Fluid flow and local heat transfer in a microchannel with single and array of pin fins have been studied. For the single pin fin case, a microchannel with a 150-µm diameter pin fin with a tip clearance was experimentally and numerically studied for three Reynolds numbers in laminar regime. Tip clearances of 0, 30, 45 and 100 µm in a 200-µm high microchannel. Experimental and numerical local temperatures and the corresponding Nusselt numbers along the centerline of the pin fin were presented and discussed. Local temperatures were measured on top of the heater surface and downstream the pin fin through micro resistance temperature detectors (RTDs). A conjugate CFD modeling capable of simulating solid/fluid conduction and convection revealed velocity, heat flux and heat transfer coefficient over the heated surface. Nusselt number and wake length for a range of tip clearances were presented and compared with full-height pin fin. Experimental and numerical results showed that a tip clearance can significantly enhance heat transfer in the wake region. Simulations revealed that tip clearance alters the flow structure by increasing the three dimensionality of the flow, promoting mixing, shortening the wake region, and increasing the velocity downstream the pin fin. A tip clearance with a height of 100 µm was found to provide the best heat transfer enhancement. For a microchannel with array of pin fins with tip clearance, an experimental study carried out with the tip clearance of 0 and 100 µm in a 200-µm high microchannel. Results revealed that introducing tip clearance in pin array can on-average almost double heat transfer coefficient compared to full height (no tip) array of pin fins.

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An experimentally obtained comparison between the breakup of water droplets in the flow field behind both a detonation wave and shock wave is considered. The experiments presented here were completed to support ongoing research efforts into droplet breakup mechanisms at different Mach and Weber numbers. The physical features of the droplets are observed using a high-speed camera and shadowgraph imagery. Droplets are roughly between 2-3 mm in diameter and are struck by detonation waves of Mach 5-6 and shock waves induced by deflagration combustion events of Mach 1-2. The Weber number of these experiments ranges from 5(10^3) to 90(10^3). These experiments were initiated in a detonation tube using four separate mixtures to allow for the creation of shock waves in the detonation tube, which consisted of hydrogen and oxygen or methane and oxygen at different equivalence ratios and once with the addition of nitrogen. Additionally, the breakup of these droplets is compared by non-dimensionalizing the displacement of fluid at the equator of the droplet, which is further compared to predictions made by the Taylor Analogy Breakup model. Attempts are made to determine the influence of factors other than Weber number on the deformation of a water droplet, while also considering the effects of Weber number.

Pressure pulsations, or thermoacoustic instabilities, as they are called in the research community, can cause extensive damage in gas turbine combustion chambers. To understand the phenomena related to thermoacoustics, a simple Rijke-type tube combustor was built and studied. Extensive experimental results, as well as theoretical analyses related to the Rijke tube are presented in this thesis. The results, attributable to both the analyses and the experiments, help explain all the phenomena affecting the acoustic pressure in the combustor. The conclusion is that there are three separate yet related physical processes affecting the acoustic pressure in the tube. The three mechanisms are as follows: a main thermoacoustic instability in accordance to the Rayleigh Criterion; a vibrating flame instability where the flame sheet exhibits mode shapes; and a pulsating flame instability driven by heat losses to the flame stabilizer. All these instabilities affect the heat released to the gas in the combustor. The energy from the oscillating heat couples with the acoustics of the volume bounded by the tube structure. The experimental results in the study are important in order to obtain model parameters for prediction as well as for achieving control of the instabilities. / Master of Science

This work investigated the application of fiber-based endoscopes (FBEs) in combustion and flow measurements, especially for multidimensional and quantitative measurements. The use of FBEs offers several unique advantages to greatly reduce the implementation difficulty and cost of optical diagnostics. However, the use of FBEs requires registering the locations and orientations of the FBEs carefully for quantitative measurements, and degrades the spatial resolution of the images transmitted. Hence this work conducted a series of controlled tests to quantify the accuracy of the view registration process and the spatial resolution degradation for FBEs. The results show that, under the conditions tested in this work, the view registration process can be accurate within ±0.5 degree and the FBEs can resolve spatial features on the order of 0.25 mm. The combined effects of such view registration uncertainty and spatial resolution degradation are reflected in the re-projection error, which was shown to be within ±0.5 pixels under typical conditions used in this work. Finally, based on these understanding, experiments were conducted to obtain instantaneous measurements of flame structures at kHz temporal resolution using FBEs, demonstrating the capability of resolving flame features on the order of 0.2~0.3 mm in three-dimensional. / Master of Science

This thesis studies how plasmas created by nanosecond repetitive pulsed discharges (NRPD) can affect and improve the combustion characteristics in a high-speed fluid flow that simulates scramjet conditions. This is done by creating a computational code that incorporates the effects of plasmas, high-speed fluid dynamics, combustion chemistry, and photoionization. Many physical effects across multiple temporal and spatial scales appear, and creating a code that efficiently and accurately models these effects was the biggest contribution of this research. A new chemical mechanism has been created that incorporates high energy states for nitrogen and oxygen. This code was applied to examine how NRPD is affected by high-speed fluid flows and different electrode geometries. In quiescent simulations, the multiple pulses couple with each other increasing the overall temperature, which can lead to ignition due to the plasma added. When there is a freestream flow the convection of the previous pulses plasma can prevent coupling between the pulses. Without modification to pulse characteristics (increase in frequency, intensity, or length), combustion may not be achieved. Next, a more applied study of a three-dimensional scramjet is conducted to examine how the plasma affects the flow by the scramjet geometry and conditions. These larger simulations add effects from turbulence by implementing an LES-EDC model. These simulations show how plasmas generated by NRPD can affect the fluid flow inside a scramjet combustor cavity. / Doctor of Philosophy / Chemical kinetics is often too slow compared to turbulent mixing in high-speed propulsion, limiting the effectiveness of conventional flame stabilization devices. This project investigates how a non-equilibrium plasma can support combustion in a turbulent supersonic combustor at Mach 2. Plasma can support both vibrational-electronic energy exchanges and radical branching boosting the ignition time-scales up to the microsecond range. This thesis centers around the development of a CFD model that incorporates the effects of high-speed convection, photoionization, and plasma effects by using the drift-diffusion equation. In addition, a novel chemistry model has been developed to model the ultra-fast chemistry of triplet nitrogen states, these states appear in the plasmas that are studied. A verification and validation process is conducted on the code and its various components. This code is then used to study how nanosecond repetitive pulsed discharges (NRPD), which are an efficient way to create plasmas, are affected by scramjet flow conditions. The results in this thesis show that these plasmas can increase the temperature and improve the conditions for combustion. Two major studies have been done in this thesis with this code. First, the physics of the energy transfer is studied for the NRPD in a computational domain containing differently shaped electrodes (both flat and curved electrodes). The flat electrodes provide the strongest energy transfer to the plasma and the fluid. Next, large-scale simulations on three-dimensional scramjet geometries are performed and compared to the experiments. The effect of the electrode placement in the cavity is discussed.