Mark Gragston

Sensitivity, stability, and precision of quantitative Ns-LIBS-based fuel-air-ratio measurements for methane-air flames at 1-11 bar.

Nanosecond laser-induced breakdown spectroscopy (ns-LIBS) is employed for quantitative local fuel-air (F/A) ratio (i.e., ratio of actual fuel-to-oxidizer mass over ratio of fuel-to-oxidizer mass at stoichiometry, measurements in well-characterized methane-air flames at pressures of 1-11 bar). We selected nitrogen and hydrogen atomic-emission lines at 568 nm and 656 nm, respectively, to establish a correlation between the line intensities and the F/A ratio. We have investigated the effects of laser-pulse energy, camera gate delay, and pressure on the sensitivity, stability, and precision of the quantitative ns-LIBS F/A ratio measurements. We determined the optimal laser energy and camera gate delay for each pressure condition and found that measurement stability and precision are degraded with an increase in pressure. We have identified primary limitations of the F/A ratio measurement employing ns-LIBS at elevated pressures as instabilities caused by the higher density laser-induced plasma and the presence of the higher level of soot. Potential improvements are suggested.

Spatially localized, see-through-wall temperature measurements in a flow reactor using radar REMPI

See-through-wall coherent microwave scattering from resonance-enhanced multiphoton ionization (REMPI) for rotational temperature measurements of molecular oxygen has been developed and demonstrated in a flow reactor at atmospheric pressure. Through limited, single-ended optical access, a laser beam was focused to generate local ionization of molecular oxygen in a heated quartz flow reactor enclosed by ceramic heating elements. Coherent microwaves were transmitted, and the subsequent scattering off the laser-induced plasma was received, through the optically opaque ceramic heater walls and used to acquire rotational spectra of molecular oxygen and to determine temperature. Both axial and radial air-temperature profiles were obtained in the flow reactor with an accuracy of ±20  K⁢(±5%). The experimental results show good agreement with a steady-state computational heat transfer model. This technique shows great potential for non-invasive, high-fidelity measurement of spatially localized temperature and radical species concentration in combustion kinetic experiments and confined combustors constructed of advanced ceramic materials in which limited or non-existing optical access hinders usage of conventional optical diagnostic techniques to quantify thermal non-uniformity.

Acoustic detection of resonance-enhanced multiphoton ionization for spatially resolved temperature measurement

In this Letter, acoustic detection of resonance-enhanced multiphoton ionization (A-REMPI) is characterized and used to measure spatially resolved O2 rotational temperature in air. The acoustic signal is generated using O2 REMPI in air and is detected by a single microphone operating within the audible range. Compared to electron number measurements by coherent microwave scattering, nonlinear light absorption and subsequent local pressure perturbation are captured by the microphone. A typical acoustic cycle of compression and rarefication of the acoustic wave is observed in the A-REMPI. Since the pressure perturbation can be regarded as close to thermodynamic equilibrium, the rotational temperature measured by A-REMPI is lower and closer to the realistic condition.

Single-shot nanosecond-resolution multiframe passive imaging by multiplexed structured image capture

The Multiplexed Structured Image Capture (MUSIC) technique is used to demonstrate single-shot multiframe passive imaging, with a nanosecond difference between the resulting images. This technique uses modulation of light from a scene before imaging, in order to encode the targets temporal evolution into spatial frequency shifts, each of which corresponds to a unique time and results in individual and distinct snapshots. The resulting images correspond to different effective imaging gate times, because of the optical path delays. Computer processing of the multiplexed single-shot image recovers the nanosecond-resolution evolution. The MUSIC technique is used to demonstrate imaging of a laser-induced plasma. Simultaneous single-shot measurements of electron numbers by coherent microwave scattering were obtained and showed good agreement with MUSIC characterization. The MUSIC technique demonstrates spatial modulation of images used for passive imaging. This allows multiple frames to be stacked into a single image. This method could also pave the way for real-time imaging and characterization of ultrafast processes and visualization, as well as general tracking of fast objects.

Time-resolved correlated measurement of laser-induced-breakdown spectroscopy and electron number density: application to high-pressure hydrocarbon flames

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.