### Recent Submissions

• #### Investigation of the friction and noise of automotive rubber belt

The objective of this research was to study the frictional properties of an automotive v-ribbed belt-pulley system. In order to evaluate the friction and noise, a new test setup was constructed. The assembly was run under various environmental and operational conditions and the results were quantified, studied, and compared among themselves. The environmental conditions included dry interface and wet interface, conducted at both room temperature (23°C) and cold temperature ( -20°C). Operational parameters varied during the experiment were wrap angle, load attached, and acceleration. Frictional forces and associated noises generated were recorded. Some of the results generated were compared with previous research work, and the setup was also used to generate new data for conditions not previously studied. Dry room temperature results show close correlation with previous research. The presence of water in liquid state in the interface induces larger adhesion as water film in the interface changes friction mechanisms in the rubber belt-pulley interface. The high stiction of wet friction can lead to stick-slip vibrations and squeal noise. The theoretical stiction model for wet belt-pulley interface is presented. The stiction-related noise test is conducted, and the result is used to identify the spectrum pattern. The belt friction under cold conditions is found to have a higher value than that in room temperature conditions. The belt noise under cold conditions is found to have much higher squeal frequency than that in room temperature conditions.
• #### The measurement of anisotropic thermal conductivity in snow with needle probes

A new method for measuring thermal conductivity is being adapted from the method of measuring isotropic thermal conductivity in snow with needle probes as used by Sturm, Johnson and others, in order to enable the determination of anisotropic thermal conductivities. This method has particular relevance to measuring thermal conductivity of natural snowpacks where conductivity can be strongly anisotropic due to structures that develop from vapor transport-induced metamorphism, self-compaction and other mechanisms, and where there are known discrepancies between density-conductivity relations empirically derived from guarded hot plate and needle probe methods. Both analytically-based solutions and finite element numerical solutions to the anisotropic case are used to calculate the expected effective thermal conductivity as a function of anisotropic thermal conductivity and needle orientation. Additionally, preliminary measurements of both anisotropic salt/sugar layered samples and of snow were taken. Both suggest that detecting anisotropy in such materials is possible, though made difficult by variability between measurements and the requirement of multiple measurements at various angles. These studies suggest that anisotropy in snow may be able to explain in part the discrepancies between guarded hot plate and needle probe measurements in certain cases.
• #### Water-in-air droplet formation in plasma bonded microchannels fabricated by Shrinky-Dink® lithography

This thesis presents the first work on water-in-air droplet microfluidics. Polymeric microchannels were prototyped to illustrate water droplet formation in air by the T-junction meditated design. The first part of the thesis is on the proof of using unfiltered air as the process gas for plasma-assisted bonding of polydimethylsiloxane (PDMS) microchannels. A series of bilayered PDMS prototypes were plasma bonded under various plasma treatment parameters to determine the optimal settings for high-strength bonding. Pressure rupture tests were conducted to measure the bonding interface strength, which were shown to be as high as 135 psi. The second part of the thesis illustrates the formation and dispersion of water droplets in a continuous air flow in microchannels, and discusses the mechanisms of how droplets are formed. The Shrinky Dinks lithography and plasma-assisted bonding were used to prototype leakage-free microcbannels for testing droplet production. Droplets are formed under the competition between the fluid viscosity and surface tension forces. The channel dimensions and the fluid flow rates dictate the mechanism of droplet formation. The major finding is that the droplet length increases and droplet velocity decreases with increasing water flow rates, but some droplets were not formed at the T-Junction. These findings are discussed.

• #### Stress-corrosion cracking susceptibility of polystyrene/TiO₂ nanocomposite coated thin-sheet aluminum alloy 2024-T3 with 3.5% NaCl

