Heat transfer inside microscale geometries is a complex and a challenging phenomenon. As supercritical fluids display large variations in their properties in the vicinity of the critical point, their usage could be more beneficial than traditional coolants. This numerical study, in two parts, primarily focuses on the physics that drives the enhanced heat transfer characteristics of carbon dioxide (CO2) near its critical state and in its supercritical state. In the first part of the study, the flow of supercritical Carbon Dioxide (sCO2) over a heated surface inside a microchannel of hydraulic diameter 0.3 mm was studied using three-dimensional computational fluid dynamics (CFD) model. The temperature of the heated surface was then compared and validated with available experimental results. Also, the heat transfer coefficients were predicted and compared with experiments. Additionally, the acceleration and pressure drop of the fluid were estimated and it was found that the available correlations for conventional fluids failed to predict the flow characteristics of the CO2 due to its supercritical nature. In the second part of the analysis, a relatively new phenomenon known as the Piston Effect (PE), also known as the fourth mode of heat transfer, was studied numerically inside a microchannel of depth 0.1 mm using a two-dimensional CFD model, and it was found that the adiabatic thermalization caused by PE was significant in microgravity and terrestrial conditions and that the time scales associated with the PE are faster than the diffusion time scales by a factor of 5 to 6400. In addition, this study revealed the presence of PE in laminar forced convective conditions. A new correlation was developed to predict the temperature raise of the bulk fluid that is farthest from the heated surface.

Understanding the combustion chemistry of ethanol is critical for continued proliferation and use in future internal combustion engines (ICEs) that will operate in a downsized, turbo-charged, high compression configuration. Detailed chemical kinetic reaction mechanisms already exist for ethanol, which have been validated over a range of operating conditions; however, capturing the conditions that may be seen in future ICEs requires extension of these conditions, namely at elevated pressure. Investigating the kinetics of ethanol existing in a combustion system first involve, for example, understanding a key global metric like ignition delay time (IDT) and measuring major or minor species in a time resolved fashion capturing both formation and decomposition stages. A shock tube facility offers ideal (thermodynamically) operation that can be used to study the high pressure kinetics across a wide range of temperatures, all the while enabling non- intrusive temporal in situ measurements within the given test time. Oxidation of ethanol was carried out behind reflected shock waves at elevated pressures by measuring IDTs and carbon monoxide (CO) time-histories, the latter of which utilized a distributed feedback quantum cascade laser centered at a wavelength in the infrared (IR). With the gathered data, various ignition regimes and sensitive chemistry were investigated for high pressure CO formation. Since CO is an important product of combustion, having an accurate prediction of its formation is necessary to preliminarily understand the efficiency and sustainability of future engine designs. Moving forward, hazardous products like CO among other harmful emissions will have stricter governmental constrains, which further supports studies as these that aid in the continued refinement of such chemical kinetic mechanisms.

Shock tubes are considered ideal reactors and are used extensively to provide valuable chemical kinetic measurements, such as ignition delay times and in-situ species time-histories. However, due to nonideal affects the combustion of fuel inside shock tubes can become nonhomogeneous, particularly at low temperatures, which complicates the acquired data. In this work, the combustion of practical fuels used by society are investigated with high-speed imaging. First, high-speed images were captured through the end wall of the shock tube for two hydrogen-oxygen systems. The combustion process was found to initiate in two modes, one that is homogeneous across the fluid medium and one that proceeds through a deflagration to detonation channel. In the second part of this work, the shock tube test section was redesigned to promote optical access from the end and side walls of the shock tube test section. Two high-speed cameras were used to capture perpendicular views of the combustion of isooctane and n-heptane, two primary reference fuels. A homogeneous and nonhomogeneous combustion process were seen for these fuels as well. Using the side view images, the impact of the sporadic ignition process was evaluated on commonly used diagnostics in shock tubes. Based on these results, it is recommended that shock tube diagnostics be confined to the homogeneous ignition modes of fuels. This is found to strongly correlate with the temperature of the combustion process, where high temperatures promote a homogeneous ignition event.

