for quantitative LIF measurements of CO in general and at elevated pressure in specific, it is necessary to control a range of properties affecting the relation between laser power and the generated signal level and the signal dependences on pressure and possible photolytic creation of C2 radicals. When probing CO, in addition to the more conventional quenching phenomenon (marked with a Q in Figure 1), properties like two-photon absorption, stimulated emission and ionization effects have to be taken into account, which are all dependent on the laser intensity. The laboratory experiments served to investigate these properties in simulated combustion environments, i.e. to investigate the dependencies on laser energy, pressure, temperature and stoichiometry, as well as, the influence from the presence of other species. A Nd:YAG (Continuum NY82-S)) pump dye laser (continuum ND60) system are utilized in the cell and the flame measurements. The dye laser (with a mixture of Rhodamine 590 and Rhodamine 610 dissolved in methanol) was pumped by the second 2 harmonic from the Nd:YAG laser, producing radiation at 587.2 nm. Frequency doubling in a KDPcrystal resulted in 293.6 nm. By mixing this with the fundamental 1064 nm from the Nd:YAG laser, approximately 4.5 mJ of the required radiation at 230.1 nm was generated. The investigated gas mixtures were kept in a stainless steel high pressure vessel equipped with quartz windows. The laser beam was focused with a single spherical lens through the cell with the focus point centred in the middle. The interaction region between the CO molecules and the laser beam was imaged with an image intensified CCD (ICCD) camera (Princeton Instruments, ICCD-5765S) perpendicular to the excitation laser beam. A long pass filter was placed in front of the ICCD to block the scattered light from the 230 nm excitation laser. Figure 2. Plot showing the CO LIF signal measured at twelve different pressures ranging from 0.1 to 3.0 bar. Shown in Figure 2 is the resulting CO LIF signal versus pressure when the cell was filled with pure carbon monoxide and the pressure varied from 0.1 bar to 3 bar. The laser was propagating from left to right and the focus point located at 0 mm. The absorption effect of the excitation laser as it transverse the cell was obvious as indicated by the decrease of the LIF signal along the laser beam and the absorption increase with the increased CO pressure in the cell. Note that even for a fix laser energy output the laser flux density will vary along the focal line, i.e. it will decrease with the distance to focus. As can be seen this results in a variation of the fluorescence intensity along the beam. The ionization potential of the CO molecule is 14.0139 eV. This gives that when the B 1 Σ + level is excited, absorption of one additional photon would be sufficient to ionize the molecule. The dip at the focus location is probably due to the increased influence of photo ionization. To understand this better, spatial resolved CO LIF measurement with 400 mbar pure CO in the cell was performed with varied excitation laser pulse energy. Shown in Figure 3 is an example of such a measurement with the laser pulse energy varied from 0.2 to 2.4 mJ. The CO LIF emission is highly
dependent on several parameters, like laser intensity, ionization, non-linear absorption which all contribute to the question of correct signal interpretation in an applied combustion situation. In order to understand the phenomena taking place and the interaction between these phenomena a simulation based on a four level rate equation system including photoionisation, quenching, two-photon absorption, and spontaneous emission was applied to calculate the spatial distribution of the CO LIF signal [22]. The thick line shown in Figure 3 is a simulation of the measured curves with 2.4 mJ per pulse the laser intensity. A general similarity between the simulated profile and the measured one was achieved, which indicates that the model are proper in prescribe the observed the phenomenon. Figure 3. The variation of the CO LIF intensity along the laser beam. The gas pressure of CO in the cell was held constant at 400 mbar, while the energy of the laser pulse was varied from 0.2 to 2.4 mJ. The thick line represents a simulated fluorescence signal along the laser beam with 2.4 mJ per pulse. Signal intensity (counts) 10 4 10 3 k = 2 k = 1 0.1 bar 0.2 bar 0.5 bar 1.0 bar 2.0 bar 3.0 bar 106 107 108 109 Laser fluence (W/cm2) Figure 4. Plot showing the measured LIF signal vs excitation laser fluxes at different CO pressures. Shown in Figure 4 are the LIF signal intensities as the laser energy was increased for different pressures of CO in the cell. Normally, the 3 fluorescence signal, S, in a two-photon excitation processes depends quadratically on the incident laser intensity, I, i.e. S ∼ I k , where k = 2. However, at higher laser energies photo-ionization of the CO molecule becomes significant. Moreover, the transition starts to saturate and also at higher laser energy two-photon absorption decreases the signal intensity. Hence, the fluorescence signal scales with k = 2 in the low laser intensity limit, and turns into k is close to one when the laser intensity is increased. If the laser intensity is further increased the signal goes through a maximum until the signal actually decreases as the laser intensity is increased further. Figure 5. LIF signal dependence on the buffer N2 pressures (100 mbar CO, N2 pressure varied). In order to estimate the delectability of using two-photon LIF at elevated pressure, e.g. in engines, measurements in the cell with and without buffer gases were carried out at pressures from 0.1 bar up to 8 bar. It is more realistic to investigate the CO LIF signal intensity while adding a buffer gas to a constant pressure of CO in the cell. Shown in Figure 5 is the signal distribution from CO at a pressure of 100 mbar while the pressure of the buffer gas, N2, was varied from 0 to 10 bar. As expected the signal decreases as the pressure is increased. Also the shape of the signal distribution changes as the pressure increases. In the flame investigations, the behaviour of spectral interference from non-resonantly excited C2 molecules, produced by photo-decomposition of fuel and fuel fragments, were studied at various stoichiometries and laser intensities. The first flame measurement was performed in a laminar, premixed, methane/air flame. During the flame measurements the cell was replaced by a premixed Bunsen-type conical burner, which has a prolonged intake duct in order to assure a laminar flow. Through a spectrometer (Acton, SpectraPro-150) equipped the ICCD camera, a spectral resolved CO LIF spectrum, as shown in Figure 6, was recorded for a Φ = 1.5 methane/air flame with 50 laser shots averaged. As shown in the Figure, some C2 interference lines are clearly shown.
Page 1: Investigation of Two-Photon Laser-I
Page 5 and 6: LIF images to be recorded at any de

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

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