Theses and Dissertations

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Open Access
Development of advanced combustion strategies for heavy duty LPG engines to achieve near-diesel efficiency
(Colorado State University. Libraries, 2024) Fosudo, Toluwalase Jude, author; Olsen, Daniel B., advisor; Windom, Bret, committee member; Wise, Dan, committee member; Grigg, Neil, committee member
As the transportation sector evolves in response to increasingly stringent emissions regulations and economic realities in the wake of the decarbonization drive, several no/low carbon fuel options have emerged as viable options for internal combustion engines. Among these fuels, Liquefied Petroleum Gas (LPG) is uniquely positioned for spark ignited engine operation due to its favorable physical and chemical properties. Currently, much of its use as an engine fuel is limited to light-duty applications, dual fuel applications, or retrofitted gasoline engines, with a lesser degree of penetration into the heavy-duty sector where diesel fuel still dominates. A key reason for this is the deficit in performance and efficiency between diesel and other low carbon fuels, including LPG, necessitating the need for targeted research aimed at bridging this gap, and positioning LPG as a fuel of choice in the heavy-duty sector. Two prominent drawbacks responsible for this gap between diesel and LPG engine performance are the dearth of specialized fuel injection hardware and tailored injection strategies, and knock, which limits the performance of spark ignited engines. This work seeks to address these and other limitations and achieve near diesel efficiency on a heavy-duty engine platform. Two engine platforms were employed in this study. A cooperative fuel research (CFR) spark-ignited engine was used to study the knock dynamics and the performance, combustion, and emissions behavior of the LPG fuel in relation to key engine parameters, the LPG fuel composition, and other low carbon fuel options. Compression ratio, engine load, exhaust gas recirculation percents, and a novel combustion control tool, the combustion intensity metric (CIM), were all varied on the CFR engine and a computational fluid dynamics (CFD) model calibrated and validated. Key findings were then transferred to a heavy-duty engine platform, the Cummins ISX15L single cylinder engine. The engine is a converted 6-cylinder diesel engine with diesel brake thermal efficiency (BTE) of 44%. A baseline evaluation was conducted with liquid LPG port-injected at 16bar and 9.3:1 compression ratio. Then the engine was switched to direct injection (DI) configuration with a fuel delivery system capable of delivering liquid LPG at pressures up to 200bar. Three principal configurations were developed for operation of the heavy-duty engine employing a gasoline direct injector (GDI) with nozzle patterns adapted for optimal distribution of the LPG fuel in the combustion chamber, a GDI modified for higher LPG flow and a double-injector port-fuel injection (PFI) system optimized for injection location, and charge cooling and distribution. The experiments and modeling contained in this study demonstrate the impact of LPG composition on engine performance, the mitigating effect of EGR on knock and NOx emissions, the potential for a better controlled combustion using the CIM tool and the advantages in terms of knock, performance, and emissions of designing an injection strategy tailored to the LPG fuel. The results show that the heavy-duty engine operated on LPG achieved the target efficiency of 44% BTE at high EGR, high compression ratio, and high load conditions for both DI and PFI configurations. The outcomes of this study advance the literature on knock, end-gas autoignition, emissions, and EGR related to LPG and its use as a choice fuel for heavy-duty applications and advances the development of specialized fuel delivery hardware and injection strategies for the LPG fuel.
Embargo
Role of right ventricular anisotropic viscoelasticity in pathophysiology of RV failure
(Colorado State University. Libraries, 2024) LeBar, Kristen, author; James, Susan, advisor; Wang, Zhijie, advisor; McGilvray, Kirk, committee member; Chicco, Adam J., committee member; Popat, Ketul, committee member
Right ventricular (RV) failure is a key contributor to the mortality and morbidity of multiple cardiovascular diseases, such as congenital heart disease, heart failure with preserved ejection fraction, and pulmonary hypertension (PH). There has still, though, been a lack of treatment for such patients, due largely to a lack of understanding of the pathology and physiology of RV failure. Right ventricular passive stiffness is significantly increased in disease progression, and this change in mechanical behavior have been shown to markedly contribute to RV diastolic and systolic function. However, the myocardium is viscoelastic, and there is both energy storage (elasticity) and dissipation (viscosity) involved in the dynamic deformation within each cardiac cycle. Therefore, the long-ignored viscous component and its impact on organ performance must be investigated. Understanding of the impact of RV viscoelasticity in RV performance will fill a key knowledge gap in RV pathophysiology. Furthermore, the microtubule (MT), a cytoskeletal component of the cardiomyocyte (CM), is known to significantly contribute to the pathophysiology of multiple cardiovascular diseases. In the pressure-overloaded RV, MT density increases, leading to a stiffening of the CM and thus potentially the entire ventricular wall. Moreover, recent cell studies have shown that the pharmaceutical removal of the MT network reduces CM viscoelasticity and increases the extent of shortening, indicating a key role of the MT network myocardial viscoelasticity and contractile function. These findings suggest a regulation of myocardial viscoelasticity and organ contractility via the MT network. Therefore, the overall goal of my study is to determine the contribution of right ventricular anisotropic viscoelasticity to organ function during PH progression. The three specific aims of my dissertation research are: determine the alterations of RV anisotropic viscoelasticity in PH; delineate the contribution of the microtubules network to RV anisotropic viscoelasticity; explore the impact of the RV viscoelasticity on organ function using experimental and computational approaches.
