A Systemic Study of Nucleate Boiling

Nucleate boiling is a heavily researched form of heat transfer due to its associated high heat transfer rates. Applying two-phase heat transfer to space systems would allow these systems to become more capable, efficient, and compact. However, a fundamental understanding of boiling dynamics in the absence of buoyancy is yet to be developed. This study intends to analyze the effects of gravity, power input, and surface geometry during successive periods of microgravity provided by NASA’s “vomit comet” through the Reduced Gravity Student Flight Opportunities Program

Microgravity Experiments for the ISS

The Get Away Special (GAS) team is a microgravity research team know for leading Utah State University to impressive distinction of flying more experiments in space than any other university in the world. The following experiments were designed by the GAS team after receiving the opportunity to develop and experiment to be performed by a Space Flight Participant aboard the International Space Station (ISS)

Switched Moving Boundary Modeling of Phase Change Thermal Energy Storage Systems

Thermal Energy Storage (TES) devices, which leverage the constant-temperature thermal capacity of the latent heat of a Phase Change Material (PCM), provide benefits to a variety of thermal management systems by decoupling the absorption and rejection of thermal energy. While performing a role similar to a battery in an electrical system, it is critical to know when to charge (freeze) and discharge (melt) the TES to maximize the capabilities and efficiency of the overall system. Therefore, control-oriented models of TES are needed to predict the behavior of the TES and make informed control decisions. While existing modeling approaches divide the TES in to multiple sections using a Fixed Grid (FG) approach, this paper proposes a switched Moving Boundary (MB) model that captures the key dynamics of the TES with significantly fewer dynamic states. Specifically, a graph-based modeling approach is used to model the heat flow through the TES and a MB approach is used to model the time-varying liquid and solid regions of the TES. Additionally, a Finite State Machine (FSM) is used to switch between four different modes of operation based on the State-of-Charge (SOC) of the TES. Numerical simulations comparing the proposed approach with a more traditional FG approach show that the MB model is capable of accurately modeling the behavior of the FG model while using far fewer states, leading to five times faster simulations.Comment: 7 pages, 6 figure

Hierarchical power management in vehicle systems

This dissertation presents a hierarchical model predictive control (MPC) framework for energy management onboard vehicle systems. High performance vehicle systems such as commercial and military aircraft, on- and off-road vehicles, and ships present a unique control challenge, where maximizing performance requires optimizing the generation, storage, distribution, and utilization of energy throughout the entire system and over the duration of operation. The proposed hierarchical approach decomposes control of the vehicle among multiple controllers operating at each level of the hierarchy. Each controller has a model of a corresponding portion of the system for predicting future behavior based on current and future control decisions and known disturbances. To capture the energy storage and power flow throughout the vehicle, a graph-based modeling framework is proposed, where vertices represent capacitive elements that store energy and edges represent paths for power flow between these capacitive elements. For systems with a general nonlinear form of power flow, closed-loop stability is established through local subsystem analysis based on passivity. The ability to assess system-wide stability from local subsystem analysis follows from the particular structure of the interconnections between each subsystem, their corresponding controller, and neighboring subsystems. For systems with a linear form of power flow, robust feasibility of state and actuator constraints is achieved using a constraint tightening approach when formulating each MPC controller. Finally, the hierarchical control framework is applied to an example thermal fluid system that represents the fuel thermal management system of an aircraft. Simulation and experimental results clearly demonstrate the benefits of the proposed hierarchical control approach and the practical applicability to real physical systems with nonlinear dynamics, unknown disturbances, and actuator delays

Bubble Behavior in Nucleate Boiling Experiment Aboard the Space Shuttle

Boiling dynamics in microgravity need to be better understood before heat transfer systems based on boiling mechanism can be developed for space applications. This paper presents the results of a nucleate boiling experiment aboard Space Shuttle Endeavor (STS- 108). The experiment utilized nickel-chromium resistance wire to boil water in microgravity, and the data was recorded with a CCD camera and six thermistors. This data was analyzed to determine the behavior of bubble formation, detachment from the heating wire, and travel in the water with effects of drag on bubble movement. Bubbles were observed to be ejected from the wire, travel through and eventually stop in the unsaturated water. The data from this experiment is in good agreement with the results of theoretical equations used to model bubble-fluid dynamics in microgravity. The primary conclusion from this experiment is that a bubble can be ejected from a heated wire in the absence of gravity, instead of the creation of a single large vapor bubble. Further conclusions from this research could be applied to the development of safe and efficient heat transfer systems for microgravity and terrestrial applications

zonoLAB: A MATLAB toolbox for set-based control systems analysis using hybrid zonotopes

This paper introduces zonoLAB, a MATLAB-based toolbox for set-based control system analysis using the hybrid zonotope set representation. Hybrid zonotopes have proven to be an expressive set representation that can exactly represent the reachable sets of mixed-logical dynamical systems and tightly approximate the reachable sets of nonlinear dynamic systems. Moreover, hybrid zonotopes can exactly represent the continuous piecewise linear control laws associated with model predictive control and the input-output mappings of neural networks with piecewise linear activation functions. The hybrid zonotope set representation is also highly exploitable, where efficient methods developed for mixed-integer linear programming can be directly used for set operation and analysis. The zonoLAB toolbox is designed to make these capabilities accessible to the dynamic systems and controls community, with functionality spanning fundamental operations with hybrid zonotope, constrained zonotope, and zonotope set representations, powerful set analysis tools, and general-purpose algorithms for reachability analysis of open- and closed-loop systems