This thesis reports an investigation into the performance of nanocomposite coatings, which consist of titanium dioxide nanoparticles within a polystyrene matrix, on the resistance to stress-corrosion cracking (SCC). The coatings are applied to compact tension specimens subject to conditions that promote failure by (SCC). It has been well documented in the literature that high-strength aluminum alloys such as 2024- T3 are prone to SCC when exposed to chloride media and sufficient levels of stress. The use of polymerbased nanocomposite coatings to protect aluminum alloy 2024-T3 has recently been shown to exhibit anticorrosion properties, which has been motivation for further study. The performance of such coatings on SCC is thus investigated here, using a fracture mechanics approach with compact tension specimens. The specimens are subject to a slow strain rate test using a constant displacement rate of 1.25 nm/s while exposed to periodically supplied 3.5% wt. sodium chloride solution. Measurements of load and crackmouth opening displacement data are recorded from the specimen throughout the test and used to characterize the response of the material to the applied mechanical loading in a corrosive environment. Results from the methods used herein showed a quantitative influence derived from the test results for several criteria of interest such as maximum load, time-to-failure, and fracture toughness. In total, four different coatings were applied; three with different titanium dioxide nanoparticle aspect ratios, and one without any titanium dioxide nanoparticles present in the polystyrene matrix. Characterization of the results showed that the shape of the titanium dioxide nanoparticle is a dominant factor that influences the susceptibility of aluminum alloy 2024-T3 to SCC.
• #### Testing and analysis of a ground source heat pump in Interior Alaska

Ground source heat pumps (GSHPs) can be an efficient heating and cooling system in much of the world. However, their ability to work in extreme cold climates is not well studied. In a heating-dominated cold climate, the heat extracted from the soil is not actively replaced in the summer because there is very little space cooling. A ground source heat pump was installed at the Cold Climate Housing Research Center (CCHRC) in Fairbanks, Alaska with the intent to collect data on its performance and effects on the soil for at least ten years. Analysis shows GSHPs are viable in the Fairbanks climate; however, their performance may degrade over time. According to two previous finite element models, the CCHRC heat pump seems to reach equilibrium in the soil at a COP of about 2.5 in five to seven years. Data from the first four heating seasons of the ground source heat pump at CCHRC is evaluated. The efficiency of the heat pump degraded from an average coefficient of performance (COP) of 3.7 to a mediocre 2.8 over the first four heating seasons. Nanofluids are potential heat transfer fluids that could be used to enhance the heat transfer in the ground heat exchanger. Improved heat transfer could lower installation costs by making the ground heat exchanger smaller. A theoretical analysis of adding nanoparticles to the fluid in the ground heat exchanger is conducted. Two nanofluids are evaluated to verify improved heat transfer and potential performance of the heat pump system. Data from the CCHRC heat pump system has also been used to analyze a 2-dimensional finite element model of the system's interaction with the soil. A model based on the first four years of data is developed using Temp/W software evaluates the ground heat exchanger for a thirty-year period. This model finds that the ground heat exchanger does not lower the ground temperature in the long term.

• #### Two-dimensional analysis of natural convection and radiation in utilidors

Central heating plants are often used on large building complexes such as university campuses or military bases. Utilidors can be used to contain heat distribution lines and other utilities between a utility station and serviced buildings. Traditional thermal analysis of utilidors is one-dimensional, with heat transfer correlations used to estimate the effects of convection, radiation, and two-dimensional geometric effects. The expanding capabilities of computers and numerical methods suggest that more detailed analysis and possibly more energy-efficient designs could be obtained. This work examines current methods of estimating the convection and radiation that occur across an air space in square and rectangular enclosures and compares them with numerical and experimental data. A numerical model was developed that solves the energy, momentum, and continuity equations for the primitive variables in two dimensions; radiation between free surfaces was also included. Physical experiments were conducted with two 10-ft-long apparatuses; one had a 1-ft $\times$ 1-ft cross section, the other was 2 ft $\times$ 4 ft. Several pipe sizes and configurations were studied with the 1-ft $\times$ 1-ft apparatus. The 2-ft $\times$ 4-ft apparatus was limited to containing 4- and 8-inch insulated pipes. Corresponding numerical studies were conducted. Difficulties in modeling large enclosures or those with large temperature differences (Rayleigh numbers above 10$\sp7$) were encountered. Results showed good agreement between numerical and experimental average heat transfer rates, and for insulated pipe cases these results also compared well with rates obtained from one-dimensional analysis. A new effective conductivity correlation for air in a square enclosure was developed, and its use was demonstrated in numerical conduction solutions and compared with full numerical convection and radiation solutions and with experimental data. Reasonably good results were achieved when there was a small temperature difference across the air gap.
• #### Pitorifices and small pumps in cold region water distribution systems