Open Access
Design and flow characterization of an indraft supersonic wind tunnel for scramjet testing
(Colorado State University. Libraries, 2024) Teeter, Spencer J., author; Dumitrache, Ciprian, advisor; Windom, Bret, committee member; Bradley, Thomas, committee member
This thesis describes the Colorado State University supersonic wind tunnel design, manufacture, assembly, and validation. The overarching goal of this research is to develop a ground testing platform for studying airbreathing hypersonic propulsion systems. Problems of interest include design of isolators, fuel injection systems, ignition and flame stabilization, shock-boundary layer interaction, and aero-thermo-elastic interactions in scramjet vehicles. An indraft-type tunnel was chosen for its simplicity, low capital investment, and low power requirement. Its main features are large windows for advanced optical flow diagnostics, modular experimental mounting system, cycle time under 15 minutes, and adjustable size up to 5.25" x 5.25" x 25". The bulk of this thesis research focuses on flow characterization using a Mach 2.5 nozzle and a test section of 5.25" x 1.57". To this effect, we determined the Mach number using shockwave schlieren and stagnation pressure measurements over longitudinal and transverse scans in the tunnel test section. Experiments show that steady-state flow consistently develops in 0.5 seconds at a uniform Mach 2.4 at the entrance of the test section decreasing to 1.5 at the exit. Nozzle outflow and shot-to-shot Mach number variation was low, while measurement deviation increased near the test section walls and exit due to boundary layer growth. By studying phenomena such as fuel mixing, ignition, and flame stability at high Mach numbers inside of a supersonic wind tunnel, research at the CSU's Aerospace Propulsion and Diagnostics Laboratory seeks to overcome the limitations of current scramjet technologies.
Embargo
Effects of nanostructured polymeric surfaces on bacterial adhesion and erythrocyte (RBCs) integrity
(Colorado State University. Libraries, 2024) Sathyanarayanan, Vignesh, author; Popat, Ketul C., advisor; Ghosh, Soham, committee member; Li, Yan Vivian, committee member
Blood-contacting devices, such as stents, artificial heart valves, vascular grafts and catheters, placed within a host body, are subjected to complications such as thrombosis, restenosis, hemolysis etc. These complications result in the frequent need for revision surgeries or long-term drug therapies post implantations. Natural and synthetic biocompatible polymers are used as potential solutions for these issues due to their superior characteristic of biodegradability. Recent advancements in nanoscale fabrication and modification of these surfaces has shown improved results with platelets, leukocytes and other whole blood components. However, disruptions in erythrocyte's cell structure, caused by the foreign body materials, can compromise their oxygen-carrying capacity. This can further affect the overall tissue oxygenation and potentially lead to myocardial ischemic conditions. Therefore, it is also vital to understand the effect of bio-implant surfaces on erythrocyte integrity and viability, to enhance their biocompatibility. In this study, PCL nanostructured surfaces, nanofibers and nanowires, were fabricated and modified with organic compounds, Tanfloc and CMKC, to investigate their antibacterial properties and their effect on erythrocyte's cell integrity. Results indicate that the modified PCL nanostructured surfaces exhibit enhanced antibacterial properties and retain erythrocyte integrity.
Open Access
Computational modeling of plasma-assisted shock wave control using the Cartesian cut cell method
(Colorado State University. Libraries, 2024) House, Elijah D., author; Dumitrache, Ciprian, advisor; Windom, Bret, committee member; Bangerth, Wolfgang, committee member
To control shock waves using plasma discharges, we have developed a numerical model focusing on improving the accuracy and efficiency of simulating supersonic channel flow. Shock waves are essential in high-speed air-breathing propulsion devices such as ramjets and scramjets. The lack of turbomachinery means that shock waves compress air prior to combustion. The shocks decelerate the high-speed flow, increasing static temperature and pressure, which is necessary for efficient combustion. However, the advantage of simplicity (no moving parts) to achieve compression is counteracted by increased wave drag, total pressure losses, and flow separation inside the engine. In this context, the generation of shock wave trains (a sequence of reflected oblique and normal shocks propagating through the engine) must be appropriately managed and optimized to reduce drag and enhance thrust. To tackle these challenges, we use the APDL-CFD code to model a Ma=2.5 supersonic flow over a 10-degree triangular wedge inside a straight channel. The wedge generates a shock wave train that is typically encountered inside the isolator of a scramjet engine. These conditions are indicative of conditions that are currently being tested in supersonic wind tunnels. The code solves the compressible Navier-Stokes equations, incorporating advective and diffusive fluxes. The advective fluxes account for mass, momentum, and energy transport, while the diffusive fluxes capture viscous stresses and thermal conduction. This formulation includes viscous dissipation and heat diffusion, ensuring accurate modeling of compressible flow behavior. Furthermore, we enhance the APDL-CFD code with the Cartesian cut cell method, which allows the representation of complex geometries on a Cartesian mesh. This research represents geometries found in wind tunnel models and internal vehicle designs. Using the cut cell method, the model can capture flow caused by geometries that do not conform to a Cartesian mesh, like the wedge that generates the oblique shock waves. This improves accuracy and significantly reduces computational costs, allowing for lower grid resolutions on a Cartesian mesh. The cut cell method is implemented to research the use of plasma actuators as an active control mechanism. The model investigates how varying key parameters, such as the location and temperature of the plasma, affect shock wave dynamics and the associated separation bubbles. Results show that the plasma kernel alters the flow and provides an effective way to shift the position and reduce the intensity of the shock waves inside the channel. The numerical simulations aim to optimize this control, showing that shocks can be dynamically managed with the proper plasma parameters to enhance flow stability and performance. The results demonstrate significant improvements in controlling shock waves and flow separation when plasma actuators are employed, showing potential for their use in high-speed propulsion systems such as scramjets. Moreover, incorporating the cut cell method has optimized the APDL-CFD code, making it more efficient and better suited for running rapid test simulations. The results can inform future experiments, such as those planned for the Colorado State University (CSU) wind tunnel. Overall, the research offers valuable insights into active flow control in supersonic and hypersonic vehicles for improving vehicle performance, efficiency, and reliability.
