Showing papers by "Terrence R. Meyer published in 2018"

Two-color volumetric laser-induced fluorescence for 3D OH and temperature fields in turbulent reacting flows.

Abstract: Single-shot, two-color, volumetric laser-induced fluorescence was demonstrated for three-dimensional (3D), tomographic imaging of the structural properties of the OH radical and temperature field in a turbulent hydrogen–air flame. Two narrowband laser sources were tuned to the Q1(5) and Q1(14) transitions of the (1,0) band in the A2Σ←X2Π system and illuminated a volumetric region of the flame. Images from eight unique perspectives collected simultaneously from each of the two transitions were used to reconstruct overlapping OH fields with different Boltzmann fractions and map the 3D temperature distribution with nanosecond precision. Key strategies for minimizing sources of error, such as detector sensitivity and spatial overlap of the two fields, are discussed.

Compact burst-mode Nd:YAG laser for kHz-MHz bandwidth velocity and species measurements.

Abstract: A compact-footprint (018 m2) flash-lamp-pumped, burst-mode Nd:YAG-based master-oscillator pulsed-amplifier laser is reported with a fundamental 1064 nm output of over 14 J per burst A directly modulated diode laser seed source is used to generate 10 ms duration arbitrary sequences of 500 kHz doublet or MHz singlet pulses for flow-field velocity or species measurements, respectively Flexible pulse widths are used to balance the energy distribution of pulse doublets and achieve second-harmonic conversion efficiencies up to 42% Burst-mode laser performance characteristics, measurement accuracies in turbulent flows, and prospects for kHz–MHz flow-field diagnostics are discussed

Interference-free hybrid fs/ps vibrational CARS thermometry in high-pressure flames.

Abstract: Interference-free hybrid femtosecond/picosecond vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen is reported for temperature measurements of 1300–2300 K in high-pressure, laminar H2–air and CH4–air diffusion flames up to 10 bar. Following coherent Raman excitation by 100 fs duration pump and Stokes pulses, a time-asymmetric probe pulse is used for the detection of spectrally resolved N2 CARS signals at probe delays as early as ∼200–300 fs. This allows for full rejection of nonresonant contributions while being independent of collisions for single-shot precision of ±2% at elevated pressures. The effects of collisions at longer probe-pulse delays are also investigated to determine the feasibility of varying the detection timing from 200 fs to 100 ps.

Generation of high-energy, kilohertz-rate narrowband tunable ultraviolet pulses using a burst-mode dye laser system.

Abstract: Typical commercial pulsed dye laser systems used in the generation of narrowband, tunable ultraviolet radiation for planar laser-induced fluorescence (PLIF) imaging are optimized for either high (∼5–10 kHz) repetition rates at comparatively low ultraviolet pulse energies (hundreds of microjoules) or high-output pulse energies (>10 mJ) at comparatively low repetition rates (∼10 Hz). In this work we use a frequency-doubled Nd:YAG burst-mode laser to pump a custom dye laser system for high pulse energies and repetition rates of 7.5, 10, and 20 kHz at 566 nm. The frequency-doubled output of over 2.2 mJ/pulse at 283 nm, which can be used for PLIF imaging of combustion radicals, is an order of magnitude higher per pulse energy as compared with continuously pulsed dye laser systems and is ∼3× higher in overall efficiency than a burst-mode optical parametric oscillator at similar wavelengths. The influence of repetition rate, pump energy, and dye concentration on the output conversion efficiency and pulse-to-pulse stability of the current system is discussed.

Femtosecond, two-photon, laser-induced fluorescence (TP-LIF) measurement of CO in high-pressure flames.

Abstract: Quantitative, kiloherz-rate measurement of carbon monoxide mole fractions by femtosecond two-photon, laser-induced fluorescence (TP-LIF) was demonstrated in high-pressure, luminous flames over a range of fuel-air ratios. Femtosecond excitation at 230.1 nm was used to pump CO two-photon rovibrational X1Σ+→B1Σ+ transitions in the Hopfield–Birge system and avoid photolytic interferences with excitation irradiance ∼1.7×1010 W/cm2. The effects of excitation wavelength, detection scheme, and potential sources of de-excitation were also assessed to optimize the signal-to-background and signal-to-noise ratios and achieve excellent agreement with theoretically predicted CO mole fractions at low and high pressure.

Burst-mode OH/CH2O planar laser-induced fluorescence imaging of the heat release zone in an unsteady flame.

Abstract: The paper presents simultaneous high-speed (7.5 kHz) planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH4/O2/N2 flame. For this purpose, a dye laser designed for high-speed operation is pumped by the second-harmonic 532 nm output of a Nd:YAG burst-mode laser to produce a tunable, 566 nm beam. After frequency doubling a high-energy kHz-rate narrowband pulse train of approximately 2.2 mJ/pulse at 283 nm is used for excitation of the OH radical. Simultaneously, CH2O is excited by the frequency-tripled output of the same Nd:YAG laser, providing a high-frequency pulse train over 10 ms in duration at high pulse energies (>100 mJ/pulse). The excitation energies enable signal-to-noise ratios (SNRs) of ~10 and ~60 for CH2O and OH PLIF, respectively, using a single high-speed intensified CMOS camera equipped with an image doubler. This allows sufficient SNR for investigation of the temporal evolution of the primary heat release zone and the local flame structure at kHz rates from the spatial overlap of the OH- and CH2O-PLIF signals.

KHz–MHz Rate Laser-Based Tracking of Particles and Product Gases for Multiphase Blast Fields

Abstract: An imaging system for analysis of particle fields and product gases in a multiphase blast is proposed. The approach combines a kHz-MHz-rate burst-mode laser with particle image velocimetry (PIV) or tomographic feature tracking velocimetry (FTV) to resolve flowfields over a wide range of time scales.

Four-Dimensional X-ray Imaging of Multiphase Flows

Abstract: Four-dimensional x-ray measurements are demonstrated in an optically complex spray using three x-ray sources and three high-speed imaging systems. Time-evolving volumes are reconstructed from the quantitative two-dimensional path length data.