A New Technique for the Spectroscopic Examination of Flames at Normal Pressures

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N₂), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H₂) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H₂ S-branch. We demonstrate H₂ thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H₂ S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm^-1. We present a pure-rotational H₂ CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N₂ spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H₂ and N₂ CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H₂ and N₂ temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H₂ CARS thermometry for probing combustion reactions.

Design and characterization of a linear Hencken-type burner

We have designed and constructed a Hencken-type burner that produces a 38-mm-long linear laminar partially premixed co-flow diffusion flame. This burner was designed to produce a linear flame for studies of soot chemistry, combining the benefit of the conventional Hencken burner's laminar flames with the advantage of the slot burner's geometry for optical measurements requiring a long interaction distance. It is suitable for measurements using optical imaging diagnostics, line-of-sight optical techniques, or off-axis optical-scattering methods requiring either a long or short path length through the flame. This paper presents details of the design and operation of this new burner. We also provide characterization information for flames produced by this burner, including relative flow-field velocities obtained using hot-wire anemometry, temperatures along the centerline extracted using direct one-dimensional coherent Raman imaging, soot volume fractions along the centerline obtained using laser-induced incandescence and laser extinction, and transmission electron microscopy images of soot thermophoretically sampled from the flame.

Spectroscopic studies of low-pressure flames - V. Evidence for abnormally high electronic excitation

Spectrum-line reversal measurements, using lines of Fe, Pb, Na and Tl, have been made for the reaction zones of many flames. For most organic flames the reversal temperatures exceed theoretical maximum temperatures, the values rising very high for lines in the ultra-violet. Lines of Fe requiring up to at least 173 kcal./mole For their excitation are emitted from the reaction zones, but not from the interconal gases. Various hydrocarbons (CH 4 , C 2 H 4 , C 2 H 6 , C 2 H 2 , C 2 H 8 ), methyl alcohol, cyanogen and ammonia burning in premixed flames with oxygen, air or nitrous oxide all show this abnormally high electronic excitation, but flames of H 2 , CO, CS 2 and formaldehyde do not. The hydrogen/nitrous oxide flame does show some abnormal excitation, but of a different character. Diffusion flames at atmospheric pressure do not show the effect. Processes by which electronic excitation may occur are examined, and the cause of the abnormal excitation is discussed. The spectrum-line reversal method of measuring flame temperatures is criticized.

Predissociation in the spectrum of OH; the vibrational and rotational intensity distribution in flames

In the main 3064 Å system of OH higher rotational levels of bands with v' = 1 and all levels with v' ≥ 2 are affected by a weak predissociation. One spin component of the 2Ʃ + levels is more strongly affected than the other. Of the three molecular states, 4II, 4Ʃ ¯ and 2Ʃ ¯ which could conceivably cause the predissociation, only the 2Ʃ ¯ could produce this effect. The predissociation is observed in the OH bands from a discharge and from the reaction zone of an oxy-acetylene flame at low pressure, and also to some extent in a flame at atmospheric pressure. The predissociation, and its reverse process, a ‘pre-association’, may also affect the vibrational intensity distribution if there is departure from equilibrium. Striking anomalies of this type in hydrogen flames are interpreted in this way. For flames at 1 atm. pressure there appears to be an excess of free atoms in the flame, and at low-pressure emission of radiation from the flame disturbs the equilibrium.

Time resolved laser induced fluorescence of the NH radical in low pressure N(2)O flames

Total removal rate constants from the NH(A(3)II(i)) electronic state have been obtained in low pressure (~14-Torr) N(2)O flames. The fluorescence decay constants and quantum yields Phi depend on the temperature and composition at each position interrogated in the flame. In similar conditions, Phi for NH(A(3)II(u)) is significantly greater than that for other radicals CH(A(2)Delta and B(2)Sigma(-)) and OH(A(2)Sigma(+)). A small decrease (~5%) in the electronic removal is observed with rotational excitation increasing from N' = 2 to 12 in the A state. Estimates of collisional quenching cross sections at flame temperatures are made for the important combustion species, H(2)O, C(3)H(8), C(2)H(4), and C(2)H(2).