Natural gas has grown in popularity as a fuel for power generation gas turbines. However, changes in fuel composition are a topic of concern since fuel variability can have a great impact on the reliability and performance of the burner design. In particular, autoignition of the premixed fuel and air prior to entering the main burner is a potential concern when using exotic fuel blends. To obtain much-needed data in this area, autoignition experiments for a wide range of likely fuel blends containing CH4 mixed with combinations of C2H6, C3H8, C4H10, C5H12, and H2 were performed in a high-pressure shock tube. However, testing every possible fuel blend combination and interaction was not feasible within a reasonable time and cost. Therefore, to predict the surface response over the complete mixture domain, a special experimental design was developed to significantly reduce the amount of 'trials' needed from 243 to only 41 using the Box-Behnkin factorial design methodology. Kinetics modeling was used to obtain numerical results for this matrix of fuel blends, setting the conditions at a temperature of 800 K and pressure of 17 atm. A further and successful attempt was made to reduce the 41-test matrix to a 21-test matrix. This was done using special mixture experimental techniques. The kinetics model was used to compare the smaller matrix to the expected results of the larger one. The new 21-test matrix produced a numerical correlation that agreed well with the results from the 41-test matrix, indicating that the smaller matrix would provide the same statistical information as the larger one with acceptable precision. iii After the experimental matrix was developed using the design of experiments approach, the physical experiments were performed in the shock tube. Long test times were created by "tailoring" the shock tube using a novel driver gas mixture, obtaining test times of 10 millisecond or more, which made experiments at low temperatures possible. Large discrepancies were found between the predicted results by numerical models and the actual experimental results. The main conclusion from the experiments is that the methane-based mixtures in this study enter a regime with a negative temperature coefficient when plotted in Arhennius form. This means that these mixtures are far more likely to ignite under conditions frequently encountered in a premixer, potentially creating hazardous situations. The experimental results were correlated as a function of the different species. It was found that the effect of higher-order hydrocarbon addition to methane is not as profound as seen at higher temperatures (>1100 K). However, the ignition delay time could still be reduced by a factor two or more. It is therefore evident that potential autoignition could occur within the premixer, given the conditions as stated in this study.

To study combustion chemistry at low temperatures in a shock tube, it is of great importance to increase experimental test times, and this can be done by tailoring the interface between the driver and driven gases. Using unconventional driver-gas tailoring with the assistance of tailoring curves, shock-tube test times were increased from 1 to 15 ms for reflected-shock temperatures below 1000 K. Provided in this thesis is the introduction of tailoring curves, produced from a 1-D perfect gas model for a wide range of driver gases and the production and demonstration of successful driver mixtures containing helium combined with either propane or carbon dioxide. The He/CO2 and He/C3H8 driver mixtures provide a unique way to produce a tailored interface and, hence, longer test times, when facility modification is not an option. The tailoring curves can be used to guide future applications of this technique to other configurations. Nonreacting validation experiments using driver mixtures identified from the tailoring curves were performed over a range of reflected-shock temperatures from approximately 800 to 1400 K, and some examples of ignition-time experiments that could not have otherwise been performed are presented.

