RESUMEN
Krypton Tagging Velocimetry (KTV) and Picosecond Laser Electronic Excitation Tagging (PLEET) velocimetry at a 100-kHz rate were demonstrated in Mach 18 flow conditions at the Arnold Engineering Development Center (AEDC) Tunnel 9 employing a burst-mode laser system and a custom optical parametric oscillator (OPO). The measured freestream flow velocities from both KTV and PLEET agreed well with the theoretical calculation. The increase in repetition rate provides better capability to perform time-resolved velocimetry measurements in hypersonic flow environments.
RESUMEN
A 100 kHz krypton (Kr) tagging velocimetry (KTV) technique was demonstrated in a Mach-6 Ludwieg tube using a burst-mode laser-pumped optical parametric oscillator system. The single-beam KTV scheme at 212 nm produced an insufficient signal in this large hypersonic wind tunnel because of its low Kr seeding (≤5%), low static pressure (â¼2.5torr), and long working distance (â¼1m). To overcome these issues, a new scheme using two excitation beams was developed to enhance KTV performance. A 355 nm laser beam was combined with the 212 nm beam to promote efficient two-photon Kr excitation at 212 nm, and increase the probability of 2 + 1 resonant-enhanced multiphoton ionization by adding a 355 nm beam. A signal enhancement of approximately six times was obtained. Using this two-excitation beam approach, strong long-lasting KTV was successfully demonstrated at a 100 kHz repetition rate in a Mach-6 flow.
RESUMEN
Rotationally resolved, broadband absorption spectra of the fundamental vibrational transition of the asymmetric C-H stretch mode of methane are measured under single-laser-shot conditions using time-resolved optically gated absorption (TOGA). The TOGA approach exploits the difference in timescales between a broadband, fs-duration excitation source and the ps-duration absorption features induced by molecular absorption to allow effective suppression of the broadband background spectrum, thereby allowing for sensitive detection of multi-transition molecular spectra. This work extends the TOGA approach into the mid-infrared (mid-IR) spectral regime, allowing access to fundamental vibrational transitions while providing broadband access to multiple mid-IR transitions spanning â¼150 cm-1 (â¼160 nm) near 3.3 µm, thereby highlighting the robustness of this technique beyond previously demonstrated electronic spectroscopy. Measurements are conducted in a heated gas cell to determine the accuracy of the simultaneous temperature and species-concentration measurements afforded by this single-shot approach in a well-characterized environment. Application of this approach toward fuel-rich methane-nitrogen-oxygen flames is also demonstrated.
RESUMEN
Multiphoton-resonance enhancement of a rare-gas-assisted nitrogen femtosecond-laser electronic-excitation-tagging (FLEET) signal is demonstrated. The FLEET signal is ideal for velocimetric tracking of nitrogen gas in flow environments by virtue of its long-lived nature. By tuning to three-photon-resonant transitions of argon, energy can be more efficiently deposited into the mixture, thereby producing a stronger and longer-lived FLEET signal following subsequent efficient energy transfer from excited-state argon to the C (3Πu) excited state of nitrogen. Such resonant excitation exhibits as much as an order of magnitude increase in this rare-gas-assisted FLEET signal, compared to near-resonance excitation of seeded argon demonstrated in previous work, while reducing the required input excitation-pulse energies by two orders of magnitude compared to traditional FLEET.
RESUMEN
10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate â¼5mJ/pulse at 282 nm and 286 nm to excite the Q1(5) and Q1(14) transitions of the A2Σ+âX2Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.
RESUMEN
Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used in the boundary layer of an ogive-cylinder model in a Mach-6 Ludwieg tube. One-dimensional velocity profiles were extracted from the FLEET signal in laminar boundary layers from pure N2 flows at unit Reynolds numbers ranging from 3.4×106/m to3.9×106/m. The effects of model tip bluntness and the unit Reynolds number on the velocity profiles were investigated. The challenges and strategies of applying FLEET for direct boundary layer velocity measurement are discussed. The potential of utilizing FLEET velocimetry for understanding the dynamics of laminar and turbulent boundary layers in hypersonic flows is demonstrated.
