Nitric oxide concentration measurements in atmospheric pressure flames using electronic-resonance-enhanced coherent anti-Stokes Raman scattering

We report the application of electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) for measurements of nitric oxide concentration ([NO]) in three different atmospheric pressure flames. Visible pump (532 nm) and Stokes (591 nm) beams are used to probe the Q-branch of the Raman transition. A significant resonance enhancement is obtained by tuning an ultraviolet probe beam (236 nm) into resonance with specific rotational transitions in the (v’=0, v”=1) vibrational band of the A²Σ⁺–X²Π electronic system of NO. ERE-CARS spectra are recorded at various heights within a hydrogen-air flame producing relatively low concentrations of NO over a Hencken burner. Good agreement is obtained between NO ERE-CARS measurements and the results of flame computations using UNICORN, a two-dimensional flame code. Excellent agreement between measured and calculated NO spectra is also obtained when using a modified version of the Sandia CARSFT code for heavily sooting acetylene-air flames (φ=0.8 to φ=1.6) on the same Hencken burner. Finally, NO concentration profiles are measured using ERE-CARS in a laminar, counter-flow, non-premixed hydrogen-air flame. Spectral scans are recorded by probing the Q₁ (9.5), Q₁ (13.5) and Q₁ (17.5) Raman transitions. The measured shape of the [NO] profile is in good agreement with that predicted using the OPPDIF code, even without correcting for collisional effects. These comparisons between [NO] measurements and predictions establish the utility of ERE-CARS for detection of NO in flames with large temperature and concentration gradients as well as in sooting environments. PACS 07.88.+y; 42.62.Fi; 42.65.Dr     

Two-point time-series measurements of minor-species concentrations in a turbulent nonpremixed flame

We report a technique that is capable of making simultaneous two-point time-series measurements of minor-species concentrations in turbulent flames. The experimental setup, which incorporates picosecond time-resolved laser-induced fluorescence, has a spatial resolution of less than 250 microm and a temporal resolution of less than 100 micros, which spatially and temporally resolve microscales in many turbulent flows. Two-point time-series data are given for a standard turbulent nonpremixed flame at Re= 10,000, including a discussion of potential implications.     

Spectroscopic,calibration and RET issues for linear laser-induced fluorescence measurements of nitric oxide in high-pressure diffusion flames

Two different strategies are compared for linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in counter-flow diffusion flames at high pressures via the A-X(0,0) system. Excitation of NO via a rovibronic transition at 226.03 nm is found to be slightly better compared to a previously utilized excitation wavelength of 225.58 nm. An indirect approach based on the computed spectral overlap fraction is verified and applied to calibrate [NO] measurements in counter-flow diffusion flames at high pressures. A five-level model for NO molecular dynamics is presented and utilized to investigate the effects of rotational energy transfer (RET) on linear LIF measurements of [NO] at pressures up to 15 atm. The results indicate that rotational relaxation effects are essentially negligible under high-pressure conditions at low laser fluences, and thus they need not be accounted for when measuring [NO] using linear LIF. The calibration technique is validated by direct comparisons to [NO] measurements made at pressures up to 5 atm via another calibration method, based on doping NO in counter-flow premixed flames at the same pressure. Using this calibration technique, LIF measurements of [NO] are obtained in a series of counter-flow diffusion flames at pressures up to 15 atm. These measurements are found to be in excellent agreement with previously reported measurements of [NO] in similar flames. PACS 07.35.+k; 33.20.Sn; 42.62.Fi     

Spectral measurements of incipient plasma temperature and electron number density during laser ablation of aluminum in air

