Edward C. Rea

Shock-tube study of pressure broadening of the A2∑+ - X2Π (0,0) band of OH by Ar and N2

Abstract A rapid-tuning, frequency-doubled ring dye laser was used to make fully-resolved lineshape measurements of OH generated behind reflected shock waves in H 2 -O 2 -Ar and H 2 -O 2 -N 2 mixtures. Values of the collisional linewidth were deduced by fitting the observed lineshapes to Voigt profiles. Lines corresponding to transitions with lower-state rotational quantum numbers ranging from 1.5 to 17.5 in the A 2 ∑ + - X 2 Π (0,0) band near 307 nm were probed in order to evaluate a possible J ″-dependence in the observed broadening. The temperature range investigated was 1400–4100 K. The collisional broadening parameter, 2γ, was measured at fixed temperature and did not exhibit a strong dependence on the rotational quantum number for either Ar or N 2 . The temperature dependence of 2γ for specific J ″-values was measured and fit to a power law with an average value of the temperature exponent of 0.8 for both species.

Laser-induced fluorescence technique for velocity field measurements in subsonic gas flows

A nonintrusive optical technique is reported for multiple-point velocity measurements in subsonic flows. The technique is based on the detection of fluorescence from a Doppler-shifted absorption line of seeded iodine molecules excited at a laser frequency fixed in the wing of the line. Counterpropagating laser sheets are used to illuminate the flow, in the present case a nitrogen round jet, thereby eliminating the need for an unshifted reference signal. The fluorescence is detected simultaneously at 10,000 points in a plane of the flow using a 100 x 100 element photodiode-array camera. The velocity at each point is computed from four successive camera frames, each recorded with a different beam direction. The measured mean velocities between 5 and 50 m/sec agree well with data from the literature.

Temperature measurements in shock tubes using a laser-based absorption technique

A laser-based technique for monitoring temperature in shock tube flows is reported. The method employs a rapid-tuning ring-dye laser to record the fully resolved absorption spectrum of the R(1)(7) and R(1)(11) lines of OH near 306.5 nm. The temperature is inferred from the ratio of the peak absorption coefficients of the two lines. Comparisons with calculated reflected shock temperatures show agreement within +/-3% for the 1300-3500 K range.

Collisional broadening of the A2Σ+ ← X2Π(0,0) band of OH by H2O and CO2 in atmospheric-pressure flames

Abstract Absorption lineshapes of OH found in hot post-flame gases were measured using a rapid-tuning single-mode laser. Collisional linewidths due to broadening by H 2 O and CO 2 were determined by fitting the observed lineshapes to standardized Voigt profiles. Lines corresponding to transitions with ground-state angular momentum quantum numbers ( J ″-values) ranging from 1.5 to 17.5 in the R 1 -branch of the A 2 Σ + ← X 2 Π (0,0) band near 307 nm were probed. The temperature-dependence of collisional broadening was investigated over the range 1470–2370 K.

Rapid laser-wavelength modulation spectroscopy used as a fast temperature measurement technique in hydrocarbon combustion

A fast temperature measurement technique is described which is suitable for use in dynamic reacting gases, particularly in situations which involve hydrocarbon-air combustion. Temperature is determined from the relative intensity of a pair of fully resolved absorption lines probed with a rapid-tuning single-frequency laser. Demonstration of the technique using 300-micros scans across the R(1) (7) and R(1)(11) lines in the (0, 0) band of the A(2)Sigma(+) ? X(2)II system of OH present in the postflame gases above a CH(4)-air flat-flame burner is reported. Fluorescence monitoring of the absorption spectra was used to provide improved spatial resolution. Temperatures inferred from simultaneous absorption and fluorescence measurements showed good agreement with radiation-corrected thermocouple scans.

Rapid-tuning frequency-doubled ring dye laser for high resolution absorption spectroscopy in shock-heated gases.

This letter describes a ring dye laser system and its initial application to fully resolved absorption line shape measurements of the OH molecule during the brief test time available following shock waves generated in a shock tube. The operating wavelength has been extended into the UV through intercavity second harmonic generation.(AIP)

Rapid extended range tuning of single-mode ring dye lasers.

A modification of commerially available single-mode ring dye lasers is described which allows scan repetition rates in excess sf 4 kHz and extends the objerved single-mode scanning range up to 75 GHz. (AIP)

Quantitative high temperature absorption spectroscopy of NCO at 305 and 440 nm

Abstract A spectral survey of NCO absorption near the P 2 + p Q 12 head of the [ A 2 Σ + (00 0 0)← X 2 Π i (00 1 0)] band was obtained at 1450°K, 0.6 atm using a remotely located cw ring dye laser source and a shock tube. Mixtures of hydrogen cyanide, oxygen and nitrous oxide diluted in argon were shock heated to provide a reproducible steady-state concentration of NCO, and narrow-line absorption was measured in repeated experiments with the laser set at different wavelengths. The peak absorption was found at 440.479 nm (vac). The experimental spectrum was compared with a theoretical model to yield an average Voigt parameter a ≅ 0.1. Additional experiments, in mixtures of cyanogen, oxygen and nitrous oxide diluted in argon, provided a known plateau level of NCO, which was used to infer an absolute absorption coefficient β(1450°K, 0.60 atm) = 110(-50, +130) cm -1 atm -1 at 440.479 nm. This value of β corresponds to an oscillator strength of 0.0026 for the (00 0 0←00 1 0) band. Similar experiments were conducted to monitor the absorption around the R 1 head of the [ B 2 Π i (10 1 0)← X 2 Π i (00 1 0)] band of NCO, using a frequency doubled cw ring dye laser. The observed spectrum displayed strong broadening, indicating predissociation of the upper state. At the peak absorption wavelength (304.681 nm, vac), we inferred β(1470°K, 0.63 atm) = 40(-19, +48) cm -1 atm -1 and a ≅ 9. This value of β corresponds to an oscillator strength of 0.0031 for the (10 1 0←00 1 0) band.

Recent developments in absorption and fluorescence laser diagnostics for high temperature gases

An overview is presented of laser-based absorption and fluorescence techniques for probing high temperature gases, including shock-heated gases and combustion flows. Techniques discussed include: remote laser absorption measurements via a fiberoptic link; wavelength modulation spectroscopy using rapid-tuning laser sources; and planar laser-induced fluorescence for imaging species, temperature and velocity.An overview is presented of laser-based absorption and fluorescence techniques for probing high temperature gases, including shock-heated gases and combustion flows. Techniques discussed include: remote laser absorption measurements via a fiberoptic link; wavelength modulation spectroscopy using rapid-tuning laser sources; and planar laser-induced fluorescence for imaging species, temperature and velocity.