Ignition and oxidation characteristics of CO/H2, H2/O2 and CO/H2/CH4/CO2/Ar fuel blends in air were studied using both experimental and computer simulation methods. Shock-tube experiments were conducted behind reflected shock waves at intermediate temperatures (825 < T < 1400 K) for a wide range of pressures (1 < P < 45 atm). Results of this study provide the first undiluted fuel-air ignition delay time experiments to cover such a wide range of syngas mixture compositions over the stated temperature range. Emission in the form of chemiluminescence from the hydroxyl radical (OH*) transition near 307 nm and the pressure behind the reflected shock wave were used to monitor reaction progress from which ignition delay times were determined. In addition to the experimental analysis, chemical kinetics calculations were completed to compare several chemical kinetics mechanisms to the new experimental results. Overall, the models were in good agreement with the shock-tube data, especially at higher temperatures and lower pressures, yet there were some differences between the models at higher pressures and the lowest temperatures, in some cases by as much as a factor of five. In order to discern additional information from the chemical kinetics mechanisms regarding their response to a wide range of experimental conditions, ignition delay time and reaction rate sensitivity analyses were completed at higher and lower temperatures and higher and lower pressures. These two sensitivity analyses allow for the identification of the key reactions responsible for ignition. The results of the sensitivity analysis indicate that the ignition-enhancing reaction H + O2 = O + OH and hydrogen oxidation kinetics in general were most important regardless of mixture composition, temperature or pressure. However, lower-temperature, higher-pressure ignition delay time results indicate additional influence from HO2- and CO- containing reactions, particularly the well-known H + O + M = HO2 + M reaction and also the CO + O + M = CO2 + M and CO + HO2 = CO2 + OH reactions. Differences in the rates of the CO-related reactions are shown to be the cause of some of the discrepancies amongst the various models at elevated pressures. However, the deviation between the models and the experimental data at the lowest temperatures could not be entirely explained by discrepancies in the current rates of the reactions contained within the mechanisms. Additional calculations were therefore performed to gain further understanding regarding the opposing ignition behavior for calculated and measured ignition delay time results. Impurities, friction induced ionization, static charge accumulation, boundary layer effects, wall reaction effects, and revised chemical kinetics were all considered to be possible mechanisms for the model and measured data disparity. For the case of wall-reaction effects, additional shock-tube experiments were conducted. For the remaining effects listed above, only detailed calculations were conducted. Results from this preliminary anomaly study are at this time inconclusive, but likely avenues for future study were identified. Additional kinetics calculations showed that the large difference between the experimental data and the chemical kinetics models predictions at low temperatures can be explained by at least one missing reaction relevant to low-temperature and high-pressure experimental conditions involving the formation of H2O2, although further study beyond the scope of this thesis is required to prove this hypothesis both theoretically and experimentally.

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The focus of this dissertation was the biomechanics of blast-induced traumatic brain injury (bTBI). This study had three specific aims. One of the specific aims was to investigate the thoracic mechanism of bTBI by characterizing the cerebral blood pressure change during local blast exposure to head or chest in a rat model. This model utilized a shock tube to simulate the blast wave. The results showed that there is a blood pressure rise with high amplitude and short duration during both Head-Only and Chest-Only exposure conditions. It was shown that cerebral blood pressure rise was significantly higher in Chest-Only exposure, and resulted in astrocyte reactivation, and infiltration of blood-borne macrophages into the brain. It was concluded that due to chest exposure to a blast wave, high amplitude pressure waves that transfer from thoracic large vessels to cerebrovasculature can lead to blood-brain barrier disruption or perivascular injury and consequently trigger secondary neuronal damage. The second and third aims were related to the viscoelasticity and heterogeneity of brain tissue respectively for blast rate loading conditions. For the second specific aim, a novel test method was developed to apply shear deformation to samples of brain tissue with strain rates in the range of 300 to 1000 s-1. The results of shear tests on cylindrical samples of bovine brain showed that the instantaneous shear modulus (about 6 kPa) increased about 3 times compared to the values reported in the literature. For the third specific aim, local viscoelastic behavior of rat brain was characterized using a micro-indentation setup with the spatial resolution of 350 mm. The results of micro-indentation tests showed that the heterogeneity of brain tissue was more pronounced in long-term shear moduli. Moreover, the inner anatomical regions were generally more compliant than the outer regions and the gray matter generally exhibited a stiffer response than the white matter. The results of this study can enhance the prediction of brain injury in finite element models of TBI in general and models of bTBI in particular. These results contribute to development of more biofidelic models that can determine the extent and severity of injury in blast loadings. Such predictions are essential for designing better injury mitigation devices for soldiers and also for improving neurosurgical procedures among other applications. / Mechanical Engineering / Accompanied by one .pdf file.

