Master of Science in Mechanical Engineering (MSME), Wright State University, 2018, Mechanical Engineering

As technology advances, the abilities of civilian and military vehicles, both air and ground, will undoubtedly increase as well. One of the main areas of improvement is in the electronics area. The new electronics are ever smaller, use ever higher amounts of electrical power, and require ever smaller temperature tolerances. This leads to the problem of effectively managing the increasing thermal loads and temperature tolerances on these systems. One electronic system that causes concern is a high energy pulse system (HEPS). These devices have very high thermal loads (100s of kW). On an air vehicle, where thermal management by legacy methods (i.e. fuel as the heat sink) is already problematic, a HEPS will certainly overload the thermal management system (TMS). HEPS performance must be understood and quantified more accurately to understand the design requirements of a TMS for this device. To aid in this understanding, the HEPS itself and a palletized system to thermally manage the HEPS will be modeled. Previous analysis of a cryogenic palletized HEPS contained a simplified power model for a HEPS that had a set efficiency and always gave a certain amount of optical power out and a certain amount of power dissipated as heat based on that set efficiency. The HEPS model developed and presented takes into account the temperature of internal HEPS components and changes the efficiency accordingly. The HEPS efficiency changes with component temperature to provide a better understanding of the consequences of not thermally managing a HEPS effectively. Along with the HEPS model, a cryogenic-based palletized TMS using Liquefied Natural Gas (LNG) for indirectly cooling the HEPS was modeled. Using LNG as a method of cooling is a possible alternative to using very large legacy systems (fuel as heat sink) to cool a HEPS. The architecture of this palletized system uses LNG to cool the heat loads. The LNG then becomes the fuel for the turbo-generator, which produces electrical po (open full item for complete abstract)

Committee: Rory Roberts Ph.D. (Advisor); Mitch Wolff Ph.D. (Committee Member); Zifeng Yang Ph.D. (Committee Member) Subjects: Aerospace Engineering; Engineering; Mechanical Engineering

Doctor of Philosophy (PhD), Wright State University, 2022, Engineering PhD

High-power laser systems (HPLS) have wide-ranging applications in many prominent areas. HPLS use laser diodes to pump fiber gain media. Understanding the functionality of both components is critical for achieving effective HPLS operation. System optical efficiency is a function of diode junction temperature. As junction temperature changes, the wavelength spectrum of the diode output shifts causing optical power losses in the fiber gain media. Optical/thermal interactions of the dynamically coupled laser diodes and fiber gain media are not fully understood. A system level modeling approach considering the interactions between optical performance and component temperature is necessary. Four distinct models were created: Diode optical, diode thermal, fiber optical, and fiber thermal. Dynamically coupling these models together provided the capability to demonstrate how HPLS electro-to-optical efficiency changes when the laser diode pump spectrum shifts due to various levels of thermal management. Subsequent studies were done to determine which parameters across all four models had the most significant impact on laser performance from a designer's perspective. Next, a statistical surrogate model was created by varying these parameters to create a parameter space. Response variables of interest were then reduced to a single equation as a function of these important parameters across the parameter space, allowing for quicker exploration of the potential design space. Lastly, laser time to steady state and laser efficiency were employed to determine when a specific diode cooling method should be used to achieve the highest laser efficiency. Understanding the optical/thermal interactions of laser operation and exploring the impact of various thermal capabilities can provide better system design and optimization guidelines. Bridging the gap between the optical and thermal aspects of laser operation in pursuit of such understanding has been made possible by the re (open full item for complete abstract)

Committee: Rory Roberts Ph.D. (Advisor); Mitch Wolff Ph.D. (Committee Member); George Huang Ph.D. (Committee Member); Amir Farajian Ph.D. (Committee Member); Soumya Patnaik Ph.D. (Committee Member) Subjects: Mechanical Engineering; Optics

Master of Science, The Ohio State University, 2020, Aero/Astro Engineering

Non-self-sustained hybrid plasmas are formed by the overlap of two separate voltage waveforms with significantly different reduced electric field values (E/N), one of them below the ionization threshold, to produce excited species and radicals selectively. In this work, a stable, capacitively coupled ns pulse – RF waveform hybrid discharge is operated in nitrogen and mixtures of nitrogen with other molecular gases at 50 – 100 Torr pressure, using a single pair of electrodes mounted externally to the reactor cell. The purpose of the ns pulse discharge is to generate ionization and electronic excitation of the mixture components, while the below-breakdown RF voltage couples additional energy to the vibrational modes of the mixture components. Based on the broadband plasma emission imaging, the plasma volume appears to be enhanced by the RF waveform, compared to ns pulse discharge, due to the drift oscillations of electrons induced by the RF waveform. Coherent Anti-Stokes Raman Spectroscopy (CARS) measurements in the hybrid discharge operated in nitrogen show that the RF waveform significantly enhances the vibrational excitation of N2 in the ground electronic state, populating vibrational levels up to at least v=3, and increasing the vibrational temperature of N2 from TV = 1210 ± 110 K in the ns pulse train plasma to TV = 1810 ± 170 K in the ns-RF hybrid discharge. The translational- rotational temperature at these conditions remains low, TR = 315 ± 15 K. To evaluate the potential of this plasma to operate in other gas mixtures, 1% of H2 is added to nitrogen. CARS measurements reveal a moderate N2 vibrational relaxation by hydrogen, reducing the vibrational temperature in the hybrid plasma to TV = 1700 ± 150 K and increasing in the translational-rotational temperature to TR = 396 ± 10 K. Time-resolved measurements of the number density of the first electronically excited state of nitrogen, N2(A3Σ), obtained using Tunable Diode Laser Absorption Spectroscopy (TDLAS) in n (open full item for complete abstract)

