Publications

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We have investigated the utility of femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman scattering(CARS)for simultaneous measurement of temperature, pressure, and velocity in hypersonic flows. Experiments were conducted in underexpanded jets of air and molecular nitrogen to assess CARS diagnostic performance in terms of signal level scaling, measurement precision, and dynamic range. Pure-rotational CARS of the Raman S branch was applied for simultaneous measurement of temperature and pressure. Thermometry was performed by fitting CARS spectra acquired under nearly collision-free conditions by introducing a picosecond CARS probe pulse at zero delay from the femtosecond pump. Pressure could be subsequently obtained by from a second CARS spectral acquisition with a picosecond probe introduced at time delay to sample molecular collisions. CARS velocimetry was attempted by monitoring the Doppler shift of the N₂ vibrational, Q-branch spectrum, with both direct spectral resolution and optical heterodyne detection schemes. Doppler shifts from the sub-I-km/s air jet flow proved too small to measure with this approach, prompting us to turn to femtosecond laser electronic excitation tagging (FLEET) for reliable single-laser-shot velocimetry and CARS temperature/pressure measurement. Scaling of the CARS signal level to very low pressure and temperature conditions expected in the Sandia hypersonic wind tunnel (HWI) was performed. CARS measurements of temperature in HWT appear to be very feasible, while prospects for HWT pressure measurements are reasonable.

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Knowledge of soot particle sizes is important for understanding soot formation and heat transfer in combustion environments. Soot primary particle sizes can be estimated by measuring the decay of time-resolved laser-induced incandescence (TiRe-LII) signals. Existing methods for making planar TiRe-LII measurements require either multiple cameras or time-gate sweeping with multiple laser pulses, making these techniques difficult to apply in turbulent or unsteady combustion environments. Here, we report a technique for planar soot particle sizing using a single high-sensitivity, ultra-high-speed 10 MHz camera with a 50 ns gate and no intensifier. With this method, we demonstrate measurements of background flame luminosity, prompt LII, and TiRe-LII decay signals for particle sizing in a single laser shot. The particle sizing technique is first validated in a laminar non-premixed ethylene flame. Then, the method is applied to measurements in a turbulent ethylene jet flame.

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Femtosecond coherent anti-Stokes Raman scattering thermometry in a solid-fuel propellant flame is demonstrated by tuning the lasers to the rovibrational Raman transitions of diatomic hydrogen (H<inf>2</inf>).

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Single-laser-shot femtosecond rotational coherent anti-Stokes Raman scattering (fs-RCARS) temperature measurements are performed across a 6-mm line in a turbulent, sooting ethylene jet flame to characterize temperature gradients. A 60- fs pulse is used to excite many rotational Raman transitions in N2, and a 160-ps pulse is used to probe the Raman coherence. The spatial resolution of the measurements is 500 μm in the direction of beam propagation and 50 μm in the transverse directions. Measurements have been performed at multiple locations in the jet flame, and the measured temperature are similar to previously recorded point measurements. Future work will include performing simultaneous laser-induced incandescence (LII) measurements to measure soot volume fraction to perform joint statistical analysis of the sooting turbulent flame.

We present spatial profiles of temperature and soot-volume-fraction statistics from a sooting, 2-m base diameter turbulent pool fire, burning a 10%-toluene/90%-methanol fuel mixture. Dual-pump coherent anti-Stokes Raman scattering and laser-induced incandescence are utilized for simultaneous point measurements of temperature and soot. The research fuel-blend used here results in a lower soot loading than real transportation fuels, but allows us to apply high-fidelity laser diagnostics for spatially resolved measurements in a fully turbulent, buoyant fire of meter-scale base size. Profiles of mean and rms fluctuations are radially resolved across the fire plume, both within the hydrocarbon-rich vapor-dome region near fuel pool, and higher within the actively burning region of the fire. The spatial evolution of the soot and temperature probability density functions is discussed. Soot fluctuations display significant intermittency across the full extent of the fire plume for the research fuel blend used. Simultaneous, spatially overlapped temperature/soot measurements permit us to obtain estimates of joint statistics that are presented as spatially resolved conditional averages across the fire plume, and in terms of a joint pdf obtained by including measurements from multiple spatial locations. Within the actively burning region of the fire, soot is observed to occupy a limited temperature range between ∼1000 and 2000 K, with peak soot concentration occurring at 1600–1700 K across the full radial extent of the fire plume, despite marked changes in the local temperature pdf across the same spatial extent. A wider range of soot temperatures is observed in the fuel vapor-dome region low in the pool fire, with detectable cold soot persisting into conditionally averaged statistics. The results yield insight into soot temperature across a wide spatial extent of a fully turbulent pool fire of meaningful size, which are valuable for development of soot radiative-emission models and for validation of fire fluid-dynamics codes.

