Understanding gas-phase combustion reactions in turbulent flows and energetic materials requires high-speed multiparameter imaging at kHz-MHz frame rates. Such measurements require energies per pulse in the range of 10s to 100s mJ at MHz rates, which led to development of burst-mode nanosecond lasers with burst durations up to 100 ms and energies per burst of up to 400 J. Burst-mode laser technology have enabled significant advances in measurement capabilities, for accessing multiple species, temperature, and velocity. However, the number of simultaneously measured parameters are limited by number of lasers and interference between modalities having similar output spectral characteristics. Here we report the laser system with three outputs with up to 1 J/pulse at 10 kHz and variable time delays between outputs to avoid interference between modalities. The laser is based on the common all-fiber oscillator and free-space Nd:YAG flashlamppumped preamplifier to reduce the cost and size of the overall system. The oscillator/preamplifier produces up to four pulses with user-selected time delays between pulses. This four-pulse sequence is repeated at user selected repetition rate up to 500 kHz. The pulse sequence is split between three free-space Nd:YAG amplifiers by two Pockels cells with time resolution of 10 ns. Special precaution is taken to minimize the crosstalk between amplifiers with less than 5% prepulse/afterpulse in the fundamental output and less than 1% in the harmonics. The system is designed for measurements of species (via planar laser induced fluorescence, PLIF) and velocity (via particle image velocimetry, PIV) at rates up to 500 kHz and total burst duration of 10 ms, with the feasibility of achieving MHz rates with further developments. The simultaneous measurements of velocity using PIV and PLIF imaging of hydroxyl and formaldehyde in a turbulent jet flame is demonstrated.
Application of time-resolved ultrashort-pulse-laser–based laser-induced breakdown spectroscopy (LIBS) is proposed for diagnostics in hydrocarbon flames at elevated pressures. Earlier study in nanosecond-laser–based LIBS for measuring fuel-to-air (F/A) ratios in stable hydrocarbon flame at high pressures is reported to have high measurement instabilities. A correlated measurement of time-resolved LIBS and electron number density measurement is carried out to develop an understanding of the source of the aforesaid measurement instability. The planned correlated measurements of LIBS and electron number density with short picosecond to femtosecond excitations have potentials to significantly reduce the instability in F/A ratio measurements at high pressure.
Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREECARS) spectroscopy, incorporating a two-color ultraviolet excitation scheme, is used to demonstrate chemically selective and sensitive detection of gas-phase species, including nitric oxide (NO) and the hydroxyl (OH) radical. The observed time-dependent, spectrally resolved CARS signal contains rich structure that depends both on the rovibronic states accessed within the bandwidth of the initial (pump) excitation pulse and the Raman-active rovibrational levels within the vibrationally excited ground electronic state that are accessed following interaction with the second (Stokes) excitation pulse. By comparing experimental spectra to computational simulations, therefore, this approach also allows simultaneous determination of local temperature associated with the thermal distribution of initial states under singlelaser- shot conditions. For OH radical detected in a reacting flow, spectral resolution of the emitted FREE-CARS signal allows simultaneous single-shot detection of relative OH mole fraction and temperature in a laminar ethylene–air flame at 1-kHz repetition rates. By comparison to previously reported OH concentration and temperature measurements, we demonstrate excellent single-shot temperature accuracies (~2% deviation from adiabatic flame temperature) and precisions (~2% standard deviation), with simultaneous relative OH concentration measurements that demonstrate high detection sensitivity (100–300 ppm).
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.
Zeolite catalysis has been exploited by the petrochemical industry since the 1940’s for catalytic cracking reactions of long chain hydrocarbons. The selectivity of zeolites strongly depends on a pore size, which is controlled by the chosen structure-directing agent (SDA) and by the SDA decomposition/removal process. Although zeolites are composed of micron-sized crystals, studies of zeolite materials typically focus on bulk (i.e., ensemble) measurements to elucidate structure-function information or to optimize catalysts and/or process parameters. To examine these phenomena on the microscale, non-linear optical microscopy is used to provide real-time imaging of chemical reactions in zeolites at temperatures exceeding 400°C. The template decomposition mechanism is studied, as elucidation of the mechanism is critical to understanding the relationship between the decomposition chemistry and the nanoscale features of the zeolite (topology, Si/Al ratio, added dopants). Forward stimulated Raman scattering (SRS), forward coherent anti-Stokes Raman scattering (CARS) and epi two-photon fluorescence (TPF) modalities are acquired simultaneously providing video-rate structural and chemical information. A high-temperature cell with gas inlet system is used for the study of reactions under various temperatures and gas environments. Examining the decomposition process with single-particle resolution enables access to ensemble-level and spatially-resolved behavior. Parallel experiments on bulk zeolite powders are conducted to enable comparison of ensemble and single-particle behavior during template decomposition. Our multi-technique approach has high potential for gaining insight into the link between nanoscale structure and catalytic activity and selectivity of zeolitic materials.
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