Browsing by Author "Diller, Thomas E."

Now showing 1 - 20 of 155
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    Additive Manufacturing of Copper via Binder Jetting of Copper Nanoparticle Inks
    Bai, Yun (Virginia Tech, 2018-06-01)
    This work created a manufacturing process and material system based on binder jetting Additive Manufacturing to process pure copper. In order to reduce the sintered part porosity and shape distortion during sintering, the powder bed voids were filled with smaller particles to improve the powder packing density. Through the investigation of a bimodal particle size powder bed and nanoparticle binders, this work aims to develop an understanding of (i) the relationship between printed part properties and powder bed particle size distribution, and (ii) the binder-powder interaction and printed primitive formation in binder jetting of metals. Bimodal powder mixtures created by mixing a coarse powder with a finer powder were investigated. Compared to the parts printed with the monosized fine powder constituent, the use of a bimodal powder mixture improved the powder flowability and packing density, and therefore increased the green part density (8.2%), reduced the sintering shrinkage (6.4%), and increased the sintered density (4.0%). The deposition of nanoparticles to the powder bed voids was achieved by three different metal binders: (i) a nanoparticles suspension in an existing organic binder, (ii) an inorganic nanosuspension, and (iii) a Metal-Organic-Decomposition ink. The use of nanoparticle binders improved the green part density and reduced the sintering shrinkage, which has led to an improved sintered density when high binder saturation ratios were used. A new binding mechanism based on sintering the jetted metal nanoparticles was demonstrated to be capable of (i) providing a permanent bonding for powders to improve the printed part structural integrity, and (ii) eliminating the need for organic adhesives to improve the printed part purity. Finally, the binder-powder interaction was studied by an experimental approach based on sessile drop goniometry on a powder bed. The dynamic contact angle of binder wetting capillary pores was calculated based on the binder penetration time, and used to describe the powder permeability and understand the binder penetration depth. This gained understanding was then used to study how the nanoparticle solid loading in a binder affect the binder-powder interactions and the printed primitive size, which provided an understanding for determining material compatibility and printing parameters in binder jetting.
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    Aerodynamic Heating of a Hypersonic Naval Projectile Launched At Sea Level
    Mabbett, Arthur Andrew (Virginia Tech, 2007-04-10)
    Hypersonic flight at sea-level conditions induces severe thermal loads not seen by any other type of current hypersonic system. Appropriate design of the hypersonic round requires a solid understanding of the thermal environment. Numerous codes were obtained and assessed for their applicability to the problem under study, and outside of the GASP Conjugate Heat Transfer module, Navier-Stokes code from Aerosoft, Inc., no efficient codes are available that can model the aerodynamic heating response for a fully detailed projectile, including all subassemblies, over an entire trajectory. Although the codes obtained were not applicable to a fully detailed thermal soak analyses they were useful in providing insight into ablation effects. These initial trade studies indicated that ablation of up to 1.25 inches could be expected for a Carbon-Carbon nosetip in this flight environment. In order to capture the thermal soak effects a new methodology (BMA) was required. This methodology couples the Sandia aerodynamic heating codes with a full thermal finite element model of the desired projectile, using the finite element code ANSYS from ANSYS, Inc. Since ablation can be treated elsewhere it was not included in the BMA methodology. Various trajectories of quadrant elevations of 0.5, 10, 30, 50, and 80 degrees were analyzed to determine thermal time histories and maximum operating temperatures. All of the trajectories have the same launch condition, Mach 8 sea-level, and therefore will undergo the same initial thermal spike in temperature at the nose-tip of approximately 3,100 K (5600R). Of the five trajectories analyzed the maximum internal temperatures experienced occurred for the 50 degree quadrant elevation trajectory. This trajectory experienced temperatures in excess of 1,000 K (1800R) for more than 80% of its flight time. The BMA methodology was validated by comparisons with experiment and computational fluid solutions with an uncertainty of 10% at a cost savings of over three orders of magnitude.