Most buried potable water distribution systems in colder regions of Alaska rely on pitorifices to provide circulation between the water main and service connections for freeze protection. Pitorifices are scoops which project into the main. When water is circulated in the main, they create a differential head which induces flow through dual service lines. Pitorifices have provided an inexpensive and simple alternative to installing a small pump at each service to provide circulation. However, very little information was available on the hydraulic performance of these devices. The objectives of this study were to: (i) develop techniques to measure pitorifice performance in the field; (ii) characterize performance of commonly used pitorifice shapes with different insertion depths and relative sizes in full-scale testing; (iii) develop an improved shape; (iv) research the competing technology of small pumps; and (v) present the information in a way that is useful to engineers. An inexpensive device for field checks of both differential head and flow rates at service lines was developed and the use of a low head loss meter was initiated. Methods and results of field studies in four different water systems are presented. Five commonly used pitorifice shapes and four new shapes were evaluated. The best shape was found to be one of the existing shapes, which is also one of the easiest to produce but not the most popular. It was also determined using a larger service line size can be cost effective. Test results are graphed and a theoretical framework is provided for designers. Smaller, energy efficient pumps may provide a cost effective alternative to pitorifices in some situations. Requirements for small pumps used for circulation in place of or to supplement pitorifices are given. Performance test results for different pumps are presented, most of which have not been used previously for service line circulation. Pumps with significantly lower operating costs than those in current use are identified. Several of these pumps were installed in services for long term testing.
• #### Size Effects In Mesoscale Mechanical Testing Of Snow

Snow is a naturally-occurring, heterogeneous material whose interactions with humans make it desirable for analysis as a geotechnical engineering material. In this study, clean, undisturbed, natural snows of two common types were collected in and around Fairbanks, Alaska and subjected to laboratory testing, and the results were compiled and analyzed. Three types of tests--flat pin indentation, unconfined compression, and cone penetration--were carried out while varying size parameters, and size effects were observed and studied. From flat-pin indentation testing, it was observed that first peak indentation strength initially fell exponentially with increasing indenter cross-sectional area, with the exponent averaging 0.84. Furthermore, the strength eventually rose to a plateau value, and the compression strength of snow could be calculated from this plateau value. This plateau, too, initially depended exponentially on the pin cross-sectional area for smaller pins. From unconfined compression testing, it was observed that as cross-sectional area of a flat pin indenter increased, plateau strength eventually reached that value found from unconfined compression testing. Furthermore, initial strength, plateau strength, and energy absorption density all increased linearly with increasing aspect ratio. From cone penetration testing, it was found that empirical values of snow strength may be obtained on both a micromechanical and macromechanical scale using cone penetration. Size effects, were also observed--smaller cone diameters and larger cone included angles yielded larger values for apparent snow strength. Some of the mechanisms behind all of these size effects are explainable from theory; others must be regarded for now as empirical in nature. In both cases, the results are quite reliable descriptors for a natural material, and may be safely interpolated from.
• #### Modeling Of A Novel Triple Turbine Solid Oxide Fuel Cell Gas Turbine Hybrid Engine With A 5:1 Turndown Ratio

Electrical production using solid oxide fuel cell gas turbine (SOFC-GT) hybrid systems has received much attention due to high-predicted efficiencies, low pollution and the availability of natural gas. Solid oxide fuel cell (SOFC) systems and hybrid variants designed to date have had narrow operating ranges due largely to the lack of control variables available to control the thermal requirements within the SOFC. Due to the higher value of peak power, a system able to meet fluctuating power demands while retaining high efficiencies is strongly preferable to only base load operation. This thesis presents results of a novel SOFC-GT hybrid configuration designed to operate over a 5:1 turndown ratio. The proposed system utilizes two control variables that allow the hybrid to maintain the SOFC stack exit temperature at a constant 1000�C throughout the turndown. The first control variable is the setting of a variable-geometry inlet nozzle turbine, which most directly influences the system airflow. The second control variable is an auxiliary combustor, which allows control of the thermal and power needs of the turbomachinery independently from that of the SOFC. At low turndown the proposed hybrid operates similarly to previous hybrids, in that roughly 80% of the power is delivered from the SOFC. However, the newly proposed hybrid uses the unique turbomachinery to drastically increase the delivered power at higher power demands. A unique aspect of the proposed hybrid is the contribution of half the rated power being supplied by the inexpensive turbomachinery with the expensive SOFC contributing the other half. This will significantly lower system capital costs compared to previous hybrid designs. The proposed hybrid has high efficiencies throughout turndown with peak efficiencies occurring at low turndown levels.
• #### Numerical Simulation Of Single Phase And Boiling Microjet Impingement

