Experimental measurement of atomic composition in sooting luminous flame by Laser-Induced Breakdown Spectroscopy☆
Introduction
Nitrogen oxide/soot particulate matter emission of Gasoline Direct Injection (GDI) engine and diesel engine severely depend on local temperature and equivalence ratio in the combustor [1], where they are strongly inhomogeneous and it is difficult to trace local and temporal properties. Thanks to enormous development of non-intrusive optical method using laser technologies for measurement in combusting flow field, we now have a lot of choices for spatially resolved measurements, e.g. Laser-Induced Fluorescensce (LIF), laser Raman scattering and many other applications. Laser-Induced Breakdown Spectroscopy (LIBS), which provides quantitative information on species concentration from the collected spectrum of laser plasma, has an advantage above other optical measurement methods in that it can simultaneously detect many species and their concentrations, as well as has much better temporal resolution with the order of micro to mili seconds [2,3].
Application of LIBS to combustion diagnostics has started only recently [[4], [5], [6], [7], [8], [9], [10]]: Ferioli et al. [5] utilized atomic emission of C, O and N to measure equivalence ratio of the engine exhaust. Phuoc and White [6] found a linear correlation of the intensity ratio H656nm/O777nm with local equivalence ratio in methane turbulent diffusion flame. Also, among several interesting works, J. Kiefer et al. [7] conducted temperature measurement utilizing the dependence of breakdown threshold on laser pulse energy. S. H. Lee et al. [8] applied LIBS measurement to gasoline droplet/spray flame. Another interesting application by B. McGann et al. [9] is to measure local fuel concentration in a supersonic combustor.
In the previous studies on flame including those described above, the correlation of the intensity ratios with local equivalence ratio Φ has been confirmed in the calibration experiment of unburned mixture extending to strongly fuel-rich composition Φ > 3–5, but actually applied flame mainly included non-luminous range of Φ < 2 without soot formation to avoid its background emission and impeding scattering of atomic/molecular emission. In situ (in flame) calibration of strongly fuel-rich flame containing soot radiation has little been addressed so far. Shi et al. [10] measured methane diffusion flame in a counter-flow arrangement. They made a calibration curve in intact mixture and compared with CHEMKIN simulation, resulting in an overall agreement. The agreement was but a little uncertain especially across the fuel-rich flame region, which was partly attributed to buoyancy. Quantitative validity should then be further pursed for improvement, but there has been little other effort to work on sooting flame. Blue flame has thus been the main concern in the LIBS measurement, while sooting flame has been always out of scope in the literature. Availability of LIBS in sooting flame is therefore remaining unclear so far, which motivated us to investigate LIBS measurement of fuel-rich region with soot formation [11,12]. Akihama et al. [11] conducted LIBS measurement in sooting counter-flow flame and achieved a good correlation of the line intensity ratios with local equivalence ratio. Iwata et al. [12] also evaluated the dependence on laser pulse energy. In situ (in flame) calibration using CHEMKIN simulation was conducted in these studies.
In this study, propane was chosen as a fuel representing many common combustion characteristics of hydrocarbon fuel. The range of local equivalence ratio of interest was taken to be roughly 0–4 which was controlled by supplying flow rate of propane/air in a counter-flow burner. In situ calibration was done with CHEMKIN simulation which provides atomic number ratios H/O and C/O as the direct indicators of local equivalence ratio. The effect of the choice of the number ratios, and the dependence on laser pulse energy were also evaluated. Another experiment supplying the same mixture from both nozzles at equivalence ratio Φ = 2.2 was conducted as the case of uniformly sooting field in predefined concentration to investigate the effect of soot radiation and verify the correlation obtained in the former case.
Section snippets
Counter-flow burner
Overall configuration of the present experimental setup is illustrated in schematic Fig. 1. A counter-flow burner with an exit diameter of 30 mm was designed. The nozzles face each other at a distance of 20 mm. Thermocouples are inserted along the centerline of the nozzles to measure the exit gas temperature. Stainless mesh and ceramic pebbles are inserted in each nozzle to rectify the outflow. A sample picture of actually observed flame in the experiment is shown in Fig. 2. A flat luminous
Calculation of flame structure
In the first place, one-dimensional flame calculation described above was done against the experimentally observed flame. Fig. 4 shows the calculated spatial distribution of temperature (red dashed line) and local equivalence ratio defined by C/O and H/O (solid lines). For comparison, observed flames in the corresponding experimental cases are shown as the clipped pictures around the centerline and rotated at a right angle for ease of direct comparison. It should be first emphasized again that
Conclusion
The present study investigated applicability of LIBS to sooting luminous flame using a counter-flow burner. Laser-induced plasma emission was collected to a spectrometer, whose spectrum was analyzed to measure the intensity ratio of atomic/molecular emission. H656nm/O777nm intensity ratio was applied in this study. Information on local equivalence ratio for comparison was provided by one-dimensional calculation by CHEMKIN-PRO including a detailed chemical kinetics of hydrocarbon-air combustion.
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This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.