With the continuous miniaturization of integrated circuit chips over the last decade, there has been a steady increase in power density of electronic devices giving rise to the need for aggressive and effective cooling systems. The rapid growth of microfabrication technology has led to the development of microelectromechanical system (MEMS) based microchannels, which can be used as miniaturized heat exchangers capable of cooling high power electronic devices. Simultaneously, studies show that carbon dioxide in supercritical state (sCO2) has an excellent ability for cooling applications due to its exceptional thermophysical properties near critical point. Implementation of pin fins in heat transfer systems requires a thorough understanding of both fluid dynamics and heat transfer mechanisms. Thus, the aim of this research is to extend the current fundamental knowledge about both thermal and hydraulic performance of supercritical carbon dioxide (sCO2) in microchannel pin fin heat sinks. Microchannel devices with micro pin fin heat sinks and sCO2 have been designed and built in three generations. The devices were microfabricated at the Cornell Nanoscale Facility (CNF). First generation is microchannel array heat sinks, followed by circular pin fin heat sinks and airfoil pin fin heat sinks, as second and third generation devices, respectively. Heat transfer characterization including heat transfer coefficient, Nusselt number, and pressure drop was performed on the devices. In microchannel array devices (first generation) the capability of sCO2 and the effect of height to width ratio (H/W) on thermal performance of the microchannel heat sink was investigated. In circular micro pin fin heat sink device (second generation), a parametric study for the effects of fin height over diameter, and inline and staggered arrangement on heat transfer coefficient and pressure drop of the system were conducted. Heat transfer experiments were performed on airfoil pin fin heat sink devices, as well. In general, the heat transfer coefficient characteristics revealed the incredibly high values of heat transfer in such systems. Comparing thermal performance of the three generations, heat transfer coefficient obtained by the circular micro pin fin heat sink is higher than microchannel array heat sinks due to higher active surface area and better flow mixing creating turbulent flow. Micro pin fin configuration also prevents relaminarization followed by flow acceleration to be aggressively dominated in the entire micro channel. However, pressure drop in the microchannel array is less than that of circular micro pin fin heat sinks. It was observed that thermal performance of airfoil micro pin fin heat sinks is better than that of two microchannel array and circular pin fin heat sinks. Among airfoil micro heat sinks, staggered arrangement has higher thermal performance than inline arrangement. It is concluded that as airfoil thickness increases, thermal performance of the heat sink increases, but comprehensive performance of the heat sink decreases. It was also found that existing conventional scale correlations and flow maps did not predict well the corresponding characteristics in micro-scale systems, and thus, new correlations have been developed.

The main objective of this study is to fundamentally investigate the flow physics and the relationship to heat transfer in the presence of roughened surfaces undergoing jet impingement cooling mechanisms in a confined channel. Thermal and fluid dynamics characteristics are directly related to the surface condition. The surface roughness is proven to play a significant role in the surface heat transfer and skin friction coefficient. In combination with the surface finish, the flow condition plays a particular role in the turbulence behavior near the wall. The magnitude of these engineering quantities tends to deviate from a smooth surface compared to a rough surface scenario. The development of accurate lower and high-fidelity models is essential in the engineering world. Predicting the heat transfer and fluid mechanics behavior inside a component is essential for a designer, such as improving wall functions within the CFD community. Usually, literature only includes well-defined rough surfaces driven by some geometric parameters, non-uniform and irregular surfaces like the one found in additive manufacturing and other physics forming phenomena is somewhat lacking. The basic geometric configuration of a single jet impingement was chosen due to the ability to create a wall jet from a stagnation region. The experimental facility was designed under no crossflow configuration, where fluid enters from the plenum passing through the orifice hole (jet) and exiting in the radial direction of a confined channel. The current research investigated the fluid dynamics associated with jet impingement over rough surfaces using non-intrusive experimental methods. Multiple jet Reynolds numbers were investigated, ranging from 21,000 to 110,000 for three different jet diameter to roughness ratios. The current research study investigated heat transfer and fluid behavior using non-intrusive experimental methods. Temperature-sensitive paint (TSP) was utilized to obtain scalar temperature field over smooth and rough surfaces. These experimental results will be compared with available literature. The flow physics was investigated by performing stereoscopic Particle Image Velocimetry. The velocity fields were further analyzed using Proper Orthogonal Decomposition (POD) and tested versus the wall similarity theory. High accuracy microphones were utilized to obtain unsteady pressure values at different rough surfaces.

Recent advancements in material technology have led to the development of non-porous negative Poisson's ratio (NPR) materials (also called auxetic structures) by making spherical inline dimples on both sides of an elastic sheet. Manufacturing technologies such as 3-D printing and additive manufacturing paved the way to realize the complex shapes needed to achieve NPR behavior. These materials are desirable in many engineering applications, especially in the gas turbines hot-gas-path, due to their unique properties. In the current study, an effort is made to understand the flow physics and surface heat transfer mechanism for channel flow having one wall with spherical dimples and protrusions. An equivalent geometry, with dimples on both sides of the flat sheet with density beyond a certain threshold, would have NPR characteristics. Furthermore, dimples and protrusions are also studied in isolation to understand what key differences are brought by the combination of these two. Flow field measurements, in-and-around these features, are done using the stereoscopic PIV. The transient TLC method is used for local surface heat transfer measurement. Numerical simulations, steady RANS, URANS, and LES, are used in conjunction with experimental data to develop a detailed understanding of the flow field and surface heat transfer. A comparison of experimental measurements and numerical simulations identifies the areas for improvement in numerical modeling of the flow field and surface heat transfer in the presence of such geometries. The friction factor measurement along with heat transfer is used to characterize the thermal performance factor (TPF) of each geometry for a range of Reynolds number. The novelty of this work is the inline arrangement of features and first-of-its-kind PIV measurement for dimples-protrusions. The understanding developed from this work can easily be utilized for designing components involving NPR materials with dimples-protrusions.

Over the years methane and natural gas (NG) as a fuel have become an important source of energy in the power generation and transportation sectors, with the latter most prominently adopting propane used in public transportation shuttles. Notable interest in using liquid methane or NG fueled rocket engines has recently gained traction and are currently in development/production at SpaceX (raptor engine, full-flow staged combustion cycle) and Blue Origin (BE-4, staged combustion cycle). Given the variety of applications, sourcing and refinement of methane/NG may be completely different to ensure a certain purity is met, especially in an advanced rocket engine cycle like the raptor engine, that may be highly sensitive to minor fuel variances. The current study seeks to understand the ignition behavior of methane (with and without CO2 dilution) and NG surrogate mixtures, including CH4/C2H6/C3H8 and CH4/C2H6/C3H8/i-C4H10/n-C4H10, using a high-pressure shock tube facility at reflected shock conditions relevant to advanced rocket engines and gas turbine cycles. This included pressures near 16, 100, and 200 bar throughout a temperature range of 1000-1621 K. Ignition delay time data were compared with predictions of a chemical kinetic mechanism, and along with performed sensitivity and pathway analyses, further aided in the understanding of the observed ignition behavior. The combined effects of lower activation energy, increased concentration of OH and CH radicals, and increased heat of combustion all play some role in the observed promotion of ignition. These combined effects are primarily a function of fuel mixture and concentration, and combustion relevant pressure and temperature conditions (reflected shock conditions T5 and P5), which in addition dictate the propensity of preignition and deflagration-to-detonation transition behavior observed in the shock tube.

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.