Date of Graduation
12-2025
Document Type
Thesis
Degree Name
Master of Science in Mechanical Engineering (MSME)
Degree Level
Graduate
Department
Mechanical Engineering
Advisor/Mentor
Huitink, David
Committee Member
Mantooth, Alan
Second Committee Member
Hu, Han
Keywords
Boiling heat transfer; Dielectric breakdown voltage; Dielectric fluids; Pool boiling and vapor-phase breakdown; Silicon carbide power electronics; Two-phase immersion cooling
Abstract
Escalating power densities in wide-bandgap semiconductor devices necessitate advanced thermal management strategies. Two-phase immersion cooling offers superior heat transfer through latent heat exploitation, but introduces a critical electro-thermal conflict: vapor generation essential for thermal performance exhibits markedly reduced dielectric strength (35–50 %) compared to liquid, potentially compromising electrical reliability in medium-voltage applications. Despite this recognized challenge, systematic characterization of breakdown voltage under realistic boiling conditions remains absent from the literature. This research addresses this knowledge gap through comprehensive experimental investigation of dielectric fluid breakdown behavior across liquid, boiling, and vapor-phase states. Custom test apparatus enabled simultaneous thermal and electrical measurements under controlled pool boiling conditions. Breakdown voltage was systematically characterized for Novec 7200 across heat flux ranges from quiescent liquid through critical heat flux using needle-to-needle and plane-to-needle electrode geometries representative of power module features. Comparative vapor-phase measurements were performed for four candidate fluids (SF10, HFE-347E, Novec 7200, Novec 7500). System-level validation employed a 1.2 kV SiC half-bridge module immersed in HFE-347E. Results demonstrate clear correlation between thermal state and dielectric degradation. Onset of nucleate boiling triggered 10–15 % breakdown voltage reduction, with total degradation reaching 21–26 % near critical heat flux depending on geometry. Needle-to-needle configurations maintained 1–2 kV mm⁻¹ advantage over plane-to-needle across all thermal regimes. All tested fluids exhibited consistent vapor degradation (35–47 %), with Novec 7500 demonstrating superior absolute performance. The 1.2 kV module achieved efficient heat removal (junction temperatures 62–67 °C) without electrical breakdown, validating design margins. This work establishes quantitative design guidelines enabling reliable two-phase immersion cooling for medium-voltage power electronics through coupled electro-thermal design accounting for vapor-phase performance.
Citation
Kokash, M. (2025). Electro-Thermal Characterization of Dielectric Fluids for Reliable Two-Phase Immersion Cooling of Medium-to-High Voltage Power Electronics. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/6074
