Clayton S. Cooper

Quantitative measurements of nitric oxide in high-pressure (2–5 atm), swirl-stabilized spray flames via laser-induced fluorescence

Abstract We report spatially resolved, linear laser-induced fluorescence (LIF) measurements of nitric oxide (NO) in preheated, high-pressure (2.09 to 5.35 atm), lean direct-injection (LDI) spray flames. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane. NO is excited via the Q 2 (26.5) transition of the γ(0, 0) band. Detection is performed in a 2-nm region centered on the γ(0, 1) band. The major goal of this study is the validation and application of a complete LIF scheme by which quantitative NO concentrations can be measured in high-pressure LDI spray flames. Standard excitation and detection scans have been performed to assess possible interferences and to validate a nonresonant wavelength so as to subtract the influence of oxygen interferences in the NO detection window. NO is doped into the reactants and convected through the flame with no apparent destruction, thus allowing an NO fluorescence calibration to be taken inside the flame environment. The in situ calibration scheme is validated by comparisons with reference flames at high pressure. Quantitative radial NO profiles are presented at 2.09, 3.18, 4.27, and 5.35 atm and analyzed so as to better understand the operation of lean-direct injectors for gas turbine combustors. Downstream NO measurements in the LDI flames indicate an overall pressure scaling corresponding to P 0.74 .

Quantitative Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide in High-Pressure (6-15 atm) Counterflow Diffusion Flames

Laser-induced fluorescence (LIF) measurements of NO concentration ([NO]) have been obtained along the centerline of methane–air counterflow diffusion flames at 6 to 15 atm. This study is an extension of our previous work involving measurements of [NO] in similar flames at two to five atm, wherein we had used a counterflow premixed flame for calibration. For the flames studied here, a method based on computed overlap fractions is developed to calibrate [NO] measurements at higher pressures. The linear LIF measurements of [NO], which are corrected for variations in the electronic quenching rate coefficient, are compared with numerical predictions from an opposed-flow flame code utilizing two Gas Research Institute (GRI) chemical kinetic mechanisms (versions 2.11 and 3.0). The effect of radiative heat loss on code predictions is accounted for by using an optically thin radiation model. The revised GRI mechanism (version 3.0) offers a significant improvement in prompt-NO predictions for these flames compared to the older version (2.11), especially at pressures below eight atm. However, a consistent discrepancy remains in the comparisons, particularly at peak NO locations for pressures lower than six atm. The measurements display a continuing trend of decreasing NO concentration with increasing pressure at 6–15 atm as expected for flames dominated by prompt NO. The discrepancy between measurements and predictions decreases with rising pressure so that the revised GRI mechanism predicts [NO] with reasonable accuracy at pressures above six atm.

Comparison of saturated and linear laser-induced fluorescence measurements of nitric oxide in counterflow diffusion flames

Abstract Quantitative measurements of NO concentrations ([NO]) have been obtained along the centerline of atmospheric ethane–air counterflow diffusion flames by using saturated and linear laser-induced fluorescence (LIF). In particular, four flames with strain rates varying from 5 to 48 s −1 were investigated while maintaining a constant fuel dilution in all cases. The utility of a broad-band laser-saturated fluorescence (LSF) technique is assessed by comparison to similar measurements of NO using linear LIF. The linear LIF measurements are corrected for variations in the local electronic quenching rate coefficient by using major species profiles generated by a diffusive flame code and available correlations for the quenching cross-sections of NO. The corrected LIF profiles compare favorably with the LSF profiles. A four-level model is used to investigate the effects of rotational energy transfer (RET) on the LSF measurements. The excellent comparison between the quenching-corrected linear LIF and the LSF measurements at locally fuel-lean to greater than stoichiometric mixture fractions verifies the validity of the LSF technique for these conditions. The slight but consistent discrepancy between the LSF and linear LIF measurements at local equivalence ratios above 1.6 may be attributed to a change in the collisional branching ratio from lean to rich stoichiometries and/or the need for further work on the electronic quenching cross-sections required for quantitative NO measurements under fuel-rich conditions.

Effect of pulsed dye-laser wavelength stabilization on spectral overlap in atmospheric NO fluorescence studies

We introduce an inexpensive application of a Fabry-Perot etalon to control long-term UV-laser line drift in atmospheric NO laser-induced fluorescence (LIF) measurements by monitoring the visible fundamental of a pulsed dye laser. A linear image sensor captures the interference pattern, and the dye grating can be adjusted to maintain a fixed wavelength through an interface with labview software. Results indicate that the laser wavelength can be fixed to an accuracy of +/-0.0001 nm in the dye fundamental and +/-0.00003 nm in the UV beam. Hence the average error in the LIF signal owing to fluctuations in spectral overlap between the laser and the NO absorption transition decreases from ~5 to ~0.05%, which results in improved measurement accuracy.

Development of the GE Aviation Low Emissions TAPS Combustor for Next Generation Aircraft Engines

GE Aviation has developed the low emissions, lean-burn, Twin Annular Premixing Swirler (TAPS) combustor for application in the next generation of aircraft engines. This paper describes the recent development of the TAPS combustor for application in the new GEnx-1B and GEnx-2B engines that power the new Boeing 787 and 747-8 wide-body aircraft, respectively. The combustor design approach, key design considerations and engine emissions data are discussed. The GEnx TAPS combustor emissions certification results showed NOx levels 52% below CAEP/6 standard while maintaining good CO, HC and smoke emissions.

