Publications

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Ultrabroadband coherent anti-Stokes Raman spectroscopy (CARS) was employed for one-dimensional imaging of temperature and major species distributions simultaneously in the near-wall region of a CH4/air flame supported on a side-wall-quenching burner. Automatic temporal and spatial overlap of the approximetaly 7 fs pump and Stokes pulses was achieved through a two-beam CARS phase-matching scheme and the crossed approximately 75 ps probe beam provides excellent spatial sectioning of the probed location. Concurrent detection of N2 O2 H2 CO CO2 and CH4 was performed. A CH4/air premixed flame at lean stoichiometric and rich conditions and Reynolds number = 5000 was probed as it quenches against a cooled steel side-wall parallel to the flow providing a persistent flame-wall interaction. An imaging resolution of better than 40 μm was achieved across the field-of-view allowing thermochemical states of the thermal boundary layer to be resolved to within approximately 30 μm of the interface.

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Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman spectroscopy (CARS) is developed utilizing a two-beam phase-matching approach for one-dimensional (1D) measurements demonstrated in an impinging jet burner to probe time-resolved head on quenching (HOQ) of a methane/air premixed flame at Φ = 1.0 and Reynolds number = 5000. Single-laser-shot 1D temperature profiles are obtained over a distance of at least 4 mm by fitting the pure-rotational N2 CARS spectra to a spectral library calculated from a time-domain CARS code. An imaging resolution of ∼61 μm is obtained in the 1D-CARS measurements. The acquisition of single-shot 1D CARS measurements, as opposed to traditional point-wise CARS techniques, enables new spatially correlated conditional statistics to be determined, such as the position, magnitude, and fluctuations of the instantaneous temperature gradient. The temperature gradient increases as the flame approaches the metal surface, and decreases during quenching. The standard deviation of the temperature gradient follows the same trend as the temperature gradient, increasing as the flame front approaches the surface, and decreasing after quenching.

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We report measurements of temperature and O2/N2 mole-fraction ratio in the vicinity of a burning and decomposing carbon-epoxy composite aircraft material samples exposed to uniform heat fluxes of 48 and 69 kW/m2. Controlled laboratory experiments were conducted with the samples suspended above a cone-type heater and enclosed in an optically accessible chimney. Noninvasive coherent anti-Stokes Raman scattering (CARS) measurements we performed on a single-laser-shot basis. The CARS data were performed with both a traditional point measurement system and with a one-dimensional line imaging scheme that provides single-shot temperature and O2/N2 profiles to reveal the quantitative structure of the temperature and oxygen concentration profiles over the duration of the 30-40 minute duration events. The measured near-surface temperature and oxygen transport are an important factor for exothermic chemistry and oxidation of char materials and the carbon fibers themselves in a fire scenario. These unique laser-diagnostic experiments provide new information on physical/chemical processes in a well-controlled environment which may be useful for the development of heat-and mass-transfer models for the composite fire scenario.

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Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15 000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm2. © 2013 AIP Publishing LLC.