This work presents results from the numerical simulation of single phase and boiling microjets primarily for high density electronics cooling. For the single phase microjets, numerical simulation results for the flow fields and heat transfer characteristics in a laminar, confined microjet (76 mum in diameter) impingement arrangement are presented. The parameters varied included the jet Reynolds Number, the fluid Prandtl Number and the ratio of the nozzle-to-plate distance to the jet diameter. Primary and secondary recirculation zones were observed in the stagnation region and the radial outflow region which had a significant impact on the local Nusselt Number distribution on the heated surface. The location and the displacement of the primary and secondary recirculation zones are of particular importance and are associated with secondary peaks in the Nusselt Number similar to those observed for turbulent jet impingement in larger conventional jets. Numerical simulation results are presented for boiling microjet impingement in a confined arrangement. The Rensselaer Polytechnic Institute (RPI) model was modified for laminar flow boiling for simulating these types of flows. The model primarily proposes three different heat transfer components, the single phase heat transfer, the quenching heat transfer and the evaporative heat transfer. The model was first validated with experimental results from the literature and then extended to study the effects of liquid subcooling, microjet Reynolds Number based on the nozzle inlet, and heat flux levels. The simulation results were in good agreement with results from comparable experiments in the literature. The average wall temperature increases as the applied wall heat flux is increased. The slopes of the temperature curves in the radial direction flatten out at higher heat fluxes and lower levels of subcooling indicating the effectiveness of boiling heat transfer. For the cases considered in this study, the single phase heat transfer component dominates the other two modes of heat transfer The liquid velocity profile has a considerable impact on the vapor bubble nucleation, vapor drag and the bubble departure diameter. Lower levels of subcooling are associated with boiling inception and more vigorous boiling in the vicinity of the stagnation zone rather than those with higher levels of subcooling. The degree of subcooling emerged as the single largest factor controlling the lateral temperature rise in an electronic chip cooled by a single, confined impinging microjet. Increases in the jet inlet Reynolds Number for the same heat flux and subcooling levels increased the dominance of forced convection heat transfer over the boiling heat transfer. Lower Reynolds Number flows are marked by partial nucleate boiling in contrast to higher Reynolds Number flows marked by forced convection boiling. For all the cases considered in this work, the single phase heat transfer component dominated the other two modes of heat transfer. The evaporative mode dominates the quenching heat transfer mode, an observation that is markedly different from those observed for turbulent evaporative jets found in the literature.
• #### Theoretical And Experimental Analysis Of Two-Phase Closed Thermosyphons

This work presents an analytical and numerical model of a long inclined two-phase closed thermosyphon, known as a hairpin thermosyphon, which is representative of a new configuration for thermosyphons used in arctic applications. A laboratory experiment and a full scale road experiment along with associated modeling are described in detail. The laboratory experiment studies the condensation heat transfer performance of carbon dioxide inside the thermosyphon condenser under conditions of limited heat flux. The operating condition is not far from the critical point for carbon dioxide, which has a significant impact on the condensation heat transfer. An experimental correlation is developed to predict the carbon dioxide condensation heat transfer performance under these specific conditions. The full scale road experiment studies the overall performance of hairpin thermosyphons under actual field conditions. The model is a quasi one-dimensional formulation based on two-dimensional two-phase flow simulations at each cross section. The proposed model is useful for predicting steady state system operating characteristics such as pressure, temperature, liquid film thickness, mass flow rate, heat flow rate, etc., at local positions as well as over the entire system. The comparison of the modeling predictions with both laboratory and field experiments showed a strong correlation between modeling predictions and experimental results.
• #### Experimental Investigations Of Fluid Dynamic And Thermal Performance Of Nanofluids