Jet Impingement and shower head cooling are critical cooling techniques used to maintain turbine blades at operational temperatures. Jet impingement is extremely effective at removing large amounts of heat flux from the target surface, the inner blade wall, through stagnation point heat transfer. Shower head cooling produces a cooling film around the exterior of the blade, in return reducing external heat flux. The current work consisted of investigating the jet impingement effectiveness with rotational effects for two different cooling schemes. The analysis was conducted numerically using STAR CCM+ with two different turbulence models, the three equation Lag Elliptic Blending K Epsilon model and the seven equation Elliptic Blending Reynolds Stress Transport (EB RST) model. The EB RST model incorporated the Generalized Gradient Diffusion method. The blade used was NASA/General Electrics E^3 row 1 blade. Two conjugate heat transfer models were developed for just the leading-edge portion of the blade, one with and one without shower head holes. The models consisted of a quarter of the blade-span to reduce computational expense and only one jet was analyzed. A flow field analysis was performed on the free jet region to analyze the potential core velocity and turbulent kinetic energy profiles. Nusselt Number spanwise distribution and external blade temperature profiles were also evaluated. The investigation showed, for both turbulence models, that rotational effects produce turbulent kinetic energy within the jet's potential core, reducing the incoming jet velocity and hence reducing impingement effectiveness. While both turbulence models illustrated an increase in turbulent kinetic energy throughout the structure of the impinging jet, the magnitudes and locations varied significantly. This is due to the well-known underprediction of turbulent dissipation in the K-Epsilon family of turbulence models, as well as the location of applications of the vorticity tensor to the transport equations.

In this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2 cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% - 85% by volume. A 10% fuel concentration was consistently used throughout the experiments. This study looked at the effects of changing the equivalence ratios (ɸ), between 0.33, 0.5, and 1.0 as well as changing the fuel ratio (θ), hydrogen to carbon monoxide, from 0.25, 1.0 and 4.0. The study was performed at 1.61-1.77 atm and a temperature range of 1006-1162K. The high-speed imaging was performed through a quartz end wall with a Phantom V710 camera operated at 67,065 frames per second. From the experiments, when increasing the equivalence ratio, it resulted in a longer ignition delay time. In addition, when increasing the fuel ratio, a lower ignition delay time was observed. These trends are generally expected with this combustion reaction system. The high-speed imaging showed non-homogeneous combustion in the system, however, most of the light emissions were outside the visible light range where the camera is designed for. The results were compared to predictions of two combustion chemical kinetic mechanisms: GRI v3.0 and AramcoMech v2.0 mechanisms. In general, both mechanisms did not accurately predict the experimental data. The results showed that current models are inaccurate in predicting CO2 diluted environments for syngas combustion.

The direct-fired supercritical CO2 (sCO2) cycle is conceptually superior to many of the trending energy production technologies due to their remarkably promising efficiency, environmental friendliness and cost. The accurate simulation of this combustion is very important because the operating conditions are very challenging to its experimentation. Hence, the current work focuses on identifying various thermal, transport, chemical kinetic models, investigating various fundamental characteristics and verifying the validity of important underlying modeling assumptions in focus to supercritical CO2 combustion. In the current work, various thermal and transport property models are identified based on accuracy, computational cost and ease of implementation for sCO2 combustion simulations. Further, a validated chemical kinetic mechanism is developed for high-pressure and high-CO2 diluted combustion by incorporating state-of-art chemical kinetic rates which are specifically calculated for sCO2 combustor conditions. Also, crucial design considerations are provided for the design of sCO2 combustors based on 0-D and 1-D reactor models. Finally, important characteristics of non-premixed sCO2 combustion are examined by a canonical counterflow diffusion flame study.