A two-beam, one-dimensional hybrid fs/ps rotational CARS scheme was applied to a coaxial dielectric barrier discharge burner to spatially resolve and simultaneously measure temperature, relative oxygen concentration, and relative hydrogen concentration. At higher applied voltages, the 1 L/min hydrogen burner produces a collapsed flame with a curved reaction zone to the surrounding quiescent air, extending roughly 5 mm above the burner surface, making this a perfect candidate for single-shot realizations of flame properties with a vertical line CARS imaging technique. Time-delayed probing of the impulsively created Raman coherence allowed for improved dynamic range in regions of high temperature gradients, but also introduced the reliance on collisional modeling. Temperature measurements proved robust with probe delay, but the higher detection limit of oxygen at longer delays encouraged the use of isolated oxygen line calibrations to Hencken burner data in place of collisional modeling. A spatial resolution of 140 μm in the axis normal to the burner surface was adequate for mapping out flame properties along the reaction zone.

Single-laser-shot femtosecond rotational coherent anti-Stokes Raman scattering (fs-RCARS) temperature measurements are performed across a 3- mm line in a turbulent, sooting ethylene jet flame to characterize temperature gradients. A 60-fs pulse is used to excite many rotational Raman transitions, and a 160-ps pulse is used to probe the Raman coherence. The spatial resolution of the measurements is 670 μm in the direction of beam propagation, 200 μm in the direction along the 1D line, and 50 μm in the transverse direction. Measurements have been performed at multiple locations in the jet flame, and the measured temperature are similar to previously recorded point measurements.

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Ultrafast pure-rotational CARS is applied to an aluminized ammonium-perchlorate propellant flame. Background-free spectra were acquired in this challenging high-temperature, particle-laden environment and successfully fit for temperature and oxygen/nitrogen ratio using a simple theoretical model.

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We apply ultrafast pure-rotational coherent anti-Stokes Raman scattering (CARS) for temperature and relative oxygen concentration measurements in the plume emanating from a burning, aluminized ammonium-perchlorate propellant strand. Combustion of these metal-based propellants is a particularly hostile environment for laserbased diagnostics, with intense background luminosity and scattering from hot metal particles as large as several hundred micrometers in diameter. CARS spectra that were previously obtained using nanosecond pulsed lasers in an aluminum-particle-seeded flame are examined and are determined to be severely impacted by nonresonant background, presumably as a result of the plasma formed by particulate-enhanced laser-induced breakdown. Introduction of femtosecond/picosecond (fs/ps) laser pulses improves CARS detection by providing time-gated elimination of strong nonresonant background interference. Single-laser-shot fs/ps CARS spectra were acquired from the burning propellant plume, with picosecond probe-pulse delays of 0 and 16 ps from the femtosecond pump and Stokes pulses. At zero delay, nonresonant background overwhelms the Raman-resonant spectroscopic features. Time-delayed probing results in the acquisition of background-free spectra that were successfully fit for temperature and relative oxygen content. Temperature probability densities and temperature/oxygen correlations were constructed from ensembles of several thousand single-laser-shot measurements with the CARS measurement volume positioned within 3 mm or less of the burning propellant surface. The results show that ultrafast CARS is a potentially enabling technology for probing harsh, particle-laden flame environments.

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