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    Analysis of Flow and Heat Transfer in the U.S. EPR Heavy Reflector
    Takamuku, Kohei (Virginia Tech, 2008-12-05)
    The U.S. Evolutionary Power Reactor (EPR) is a new, large-scale pressurized water reactor made by AREVA NP Inc. Surrounding the core of this reactor is a steel wall structure sitting inside called the heavy reflector. The purpose of the heavy reflector is to reduce the neutron flux escaping the core and thus increase the efficiency of the reactor while reducing the damage to the structures surrounding the core as well. The heavy reflector is heated due to absorption of the gamma radiation, and this heat is removed by the water flowing through 832 cooling channels drilled through the heavy reflector. In this project, the temperature distribution in the heavy reflector was investigated to ascertain that the maximum temperature does not exceed the allowable temperature of 350 C, with the intent of modifying the flow distribution in the cooling channels to alleviate any hot spots. The analysis was conducted in two steps. First, the flow distribution in the cooling channels was calculated to test for any maldistribution. The temperature distribution in the heavy reflector was then calculated by simulating the conjugate heat transfer with this flow distribution as the coolant input. The turbulent nature of the flow through the cooling channels made the calculation of the flow distribution computationally expensive. In order to resolve this problem, a simplification method using the "equivalent flow resistance" was developed. The method was validated by conducting a few case studies. Using the simplified model, the flow distribution was calculated and was found to be fairly uniform. The conjugate heat transfer calculation was conducted. The same simplification method used in the flow distribution analysis could not be applied to this calculation; therefore, the computational cost of this model was reduced by lowering the grid density in the fluid region. The results showed that the maximum temperature in the heavy reflector is 347.7 C, which is below the maximum allowable temperature of 350 C. Additional studies were conducted to test the sensitivity of the maximum temperature with change in the flow distribution in the cooling channels. Through multiple calculations, the maximum temperature did not drop more than 3 C; therefore, it was concluded that the flow distribution in the cooling channels does not have significant effect on the maximum temperature in the heavy reflector.
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    An analytical and experimental investigation of respiratory dynamics using P/D control and carbon dioxide feedback
    Thompson, Christopher David (Virginia Tech, 1988-12-05)
    This thesis addresses the problem of defining the control law for human respiration. Seven different drivers have been identified as possibly having an input to the respiratory controller. These seven represent a combination of feedforward and feedback inputs arising from neural and humoral mechanisms. Using the assumption that carbon dioxide concentrations in the arterial blood have the strongest effect, a control equation with proportional and derivative components based on this driver was evaluated. The methodology for the evaluation was to create a model of the respiratory system incorporating the P/D controller, obtain experimental data of one test subject's respiratory response to exercise, then compare model generated output with experimental data, and adjust the parameters in the control equation to yield optimal model performance. The usual practice of testing controller performance has been to apply single step loads to a model and evaluate its response. A multi-step protocol was used here to provide a better, more generalized test of controller performance. This thesis may represent the first documented use of an approach of this type for evaluating respiratory controller performance. Application of a multi-step protocol revealed a non-linear controller was needed to keep pace with system changes. Respiratory system operation was effectively managed using a controller of the form: VENTILATION = F(dCO₂/dT,Q) + F(CO₂,Q) + CONSTANT.
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    The Application of BioHeat Perfusion Sensors to Analyze Preservation Temperature and Quantify Pressure Ischemia of Explanted Organs
    O'Brien, Timothy J. (Virginia Tech, 2015-03-09)
    The development of an organ preservation system (primarily kidneys and livers, but could be adapted to fit hearts, lungs, and even limbs in the future) that can provide surgeons and doctors with real-time quantitative feedback on the health of the organ would be a significant improvement on current transplant practices. This organ transport system will provide surgeons and doctors the opportunity to make more educated decisions towards whether or not to proceed with organ transplantation. Here, we discuss the use Smart Perfusion's organ preservation system as a platform for determining the optimal perfusion temperature of an organ. Porcine kidneys were procured and perfused with a modified PBS solution on the Vasowave™. While on this organ preservation system, a heart emulating pressure waveform (90/50 mmHg) was generated and sent to the specimen. The pressure response, flow rate, temperature, pH, dissolved oxygen content, and conductivity of the fluid stream were all monitored throughout the duration of experimentation. In addition to inline sensors, IR imaging captured the surface temperature of the organ while on the system. Lastly, the use of a combined heat flux-temperature (CHFT) sensor, previously developed at Virginia Tech, was applied for the first time to monitor and measure local tissue perfusion of an explanted organ. A total of 12 experiments were performed (6 at a set fluid temperature of 15°C, and 6 at 20°C). All system data was collected, statistically evaluated and finally compared against blind histological readings (taken at the termination of each experiment at the hilum and pole) to investigate the effects of temperature on organ vasculature. The results of this experiment indicated that the effects of temperature on explanted kidneys can be affectively measured using a non-invasive bioheat perfusion sensor. Specifically, the lower temperature group of kidneys was measured to have lower perfusion. Furthermore, an enhancement to the CHFT sensor technology (CHFT+) was developed and tested for compliance. A controllable thin filmed heat resistor was added to the CHFT assembly to replace the current convective thermal event. This enhancement improved the measured heat flux and temperature signals and enables autonomy. Also, the thin and semi-flexible nature of the new CHFT+ sensor allows for perfusion measurements to be taken from the underside of the organ, permitting a quantitative measure of pressure ischemia. Results from a live tissue test illustrated, for the first time, the effects of pressure ischemia on an explanted porcine kidney.