Open Access
Development, testing, and validation of a heat transfer model for bi-propellant liquid rocket engines
(Colorado State University. Libraries, 2024) Roberts, Jadon A., author; Windom, Bret, advisor; Wise, Dan, committee member; Adams, Jim, committee member
Accurately modeling the heat transfer characteristics in a bi-propellant liquid rocket engine is a time and resource intensive process. The highly unpredictable and turbulent nature of the combustion requires complex modeling to predict the temperatures and fluid properties. These properties are required to evaluate material requirements and thermal performance. The primary objective of this project was to determine the effectiveness of an adaptable analytical heat transfer model implemented in MATLAB. The analytical model was pursued for the dramatic speed increase over numerical techniques such as computational fluid dynamics (CFD). The effectiveness of the model is determined by comparing results to CFD simulations as well as data obtained from testing. Strong correlations can be drawn with variations a low at 1\% between the CFD and analytical models. Three separate engines were analyzed to gauge the effectiveness of the analytical model across various engine and cooling configurations. A 10 N, 250 N and 2.9 kN thrust engines were developed. Extensive analysis was done on all engines using both the analytical model and CFD. These engines were designed with a wide range of cooling methods including radiative, ablative and regenerative cooling. A test stand previously only capable of testing hybrid rocket engines, was modified to allow for the testing of liquid bi-propellant rocket engines. The needed modifications included the addition of a fuel tank with mass measurement, venting and control valves, and fuel line sensing equipment. Upgrades were completed on the data acquisition system to incorporate additional sensors and controls. Further work was done to improve the safety of the test stand through redundancy and automation. These modifications culminated in two successful static fires of the 2.9 kN engine. The predicted temperatures of the 2.9 kN engine were compared to the test results from the static fires.
Open Access
High efficiency air delivery system for solid oxide fuel cell power generation
(Colorado State University. Libraries, 2024) Mitchel, Lars Jared-Brian, author; Bandhauer, Todd M., advisor; Windom, Bret C., committee member; Cale, James, committee member
Distributed power generation systems can be used in the electric grid to reduce peak loads, raise power quality, and reduce/eliminate transmission losses. One distributed energy system with distinct advantages is a Solid Oxide Fuel Cell (SOFC) integrated with an Internal Combustion Engine (ICE) which has the capability to operate at electric efficiencies as high as 70%. This research aimed to produce and test a high efficiency air delivery system that supports the SOFC-ICE to generate power on the scale of 80 kW. The air balance of plant (BOP) system utilized low speed scroll-type rotating compressors and brazed plate and frame heat exchangers for efficient preheating. The scroll compressors were modeled in GT-Suite and the remaining air BOP system was modeled with thermodynamic and heat transfer equations. Then testing was done on the compressors and heat exchangers to validate the model so that the air BOP system performance could be accurately predicted within a range of conditions. Both compressors were run from a range of 20 g/s to 60 g/s with the heat through the system being swept from 100°C to 600°C which yielded compressor efficiencies over 60% and heat exchanger effectiveness over 0.90. The validated model was then used to make predictions about system performance at on and off-design conditions.