Graphene-Based Electromechanical Thermal Switches

Thermal management is an important challenge in modern electronics, avionics, automotive, and energy storage systems. While passive thermal solutions (like heat sinks or heat spreaders) are often used, actively modulating heat flow (e.g. via thermal switches or diodes) would offer additional degrees of control over the management of thermal transients and system reliability. Here we report the first thermal switch based on a flexible, collapsible graphene membrane, with low operating voltage, < 2 V. We also employ active-mode scanning thermal microscopy (SThM) to measure the device behavior and switching in real time. A compact analytical thermal model is developed for the general case of a thermal switch based on a double-clamped suspended membrane, highlighting the thermal and electrical design challenges. System-level modeling demonstrates the thermal trade-offs between modulating temperature swing and average temperature as a function of switching ratio. These graphene-based thermal switches present new opportunities for active control of fast (even nanosecond) thermal transients in densely integrated systems

Thin Wire Nucleate Boiling of Water in Sustained Microgravity

The advancement of small satellite technology relies on the development of effective thermal management systems that can be made smaller, safer, and more robust. This paper presents the results and analysis of a nucleate boiling experiment in sustained microgravity aboard the Space Shuttle Endeavor (STS-108). Bubble growth and departure were observed from a single and a braid of three 0.16 mm diameter and 80 mm long nickel-chromium resistive wires. Analysis showed that the braided wire provides a unique surface configuration to enhance the onset of boiling. The braid of wires was also observed to produce several bubble explosions; this is the first observation of such phenomenon under microgravity conditions. Bubble explosions are being researched on Earth due to their ability to remove large amounts of heat. Large spherical bubbles enclosing the wire were not observed, in contrast to many previous thin wire microgravity boiling experiments which often lead to the burnout of the heating element in microgravity. Measured bubble propagation was in good agreement with several prediction models based on drag forces. The effects of bubble formation, departure, and propagation on the temperature gradients in the fluid were analyzed. Applications for the development of microgravity heat transfer systems based on boiling mechanisms are discussed, along with the potential for further research utilizing small satellite technology

A decentralized control design approach to a class of large-scale systems

Large-scale systems present a unique control challenge. The large number of states, actuators, and control objectives for these systems often restricts the ability to analyze and control the system as a whole. Typically, these large systems are decomposed into multiple smaller subsystems which can be analyzed and controlled separately using a decentralized control approach. However, if the interactions between subsystems significantly affect the dynamics of the system, a decentralized control approach may prove to be ineffective and even result in unstable behavior. This thesis develops a control strategy for a class of systems with a particular hierarchical structure known as a Block Arrow Structure (BAS). Many real world systems naturally exhibit this two-level hierarchical structure, where a common subsystem at the higher, global, level interacts with multiple subsystems at the lower, local, level. There is no direct interaction among the lower level subsystems. A standard decentralized control approach would control each subsystem separately, ignoring the interactions between the higher and lower level subsystems. However, the interaction between the two levels may significantly affect the system dynamics, rendering the decentralized control approach ineffective. The proposed control strategy, referred to as the BAS control strategy, retains the scalability of the decentralized control approach but is also able to directly consider the interactions between the higher and lower level subsystems. This allows the BAS control approach to perform significantly better than a decentralized approach. Model predictive control (MPC) is used to evaluate the performance of the BAS control strategy relative to both centralized and decentralized approaches for two different BAS systems. In addition to the BAS control approach, this thesis develops an extremum seeking control (ESC) strategy which is used to improve the overall efficiency of the BAS system. In addition to performance objectives such as tracking a desired value for a state of the system, many systems have an efficiency objective. This objective seeks to control the system in the most efficient way possible, while still meeting the performance objectives. Minimizing the total energy use of all the actuators in the system is a common example of such an efficiency objective. In this work, ESC is used to augment the BAS control strategy at the global level to further improve the efficiency of the overall system. The model-free nature of ESC makes this control strategy especially effective in the presence of unknown disturbances and system nonlinearity, which may not be captured by the models used for the MPC controllers of the BAS control strategy. A linear example system is used to demonstrate the concepts and ideas presented throughout this thesis. For this example system, the BAS control architecture with ESC is able to achieve a control performance very similar to that of the centralized control approach while retaining the scalability of the decentralized approach. The benefits of the BAS control approach are also demonstrated for a more realistic system: a variable refrigerant flow (VRF) air-conditioning and refrigeration system for a building. Through a gray-box modeling approach, it is shown that VRF systems naturally exhibit a BAS structure and, therefore, can benefit from a BAS control approach. VRF systems are becoming widely used to meet the air-conditioning and refrigeration needs of buildings because of their greater efficiency in removing heat versus the conventional forced air systems. For these systems, it is very important to meet both the performance objectives, such as maintaining a desired air temperature in a room, as well as the efficiency objective of minimizing the total energy consumed by the system. Through a series of simulation examples, the BAS control approach is found to be a very effective control strategy for meeting both of these objectives