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    Artificial Anisotropy for Transverse Thermoelectric Heat Flux Sensing
    Derryberry, Rebekah Ann (Virginia Tech, 2007-04-05)
    Thermoelectric phenomenon describes the relationship between the flow of heat and electricity. Two main categories encompassed in thermoelectric theory are the Seebeck and Peltier effects. The Seebeck effect is the generation of a voltage in a device that consists of two different materials in the presence of a temperature gradient, while the Peltier effect is the generation of a temperature gradient across a device of two different materials in the presence of an electrical current. This project focuses on the first of these two phenomena, where the Seebeck effect is used in a novel heat flux sensor that is transverse in nature. Transverse thermoelectric devices are characterized by their anisotropy, meaning that a temperature gradient generated across a device will be perpendicular to the flow of electricity through the device. This orthogonal arrangement allows for the manipulation of material properties, device arrangement, and construction methods for device optimization. This project characterizes the heat flux sensing capabilities of an artificially anisotropic transverse thermoelectric device via experimental and theoretical methods. The device tested is constructed out of bismuth telluride and titanium grade 5. Bismuth telluride is a standard thermoelectric material, while the titanium is used because of its high melting point and good electrical conductivity. The device is constructed by alternating rectangular pieces of these two materials. These pieces are bonded together at a given angle to simulate anisotropy. Several devices are constructed in a range of angles from 59 to 88°. These devices are each tested in a vacuum chamber where a heater heats one side of the device. This heat flux into the device creates a temperature gradient across the device and the device generates a voltage perpendicular to this temperature gradient. Steady state data are collected for both the temperature difference between the two sides of the device and the voltage generated by the device. This procedure is repeated on each device for a range of heat fluxes from 0 to 2.6 W/cm². This range generates voltage signals up to 14341 µV for an angle of 59°. Data collected are then used to generate a linear trend line that describes the devices response to a given heat flux. These experimental results are compared to theoretical predictions using thermoelectric theory. The results indicate that the device does exhibit transverse thermoelectric characteristics and the experimental data follow the predicted trends. This thesis documents the process of constructing, testing, and analyzing this device.
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    Assessment of a Leading Edge Fillet for Decreasing Vane Endwall Temperatures in a Gas Turbine Engine
    Lethander, Andrew Tait (Virginia Tech, 2003-12-01)
    The objective of this investigation was to improve the thermal environment for a turbine vane through reduction of passage secondary flows. This was accomplished by modifying the vane/endwall junction to include a leading edge fillet. The problem approach was to integrate optimization methods with computational fluid dynamics to optimize the fillet design. The resulting leading edge fillet was then tested in a large-scale, low speed cascade to verify thermal performance. A combustor simulator located upstream of the cascade was used to generate realistic inlet conditions for the turbine vane. Both computational and experimental results underscore the importance of properly modeling the inlet conditions to the turbine. Results of the computational optimization process indicate that significant reductions in adiabatic wall temperature can be achieved with a leading edge fillet. While the intent of the initial fillet design was to improve the thermal environment for the vane endwall, computational results also indicate thermal benefit to the vane surfaces. Flow and thermal field results show that a fillet can enhance coolant effectiveness, prevent formation of the leading edge horseshoe vortex, and preclude full development of a passage vortex. In experimental testing, four cascade inlet conditions were investigated to evaluate the effectiveness of the fillet in reducing endwall temperature levels. Two tested conditions featured a flush combustor/cascade interface, while the remaining two included coolant injection through a backward-facing slot. With the flush interface, fillet thermal performance was evaluated for two inlet total pressure profiles. For the design profile, the fillet had a positive impact on the endwall temperature distribution as well as on the passage thermal field. For the off-design profile, the fillet was observed to have a slightly detrimental impact on the endwall adiabatic temperature distribution; however, passage thermal field results indicate a thermal benefit for the vane suction surface. With the backward-facing slot, thermal tests were conducted for two slot coolant flow rates. For both slot flow rates, the fillet improved endwall thermal protection and prevented coolant lift-off. While increasing the flow rate of slot coolant enhanced endwall effectiveness, fillet thermal performance was similar for the two slot flow rates.