Open Access
Experimental evaluation of a standalone hollow cathode apparatus with a magnetic field
(Colorado State University. Libraries, 2024) Ku, Emily X., author; Williams, John, advisor; Dumitrache, Ciprian, committee member; Thornton, Christopher, committee member
Testing hollow cathode assemblies independently from their use in Hall or gridded ion thrusters offers advantages such as reduced test facility size, lower power requirements, and improved diagnostic access. Standalone tests can reveal important cathode characteristics like ignition time, keeper ignition voltage, tip temperature, and current capability. Replicating the plasma phenomena that occur when a cathode operates within a thruster is challenging but essential, as these phenomena can generate energetic ions that erode cathode and keeper surfaces, limiting thruster lifespan. The primary challenge is to accurately emulate thruster conditions in standalone tests and verify this emulation through comparison with cathode-thruster operations. This thesis presents data on a standalone hollow cathode operated with magnetic fields that emulate those in electric propulsion devices, testing it both without an applied magnetic field and with permanent and solenoidal magnetic fields. Measurements of keeper, anode, and cathode-to-ground voltages were conducted over a range of anode currents and flow rates. At certain conditions, the plasma discharge transitioned to a less stable mode known as plume mode, with higher flow rates shifting this transition to higher anode currents. Introducing a magnetic field decreased the anode current at which this voltage shift occurred. Important findings in this work include: (1) Repeat tests with no magnetic field show that the transition behavior was different from one test to another, indicating that transition behavior may be affected by minute changes in cathode apparatus, or there are significant uncertainties associated with the transition and (2) Significant hysteresis in plume mode transition was observed when increasing and then decreasing anode current. These two findings along with the deleterious effects of the magnetic field have important implications on cathodes operating with Hall thrusters, which often exhibit large, rapid oscillations in discharge current.
Open Access
Single power supply operation of a Hall thruster
(Colorado State University. Libraries, 2024) Robertson, Zachary K., author; Williams, John, advisor; Fankell, Doug, committee member; Roberts, Jacob, committee member
Interest in operating Hall thrusters with a single power supply, facilitated by heaterless hollow cathodes, has motivated this research. Initial investigations into the Safran PPS 1350 confirmed the potential for this configuration. Building on these findings, modifications were made to a laboratory Hall thruster to enable operation with a single power supply and changes were made to the electrical configuration to promote smooth cathode ignition and decrease the influence of the inductance in the magnetic coils. In addition to operating with a laboratory power supply, CisLunar Industries supplied a prototype anode supply that demonstrated capability of running the laboratory Hall thruster under these conditions without efficiency losses, as verified by thrust stand data. A phenomenological efficiency analysis was performed using a Faraday probe and a retarding potential analyzer which supported these results, while also providing pertinent sub-efficiencies. The study concludes that a single power supply configuration is a viable approach to starting and operating a Hall thruster equipped with a heaterless hollow cathode.
Open Access
Evaluating thermal efficiency and economic impacts in supplying energy demands for direct air capture
(Colorado State University. Libraries, 2024) Siegel, Madeleine Hope, author; Bandhauer, Todd, advisor; Quinn, Jason, committee member; Herber, Daniel, committee member
Direct Air Capture (DAC) technologies that remove CO2 directly from the atmosphere are needed to meet international goals of limiting atmospheric temperature increase before 2100. Operating costs, including the cost of energy inputs, currently limit the rapid deployment of DAC systems. An abundance of untapped and abandoned geothermal resources provides an opportunity to utilize this thermal energy beneath the Earth's surface to reduce the financial and energy costs of DAC. In this study, thermodynamic models of applicable renewable energy scenarios for fulfilling heating and electrical requirements of DAC were analyzed using ASPEN Plus. Individual components were optimized within the geothermal-DAC coupled systems to quantify specific costs of implementation. The results were integrated into a techno-economic analysis (TEA) to provide a holistic perspective to optimize DAC coupled renewable energy systems. The levelized cost of energy (LCOE) for DAC was reduced from $175/t-CO2 removed to as low as $66/t-CO2 removed, guiding large-scale deployment of DAC, and supporting decision-making in the future.
Open Access
Diurnal and seasonal predictability of envelope pressures driving natural infiltration in residential buildings
(Colorado State University. Libraries, 2024) Bledsoe, Dominic, author; Bond, Tami, advisor; L'Orange, Christian, committee member; Farmer, Delphine, committee member
This study examines the dynamics of residential building envelope pressures by predicting and comparing time series site-specific weather conditions at minute-level resolution. Utilizing theoretically established relationships of both stack and wind effects, this research examines the predictability and accuracy of envelope pressures under different weather conditions. When high wind effects are removed, the Mean Absolute Error (MAE) in stack pressure predictions are minimized, typically falling below 0.24 Pa. The use of airport weather data, even after correcting for height difference and terrain, was found to be inconducive to prediction, highlighting the preference for site-specific measurements to enhance prediction accuracy. This research utilizes minute-level data for real-time environmental monitoring, aiming to inform pressurization or integrate predictive models for dynamic indoor air quality management. The findings contribute to the field by offering a practical approach to measuring and predicting residential air exchange rates, providing insights that could lead to improved health outcomes and energy efficiency in homes.