The goal of this research was to investigate the fluid dynamic and thermal performance of various nanofluids. Nanofluids are dispersions of metallic nanometer size particles (<100 nm) into the base fluids. The choice of base fluid is an ethylene or propylene glycol and water mixture in cold regions. Initially the rheological characterization of copper oxide (CuO) nanofluids in water and in propylene glycol was performed. Results revealed that higher concentrations of CuO nanoparticles (5 to 15%) in water exhibited time-independent pseudoplastic and shear-thinning behavior. Lower concentrations (1 to 6%) of CuO nanofluids in propylene glycol revealed that these nanofluids behaved as Newtonian fluids. Both nanofluids showed that viscosity decreased exponentially with increase in temperature. Subsequent correlations for viscosities as a function of volume concentration and temperature were developed. Effects of different thermophysical properties on the Prandtl number of CuO, silicon dioxide (SiO2) and aluminum oxide (A12O 3) nanofluids were investigated. Results showed that the Prandtl number increased with increasing volume concentrations, which in turn increased the heat transfer coefficients of the nanofluids. Various nanofluids were compared for their heat transfer rates based on the Mouromtseff number, which is a Figure of Merit for heat transfer fluids. From this analysis, the optimal concentrations of nanoparticles in base fluids were found for CuO-water nanofluids. Experiments were performed to investigate the convective heat transfer enhancement and pressure loss of CuO, SiO2 and A12O 3 nanofluids in the turbulent regime. The increases in heat transfer coefficient by nanofluids for various volume concentrations compared to the base fluid were determined. Pressure loss was observed to increase with nanoparticle volume concentration. It was observed that an increase in particle diameter increased the heat transfer coefficient. Calculations showed that application of nanofluids in heat exchangers in buildings could result in volumetric flow reduction, reduction in the mass flow rate and size, and pumping power savings. Experiments on a diesel electric generator with nanofluids showed a reduction of cogeneration efficiency due to the decrease in specific heat compared to the base fluids. However, it was found that the efficiency of the waste heat recovery heat exchanger increased for nanofluids.
• #### Nanotribological Characterization Of Dynamic Surfaces

This dissertation research includes three fundamental areas: utilizing an atomic force microscope (AFM) to study the nanomechanical and tribological properties, to understand friction and wear at nanometer length, and to study wear mechanisms of boride coatings for biological applications. This was the first time that an AFM was used to study the nanomechanical and tribological properties and the performance of the materials. The AFM enables detailed investigation of the wear modes at multi-length scales as well as the surface mechanical properties. Surface analysis using an AFM included the surface texture, profile of indents, wear tracks, and wear scars. The friction force microscope (FFM) revealed the relationship between surface texture and frictional properties, thus contributing to the fundamental understanding of nanotribology. A new wear model was proposed. Also, hardening was discovered under the indents. The multi-scale wear study was focused on fundamental wear mechanisms. New wear modes, different than the traditional ones, were proposed. In this research, nanocracks and other damage (hardening and plastic flow) were found at different scales. Boride coatings on refractory metals were investigated for biological applications. Tribological performance of these coatings was studied in dry and wet (biofluid) conditions. It was found that boron plays an important role in forming amorphous and crystalline wear debris.
• #### Maximum weight lifting prediction considering dynamic joint strength

This thesis describes an efficient optimization method for predicting the maximum lifting weight considering dynamic joint strength in symmetric box lifting using a skeletal model. Dynamic joint strength is modeled as a three-dimensional function of joint angle and joint angular velocity based on experimentally obtained joint strength data. The function is further formulated as the joint torque limit constraint in an inverse dynamics optimization formulation to predict the lifting motion. In the proposed optimization formulation, external load is treated as design variables along with joint angle profiles, which are represented by control points of B-spline curves. By using this new formulation, dynamic lifting motion and strategy can be predicted for a symmetric maximum weight box lifting task with given initial and final box locations. Results show that incorporating dynamic strength is critical in predicting the lifting motion in extreme lifting conditions. The prediction outputs in joint space are incorporated in OpenSim software to find out muscles force and activity during the movement. Electromyography data are collected for a regular weight lifting to validate the integration process between the predictive model (joint model) and OpenSim model (muscle model). The proposed algorithm and analysis method based on motion prediction and OpenSim can be further developed as a useful ergonomic tool to protect workers from injury in manual material handling.