The objective of this work was to design, analyze and test a fast response fiber-optic temperature probe and sensor. The sensor is intended for measuring rapid temperature changes such as produced by a blast wave formed by a detonation. This work was performed in coordination with Luna Innovations Incorporated, and the design is based on extensions of an existing fiber-optic temperature sensor developed by Luna. The sensor consists of a glass fiber with an optical wafer attached to the tip. A basic description of the principles behind the fiber-optic temperature sensor and an accompanying demodulation system is provided. For experimental validation tests, shock tubes were used to simulate the blast wave experienced at a distance of 3.0 m from the detonation of 22.7 kg of TNT. The flow conditions were predicted using idealized shock tube theory. The temperature sensors were tested in three configurations, flush at the end of the shock tube, extended on a probe 2.54 cm into the flow and extended on a probe 12.7 cm into the flow. The total temperature was expected to change from 300 K to 1130 K for the flush wall experiments and from 300 K to 960 K for the probe experiments. During the initial 0.1 milliseconds of the data the temperature only changed 8 K when the sensors were flush in the end of the shock tube. The sensor temperature changed 36 K during the same time when mounted on a probe in the flow. Schlieren pictures were taken of the flow in the shock tube to further understand the shock tube environment. Contrary to ideal shock tube theory, it was discovered that the flow did not remain stagnant in the end of the shock tube after the shock reflects from the end of the shock tube. Instead, the effects of turbulence were recorded with the fiber-optic sensors, and this turbulence was also captured in the schlieren photographs. A fast-response thermocouple was used to collect data for comparison with the fiber-optic sensor, and the fiber-optic sensor was proven to have a faster response time compared to the thermocouple. When the sensors were extended 12.7 cm into the flow, the fiber-optic sensors recorded a temperature change of 143 K compared to 38 K recorded by the thermocouple during the 0.5 millisecond test. This corresponds to 22% of the change of total temperature in the air recorded by the fiber-optic sensor and only 6% recorded by the thermocouple. Put another way, the fiber-optic sensor experience a rate of temperature change equal to 2.9x105 K/s and the thermocouple changed at a rate of 0.79x105 K/s. The data recorded from the fiber-optic sensor also contained much less noise than the thermocouple data. An unsteady finite element thermal model was created using ANSYS to predict the temperature response of the sensor. Test cases with known analytical solutions were used to verify the ANSYS modeling procedures. The shock tube flow environment was also modeled with Fluent, a commercially available CFD code. Fluent was used to determine the heat transfer between the shock tube flow and the sensor. The convection film coefficient for the flow was predicted by Fluent to be 27,150 W/m2K for the front of the wafer and 13,385 W/m2K for the side. The Fluent results were used with the ANSYS model to predict the response of the fiber-optic sensor when exposed to the shock tube flow. The results from the Fluent/ANSYS model were compared to the fiber-optic measurements taken in the shock tube. It was seen that the heat flux to the sensor was slightly over-predicted by the model, and the heat losses from the wafer were also over-predicted. Since the prediction fell within the uncertainty of the measurement, it was found to be in good agreement with the measured values. Inverse heat transfer methods were used to determine the total temperature of the flow from the measured data. Both the total temperature and the film coefficient were determined simultaneously during this process. It was found that for short testing times, there were many possible solutions. In order to obtain ultimate success with this method, the uncertainty of the demodulation system must be improved and/or the simple analytical thermal model used to predict the response of the sensor needs to match the physical sensor. Whenever possible, longer testing times should be employed. Promising suggestions for extending this approach are provided. / Ph. D.

The threat of terrorist bombings and accidental industrial explosions motivates the need for more economical and efficient blast-resistant construction techniques that offer enhanced levels of protection at reduced component damage levels. Despite having a high strength-to-weight ratio and being chemically inert, fiber reinforced polymer (FRP) reinforcing bars are not currently used in blast-resistant reinforced concrete due to their brittle nature and lack of ductility. However, the innovative use of blended mixtures of FRP and steel rebar as tensile reinforcement promises to address these limitations through self-centering behavior that provides reductions in residual damage and enhancements in flexural performance. This thesis presents the results of an experimental and analytical investigation on the effect of hybrid arrangements of glass fiber reinforced polymer (GFRP) and conventional mild steel reinforcement on the blast performance of reinforced concrete beams. Seven large-scale reinforced concrete beams with different combinations of tensile steel and GFRP rebar were designed, constructed, and tested under progressively increasing blast loading generated using the Virginia Tech Shock Tube Research Facility. The effect of hybrid reinforcing on the blast performance of the beams was evaluated based on the global response, failure mode, damage pattern, mid-span displacement, and support reactions of the tested beams. The results demonstrated several benefits in using hybrid arrangements of steel and GFRP reinforcement. Beams with hybrid reinforcing experienced reduced overall residual displacements compared with similar conventionally reinforced concrete members. This was attributed to the elastic nature of GFRP rebar which was found to produce a self-centering behavior that assisted in returning the hybrid members to their original undeformed position. This permitted the hybrid beams to safely experience larger maximum displacements at substantially less damage than all-steel construction. Furthermore, if the GFRP reinforcement did rupture, the presence of steel arrested hazardous component failure and provided additional energy dissipation and redundancy. Accompanying the experimental tests was an inelastic single-degree-of-freedom analysis to predict the displacement time-history response of the beams. Reasonably good predictions of response were obtained when the advanced material models and the effects of accumulated damage due to repeated blast testing were incorporated into the analytical predictions. Finally, a series of protective design recommendations and a new proposed response limit, that describes the level of damage achieved after a blast event, were established to encourage use of hybrid GFRP/steel reinforcement in blast-resistant construction. / Master of Science / The threat of terrorist bombings and accidental industrial explosions motivate the need for new blast resistant construction techniques. Despite having a high strength-to-weight ratio and being chemically inert, fiber reinforced polymer (FRP) reinforcing bars are not currently used in blast-resistant reinforced concrete due to their brittle nature and lack of ductility. However, the innovative use of blended mixtures of FRP and steel rebar as tensile reinforcement promises to address these limitations through self-centering behavior that provides reductions in residual damage and enhancements in flexural performance. Large-scale reinforced concrete beams with different combinations of steel and GFRP rebar were designed, constructed, and tested under progressively increasing blast loads, gen-erated by the Virginia Tech Shock Tube Research Facility. The results demonstrated that beams with hybrid reinforcing experienced reduced overall residual damage in comparison with similar conventionally reinforced concrete members. Additionally, if the GFRP rebar ruptured, the presence of steel prevented a brittle failure and provided additional energy dissipation and redundancy. The inelastic single degree of freedom model developed for this investigation resulted in an adequate prediction of the load-deflection characteristics record-ed from experimental testing. To encourage the use of hybrid FRP/steel reinforcement in blast-resistant construction, a series of protective design recommendations and a proposed response limit, that describes the level of damage achieved after a given blast event, were established.