Open Access
Barium sensing in hollow cathode plasma using cavity ring-down spectroscopy (CRDS)
(Colorado State University. Libraries, 2024) Antozzi, Seth, author; Yalin, Azer, advisor; Dumitrache, Ciprian, committee member; Aristoff, David, committee member
Hollow cathodes (HCs) are ion propulsion devices commonly paired with Hall Effect Thrusters (HETs), which are devices of increasing importance in the ion propulsion community. Barium Oxide (BaO) cathodes are known to emit barium when operating under high-temperature conditions. Understanding barium densities in the cathode plasma provides experimental guidance for NASA barium modeling, including understanding of the physical characteristics and lifetime of the cathode. Based on modeling work, expected barium densities are ~1010 cm-3. A sensitive diagnostic is required such as CRDS. In this work, the detection of barium from the thermionic emitter of the Mark II 25 A BaO HC using the laser diagnostic technique of cavity ring-down spectroscopy (CRDS) is presented. CRDS detects ground state neutral barium via absorption of the probe laser beam in the vicinity of 553.548 nm (air wavelength). The cathode CRDS measurements are performed along the axis of the cathode since that is the control volume of interest. We report barium density as a function of heater current (plasma off) with results showing an approximately exponential density increase with current. Further parameters of study include keeper current, anode current (with the cathode operating), and propellant flow values. The measured signal-to-noise allows estimation of the barium density detection limit as ~106 cm-3 in the present configuration. An appendix to this work addresses the need for a diagnostic technique to measure krypton neutrals in HC plumes. In the krypton study, we enhance the krypton Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique and apply it to a BaO HC plasma. We utilize a dye laser at 212.6 nm to excite TALIF fluorescence within the plume, with the fluorescence detected at 758.7 nm. We present spatial maps for krypton neutral densities at a cathode flow rate of 7.5 sccm and anode currents of 5A and 13A. These measurements provide insights into facility effects related to cathode coupling and cathode physics, such as the collisional damping of instabilities. Additionally, we discuss how plasma characteristics, including spot versus plume mode, and plasma luminosity, are influenced.
Open Access
Bistable prestressed spring steel grippers for aerial perching and grasping
(Colorado State University. Libraries, 2024) Jones, Bryce, author; Zhao, Jianguo, advisor; Ciarcia, Marco, committee member; Simske, Steve, committee member
Quadcopter drones are popular in both the consumer and commercial markets, with a wide range of uses and applications, including inspections, research, natural disaster response, and filming and photography. These uses and applications are currently limited, however, by the limited battery power and range of current drones. Aerial perching can extend the useful flight time of a drone by allowing for passive perching in a location for a desired amount of time. Compliant bistable mechanisms are well-suited for this application because of their adaptability in a wide range of environments while utilizing bistability to reduce energy consumption and complexity. Current research into aerial perching with compliant mechanisms is limited to heavy, rigid grippers with limited applications in a wide variety of environments and grippers with complicated pneumatic controls. In this thesis, we propose a novel solution to this gap in research through the use of prestressed spring steel bands (PSSB) to create compliant bistable grippers for aerial perching and grasping. We investigate multiple different PSSB configurations. We first investigate two single PSSB gripper designs, a single band gripper with a cable driven opening system, then an improved silicone encased single PSSB gripper design. The first single band gripper is experimentally tested to determine the triggering force, effect of offset on triggering force, effect of spring pretension on triggering force, opening force, grasping force, and activation time. This design had some issues with opening reliably and tangling. The improved silicone encased gripper is experimentally tested for triggering force, the effect of varying contact points and angles, activation time, reduction of triggering force with springs, and actual flight tests on a drone done in partnership with IIIT Hyderabad. The single band gripper designs can grasp a variety of objects, especially cylindrical ones, but are limited in grasping spherical and heavier objects, and vertical grasping. We then design a cross-shaped gripper based on the silicone encased PSSB gripper. This gripper is experimentally tested in the same manner as the silicone encased single band gripper and performs well in grasping spherical objects and vertical grasping. It does, however, struggle to grasp longer cylindrical objects. These gripper designs have a fixed triggering force based on the design that limited the applications for drone applications with high acceleration causing inadvertent activation, as well as for grasping lightweight objects. Being able to actively control the triggering force of the grippers would give the ability to tune the gripper for ideal performance in a wide range of applications. To actively tune the triggering force, we investigate the use of on the fly tuning with Nitinol Shape Memory Alloy springs. We first attempted closed-loop control by using a PID controller to control the resistance of the springs. Then we used an open loop control method where constant voltage is applied to the springs that allows for precise tuning of the triggering force to a set range for the desired application, and experimentally verify the reduction in triggering force and show the application of on the fly triggering force tuning.
