Investigation on heat transfer characteristics of flow-enhanced immersion cooling system for UUV battery packs under high-rate discharge

To address the thermal management requirements of lithium-ion battery packs in UUV battery compartments under high-rate discharge conditions, a flow-enhanced immersion cooling method based on a Fan-boundary-induced forced convection mechanism was proposed, enabling active coolant circulation within sealed compartments without external inlet or outlet connections. A three-dimensional transient CFD model was developed to investigate the coupled flow and heat transfer characteristics of the battery system under varying Fan boundary strength, height, configuration and inclination angle. The results demonstrate that, compared with air cooling and natural convection immersion cooling, the proposed method significantly enhances thermal performance, reducing the maximum battery temperature by more than 33 K and 6 K, respectively, with corresponding temperature reduction rates of 53.4% and 17.21%. Parametric analysis reveals that heat dissipation capability improves progressively with increasing Fan boundary strength and tends to stabilize when the Thrust-weight Number (T_w, defined as the ratio of fan-induced pressure force to coolant gravitational force) reaches 0.15. In terms of structural design, an optimal installation height of 74 mm is identified and the dual-Fan configuration achieves superior cooling performance. Furthermore, the system reaches optimal thermal management efficiency at an inclination angle of 120°. These findings provide new insights into the development of advanced immersion cooling systems for UUV battery packs, particularly under fully sealed operating conditions where enhanced internal flow regulation is required.