MS, Kent State University, 2024, College of Aeronautics and Engineering

Understanding thermal performance is essential to optimizing system performance, reducing damage, and enhancing overall reliability and safety, especially in high-powered heat generating applications. In the growing eVTOL (electric Vertical Take-Off and Landing) ecosystem, being able to provide a means to quantify and characterize the thermal performance of new propulsion platforms has resorted to complex, numerical solutions via commercial software. One reason for this is how thermal performance is affected by a phenomenon called thermal soak. Thermal soak is the sudden increase of internal component temperature due to terminating any forced convective cooling from a heat-producing system (i.e., landing and shutting down after hovering.) A more accessible thermal soak model was developed and explored with the aim of better understanding heat transfer properties of electric/hybrid motors under VTOL power conditions. The purpose of this thesis is to discover if thermal soak as a lumped parameter 2nd order system can be reliably modeled with defined damping ratios, natural frequencies, and initial conditions that are functions of system thermal parameters. Comparisons between the thermal soak model and experimental data from a multitude of eVTOL motor and propeller configurations at different power and ambient conditions exhibit this. The importance of this model lies in its predictive capability for evaluating thermal soak effects on mechanical components, especially in the initial stages of design or for developing generalized rules-of-thumb for new thermal systems. This contributes to the framework of an eHETR (eVTOL Hybrid Electro-Thermal Rotorcraft) model capable of simulating high-power (>100kW) electric/hybrid rotary-wing propulsion systems and the heat generation experienced during rotorcraft flight.

Committee: Ali Abdul-Aziz Dr. (Committee Member); Kelsen LaBerge Dr. (Committee Chair); D Blake Stringer Dr. (Advisor); D Blake Stringer Dr. (Committee Member) Subjects: Aerospace Engineering

Master of Science in Engineering, University of Akron, 2018, Mechanical Engineering

Battery Thermal Management Systems are necessary for the overall efficiency and life cycle of vehicle as well safety of passengers and vehicle, as elevated operating temperatures have adverse effect on the efficiency, life cycle and safety of the Li-ion battery packs. The operating temperature prescribed by many studies for improved performance and better life cycle of Li-ion batteries is around 25°C. In some worst case scenarios, high operating temperature may lead to thermal runaway in the battery, causing immense amount of heat generation, even leading to an explosion. To avoid all these dangerous events, battery thermal management is utilized to regulate the temperature of the battery pack. In this thesis, a novel battery thermal management system based on liquid cooling principle is proposed. The system involves dual purpose cooling plate for prismatic Li-ion batteries, which can maintain the temperature under normal conditions as well as mitigate the heat generated during thermal runaway. An experiment was performed on the prismatic Li-ion battery to measure the heat generation trends. The battery was discharged at 5C to replicate aggressive conditions. The data for maximum heat flux generated in the Li-ion battery was obtained. An estimated amount of heat generated during thermal runaway was calculated. A conjugate heat transfer method was used to simulate the cooling plates for normal operation and thermal runaway. The plates were simulated with two flow rates for normal operation and three flow rates for thermal runaway. Three different designs of cooling plates were compared on the basis of surface temperature, pressure drop and flow rate. The final design was selected based on the comparison with the rest of the cooling plates. The selected cooling plate can: • Maintain the temperature of battery below 25°C during normal operation. • Dissipate the maximum possible heat generated during thermal runaway and bring the temperature to less than 80°C. • M (open full item for complete abstract)

Committee: Siamak Farhad (Committee Chair); Gopal Nakarni (Committee Co-Chair); Alper Buldum (Committee Member); Reza Madad (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 2023, EMC - Mechanical Engineering

This bench-scale experimental research studies the ignition mechanism by simulated firebrands in wildfire and thermal runaway of lithium-ion battery cells. The wildfire research investigates the effects of spatial distribution of multiple firebrands on ignition and burning characteristics of a combustible fuel substrate. The spatial distribution of firebrands is controlled by varying spacing between the firebrands. The research consists of two steps. In the first step, a square array of three-by-three birch wood cubes is ignited using hot electrical coils to generate flaming firebrands. These samples are positioned on suspension wires without a substrate to focus on burning and smoldering behavior of the firebrands only. The spacing is varied from 0 to 30 mm. It shows a non-monotonic dependency of burning intensity on firebrand spacing due to enhancement of flame heat intensity and decrease in air entrainment to flames when the spacing decreases. It is also found that the smoldering temperature and duration significantly increase when the firebrands are in proximity. The second step investigates the ignition of a fuel substrate caused by multiple firebrands. The firebrands are deposited on a 6.35 mm thick birch plywood under controlled wind conditions. An infrared camera monitors the temperature of firebrands and the plywood substrate. The experimental results exhibit that firebrands with 10 mm spacing result in the most severe fire damage of the plywood, whereas those with 20 mm spacing burn the largest area. A steady-state radiation model shows that the radiation heat flux from the firebrands is dominant during the heating process. Lastly, experimental method and apparatus are developed to collect time-resolved data on the gas compositions and fire characteristics during and post-thermal runaway of LIB cells. The cell at a desired state-of-charge (SOC) is forced into thermal runaway by an electrical heating tape at a constant heating rate. From the experiments (open full item for complete abstract)

Committee: Ya-Ting Liao (Committee Chair); Fumiaki Takahashi (Committee Member); Chris Yingchun Yuan (Committee Member); Gary Wnek (Committee Member) Subjects: Mechanical Engineering