The emissions characteristics of two combustion platforms, a T63 turboshaft engine and an atmospheric swirl-stabilized research combustor, fueled with conventional military jet fuel (JP-8), a natural-gas-derived Fischer−Tropsch synthetic jet fuel (also referred herein as synjet or FT), and blends of the two were investigated. Nonvolatile particulate matter (PM) and gaseous emissions were analyzed to assess the impacts of the aromatic- and sulfur-free synjet fuel on the combustion products of the two platforms. The engine was operated at two power settings, and the combustor at several equivalence ratios, to evaluate the emission production over a wide range of combustion temperatures. Conventional aerosol instrumentation was used to quantify particle number (PN), size, and PM mass emissions, while a Fourier Transform Infrared analyzer was used to quantify the gaseous species. Planar laser-induced fluorescence and laser-induced incandescence techniques were employed on the research combustor to study the effects of the FT fuel on the formation and oxidation of particles in the combustor primary zone. Test results show dramatic reductions in particle concentrations and mean size on both combustion platforms with the neat FT and synjet fuel blends relative to operation with JP-8. Reductions of over 90% in PN were observed on both platforms for several operating conditions with neat FT fuel. For the engine, over an 80% reduction in smoke number was observed with neat synjet relative to operation on JP-8. As expected, reductions in sulfur oxide emissions and slight increases in water vapor (measured only in the atmospheric combustor) resulted due to the sulfur-free nature and higher hydrogen-to-carbon ratio of the synthetic fuel. Minor impacts were observed for other gaseous emissions. American Society for Testing and Materials fuel specification tests showed that JP-8/synjet blends up to 50/50% by volume satisfied the JP-8 military fuel requirements and that only the minimum specific gravity requirement was not satisfied at higher synjet concentrations. Impacts of the synjet fuel on the emissions of the atmospheric combustor and the T63 engine, a comparison of emissions between the two platforms, and results of in situ laser-based measurements in the combustor reaction zone are discussed.

High-repetition-rate, single-laser-shot measurements are important for the investigation of unsteady flows where temperature and species concentrations can vary significantly. Here, we demonstrate singleshot, purerotational, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) thermometry based on a kHz-rate fs laser source. Interferences that can affect nanosecond (ns) and ps CARS, such as nonresonant background and collisional dephasing, are eliminated by selecting an appropriate time delay between the 100-fs pump/Stokes pulses and the pulse-shaped 8.4-ps probe. A time-and frequencydomain theoretical model is introduced to account for rotational-level dependent collisional dephasing and indicates that the optimal probe-pulse time delay is 13.5 ps to 30 ps. This time delay allows for uncorrected best-fit N 2 -RCARS temperature measurements with ~1% accuracy. Hence, the hybrid fs/ps RCARS approach can be performed with kHz-rate laser sources while avoiding corrections that can be difficult to predict in unsteady flows. KeywordsFriedrich-Alexander University Erlangen-Nürnberg, aerodynamics, probes, Raman scattering, Raman spectroscopy, temperature measurement, thermometers, dephasing, femtoseconds, frequency domains, Fs laser, gasphase, high repetition rate, laser sources, species concentration, equipment design, optics and photonics, Fourier analysis Abstract: High-repetition-rate, single-laser-shot measurements are important for the investigation of unsteady flows where temperature and species concentrations can vary significantly. Here, we demonstrate singleshot, pure-rotational, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) thermometry based on a kHz-rate fs laser source. Interferences that can affect nanosecond (ns) and ps CARS, such as nonresonant background and collisional dephasing, are eliminated by selecting an appropriate time delay between the 100-fs pump/Stokes pulses and the pulse-shaped 8.4-ps probe. A time-and frequency-domain theoretical model is introduced to account for rotational-level dependent collisional dephasing and indicates that the optimal probe-pulse time delay is 13.5 ps to 30 ps. This time delay allows for uncorrected best-fit N 2 -RCARS temperature measurements with ~1% accuracy. Hence, the hybrid fs/ps RCARS approach can be performed with kHz-rate laser sources while avoiding corrections that can be difficult to predict in unsteady flows.

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Gas-phase temperatures and concentrations are measured from the magnitude and decay of the initial Raman coherence in femtosecond coherent anti-Stokes Raman scattering (CARS). A time-delayed probe beam is scattered from the Raman polarization induced by pump and Stokes beams to generate CARS signal; the dephasing rate of this initial coherence is determined by the temperature-sensitive frequency spread of the Raman transitions. Temperature is measured from the CARS signal decrease with increasing probe delay. Concentration is found from the ratio of the CARS and nonresonant background signals. Collision rates do not affect the determination of these quantities.

Published by the AIP PublishingArticles you may be interested in Analysis of time resolved femtosecond and femtosecond/picosecond coherent anti-Stokes Raman spectroscopy: Application to toluene and Rhodamine 6G

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The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N(2) and O(2) from 1-20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N(2) and O(2) from ~0.06 to 2.2 cm(-1) and the line separation of O(2) triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures.

A narrowband, time-asymmetric probe pulse is introduced into the hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering (fs/ps RCARS) technique to provide accurate and precise single-shot, high-repetition-rate gas-phase thermometric measurements. This narrowband pulse—generated by inserting a Fabry-Pérot étalon into the probe-pulse beam path—enables frequency-domain detection of pure-rotational transitions. The unique time-asymmetric nature of this pulse, in turn, allows for detection of resonant Raman-active rotational transitions free of signal contamination by nonresonant four-wave-mixing processes while still allowing detection at short probe-pulse delays, where collisional dephasing processes are negligible. We demonstrate that this approach provides excellent single-shot thermometric accuracy (<1% error) and precision (∼2.5%) in gas-phase environments.