Plasmas generated during incipient laser ablation of aluminum in air were studied using emission spectroscopy. A plasma emission model was developed, invoking one-dimensional radiative transfer, to describe the observed emission spectra, while taking into account the effects of continuum radiation. Theoretical spectra were calculated and compared to experimental spectra in the range 387–406 nm. Satisfactory agreement was found between the experimental and predicted spectra, especially at delay times of 30–200 ns, thus providing plasma temperatures and electron number densities as functions of delay time and laser irradiance (1.8–8.0 GW/cm²). In general, both the plasma temperature and electron number density rise at greater laser irradiances but drop at increasing delay times, with a more rapid drop for delay times less than 60 ns. PACS 42.62.Fi; 52.25.-b     

PRESSURE-VELOCITY EQUILIBRIUM HYDRODYNAMIC MODELS

This article describes mathematical models for phase separated mixtures of materials that are in pressure and velocity equilibrium but not necessarily temperature equilibrium. General conditions for constitutive models for such mixtures that exhibit a single mixture sound speed are discussed and specific examples are described.     

Photon-counting technique for rapid fluorescence-decay measurement

We report on a novel laser-induced fluorescence triple-integration method (LIFTIME) that is capable of making rapid, continuous fluorescence lifetime measurements by a unique photon-counting technique. The LIFTIME has been convolved with picosecond time-resolved laser-induced fluorescence, which employs a high-repetition-rate mode-locked laser, permitting the eventual monitoring of instantaneous species concentrations in turbulent flames. We verify the technique by application of the LIFTIME to two known fluorescence media, diphenyloxazole (PPO) and quinine sulfate monohydrate (QSM). PPO has a fluorescence lifetime of 1.28 ns, whereas QSM has a fluorescence lifetime that can be varied from 1.0 to 3.0 ns. From these liquid samples we demonstrate that fluorescence lifetime can currently be monitored at a sampling rate of up to 500 Hz with less than 10% uncertainty (1sigma) .     

Comparison of laser-induced and planar laser-induced fluorescence measurements of nitric oxide in a high-pressure, swirl-stabilized, spray flame

We report spatially resolved linear laser-induced fluorescence (LIF) and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) in a pre-heated, high-pressure (4.27 atm), lean direct-injection (LDI) spray flame. The feasibility of using PLIF in lieu of LIF is assessed with respect to measuring NO concentrations in high-pressure LDI spray flames. NO is excited via the resonant Q₂(26.5) transition of the γ(0,0) band while a non-resonant wavelength is employed to subtract background interferences. LIF detection is performed in a 2-nm region centered on the γ(0,1) band. PLIF detection is performed in a 68-nm window that captures fluorescence from several vibrational bands. An in situ NO doping scheme for fluorescence calibration is successfully employed to quantify the LIF signals. However, a similar calibration scheme for the reduction of PLIF images to quantitative field measurements is plagued by the laser-excited background. Excitation scans and calibration comparisons have been performed to assess the background contribution for PLIF detection. Quantitative radial NO profiles measured by LIF are presented and analyzed so as to correct the PLIF measurements to within the accuracy bars of the LIF measurements via a single-point scaling of the PLIF image. Received: 23 November 1999 / Revised version: 17 January 2000 / Published online: 27 April 2000     

Determination of mixed polynuclear aromatic hydrocarbons in the vapor phase by laser-induced fluorescence spectrometry

Laser-induced fluorescence spectromety is shown to be a viable technique for quantifying polynuclear aromatic hydrocarbons (PAH) in the vapor phase. Mixtures of anthracene, fluoranthene, and phenanthrene were analyzed in a vapor cell at temperatures ranging from 50° to 90°C. The individual concentrations of PAH's in these mixtures were calculated by a least-squares matrix technique and were accurate to within 3–5% at levels of about 5 × 10¹²?3 × 10¹⁵ molecules cm^?3.     

Acetone as a tracer for mixture fraction time-series measurements in turbulent non-reacting jets

Mixture fraction time series were measured using laser-induced fluorescence of seeded acetone in turbulent non-reacting jets. Power spectral densities obtained using the seeding technique, especially in the shear layer, appear to be consistent with turbulence theory. The technique can be used for many gaseous mixtures of interest. Received: 19 October 1999/Accepted: 17 October 2000