Speaker
Description
A crucial aspect of electric aircraft propulsion is the development of higher power density motors and generators. One promising approach to achieving this is using cryogenic flow cooling, which reduces the weight and losses in conductors in motors. The resistivity of aluminum decreases significantly with lower temperatures, dropping by a factor of 10 from room temperature to 77.2 K (liquid nitrogen). However, continuous cooling is required due to the joule heating. To address this, we developed a motor with liquid cryogen-cooled aluminum bar windings for the stator. An experimental single-slot fixture was designed to demonstrate thermal hydraulic instability within the stator. This fixture contains two parallel aluminum bars with cross-sectional areas of 1.6 mm × 4.6 mm and 3.2 mm × 4.6 mm, separated by a 1.6 mm gap to allow cryogen flow. The total length of the channel is 120 mm. In previous study, we demonstrated experimentally that the ampacity of the aluminum stator bars could reach 75 A/mm² with liquid nitrogen flowing. We ascribed the maximum current density was limited by thermal hydraulic instability in two-phase boiling flow cooling, but this phenomenon was not analyzed in detail. In this study, we developed analytical and numerical models to investigate thermal hydraulic instability in two-phase flow cooling. Our analysis shows that a sudden pressure increase, caused by phase change in the cooling flow, can impede the flow and lead to thermal runaway. A computational fluid dynamics (CFD) model was created to simulate and demonstrate the thermal runaway phenomenon. This study is funded by ARPA-E program.
