Application of Resonant and Non-Resonant Laser-Induced Plasmas for Quantitative Fuel-to-Air Ratio and Gas-Phase Temperature Measurements

In this work, two laser-induced plasma techniques are used for gas-phase chemical and temperature measurements. The first technique, laser-induced breakdown spectroscopy (LIBS) is applied for fuel-to-air ratio (FAR) measurements in a well calibrated Hencken flame. In Chapter I, relevant technical and background information for each technique is provided. In Chapter II, measurements are first performed for high-pressure (1-11 Bar) methane-air flames, for which calibration curves are generated using the emission ratio of hydrogen at 656 nm and ionic nitrogen at 568 nm. The effect of pressure on the sensitivity and precision of the resulting calibrated curves is evaluated. Results indicate a degradation of measurement precision as environmental pressure increases, with data indicating that fluctuations of the plasma play a major part in this behavior. Expanding upon this work with LIBS, a comparison of FAR calibration curve results for atmospheric methane-air Hencken flame using three different laser pulse widths, femto-, pico-, and nanosecond regimes, is done in Chapter III. The results are discussed in the context of potential advantages for high-pressure LIBS-based FAR measurements. Results indicate that while nanosecond duration pulses provide better precision at 1 Bar conditions, femtosecond duration pulses might be better suited for high-pressure measurements.In Chapter IV, the radar REMPI technique, which uses microwave scattering from a plasma created by selective multiphoton ionization of molecular oxygen, is used for gas-phase temperature measurements through the wall of ceramic-enclosed environments. Specifically, measurements are done through the wall of a heated laboratory flow reactor and through the wall of a ceramic well-stirred reactor. Results show good agreement with thermocouple and/or computational modeling and the effectiveness of radar REMPI for through-the-wall measurements.In Chapter V, a new technique is discussed, namely acoustic REMPI, which utilizes the pressure wave generated from the creation of the REMPI plasma for diagnostics. The acoustic emission from the plasma is characterized and used for gas-phase temperature measurements. Comparison, with radar REMPI shows a high-level of agreement.Finally, in Chapter VI, a summary of the work in this dissertation is provided along with a discussion of potential for work in the future

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 target’s 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

Measurement of Electron Density and Temperature from Laser-induced Nitrogen Plasma at Elevated Pressure (1–6 bar)

Laser-induced plasmas experience Stark broadening and shifts of spectral lines carrying spectral signatures of plasma properties. In this paper, we report time-resolved Stark broadening measurements of a nitrogen triplet emission line at 1–6 bar ambient pressure in a pure nitrogen cell. Electron densities are calculated using the Stark broadening for different pressure conditions, which are shown to linearly increase with pressure. Additionally, using a Boltzmann fit for the triplet, the electron temperature is calculated and shown to decrease with increasing pressure. The rate of plasma cooling is observed to increase with pressure. The reported Stark broadening based plasma diagnostics in nitrogen at high pressure conditions will be significantly useful for future studies on high-pressure combustion and detonation applications.Abstract © OSA

Time-Gated Single-Shot Picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) for Equivalence-Ratio Measurements

Time-gated picosecond laser-induced breakdown spectroscopy (ps-LIBS) for the determination of local equivalence ratios in atmospheric-pressure adiabatic methane–air flames is demonstrated. Traditional LIBS for equivalence-ratio measurements employ nanosecond (ns)-laser pulses, which generate excessive amounts of continuum, reducing measurement accuracy and precision. Shorter pulse durations reduce the continuum emission by limiting avalanche ionization. Furthermore, by contrast the use of femtosecond lasers, plasma emission using picosecond-laser excitation has a high signal-to-noise ratio (S/N), allowing single-shot measurements suitable for equivalence-ratio determination in turbulent reacting flows. We carried out an analysis of the dependence of the plasma emission ratio Hα (656 nm)/NII (568 nm) on laser energy and time-delay for optimization of S/N and minimization of measurement uncertainties in the equivalence ratios. Our finding shows that higher laser energy and shorter time delay reduces measurement uncertainty while maintaining high S/N. In addition to atmospheric-pressure flame studies, we also examine the stability of the ps-LIBS signal in a high-pressure nitrogen cell. The results indicate that the plasma emission and spatial position could be stable, shot-to-shot, at elevated pressure (up to 40 bar) using a lower excitation energy. Our work shows the potential of using ps-duration pulses to improve LIBS-based equivalence-ratio measurements, both in atmospheric and high-pressure combustion environments

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Beam focus of both lasers separated into two subplots

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Beam focus of both lasers overlayed on each other

Effects of Surface Roughness on Shock-Wave/Turbulent Boundary-Layer Interaction at Mach 4 over a Hollow Cylinder Flare Model

Although it is understood that surface roughness can impact boundary layer physics in high-speed flows, there has been little research aimed at understanding the potential impact of surface roughness on high-speed shock-wave/boundary-layer interactions. Here, a hollow cylinder flare model was used to study the potential impact of distributed surface roughness on shock-wave/boundary-layer interaction unsteadiness. Two surface conditions were tested—a smooth steel finish with an average roughness of 0.85 μm and a rough surface (3K carbon fiber) with an average roughness value of 9.22 μm. The separation shock foot from the shock-wave/boundary-layer interaction on the hollow cylinder flare was tracked by analyzing schlieren images with a shock tracking algorithm. The rough surface increased boundary layer thickness by approximately a factor of 10 compared to the smooth case, significantly altering the interaction scaling. Despite normalizing results, based on this boundary layer scaling, the rough surface case still exhibited mean shock foot positions further upstream more than the smooth surface case. Power spectra of the unsteady shock foot location data demonstrated that the rough surface case exhibited unsteady motion with attenuated energy relative to the smooth-wall case