Particle-induced micro-convection and heat transfer enhancement in nanofluids: Roles of Brownian motion and aggregation

Particle dynamics play a pivotal role in the heat transfer performance of nanofluids, yet the mechanisms of particle-induced disturbances remain insufficiently understood and actively debated. A particle-scale perspective is essential for elucidating how Brownian motion and aggregation modulate fluid convection and heat transfer. In this study, the discrete element method coupled with the lattice Boltzmann method and immersed moving boundary is employed to resolve particle motion and its effects on convective heat transfer. Analysis of the Péclet (Pe) and Nusselt (Nu) numbers across different flow regions and inlet velocities reveals that the contribution of particle-induced disturbances is inversely proportional to fluid velocity. Heat transfer is markedly enhanced in boundary layers with low fluid velocity, achieving up to a 33.3% increase in Nu, where particle motion disrupts weak gradients and strengthens local fluctuations. In contrast, the effect becomes negligible in central regions or at high inlet velocities, where bulk convection dominates. Moreover, variations in the Brownian number (Br) and adhesion number (Ad) demonstrate that strong Brownian motion leads to dispersed particles, which promote widespread velocity fluctuations, whereas large-scale aggregation suppresses them. Finally, a phase diagram of convective heat transfer that integrates the two governing dimensions of fluid velocity (Pe0) and particle dynamics (Br and Ad) is proposed, providing insight into the conditions under which particle-induced disturbances predominantly influence convective heat transfer in nanofluids.