Numerical Investigation of Windage Loss and Heat Transfer in a High-Speed Flywheel Energy Storage System with Slit Wall

Energy storage has recently emerged as a topic of discussion in the context of the energy transition away from fossil fuels. The flywheel energy storage system (FESS) is gaining popularity due to its distinct advantages, which include long life cycles, high power density, and low environmental impact. However, windage loss significantly contributes to total losses in high-speed rotating machines such as the flywheel. Windage loss increases self-discharge, rendering FESS unsuitable for long-term energy storage applications. In the FESS application, the enhancement of heat transfer by the medium within the annulus is crucial. The FESS with a slit wall in the outer cylinder is examined in this investigation to assess the flow field distribution, windage losses, and heat transfer characteristics. The effect of four slit wall geometrical configurations, triangular, trapezoidal, rectangular, and half circle, are investigated in this study. The computational fluid dynamics (CFD) technique is used to model the flow in the annulus of a solid FESS. The results show that the slit wall geometry significantly impacts the distribution of the flow field, the heat transfer process, and windage loss. The fluid is mixed more quickly between the hot and cold sides due to the geometry of the slit wall, resulting in a more homogeneous temperature. As a result, the slit geometry with triangular and rectangular slit walls resulted in the best overall system performance through windage loss reduction and enhanced heat transfer. The findings of this study can assist with the design and optimisation of other rotating machinery with similar flow characteristics.