Investigation of self-adjusting cooling system for the autonomous underwater vehicle propulsion motor

Permanent magnet (PM) motors are widely employed in the autonomous underwater vehicle (AUV) propulsion because of their high efficiency and torque density. Since the AUV housings are sealed and narrow, motors are vulnerable to the risk of overheating, leading to dissolution of the winding insulation and demagnetization of the permanent magnets. In this study, a self-adjusting cooling scheme was proposed, which uses the hydrodynamic pressure to pump seawater into the cooling channel automatically. Firstly, the mathematical model of the flow field and heat transfer was established using equation derivations. Subsequently, to clarify the characteristics of the self-adjusting cooling system, the effect of the AUV speed on the thermal and fluid characteristics was investigated using computational fluid dynamics. Results show that the flow rate in the channel increases from 2 to 8.1 L/min as the AUV speed increases from 15 to 50 kn, resulting in the improvement of the heat transfer coefficient from 41 to 764 W/(m²∙K). Subsequently, the influences of inlet and outlet on the thermal characteristics were studied and found to be beneficial for improving the cooling performance. To investigate the spiral flow channels, the distribution coefficient k_C and shape factor R_f were defined. Compared to the closed-cycle cooling system for the AUV propulsion motor, the proposed cooling system eliminated auxiliary devices such as pumps and heat exchangers, which makes it more compact and efficient for AUVs. However, introducing seawater into the AUV housing poses a risk of liquid leakage.