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Show This then provides the best overall mechanism to explain all the combustion properties at once. This mechanism is rapidly being adopted by combustion researchers and designers worldwide; continuous updates are available at a Website2 and the most recent innovations are described in a separate paper at this conference.3 It is important to select measured targets which test all facets of the mechanism. Three years ago the only available target for N O x production chemistry was our measurement of N O in a low pressure flame.4 This required adjustment of the C H + N 2 rate coefficient beyond what appeared realistic. W e undertook development of a n e w target, the measurement of absolute concentrations of the C H radical, reported at the last IGRC.5 In this work, w e showed that the problem was not the earlier GRI-Mech's prediction of N O formation rate, but rather an inability to correctly predict C H concentrations. This problem has been subsequently solved through a remeasurement of the C H + O 2 reaction rate at high temperature, crucial to a correct prediction of C R ABSOLUTE LASER-INDUCED FLUORESCENCE MEASUREMENTS OF FLAME RADICALS Laser-induced fluorescence (LEF) has become a mature technology for the study of combustion. In the last international Combustion Institute symposium,6 held in Naples, 4 7 % of the papers reporting experimental results used a laser technique, and nearly 4 0 % of these used LIF. LIF is a sensitive, selective means of detecting free radicals in many types of flame environments.7 It can be made highly spatially precise, an essential attribute to the tests of flame chemistry described here. In LIF, one tunes the laser to match an absorption line of the molecule of interest, promoting it to an electronically excited state, from which it radiates. That emitted fluorescence is detected as the signal. In the present work, w e make absolute measurements of the concentrations of the C H , N O , O H , and H C O radicals. In addition, relative measurements on different rotational lines in O H determine the temperature, which governs the distribution of population amongst rotational levels. The absolute measurements of the radicals require knowledge of all terms in the equation for signal strength S F SF = BILrTLNfBOFfl(Q/47t)eriV (1) Here, B is the Einstein absorption coefficient, II is the laser energy, T a linewidth integral reflecting overlap between laser and absorption lines, TT_ the temporal laser pulse length, N the radical concentration (the desired quantity), fr$ the fraction of the total radical population in the absorbing level, O the fluorescence quantum yield which includes the Einstein emission rate, predissociation and collisional rates, Fn the detected fraction of fluorescence with the experimental detection spectral bandwidth, Q. the detection solid angle, e and rj transmission and photoelectron efficiencies, and V the interaction volume observed. Many of these quantities can be determined experimentally or taken from the literature; a detailed discussion in the case of C H (including extensive studies to establish new accurate values for Einstein A and B coefficients) is given in Ref. 5. <E> is a quantity that varies from place to place in the flame; its determination is discussed below. The last terms, (Q/47c)enV, are crucial to accurate absolute measurements; the approach used to determine this detection efficiency and flame volume is quite different for each of the four radicals, and is discussed below. EXPERIMENTAL DETAILS All of the experiments were conducted on a low pressure flat flame porous plug burner (McKenna). Flames of different mixing ratio were burned at pressures ranging from 15 to 30 Torr. The stoichiometry is determined by gas flow controllers and the chamber pressure is stabilized with feedback control of the conductance of a valve in the exhaust line. The 'air" is made up of separate N 2 and O 2 inlets; w e sometimes find O2/N2 ratios slightly different from air are necessary for achieving stable flames at the lowest pressures. For the study of reburn 3 |
