Heat transfer and fluid dynamics of offset unsubmerged axial and tangential jets impinging on a confined heated rotating disk

Abstract

This study numerically investigates offset unsubmerged axial and tangential jets impingement on a confined heated rotating disk for electric motor cooling applications. The motor's rotor and stator are modeled as rotating and stationary solid regions, respectively. The multi-phase flow and heat transfer characteristics are analyzed over a range of rotational Reynolds numbers from 1 × 10⁵ to 3.7 × 10⁶ and jet Reynolds numbers ranging from 4.5 × 10² to 7.3 × 10³ in a confined space. The jet nozzle diameter is 1.5 mm with the jet location fixed at an offset of 70 % of the disk radius. For axial and tangential jets, the ratio of jet impingement distance to nozzle diameter is held constant at 12 and 14.7, respectively. The Volume of Fluid method and a moving mesh rotation model are used to simulate the two-phase flow dynamics. The results show that axial jets achieve effective rotor cooling at a mid-range rotational Reynolds number of 2 × 10⁶ and a jet Reynolds number of 7.3 × 10³ but struggle to cool the stator due to limited oil distribution. Axial jet efficiency improves with higher jet Reynolds numbers; however, performance reduces at extreme rotational speeds, as oil contact with critical areas is reduced. Axial jets are thus most suitable for high rotor heat loads and oil flow rates, as their direct impingement enhances cooling effectiveness. In contrast, tangential jets rely heavily on an optimal velocity ratio between jet exit velocity and rotor speed to achieve efficient cooling. At rotational Reynolds number of 6.1 × 10⁵, tangential jets deliver superior heat transfer and temperature uniformity with a lower jet Reynolds number of 3.7 × 10³ and an ideal velocity ratio of 1, which promotes oil-air mixing and helical impingement. Tangential jets also exhibit up to 23 % lower drag losses at rotational Reynolds number of 6.1 × 10⁵, and maintain lower pressure losses than axial jets, with an 15 % reduction at rotational Reynolds number of 3.7 × 10⁶ due to better alignment with the rotating air. Overall, tangential jets are more efficient for lower flow rates and stator-focused cooling, while a mid-range rotational Reynolds number of 2 × 10⁶ optimally balances oil distribution and cooling efficiency for both jet types.