Numerical simulation and modeling of nanoparticle aggregation effect on anisotropic effective thermal conductivity of nanofluids

Particle aggregation is ubiquitous in nanofluids and significantly affects the effective thermal conductivity (ETC) of nanofluids. However, it is difficult to quantitatively characterize the effect of aggregation on the anisotropic ETC due to its complex and elusive morphological features. Herein, the orientation tension T is innovatively proposed to quantitatively describe the nanoparticle aggregations. The microstructures of aggregations with different T , volume fraction phi , thermal conductivity ratio k p /k f and nanoparticles morphology (aspect ratio, AR ) are reconstructed, and their effects on ETC are constitutively simulated using the thermal lattice Boltzmann method (T-LBM). Simulation results indicate that T can effectively measure the thermal conductivity contribution of particle chains within aggregations to various directions, thereby showing a strong positive correlation with ETC of nanofluids. Additionally, the parameters (phi, k p /k f and AR ) have synergistic effects with T on the ETC of nanofluids to varying degrees. Based on these, a comprehensive model is established and further validated to have good predictive capabilities for anisotropic ETC of nanofluids with various parameters, notably for nano- fluids with significant orientation arrangements.