Open Access
Open-path cavity ring-down spectroscopy for the detection of hydrogen chloride gas and particles in a cleanroom environment
(Colorado State University. Libraries, 2024) Khan, Muhammad Bilal, author; Yalin, Azer P., advisor; L'Orange, Christian, advisor; Yost, Dylan, committee member
Semiconductor chips are the driving force behind the electronics industry, and modern technology depends on these vital chips to function. The size of these semiconductors has been steadily decreasing in accordance with Moore's law. The increasingly smaller feature sizes require very pristine cleanroom manufacturing environments to ensure minimal contamination from unwanted gases and particles. Two of the main contaminants to monitor in a cleanroom are gaseous hydrogen chloride (HCl) and airborne particles. HCl is a corrosive gas that affects the lifespan of equipment, infrastructure, and ventilation systems while also negatively impacting human health. Likewise, the presence of airborne particles is problematic since it can result in yield loss due to blockage of the inscription of miniature circuits on the wafers. Manufacturing must occur in spaces between ISO 3 to 8 (International Organization for Standardization) requiring precise monitoring of particles. The overarching goal of the present research is to develop new cleanroom monitoring methods for HCl and particles based on novel laser instrumentation. An acrylic chamber with controlled inlet and outlet flow was constructed and utilized to simulate cleanroom conditions. This chamber allowed for controlled air flows mixed with HCl gas in the range of ~0-100 parts-per-million (ppm) or particles at ISO levels of ~≤3-9. Detection of both HCl and particles uses a single continuous-wave 1742 nm near-infrared laser as a light source for open path cavity ring-down spectroscopy (CRDS). The compact laser system consists of a 60 cm cavity. High sensitivity detection of HCl is achieved by probing the 2-0 vibrational band of HCl (R(3) line). The CRDS system can accurately detect HCl with an Allan deviation of 0.15 ppb over a 10-minute duration. Several approaches for particle detection based on analyzing the small fluctuations in ring-down times caused by Mie scattering are examined. The most sensitive particle detection uses statistical analysis of ring-down times based on the 3rd and 4th standard moments allowing the detection of particles (diameter > 1 μm) at low concentrations down to ISO of approximately 5. The results provide a promising foundation for the development of open-path CRDS laser instrumentation for cleanroom monitoring.
Open Access
Transient modeling of an ambient temperature source centrifugal compressor steam generating heat pump
(Colorado State University. Libraries, 2024) Ryan, Kelly Patrick, author; Bandhauer, Todd M., advisor; Windom, Bret C., committee member; Herber, Daniel R., committee member
As the US electricity grid transitions to renewable power generation, electrifying end-uses that are currently fossil fuel fired presents a promising path towards deeper decarbonization, and next generation high temperature heat pumps are a viable solution for decarbonizing fossil fuel fired steam boilers. These next-gen systems require a higher degree of design complexity and more finely tuned control strategies than existing systems, and therefore can benefit from complex transient modeling that has not been previously implemented for these types of systems. A transient study of a novel steam-generating heat pump with steam delivery temperature of 150°C was conducted using physics-based simulation software. The model used manufacturer supplied performance data to calibrate each competent, providing reliable preliminary validation to the model. The model was set up to match the configuration of a prototype system constructed at Colorado State University. It was found that the results of the transient model agreed well with the steady state model of the heat pump at the design point. Transient conditions including cold startup to full load operation, full load operation to part load operation, and part load operation back to full load operation were modeled and the system was found to operate with stability. Compressor and expansion valve performance was investigated. Compressors were found to operate within their performance maps for both steady and transient operation. A control strategy was developed for the expansion valves to prevent liquid ingestion when transitioning to turndown operation. The system COP was predicted for both full and part load operation and in transition between them.
Open Access
Thermal management of discretized heaters using CuW microchannel heat sinks and FC3283 for laser diode applications
(Colorado State University. Libraries, 2024) Amyx, Isabella Gascon, author; Bandhauer, Todd M., advisor; Dumitrache, Ciprian, committee member; Venayagamoorthy, Subhas Karan, committee member
Single-phase cooling using microchannel heat sinks (MCHS) has become a popular approach for overcoming the thermal challenges associated with high-powered microelectronic devices. Thermal management is one of the largest barriers to higher power densities in electronics and frequently limits overall device performance. The implementation of forced convective cooling via single-phase liquid cooling in MCHS reduces the thermal resistance resulting in lower device temperatures at high-power conditions, which can decrease the package size and extend the lifespan of devices. The goal of this effort was to investigate practical cooling solutions for laser diode bars. This study examined the effectiveness of a copper tungsten (CuW) microchannel heat sink paired with a dielectric coolant (FC3283) for dissipating both discrete and uniform heat fluxes up to 600 W/cm2 across a 0.25 cm2 surface area through a numerical and experimental study. CuW was chosen as the MCHS material because it is thermal expansion matched to GaAs, which is a common laser diode substrate. FC3283 serves as a dielectric coolant that is compatible with power electronics cooling. Reasonable agreement was found between the numerical model and the experimental results. The resulting thermal resistance ranged from 0.15 cm2 K/W at the highest flow rate to 0.26 cm2 K/W at the lowest flow rate. The resulting thermal performance from this study proved to be insufficient for maintaining optimal temperatures for laser diode applications. Using the validated model, cooling fluid and geometry modifications proved to have a significant impact on the heat transfer coefficients. This study revealed the importance of considering discrete heat sources separately from uniform heat sources and proved that CuW microchannels can be a promising cooling option toward future advancements of laser diode bars and other high-power microelectronics when using a low viscosity and high thermal conductivity, dielectric cooling fluid and an optimized geometry.