Supercritical carbon dioxide (sCO2) can be utilized as a working or heat transfer fluid in various thermal systems with applications in large-scale power cycles; portable power production units, coolant systems and devices. However, there are no sufficient methods and equations of heat transfer coefficient correlations, and in addition insufficient research studies about the mechanisms controlling heat transfer processes for sCO2. This study is motivated by the need to understand the intricate properties of sCO2 heat transfer and fluid dynamics with an emphasis on flow direction and inclination effects. This paper presents the study on effects of gravity, buoyancy on sCO2 flow at temperatures near and away from the pseudocritical temperature. The experimental setup consists of a high temperature and pressure sCO2 heat transfer loop and flow testing facility. Recently researched sCO2 heat exchangers can have tubes oriented at different angles such as 45° or 90° to horizontal. For the optimized design of efficient and cost-effective turbomachinery components utilizing sCO2 as the heat transfer fluid, an understanding of convective heat transfer inside a tube/pipe is equally as important as external heat transfer. A study on sCO2 heat transfer at various inclinations with angles ranging from 0°(horizontal) to 90°(vertical) along with upward and downward flow directions with different inlet temperatures is conducted. Thermocouple-based temperature measurement is utilized at multiple locations within the tube test section axially and circumferentially to study the temperature distributions on the tube surface. Volumetric heat generation is utilized to heat the external wall of the tube test section, Nusselt and Richardson numbers are calculated at circumferential wall location to show the effects of buoyancy and gravity. These Non-dimensional parameters are plotted from experimental data to show the effect of the varying parameters on heat transfer and fluid dynamics properties of the flow. it can be seen that for inlet bulk temperatures near the pseudocritical temperature, buoyant force are stronger but reduce as the inlet temperature and inclination angle is increased the buoyant forces become negligible.

As detonations are being implemented into modern combustion technologies to benefit from the efficiency gain, their properties need to be fully characterized. Of main interest is hydrocarbon fuels given the substantially higher energy density over hydrogen. In thin channels detonations have been known to appear nominally 2D allowing for higher detail line-of-sight imaging techniques. Many studies have investigated hydrocarbon detonations in this mode but have not evaluated the consistency of the key detonation properties. A statistical approach is used in this study by using ensemble averaging over many realizations of the detonation to determine these properties. The experimental data was collected by igniting a pre-mixed Methane-Oxygen-Nitrogen mixture in a confined channel. The detonating wave travels through a converging section to reduce the channel width to the test condition. The detonation is then observed through a combination of high-speed schlieren imaging and a pressure transducer array. This data is then processed to provide quantified statistics for the detonation cell size, Chapman-Jouguet velocity and pressure, and the Von-Neumann pressure spike helping to further the understanding of detonations.

Combustion is a complex physical phenomenon that occurs under various temperature and pressure conditions. Depending on the combustion environment, the reaction pathways of fuels and oxidizers can differ, leading to the formation of different end products. Internal combustion engines and gas turbines typically operate under high temperature and high-pressure conditions. However, in the case of rocket exhaust afterburning, unburned hydrocarbons can undergo combustion in an extreme environment characterized by high temperatures but very low pressures due to the high altitude. Understanding the reaction kinetics in this unique environment is crucial as it can impact the efficiency of supersonic retro propulsion, particularly with regards to flame impingement on spacecraft surfaces. Validating chemical kinetic mechanisms with experimental data is essential for improving their predictive capabilities in both scenarios. This doctoral study aims to validate fuel oxidation mechanisms by providing targets such as ignition delay time, temperature profiles, and temporal evolution of multi-species concentrations under two types of extreme combustion environments: high-temperature/high pressure and high-temperature/low-pressure conditions. The experiments were conducted at the UCF shock tube facility using fixed/scanned wavelength laser absorption spectroscopy. The temperature range varied from 1100 K to 2400 K, and the pressure range from 0.25 atm to 10 atm. The fuels investigated include methane, acetylene, 1,3-butadiene, and three isomers of methyl butene. State-of-the-art reaction mechanisms were employed for chemical kinetics simulations to analyze reaction pathways, species sensitivity, and to compare different models. The findings of this research will assist modelers in refining their reaction mechanisms and improving the overall accuracy.

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.