Integrated hybrid cooling of lithium-ion battery packs using graphene-enhanced PCM and dual fluid convection

In this study, the thermal performance of a hybrid cooling system for a lithium-ion battery pack consisting of 20 cylindrical cells was investigated. The system integrates three components: airflow in the outer chamber, nanofluid flow through a wavy channel between the cells, and a phase change material (PCM) containing graphene nanoparticles surrounding the batteries. The main objective was to evaluate the effect of air velocity, nanofluid velocity, and nanoparticle volume fraction on temperature distribution and heat transfer coefficients under steady conditions. The results showed that increasing fluid velocities directly enhanced heat dissipation. As the airflow velocity increased from 1 to 10 mm/s, the maximum battery temperature decreased from 304.98 K to 302.14 K, while the mean temperature dropped from 302.81 K to 299.67 K , indicating a notable enhancement in convective heat removal. Increasing the nanofluid velocity from 1 to 10 mm/s produced an even stronger effect, reducing the maximum temperature from 304.86 K to 301.07 K (nearly 1.24%) and the mean temperature from 302.67 K to 299.15 K (roughly 1.16%) as well, which confirms the superior influence of the internal liquid-cooling loop compared to the external airflow. Enhancing the graphene nanoparticle volume fraction in the PCM from 0% to 2% resulted in a moderate reduction from 304.98 K to 303.29 K in maximum temperature and from 303.09 K to 301.52 K in mean temperature, showing that nanoparticle addition primarily improves PCM thermal conductivity and melting uniformity rather than acting as a dominant cooling mechanism. Overall, nanofluid velocity exhibits the strongest impact on thermal performance, followed by airflow velocity, while nanoparticle-enhanced PCM plays a stabilizing and supporting role in temperature regulation.