Committee: Igor Adamovich (Advisor); Jeffrey Sutton (Committee Member) Subjects: Chemistry; Energy; Engineering; Environmental Engineering

Master of Science (MS), Ohio University, 2014, Electrical Engineering (Engineering and Technology)

Solid-state white lighting devices (SSWLDs) commonly use III-nitride near-UV or blue light emitting diodes (LEDs), combined with one or more phosphors, to generate white light. These devices already offer many advantages over traditional incandescent and fluorescent light sources, including long lifetimes, environmentally friendly designs without the need for mercury, and enormous energy savings. Despite unquestionable recent commercial success and the future potential for further development, current SSWLDs suffer from the droop effect limiting the overall efficacy and a thermally-induced shift in the peak emission wavelength of the phosphor. Thus, the overall efficiency of these devices can still be improved. One such example is to control the operating temperature of the device. When operating an LED, the temperature inevitably increases, yet the phosphor particles exhibit a loss in efficiency as the temperature of the device increases. In addition, LEDs suffer from efficiency loss and color instability with increased operating current, making high-power devices not achievable using LEDs as the excitation source. Recently, a new concept for developing SSWLD, based on laser diode (LD) substituting for LED as a pump source for exciting colour-stable phosphors, was proposed. In contrast to LEDs, laser diodes do not exhibit efficiency loss; many exhibit increased efficiency as current increases, and maintain color stability. Thus, there is a need in the art for improved solid-state white lighting devices that rely on laser diodes. In this project we have characterized individual Eu(WO4)2 (red phosphor), BaMg2Al16O27:Eu,Mn (green phosphor) and (Sr,Mn)2SiO4:Eu (blue phosphor) and trichromatic white light phosphors. Characteristics of light produced with each phosphor, variations with incident light power and phosphor temperature, as well as effects from phosphor ageing, are described. Results of comparison between pumping with coherent and incoherent light at the same (open full item for complete abstract)

Committee: Wojciech Jadwisienczak (Advisor) Subjects: Electrical Engineering

MS, University of Cincinnati, 2010, Engineering : Aerospace Engineering

An investigation of the viability of tunable diode laser absorption spectroscopy for use as a flow measurement device in a scramjet engine was completed. First, the effects on TDLAS measurements across a temperature jump that is common in scramjet combustor flow-paths was studied using a flat flame burner designed with four independently fueled quadrants. Rigorous thermocouple mapping of the burner was performed and a discussion of multi-thermocouple radiation correction techniques is presented. The fundamental mass capture measurements (temperature, water number density, pressure, and velocity) were then made in the isolator section of a direct-connect scramjet engine and compared to a scramjet performance analysis code. Post-combustion measurements (temperature and water number density) were measured in the exhaust section of the model engine. The results of the measurements and an in-depth discussion of analysis routines used in the processing of raw absorption measurements is presented.

Committee: San-Mou Jeng PhD (Committee Chair); Ephraim Gutmark PhD, DSc (Committee Member); Michael Brown PhD (Committee Member) Subjects: Aerospace Materials

Master of Science, Miami University, 2007, Physics

In this thesis, I experimentally investigated Electromagnetically Induced Transparency (EIT) using the Hanle effect in Rubidium (Rb) atomic vapor. The Hanle effect results in a sharp coherent feature in the absorption of the incident beam at zero magnetic field. We have observed this Hanle peak with good signal-to-noise ratio in the Rb87 transition. Hanle features in this particular transition of Rb have never been previously observed. Our goal is to use the Hanle effect as a sensitive measure of radiation trapping.

Committee: Samir Bali (Advisor) Subjects:

Master of Science, Miami University, 2005, Physics

We have constructed an external cavity diode laser (ECDL) with an easily adjustable, variable cavity length in the Littman-Metcalf configuration to improve the poor beam quality of a 2 W Coherent Inc. broad-area laser (BAL) as well as study the effect of varying dispersive feedback. Our results at four different cavity lengths show that the individual modal linewidth varies with an L -1/2 cavity length dependence while the effective bandwidth of the ECDL was not altered. This suggests that BALs behave differently in external cavities. We describe the detailed characteristics of our ECDL and the passive stabilization techniques employed to keep the laser at a fixed wavelength for long periods, which is important for our intended application to spin-exchange optical pumping experiments using a Rb- 129 Xe mixture. We discuss other applications of this laser that include high-resolution spectroscopy and laser cooling, and provide a review of BAL coupling techniques.

Committee: Burcin Bayram (Advisor) Subjects:

Doctor of Philosophy, University of Akron, 2008, Chemistry

A diode laser system was utilized to obtain spatially and temporally resolved absorption spectra during flame spread through non-homogenous fuel-air mixtures. The diode laser was wavelength-modulated in order to increase detection sensitiviy of the system. Gas concentrations were determined from the absorption spectra using regression models.Two fundamentally different systems were studied in this work. Methane concentrations were determined from absorption measurements in a buoyant plume of methane and air formed in a vertical low speed flow tunnel. Methanol and water concentrations were determined from absorption measurements in a system in which a non-uniform methanol-air layer was formed by evaporation along a horizontal gallery floor. In both systems, the layered mixture was ignited and measurements were obtained as the flame propagated. Measurements in the gallery were made in both terrestrial and microgravity environments. Thus, the ability to measure gas concentration using a system that meets the severe restrictions of microgravity research was demonstrated.

Committee: David Perry PhD (Advisor); Fletcher Miller PhD (Advisor); James Hardy PhD (Committee Member); Jun Hu PhD (Committee Member); Matthew Espe PhD (Committee Member); Rex Ramsier PhD (Committee Member) Subjects: Chemistry