RESUMEN
Krypton (Kr)-based tagging velocimetry is demonstrated in a Kr/N2 jet at 100 kHz repetition rate using a custom-built burst-mode laser and optical parametric oscillator (OPO) system. At this repetition rate, the wavelength-tunable, narrow linewidth laser platform can generate up to 7 mJ/pulse at resonant Kr two-photon-excitation wavelengths. Following a comprehensive study, we have identified the 212.56 nm two-photon-excitation transition as ideal for efficient Kr-based velocimetry, producing a long-lived (â¼40µs) fluorescence signal from single-laser-pulse tagging that is readily amenable to velocity tracking without the need for a second "read" laser pulse. This long-lived fluorescence signal is found to emanate from N2-rather than from Kr-following efficient energy transfer. Successful flow velocity tracking is demonstrated at multiple locations in a high-speed Kr/N2 jet flow. The 100 kHz repetition rate provides the ability to perform time-resolved velocimetry measurements in high-speed and even hypersonic flow environments, where standard velocimetry approaches are insufficient to capture the relevant dynamics.
RESUMEN
Krypton planar laser-induced fluorescence (Kr PLIF) was demonstrated at a repetition rate of 100 kHz. To achieve this increased rate, a custom injection-seeded optical parametric oscillator was built to efficiently convert the 355 nm output of a high-energy, high-repetition-rate nanosecond burst-mode laser to 212.56 nm to excite Kr from the ground to the 5p[1/2]0 electronic state. Successful tracking of flow structures and mixture fraction was demonstrated using detection speeds 100 times greater than previously attained with a femtosecond laser source. The increase in repetition rate makes time-resolved Kr PLIF relevant for high-speed flows in particular.
RESUMEN
Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements were used to determine the temperature and major species mole fractions in laminar, premixed, ethylene-air flames operating at atmospheric pressure. Conventional ultrabroadband dye lasers for WIDECARS, which use Pyrromethene dyes, have historically suffered from day-to-day wavelength shifting. To overcome this problem, a new ultrabroadband dye laser was developed in this study to provide a stable wavelength and power generation. A new dye laser pumping scheme and a mixture of Sulforhodamine 640, Kiton Red 620, and Rhodamine 640, was used to generate the desired FWHM ${\sim}{15}\;{\rm nm}$â¼15nm (${410}\;{{\rm cm}^{ - 1}}$410cm-1) bandwidth. The WIDECARS measured mole fraction ratios of ${{\rm CO}_2}$CO2, CO, and ${{\rm H}_2}$H2 with ${{\rm N}_2}$N2 agreed well with chemical equilibrium calculations.
RESUMEN
We report the high-speed imaging of multi-species and multi-parameter combustion diagnostics for turbulent non-premixed jet flames using a three-legged burst-mode laser system. Simultaneous OH/CH2O planar laser-induced fluorescence and Rayleigh-scattering imaging measurements at a 10-kHz rate are obtained. OH and CH2O concentrations, flame temperatures, and heat-release rates are simultaneously acquired in two-dimensions at 10 kHz.
RESUMEN
Two-dimensional, Rayleigh-scattering-based temperature measurements utilizing a turbulent jet flame were performed in this study at 100-kHz frequency. This tenfold increase in measurement speed-compared to the 10-kHz frequency considered previously-facilitated identification and tracking of several highly dynamic flow features. Findings of this study demonstrate that flow-feature dynamics become uncorrelated qualitatively and quantitatively prior to an elapse of 100 µs between successive measurements, thereby necessitating the temperature-measurement frequency to exceed 10 kHz. At the proposed 100-kHz measurement frequency, resolution of the Taylor microscale and integral scales have been demonstrated in both space and time for this flow.
RESUMEN
Two-photon-absorption laser-induced fluorescence of Kr was explored using both nanosecond- and femtosecond-duration laser excitation sources. Fluorescence signals following two-photon excitation at two wavelengths (212.56 nm and 214.77 nm) were compared while varying laser pulse duration, energy, and excitation wavelength as well as pressure and Kr mole fraction in mixtures with nitrogen. Our findings show that stronger fluorescence was observed when the excitation wavelength was tuned to 212.56 nm, regardless of the excitation-pulse duration. Moreover, an approximate 100-fold signal enhancement from nanosecond excitation (â¼3 mJ/pulse, 10 ns duration) was observed as compared to femtosecond excitation (â¼6 µJ/pulse, 90 fs duration).