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    Assessment of the Measurement Repeatability and Sensitivity of a Noninvasive Blood Perfusion Measuring Probe
    Comas, Caroline Marie (Virginia Tech, 2005-07-07)
    Blood perfusion is the local, non-directional blood flow through tissue. It is measured as the volumetric flow rate of blood through a given volume of tissue. One method that has been developed for measuring blood perfusion is a probe that measures the temperature response of the tissue when a thermal event is applied. From the temperature response, the blood perfusion and contact resistance can be estimated by comparing the experimental response to a predicted response, and employing Gaussian minimization techniques to estimate the blood perfusion and contact resistance. The objective of this research was to assess the measurement repeatability and sensitivity of the blood perfusion probe by testing the probe on phantom tissue, such that the effects of physiologic or pathologic conditions on the blood perfusion could be eliminated. Another objective was to conduct a preliminary in vivo study using rats for the purpose of establishing proper experimental protocols for future testing of the blood perfusion probe. A phantom tissue test stand comprised of porous material and water to simulate tissue and blood, respectively, was constructed for the phantom study. Inlet flow rates into the porous media ranging between 0 cc/min and 30 cc/min were tested. To test the measurement repeatability 7 flow rates (0, 5, 10, 15, 20, 25 and 30 cc/min) were tested on two different days. To test the measurement sensitivity of the probe, flow rates between 0 and 10 cc/min, and 15 and 20 cc/min were tested at intervals of 1 cc/min. From the phantom study it was concluded that the probe displayed good measurement repeatability, as the trend in perfusion as a function of inlet flow rates for both days was found to be the same. It was also found that the data collected using the probe yielded significantly different perfusion estimates for different flow rates, as statistical analyses show that the average perfusion differences between flow rates are truly independent within a 90% confidence interval, for flow differences above 4 cc/min. It was found that for flow rates below 4 cc/min the probe sensitivity was significantly reduced. For the in vivo study it was concluded that the probe can be used to obtain estimates of perfusion from rats. This preliminary study also served to establish proper experimental protocols for future tests.
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    Assessment of the Repeatability and Sensitivity of the Thermoelectric Perfusion Probe
    Ellis, Brent Earl (Virginia Tech, 2006-12-06)
    The Thermoelectric Perfusion Probe is a completely electronic system that cyclically heats and cools tissue to measure blood perfusion. The probe produces the thermal event with a thermoelectric cooler and then measures the resulting heat flux and temperatures: the arterial temperature and the sensor temperature (the temperature between the heat flux gage and the skin). The Thermoelectric Perfusion Probe was validated and calibrated on a phantom tissue test stand, a system that simulates perfusion with known, controlled flow. With the new pressed sensor technology, a thermocouple sealed to a heat flux gage, the sensor temperature and the heat flux are simultaneously recorded. The pressed sensor tests validated the program used to predict perfusion for the Thermoelectric Perfusion Probe. This perfusion estimation program can determine the tissues perfusion regardless of how the thermal event is created (i.e. convective cooling, convective heating, conductive heating). Based on experimentation, the Thermoelectric Perfusion Probe displays good repeatability and sensitivity for continuously measuring perfusion. The sensitivity and repeatability of the Thermoelectric Perfusion Probe was proven when the perfusion estimates were compared to the perfusion estimates predicted by the Convective Perfusion Probe, a previously validated perfusion probe, and the CFD Flow Model, a computational model of the phantom tissue test stand.
