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.

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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.

The purpose of this research is to develop a dataset for some of the most important fire characteristics, namely CO2 yield, CO yield, HCN yield, soot yield and heat of combustion for probabilistic analysis and modelling. Raw data in time series are required to mechanically reduce experimental data into yields (kg/kg) and effective heats of combustion (MJ/kg), which are expressions for the amount of products generated per unit mass of fuel. Mass loss rate thresholds were applied to all tests to define the beginning and end of tests. These species yields and heat of combustions were then grouped by material compositions and fitted with distribution functions to produce distributions curves. As fire species productions and heat of combustions are dependent on the fire conditions as it develops, different yields are expected at different fire stages. These have been identified as the growth (G), transition (T), and smouldering (S) stages in this research. These values are also compared against, and are generally in agreement with, other research data. Nonetheless, some discrepancies have occurred and require further information to ascertain the material characteristics and combustion conditions. In conclusion, design recommendations for these fire characteristics have been made for several material groupings and verified against other research results. Certain physical and chemical limitations exist for combustions and have not been reflected in the fitted distribution, including stoichiometric yields and unlimited air yields. As such, species yields and heat of combustions beyond these values should not be considered in fire engineering design and analysis. Research results on HCN including all required data parameters for yield conversions were difficult to obtain and require further research efforts. Tube furnace results were initially investigated. Unfortunately, without a continuous mass record, has proved to be challenging in producing reliable mass loss rate profiles for yield conversions. A semi-automated data reduction application UCFIRE was also used. However, certain technical difficulties were encountered and require modifications to broaden its applicability.

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Thesis (Ph. D.)--West Virginia University, 2005 / Title from document title page. Document formatted into pages; contains ix, 183 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 178-183).

Detonation-based combustion systems are desired for propulsion and power systems due to their ability to provide high thermal efficiency and enable supersonic flight. Detonation combustion in hypersonic flows has traditionally been realized using an oblique detonation wave. However, oblique detonation realization and stabilization in combustion systems is challenging. This communication presents an alternative realization of a detonation mode of combustion through a reacting Mach stem. The detonation is experimentally realized in a hypersonic reacting facility, which is optimized for Mach 5 flow at the combustor inlet and includes a 2D-wedge to stabilize hypersonic reactions at high-enthalpy flow conditions. The Mach stem detonation is analyzed with simultaneous 30 kHz schlieren and chemiluminescence imaging, which reveals the coupling between the Mach stem and the reaction. Further confirmation is provided by comparing the Mach number of the reacting Mach stem with the Chapman-Jouguet (CJ) detonation Mach number. It is found that the Mach number of the reacting Mach stem reaches 94% of the CJ detonation Mach number, confirming that the reacting Mach stem realizes a detonation mode of combustion.