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.

As the flight speeds of aircraft are increased above Mach 5, efficient methods of propulsion are needed. Scramjets may be a solution to this problem. Supersonic combustion is one of the main challenges involved in the operation of a Scramjet engine. In general, both an igniter and a flameholder are needed to achieve and maintain supersonic combustion. The current work examines a plasma torch-strut combination as an igniter-flameholder. The plasma torch-strut combination was tested in the Virginia Tech unheated supersonic wind tunnel at Mach 2.4. Pressure and temperature sampling, filtered photography, and spectroscopic measurements were used to compare different test cases. These results provide both qualitative and quantitative results on how the combination responds to changes in the mass flow rate of fuel and the power to the plasma torch. The key conclusions of the work were the following: 1. Tests showed that an exothermic reaction takes place. 2. The amount of heat release increases with an increase in the mass flow rate of fuel. 3. The plasma torch-fuel injector interaction caused the heat release to be well above the tunnel floor and sometimes off the strut centerline 4. One change in the fuel injector pattern caused more temperature rise near the floor of the tunnel. 5. The flow penetration height of the plasma torch alone was reduced by the fuel-plasma torch interaction. 6. Moving the strut upstream reduced the measured temperature rise at a fixed downstream location, but increased the penetration height of the plasma torch. 7. The computed heat release was found to be small compared to the potential heat release from all the fuel burning. 8. The amount of temperature rise caused by the fuel is not greatly affected by the power to the plasma torch. / Master of Science

To realize supersonic combustion of hydrocarbons, an effective atomizer-igniter combination with the capabilities of fuel preheating, atomization, penetration, mixing, ignition and flameholding is desired. An original design concept incorporating these capabilities was built and tested at Virginia Tech, and was found to provide good penetration, effective atomization, and robust ignition and flameholding. Quiescent testing with kerosene and JP-7 provided initial performance data. The atomizer-injector design was then modified for insertion into a supersonic wind tunnel, and tested with kerosene in an unheated Mach 2.4 flow with typical freestream conditions of To = 280 K and Po = 360 kPa. Water injection was utilized in both cases for comparison and to analyze atomization behavior. In the quiescent environment, the regeneratively cooled plasma torch igniter was found to significantly increase electrode life while heating, atomizing, and igniting the liquid fuel. Jet breakup length was measured and characterized, and mean droplet size was estimated using an existing correlation. Several qualitative observations regarding quiescent combustion were made, including torch power effects and the process of flame formation. In the supersonic environment, the effect of fuel injection direction was analyzed. Best results were obtained when fuel was injected with a velocity component opposite to the direction of main tunnel flow. Repeatable ignition occurred in the supersonic boundary layer at the fuel stagnation location near the plasma torch plume. Direct, filtered, shadowgraph, and schlieren photographs, temperature measurements, and visible emission spectroscopy provided evidence of combustion and the details of the flame structure. The new atomizer-igniter design provided robust and reliable ignition and flameholding of liquid hydrocarbon fuels in an unheated supersonic flow at M=2.4, with no ramp, step, or other physical penetration into the flowpath. / Master of Science

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.

This thesis details a process for the development of reliable metrics that could be used to assess the quality of combustion pressure measurement data - important data used in the development of internal combustion engines. The approach that was employed in this study was a model based technique, in conjunction with a simulation environment - producing data based models from a number of strategically defined measurement points. A simulation environment was used to generate error data sets, from which models of calculated result responses were built. This data was then analysed to determine the results with the best response to error stimulation. The methodology developed allows a rapid prototyping phase where newly developed result calculations may be simulated, tested and evaluated quickly and efficiently. Adopting these newly developed processes and procedures, allowed an effective evaluation of several groups of result classifications, with respect to the major sources of error encountered in typical combustion measurement procedures. In summary, the output gained from this work was that certain result groups could be stated as having an unreliable response to error simulation and could therefore be discounted quickly. These results were clearly identifiable from the data and hence, for the given errors, alternative methods to identify the error sources are proposed within this thesis. However, other results had a predictable response to certain error stimuli, hence; it was feasible to state the possibility of using these results in data quality assessment, or at least establishing any boundaries surrounding their application for this usage. Interactions in responses were also clearly visible using the model based sensitivity analysis as proposed. The output of this work provides a solid foundation of information from which further work and investigation would be feasible, in order to achieve an ultimate goal of a full set of metrics from which combustion data quality could be accurately and objectively assessed.