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    Augmentation of Jet Impingement Heat Transfer on a Grooved Surface Under Wet and Dry Conditions
    Alsaiari, Abdulmohsen Omar (Virginia Tech, 2018-11-27)
    Array jet impingement cooling experiments were performed on flat and grooved surfaces with the surface at a constant temperature. For the flat surface, power and temperature measurements were performed to obtain convection coefficients under a wide range of operating conditions such as jet speed, orifice to surface stand-of distance, and open area percentage. Cooling performance (CP) was calculated as the ratio between heat transfer and fan power. An empirical model was developed to predict jet impingement heat transfer taking into account the entrainment effects. Experimental results showed that jet impingement can provide high transfer rates with lower rates of cooling cost in comparison to contemporary conventional techniques in the industry. CP values over 279 were measured which are significantly higher than the standard values of 70 to 95 in current technology. The model enhanced prediction accuracy by taking into account the entrainment effects; an effect that is rarely considered in the literature. Experiments on the grooved surfaces were performed at dry and wet surface conditions. Under dry conditions, results showed 10%~55% improvement in heat transfer when compared to the flat surface. Improvement percentage tends to be higher at wider gaps between the array of orifices and the grooved surface. An improvement of 30%~40% was observed when increasing Re either by increasing orifice diameter or jet speed. Similar improvement was observed at higher flow open area percentages. No significant improvement in heat transfer resulted from decreasing the size of the grooves from 3.56mm to 2.54mm. Similarly, no noticeable change in heat transfer resulted from changing the relative position of the jets striking the surface at the top of the grooves to the bottom of the grooves. Deeper grooves with twice the depth gave statistically similar average heat transfer coefficients as shallower grooves. Under wet conditions, a hybrid cooling technique approach was proposed by using air jets impinging on a grooved surface with the grooves containing water. The approached is proposed and evaluated experimentally for its feasibility as an alternative for cooling towers of thermoelectric power plants. Convection heat and mass transfer coefficients were measured experimentally using the heat mass transfer analogy. Results showed that hybrid jet impingement provided high magnitudes of heat flux at low jet speeds and flow rates. High coefficients of performance CP > 3000, and heat fluxes > 8,000W/m2 were observed. Hybrid jet impingement showed 500% improvement as compared to jet impingement on a dry flat surface. CP values of hybrid jet impingement is 600% to 1,500% more as compared to performance of air-cooled condensers and wet cooling towers. Water use for hybrid jet impingement cooling is efficient since evaporation energy is absorbed from the surface directly instead of cooling air to near wet-bulb temperature.
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    Bed dynamics and heat transfer in shallow vibrated particulate beds
    Mason, Mark Olin (Virginia Polytechnic Institute and State University, 1990)
    A vibrated bed is a mobile layer of solid particles contained in a vessel that is vibrated vertically. This study investigates bed dynamics and heat transfer from a vertical surface in shallow vibrated beds in absence of aeration. In general, "shallow" means a depth-to-width ratio less than one. In this study, bed depth is 30 mm, and this ratio is about 0.2. All experiments are at 25 hertz and at vibrational amplitudes affording peak accelerations between 2 and 7 times gravity. The study uses spherical glass beads of two densities and "Master Beads," nearly spherical particles of a crude, dense alumina, in size fractions from 63 to 707 micrometers. A disc embedded in the vessel floor, vibrated at 4.5 kilohertz, gives data on bed-vessel separation, showing it to occur later than predicted by plastic, single-mass models. The delay is attributed to bed expansion, monitored by piezoelectric force gauges mounted on floor and wall of the vessel. In large-particle beds, bed-vessel collision occurs simultaneously everywhere. In small-particle beds, exhibiting an uneven top surface, collision occurs first at the side walls and moves toward the center. In small-particle beds, pressure gradients appearing during the bed's free flight drive a horizontal component of particle circulation from the vessel's side walls toward its center. An apparent viscosity of the bed, estimated crudely by pulling a rod through it, influences this component's velocity. In beds of large particles, circulation is almost entirely vertical, a layer of two or three particles moving downward at a wall, and a slow return flow moving upward elsewhere. The study confirms the downward wall motion to be driven by friction. Heat transfer closely follows trends in rate of circulation. Greater dependence upon vibrational intensity is seen in small-particle beds. Values as high as 578 W/m²-K are measured. Comparison of vertical-surface heater geometry with an earlier horizontal tube shows the former to be generally superior for surface-to-bed heat transfer.