Open Access
Optical performance of cylindrical absorber collectors with and without reflectors
(Colorado State University. Libraries, 1994) Menon, Arun B., author; Duff, William, advisor; Burns, Patrick J., committee member; Zachmann, David W., committee member
The optical efficiency of a solar collector, which depends on the collector geometry and material properties (i.e., geometry and radiative properties of the cover, absorber and any reflector), contributes significantly towards its overall performance. This optical efficiency is directly proportional to the transmittance-absorptance or τα product for all possible angles of incidence. A 3-D Monte Carlo ray tracing technique is used to determine this τα product for evacuated tubular collectors (ETCs) with cylindrical absorbers in an effort to identify the most efficient optical design parameters. These collectors are asymmetric with respect to the incident solar radiation and their optical efficiencies are therefore difficult to estimate using any other method. The collector geometry is modeled using constructive solid geometry (CSG). CSG allows the generation of complex collector shapes by combining simple primitive objects. The ray tracing algorithm tracks individual photons through the collector geometry to provide a means of obtaining the absorbed fraction for a particular angle of radiation incident on the collector plane. Incidence angle modifiers (IAMs), the ratio of the τα product at a particular set of longitudinal and transverse radiation incidence angles to the τα product at normal incidence are thereby obtained. IAMs are calculated for variations in five different design parameters to determine the most advantageous geometries. It is found that diffusely reflecting back planes significantly enhance optical performance of tubular collectors. Verification of the ray trace calculations is made by comparing with experimental results from the indoor solar simulator at CSU. TRNSYS predicted values of τα are within 1% of the ray trace results for normal incidence tests and within 7% for off-normal tests. Inaccuracies resulting from the use of a multiplicative technique wherein off-axis IAMs are obtained by a multiplicative combination of the biaxial IAMs are also addressed. The multiplicative approach is found to be very inaccurate for angles of incidence greater than 40°. To further assess the relative advantages of tubular collectors over flat plate collectors and whether a reflective back plane is really necessary, the two types of collectors are modeled in a simple fashion and the amount of radiation that is available for collection by each is determined. Calculations show that reflectors would probably not be required for collector slopes in excess of 50°. However, for slope angles less than 50°, a reflector placed behind the tubes is beneficial.
Embargo
Computational methods for the analysis of cell migration and motility
(Colorado State University. Libraries, 2024) Havenhill, Eric Colton, author; Ghosh, Soham, advisor; Heyliger, Paul, committee member; McGilvray, Kirk, committee member; Zhao, Jianguo, committee member
Collective cell migration (CCM) is necessary for many biological processes, such as in the formation or regeneration of tissue, fibroblast movement in wound healing, and the movement of macrophages and neutrophils in the body's immune response, to name a few. CCM is commonly modeled with PDEs, however these equations usually model the population density, rather than the displacement field describing the movement of any arbitrary cell. One unknown aspect of this movement is the various methods that cells use to facilitate communication to each other. Chemical communication plays a substantial role in directed cell movement, however, other mechanical methods, such as the propagation of stresses through a shared substrate to neighboring cells and cell behavior in a crowded environment, also play an important role which is less understood. The quantification of the kinematic and dynamic characteristics in CCM would present several novel advancements in understanding the collective cell behavior. First, the dynamic mode decomposition (DMD) framework is utilized. DMD allows for the recovery of a dynamic system, in the form of an ODE or PDE, by sampling the states of a system. In the context of the cell migration, the displacements of fibroblasts during a scratch-wound assay are obtained, which result in a governing PDE through the DMD process. This PDE is used in conjunction with modern optimal control theory to develop a 2D and 3D trajectory for the migration of controllable cells to a target. On an individual level, with the hybrid use of modern static structural optimization and simple non-linear control, a cell's cytoskeleton during migration can be studied, providing for the quantification of the traction force exerted on the substrate. The results of this analysis are compared with stress and structural optimization models in ANSYS and FEBio, which uses the finite element method, so that a reasonable range of these stresses during CCM can be provided. To further study the individual mechanics of cell migration, the proposed hybrid model is extended to a fully dynamic model which predicts the cytoskeletal stress fiber formations over time that require the minimal amount of material with the use of optimal control theory. The results of this research could provide useful applications in many real-world situations, from the generating of a trajectory for microrobots during drug delivery to the study of the collective migration of organisms including cells.