Laser-induced fluorescence measurements in lean direct-injection spray flames: technique development and application

The combustion diagnostics community has recently begun to focus its efforts toward practical combustion devices. One impetus behind this effort is the need to develop aeropropulsion gas turbine combustors with ultra-low NOx emissions. For the past several years, the Flame Diagnostics Laboratory at Purdue University has been advancing optically non-intrusive techniques to measure concentrations of nitric oxide [NO] in lean direct-injection (LDI) spray flames. LDI flames offer the possibility of reducing NOx emissions from gas turbines by rapid mixing of the liquid fuel and air so as to drive the flame structure toward partially premixed conditions. In this paper, we review the technical approach required to utilize laser-induced fluorescence (LIF) methods for quantitatively measuring [NO] in LDI spray flames. In the progression from atmospheric to high-pressure measurements, the LIF method requires a shift from the saturated to the linear regime of fluorescence measurements. As such, we discuss quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF) and planar laser-induced fluorescence (PLIF) measurements of NO concentration in atmospheric, LDI spray flames. In general, the results are comparable, although novel filtering techniques are required at higher flame pressures.

Comparisons of laser-saturated, laser-induced, and planar laser-induced fluorescence measurements of nitric oxide in a lean direct-injection spray flame

We report quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF), and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) concentration in a preheated, lean direct-injection spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane, and the overall equivalence ratio is unity. NO is excited by means of the Q(2)(26.5) transition of the gamma(0, 0) band. LSF and LIF detection are performed in a 2-nm region centered on the gamma(0, 1) band. PLIF detection is performed in a broad ~70-nm region with a peak transmission at 270 nm. Quantitative radial NO profiles obtained by LSF are presented and analyzed so as to correct similar LIF and PLIF profiles. Excellent agreement is achieved among the three fluorescence methodologies.

Quantitative laser-saturated fluorescence measurements of nitric oxide in a heptane spray flame

Abstract We report spatially resolved laser-saturated fluorescence measurements of NO concentration in a pre-heated.lean-direct injection (LDI) spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane. NO is excited via the Q2(26.5) transition of the γ(O, 0) band. Detection is performed in a 2-nm region centered on the γ(O, 1) band. Because of the relatively close spectral spacing between the excitation (226 nm) and detection wavelengths (236 nm), the γ(O, 1) band of NO cannot be isolated from the spectral wings of the Mie scattering signal produced by the spray. To account for the resulting superposition of the fluorescence and scattering signals, a background subtraction method has been developed that utilizes a nearby non-resonant wavelength. Excitation scans have been performed to locate the optimum off-line wavelength. Detection scans have been performed at problematic locations in the flame to determine possible fluorescence i...

Parametric study of no production via quantitative laser-induced fluorescence in high-pressure, swirl-stabilized spray flames

We report the influence of equivalence ratio and air preheat temperature on nitric oxide (NO) concentration in high-pressure heptane spray flames. The burner used was based on the lean direct-injection design and incorporated a pressure-atomized hollow-cone spray nozzle. Helical vanes in the air passage coupled with a divergent exit and preheated air produced a strongly swirling, clean blue flame. NO concentration profiles were measured at 4.27 atm using laser-induced fluorescence (LIF) at five axial heights above the burner. The uniformity of NO throughout the central region of the flame demonstrated the well-mixed nature of the recirculation zone. Measurements were taken at the 40 mm centerline height to determine the effects of primary equivalence ratio (φp=1.0 to 0.8) and air preheat temperature (Tair=375 K to 575 K). The results strongly suggest that NO formation occurs in near stoichiometric regions of the flame and is subsequently diluted with excess air. A residence time effect is evident and apparently scales as the mass flow rate of air relative to that for stoichiometric combustion, yielding a φP2 scaling of the NO (ppm). Moreover, moderate increases in preheat temperature produce significant increases in NO (ppm) levels, suggesting thermal NO production.

In-Situ Calibration Technique for Laser-Induced Fluorescence Measurements of Nitric Oxide in High-Pressure, Direct-Injection, Swirling Spray Flames

We report spatially resolved laser-induced fluorescence (LIF) measurements of nitric oxide (NO) in a preheated, two-atmosphere, lean direct-injection (LDI) spray flame. The spray is produced by a hol-low-cone, pressure-atomized nozzle supplied with liquid heptane. NO is excited via the Q2(26.5) transition of the γ(0,0) band. Detection is performed in a 2-nm region centered on the γ (0,1) band. A complete scheme is developed by which quantitative NO concentrations in high-pressure LDI spray flames can be measured by applying linear LIF. Standard excitation and detection scans are performed to assess possible interferences and to verify a non-resonant wavelength for subtracting the influence of oxygen interferences and Mie scattering in the NO detection window. NO is doped into the reactants and convected through the flame with no apparent destruction, thus allowing an NO fluorescence calibration to be taken inside the flame environment. The in-situ calibration scheme is validated by comparisons to a reference flame. Relative axial calibration slopes are utilized in order to obtain radial profiles of absolute NO concentrations. These quantitative NO profiles are presented and analyzed so as to better understand the operation of lean-direct injectors for gas turbine combustors.