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    Boundary-layer analysis and measurement of Newtonian and non-Newtonian fluids
    Kim, Byung Kyu (1984)
    The velocity fields around a circular cylinder in a crossflow of drag-reducing polymeric solutions and water were experimentally investigated using a laser-Doppler velocimeter. Measured boundary-layer velocity profiles indicated that the flow parameter controlling the drag on a bluff body in drag-reducing flows is the turbulence intensity rather than the Reynolds number. For turbulence intensity less than 0.7% polymer addition induced delayed separation. For turbulence intensity over 1% the opposite effect was true. Time-averaged velocity profiles of water did not show any significant difference between self-induced and forced oscillatory flows. Heat, mass and momentum transfer of Newtonian and power-law non-Newtonian fluids were theoretically investigated using an implicit finite-difference scheme. The results clearly· indicated that shear-dependent non-Newtonian viscosity controls the entire transport processes of the power-law fluids. For the major portion of the boundary layer, it was found that the more shear thinning the material exhibits, the lower the skin friction and the higher the heat transfer result. Accounting for the motion of the stagnation point provided an improved prediction of heat transfer for Newtonian fluid.
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    Burnthrough Modeling of Marine Grade Aluminum Alloy Structural Plates Exposed to Fire
    Rippe, Christian M. (Virginia Tech, 2015-11-13)
    Current fire induced burnthrough models of aluminum typically rely solely on temperature thresholds and cannot accurately capture either the occurrence or the time to burnthrough. This research experimentally explores the fire induced burnthrough phenomenon of AA6061-T651 plates under multiple sized exposures and introduces a new burnthrough model based on the near melting creep rupture properties of the material. Fire experiments to induce burnthrough on aluminum plates were conducted using localized exposure from a propane jet burner and broader exposure from a propane sand burner. A material melting mechanism was observed for all localized exposures while a material rupture mechanism was observed for horizontally oriented plates exposed to the broader heat flux. Numerical burnthrough models were developed for each of the observed burnthrough mechanisms. Material melting was captured using a temperature threshold model of 633 deg C. Material rupture was captured using a Larson-Miller based creep rupture model. To implement the material rupture model, a characterization of the creep rupture properties was conducted at temperatures between 500 and 590 deg C. The Larson-Miller curve was subsequently developed to capture rupture behavior. Additionally, the secondary and tertiary creep behavior of the material was modeled using a modified Kachanov-Rabotnov creep model. Thermal finite element model accuracy was increased by adapting a methodology for using infrared thermography to measure spatially and temporally varying full-field heat flux maps. Once validated and implemented, thermal models of the aluminum burnthrough experiments were accurate to 20 deg C in the transient and 10 deg C in the steady state regions. Using thermo-mechanical finite element analyses, the burnthrough models were benchmarked against experimental data. Utilizing the melting and rupture mechanism models, burnthrough occurrence was accurately modeled for over 90% of experiments and modeled burnthrough times were within 20% for the melting mechanism and 50% for the rupture mechanism. Simplified burnthrough equations were also developed to facilitate the use of the burnthrough models in a design setting. Equations were benchmarked against models of flat and stiffened plates and the burnthrough experiments. Melting mechanism burnthrough time results were within 25% of benchmark values suggesting accurate capture of the mechanism. Rupture mechanism burnthrough results were within 60% of benchmark values.
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    Comparison of heat flux standards for calibrating heat flux gages at elevated temperatures and high heat flux levels
    Horn, Thomas J. (Virginia Tech, 1993-12-05)
    The goal of this thesis is to develop a set of standards for use in calibrating heat flux gages at elevated temperatures in a radiant heat transfer environment by comparing several "trial" standards. Ideally, the same incident heat flux is derived from each standard when exposed to the same heat source. Three heat flux standards are proposed and evaluated. The standards are based on temperature measurements, material properties, and electrical measurements. The theory and design of each standard are described, as are the calibration procedures used. For experimental comparisons, two standards are simultaneously exposed to heat fluxes of up to 220 W/cm² by placing one standard on each side of a graphite flat plate heater. The temperature measurement based standard derives incident heat flux from the temperature of a blackbody heat source and the Stefan-Boltzmann law. The heat flux gage employed in this standard is a water-cooled Gardon gage. This standard does not operate at high temperatures. The calibration of this standard produced highly repeatable results.