Open Access
Prediction and mitigation strategies for the transient thermal performance of low thermal resistance microchannel evaporators
(Colorado State University. Libraries, 2024) Anderson, Caleb Del, author; Bandhauer, Todd M., advisor; Venayagamoorthy, Karan, committee member; Windom, Bret, committee member; Wise, Daniel, committee member
Microchannel flow boiling heat transfer offers an effective thermal management solution for high heat flux microelectronic devices such as laser diodes. The high heat transfer rates, nearly isothermal flow conditions, high surface area-to-volume ratios, and lower required pumping powers facilitate smaller component systems while more efficiently cooling devices and reducing packaging stresses associated with thermal expansion when compared with single-phase cooling systems. Although much study has been dedicated to optimizing steady state flow boiling performance, the typically highly transient operation of these microelectronic devices leads to unsteady spikes in heat flux and, subsequently, in device temperatures and may potentially exacerbate flow instabilities present at steady state. The low thermal capacitance of the package that often accompanies the low thermal resistance of microchannel evaporators increases the potential for device damage and failure since large temperature swings are more likely. Predicting and mitigating the transient response of a low thermal resistance microchannel evaporator is paramount to practical application as a thermal management technique. In this work, temperature, pressure, and flow visualization measurements during stepped heat loads on two, low thermal resistance, microchannel evaporators revealed the presence of severe vapor backflow, large temperature overshoots, and impacted flow dynamics at the onset of nucleate boiling (ONB) despite the stability and high performance of the device under steady state heating conditions. These overshoots were exacerbated with higher heating rates and reduced subcooling but were generally improved with higher flow rates. Applying a slower heating rate greatly improved the transient thermal response, reducing both peak temperature and vapor backflow. Channel and inlet orifice geometry were found to greatly impact the performance, with smaller channels and smaller orifice-to-channel restriction ratios resulting in intensified vapor backflow and temperature spikes, despite offering improved steady state performance. A computational model embedded in a reduced order design tool was created and validated with the experiments. Two separate models were created due to the different transient conditions observed between the two tested microchannel evaporators. The models allow predictive modeling of these evaporators to determine the impact of the transient heating behavior on microchannel evaporator devices. The effect of incorporating gallium-based, solid-liquid Phase Change Materials (PCMs) was studied semi-empirically by simulating the performance of a virtual test section with predicted properties of a microchannel evaporator combined with gallium and gallium-composite foam PCMs. Properties of the PCMs were estimated and used to predict the test section thermal response under a range of PCM volumes. Models assuming single phase performance were conducted initially and the resulting predicted heat rate to the fluid applied experimentally to the test section heater to determine the temperature response. It was found that the simulated addition of the PCM slightly reduced the ONB temperatures but did not affect the peak temperature experienced by the device. The applied heating rate, however, did not consider the increased thermal resistance to the refrigerant fluid during the transient vapor backflow regime. The effect was most pronounced in the PCMs with the largest exposed surface area and with thermal conductivity-enhanced PCM composites comprised of gallium infiltrated in a copper foam matrix. Additional PCM models utilizing the transient flow boiling model were subsequently run on a series of representative heat load test cases comparing the performance of a gallium-nickel and gallium-copper composite with similar dimensions to the earlier simulations. Key assumptions included the same ONB temperatures and vapor backflow conditions as the baseline cases without PCMs. The models predicted significantly lowered peak device temperatures due to the heat absorption into the PCM during the transient vapor backflow phase. The effect was dependent on the PCM thickness, latent heat, and thermal conductivity, reflecting trade-offs in material. In addition, peak temperature variability observed experimentally across multiple trials at the same nominal testing conditions was greatly reduced with the inclusion of a PCM.
Embargo
Towards automated manufacturing of composites via thermally assisted frontal polymerization
(Colorado State University. Libraries, 2024) Jordan, Walter Patrick, author; Yourdkhani, Mostafa, advisor; Zhao, Jianguo, committee member; Simske, Steve, committee member
Current methods for the manufacturing and repair of fiber-reinforced thermoset composites are energy-intensive, slow, and costly due to extensive processing steps and expensive equipment required to achieve complete cure. This is especially true for large, complex geometries that require autoclaves and prolonged cure times. As a result, there is a need to develop faster, cost-effective, energy-efficient processes. With the implementation of rapid curing thermoset resins, the cure cycle can be reduced from hours to minutes. This research focuses on the development, implementation, and testing of these resin systems in the established fields of mobile additive manufacturing and filament winding to demonstrate unprecedented, rapid manufacturing of composite parts. Additive manufacturing of fiber-reinforced thermoset composites is desirable due to its inherent ability to produce custom, complex parts quickly, with minimal required tooling. By printing and simultaneously curing the composite as it is deposited, freeform unsupported structures with high mechanical properties can be created. One limitation of current additive manufacturing methods is the print volume associated with traditional gantry style additive manufacturing systems. By combining the highly desirable properties of additive manufacturing using rapid, thermally curable resin systems with the mobility of a mobile additive manufacturing system, large, mechanically sound structures with virtually no limitations on print volume can be created. Moreover, rapid curing thermoset resin systems have the potential to revolutionize traditional composite manufacturing processes. Due to its wide range of applications and its ubiquitous nature, filament winding serves as a natural starting point to do so. Traditional filament winding is typically a two-step manufacturing process, where the composite part is first wound on a rotating mandrel and then cured using autoclaves or ovens. By combining these processes on the winding machine, the labor involved in manufacturing, the energy required for curing, and the overall production time are significantly reduced. In this research, a mobile additive manufacturing robot is designed, validated, and optimized for accurate locomotion and fast, dimensionally accurate printing of composite structures with high fiber alignment and degree of cure. The capabilities of this system are exhibited throughout several demonstrations that involve printing unsupported structures upside-down, the manufacturing of a bridge strong enough for the robot to pass over, and bridging the material across a 60 cm gap. Additionally, a pre-existing filament winding machine is optimized for the manufacturing of large, geometrically unconstrained composite structures. Improvements in fiber volume fraction are achieved through processing changes and a thermal profile for dry fibers is established to facilitate identification of frontal polymerization.