Hydroxyl-terminated polybutadiene (HTPB) is a common solid rocket fuel that is used in solid-fueled ramjets (SFRJs) for hypersonic propulsion. Renewed interest in hypersonic systems has developed a need to simulate HTPB combustion using computational fluid dynamics (CFD). However, the current chemical kinetic mechanisms lack the experimental data necessary to accurately simulate SFRJ conditions. Shock tube experiments at the University of Central Florida (UCF) were carried out to study the interaction of HTPB pyrolysis products (ethylene, propene, 1,3-butadiene, benzene, and toluene) with air. Experiments were conducted at 5 ± 0.5 atm and 1100 – 1800 K. Mixtures ranged from fuel lean to fuel rich (Φ = 0.5, 1.0, 4.0, 4.76) and contained individual fuel components or a fuel blend of the HTPB pyrolysis products. Species time-history of carbon monoxide (CO) and time-resolved temperature were measured with a quantum cascade laser (QCL) in the mid-infrared (MIR) region and compared against chemical kinetics models: AramcoMech3.0, HyChem, and NUIG 1.3. Additional OH* measurements were taken with an emission detector to capture ignition delay time (IDT). The data is used to develop an improved chemical kinetic mechanism for simulating HTPB combustion in air at SFRJ conditions.

Both experimental and numerical investigations of converging shock waves have been performed. In the experiments, a shock tube was used to create and study converging shock waves of various geometrical shapes. Two methods were used to create polygonally shaped shocks. In the first method, the geometry of the outer boundary of the test section of the shock tube was varied. Four different exchangeable shapes of the outer boundary were considered: a circle, a smooth pentagon, a heptagon, and an octagon. In the second method, an initially cylindrical shock wave was perturbed by metal cylinders placed in various patterns and positions inside the test section. For three or more regularly spaced cylinders, the resulting diffracted shock fronts formed polygonal shaped patterns near the point of focus. Regular reflection was observed for the case with three cylinders and Mach refection was observed for cases with four or more cylinders. When the shock wave is close to the center of convergence, light emission is observed. An experimental investigation of the light emission was conducted and results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The shock wave focusing was also studied numerically using the Euler equations for a gas obeying the ideal gas law with constant specific heats. Two problems were analyzed; an axisymmetric model of the shock tube used in the experiments and a cylindrical shock wave diffracted by cylinders in a two dimensional test section. The results showed good agreement with the experiments. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there. As a practical example of shock wave focusing, converging shocks in an electrohydraulic lithotripter were simulated. The maximum radius of a gas bubble subjected to the pressure field obtained from the lithotripter was calculated and compared for various geometrical shapes and materials of the reflector. Results showed that the shape had a large impact while the material did not influence the maximum radius of the gas bubble. / QC 20100706