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    Computational and Experimental Modeling of the Bioheat Transfer Process of Perfusion in Tissue Applied to Burn Wounds
    Al-Khwaji, Abdusalam (Virginia Tech, 2013-04-29)
    A new mathematical model has been developed along with a new parameter estimation routine using surface temperature and heat flux measurements to estimate blood perfusion and thermal resistance in living tissue. Dynamic thermal measurements collected at the surface of the sensor before and after imposing a dynamic thermal cooling event are used with the model to estimate the blood perfusion, thermal resistance and core temperature. The Green\'s function based analytical solution does not require calculation of the whole tissue temperature distribution, which was not the case for the previous models. The result from the new model was proved to have better and more consistent results than previous models. The new model was validated to solve one of the unsolved biomedical problems which is the ability of detecting burn severity. The method was tested with a phantom perfusion system. The results matched known blood perfusion and thermal resistance values. The method was also tested with burns on animal models. Inflammation effects associated with the burns were studied using a newly developed term called the Burn Factor. This correlated with the severity of imposed burns. This work consists of three journal papers. The first paper introduces the mathematical model and its validation with finite-difference solutions. The second paper validates the physical aspects of the usage of the model with thermal measurement in detecting simulated burned layers and the associated perfusion. The third paper demonstrates the ability of the model to use thermal measurements to detect different burn severity of an animal model and to study the healing process.
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    A Computational Approach For Investigating Unsteady Turbine Heat Transfer Due To Shock Wave Impact
    Reid, Terry Vincent (Virginia Tech, 1998-04-30)
    The effects of shock wave impact on unsteady turbine heat transfer are investigated. A numerical approach is developed to simulate the flow physics present in a previously performed unsteady wind tunnel experiment. The windtunnel experiment included unheated and heated flows over a cascade of highly loaded turbine blades. After the flow over the blades was established, a single shock with a pressure ratio of 1.1 was introduced into the wind tunnel test section. A single blade was equipped with pressure transducers and heat flux microsensors. As the shock wave strikes the blade, time resolved pressure, temperature, and heat transfer data were recorded.
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    Computational Study of Highway Bridges Structural Response Exposed to a Large Fire Exposure
    Nahid, Mohammad N. (Virginia Tech, 2015-07-08)
    The exposure from a localized vehicle fire has been observed to produce excessive damage onto highway bridge structural elements including complete collapse of the infrastructure. The occurrence of a fire beneath a bridge can lead to significant economic expense and loss of service even if the bridge does not collapse. The focus of the current research is to assess and evaluate the effect of realistic localized fire exposures from vehicles on the bridge structural integrity and to guide future development of highway bridge design with improved fire resistance. In this research, the bridge structural element response was predicted through a series of three loosely coupled analyses: fire analysis, thermal analysis, and structural analysis. Two different types of fire modeling methodologies were developed in this research and used to predict the thermo-structural response of bridge structural elements: one to model the non-uniform exposure due to a vehicle fire and another to predict response due to a standard uniform furnace exposure. The vehicle fire scenarios required coupling the computational fluid dynamics (CFD) code Fire Dynamics Simulator (FDS) with Abaqus while the furnace exposure scenarios were all done within Abaqus. Both methodologies were benchmarked against experimental data. Using the developed methodologies, simulations were initially performed to predict the thermo-structural response of a single steel girder-concrete deck composite assembly to different local, non-uniform fires and uniform standard furnace fire exposures. The steel girder-concrete deck composite assembly was selected since it is a common bridge design. Following this, a series of simulations were performed on unprotected highway bridges with multiple steel plate girders and steel tub girders subjected to localized fires. The analyses were used to evaluate the influence of a fire scenario on the bridge element response, identify the factors governing the failure of bridge structural elements subjected to a localized fire exposure, and provide guidance in the design of highway bridge structural elements against fire hazards. This study demonstrates that girder geometry affected both the dynamics of the fire as well as the heat transfer to the bridge structural elements which resulted in a different structural response for the bridge. A heavy goods vehicle (heat release rate of 200 MW) and tanker fires (heat release rate of 300 MW) were predicted to cause the bridge to fail due to collapse, while smaller fires did not. The geometric features of the plate girders caused the girder elements to be exposed to higher heat fluxes from both sides of the girder resulting in collapse when exposed to a HGV fire. Conversely, the closed feature of the box girder does not allow the interior surfaces to be in direct contact with the flames and are only exposed to the internal reradiation from surfaces inside the girder. As a result, the single and double lane tub girder highway bridge structure does not fail due to a heavy goods vehicle fire exposure.
