Experimental Characterization of Methane-Air Flame Structures using Laser-Induced Breakdown Spectroscopy

Laser-induced Breakdown Spectroscopy (LIBS) is a highly sensitive technique for elemental analysis that can be used to obtain temporally and spatially resolved atomic species concentrations in combustion systems. In this paper LIBS is employed to quantify equivalence ratios in laminar premixed methane-air flames. A microplasma is generated by a pulsed Nd:YAG laser with pulse duration 5 ns operated at the fundamental wavelength of 1064nm. The intensities of the analytical spectral lines of hydrogen, nitrogen and oxygen atoms were measured. The spatial distribution of equivalence ratio is then quantified through analysis of the H/O and H/N atomic ratios. The sensitivity and accuracy of the technique is further discussed. INTRODUCTION The use of Laser Induced Breakdown Spectroscopy (LIBS) as an analytical tool has grown significantly over the past 10 years. Spectrochemical analysis using LIBS has proven to be extremely versatile, providing multi-element analysis in real time. LIBS can be regarded as a universal sampling, atomization, excitation and ionization source, since laser-induced plasmas can be produced in gases [1, 2] and liquids [3, 4] as well as on solid samples [5-7]. The basic features of the LIBS technique have been reviewed in several publications [e.g., 810]. Plasma characteristics and analytical performances of plasmas produced by laser strongly depend on experimental parameters such as the laser intensity, the laser wavelength and pulse duration, the properties of the material to analyze and also on the surrounding atmosphere [11]. Recently, the LIBS technique has also been applied to combustion systems. Phuoc and White [12] used simultaneous measurements of the Hα line at 656.3 nm and the Oxygen triplet near 777 nm in order to determine averaged equivalence ratio in non-reacting and reacting methane-air jets. Ferioli et al. [13] used LIBS to determine the averaged and time resolved equivalence ratio in the exhaust stream of a spark-ignition engine through measurement of C, O and N atomic lines. In the present work, the LIBS technique is used in order to obtain, real time and in situ, analytical spectral lines of the H, N and O atoms in laminar premixed methane-air flames and quantify their variation as a function of the equivalence ratio. The technique is further used to map a fuel-rich flame through use of the H/O and H/N atomic ratios. Joint Meeting of the Greek and Italian Sections of The Combustion Institute 2 EXPERIMENTAL The experimental arrangement used is schematically depicted in Fig. 1. The energy source used in our experiments was a Q-switched Nd:YAG laser delivering 5ns pulses, operating at its fundamental frequency at 1064 nm, with a repetition rate of 10 Hz. This laser can supply up to 500mJ per pulse. The energy was controlled by a previously calibrated energy meter. Sparks were produced by focusing the laser beam using a 20 cm focal length quartz lens. The light emitted from the plasma was collected by a 15 cm focal length quartz lens which was coupled with a quartz fiber bundle. The fiber bundle was transmitting the light to a 0.46-m spectrograph where the signal is spectrally resolved. The spectrograph was equipped with two interchangeable holographic gratings of 150 lines/mm and 600 lines/mm covering the spectral area 190-800 nm. An intensified 1024×1024 ICCD was used as the detector. In order to obtain an optimum signal to noise ratio, a delay of 1 μs and a gate width of 3 μs was used for all measurements. The gate delay and width were constantly monitored by a digital 500 MHz oscilloscope. Laminar premixed methane-air flames (φ = 0.5 – 1.6) were stabilised in a Bunsen-type burner with nozzle diameters in the range 8 – 20mm. Both air and methane were supplied from bottles and flow was regulated by calibrated flowmeters.