Effect of a multi-pass alumina water nanofluid tube embedded in PCM on the thermal management of a lithium-ion battery pack

In this study, a three-dimensional thermal analysis is performed for a 5 × 5 square-arranged lithium-ion battery pack. The entire pack is embedded in phase change material (PCM), within which a centrally located tube containing a slow, reciprocating alumina-water nanofluid is placed. The combined battery-PCM-nanofluid structure is enclosed inside an air duct, where air velocities between 10 and 40 mm/s are introduced. A 2000-second thermal cycle is simulated using COMSOL Multiphysics, considering variations in nanoparticle volume fraction under both charging and non-charging conditions. The results demonstrate that airflow velocity is the most influential parameter in the system's thermal response. Increasing the air velocity from 10 to 40 mm/s reduces the time-averaged outlet air temperature by 7.85%, while the maximum instantaneous drop reaches 11.38% at 2000 s, highlighting a substantial enhancement in forced convection and heat removal. The average battery temperature also decreases by 4.27% (time-averaged) and up to 6.52% in maximum values, confirming that even low-velocity airflow can meaningfully improve pack cooling. In contrast, the PCM-related parameters remain largely stable. The molten fraction shows only 1.42% variation, and the average PCM temperature changes, indicating that the PCM layer with its high latent heat maintains thermal stability despite airflow changes. The outlet nanofluid temperature also decreases with higher nanoparticle concentrations, with reductions of 1.43% in the time-averaged value over 2000 s.