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    Controlling Object Heat Release Rate using Geometrical Features
    Kraft, Stefan Marc (Virginia Tech, 2017-06-08)
    An experimental study was conducted to determine the effect of complex geometries on the burning rate of materials made using additive manufacturing. Controlling heat release rate has applicability in limiting fire hazards as well as for designing fuels for optimal burning rate. The burning rate of a structure is a function of the material properties as well as the airflow through it, which is dictated by the geometry. This burning rate is generally proportional to the porosity for objects in which the flow is limited by the path constriction. The relations between porosity and burning rate are well studied for wood cribs, which are layers of wood sticks. Crib and other objects with various geometric features were constructed of ABS plastic and coal powder using additive manufacturing processes. A cone calorimeter using oxygen calorimetry was used to measure the heat release rate of the crib specimens. Within the flow limited burning regime, the burning rate of an object is proportional to the porosity factor. Porosity factors calculated from a 1-D theoretical burn rate model as well as from two empirical models were found to correlate the heat release rate results for the crib samples. The heat release rate results of the complex geometries generally correlated to the same porosity factor; however, the model was modified to account for differences between regularly shaped cribs and objects with different sized flow areas. Using the empirical models provides good correlation for the crib burning data and gives a clearer delineation between the flow-limited and surface area controlled regimes.
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    Convection Calibration of Schmidt-Boelter Heat Flux Gages in Shear and Stagnation Air Flow
    Hoffie, Andreas Frank (Virginia Tech, 2006-12-19)
    This work reports the convection calibration of Schmidt-Boelter heat flux gages in shear and stagnation air flow. The gages were provided by Sandia National Laboratories and included two one-inch diameter and two one-and-one-half-inch diameter Schmidt-Boelter heat flux gages. In order to calibrate the sensors a convection calibration facility has been designed, including a shear test stand, a stagnation test stand, an air heater and a data acquisition system. The current physical model for a combined radiation and convection heat transfer environment uses an additional thermal resistance around the heat flux gage. This model clearly predicts a non-linear dependency of the gage sensitivity over a range of heat transfer coefficients. A major scope of this work was to experimentally verify the relation found by the model assumptions. Since the actual heat sink temperature is not known and cannot be measured, three different cases have been examined resulting in three different sensitivities for one pressure value, which is the gage sensitivity for the not cooled case and the gage sensitivity for the cooled case, based on the plate temperature or on the cooling water temperature. All of the measured sensitivities for shear as well as for stagnation flow fit well in the theory and show the non-linear decay for increasing heat transfer coefficient values. However, the obtained data shows an offset in the intersection with the sensitivity at zero heat transfer coefficient. This offset might arise from different radiation calibration techniques and different surface coatings of test gage and reference standard.
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    Design and Benchmarking of a Combustor Simulator Relevant to Gas Turbine Engines
    Barringer, Michael David (Virginia Tech, 2001-09-20)
    An experimental facility was designed and benchmarked that could simulate the non-uniformities in the flow and thermal fields exiting real gas-turbine combustors. The design of the combustor simulator required analyses of the flow paths within a real combustor in a gas turbine engine. Modifications were made to an existing wind tunnel facility to allow for the installation of the combustor simulator. The overall performance of the simulator was then benchmarked through measurements of velocity, pressure, temperature, and turbulence using a straight exit test section to provide a baseline set of data. Comparisons of the measured quantities were made between two test cases that included a flow field with and without dilution flow.One of the major findings from this study was that the total pressure profiles exiting the combustor simulator in the near-wall region were different from a turbulent boundary layer. This is significant since many studies consider a turbulent boundary layer as the inlet condition to the turbine. Turbulent integral length scales were found to scale well with the dilution hole diameters and no dominant frequencies were observed in the streamwise velocity energy spectra. Dilution flow resulted in an increase in turbulence levels and mixing causing a reduction in the variation of total pressure and velocity. Adiabatic effectiveness levels were significantly reduced for the case with dilution flow in both the near combustor exit region and along the axial length of the straight exit test section.