Effect of particle aggregation on thermal conductivity of nanofluids: Enhancement of phonon MFP

Particle aggregation in nanofluids is a common phenomenon. Some studies indicated that aggregation can increase the thermal conductivity of nanofluids due to the formation of an "efficient heat channel," while the underlying mechanism is still unclear. In this study, an effective medium model considering phonon scattering is applied to explore the effect of particle aggregation on nanofluid thermal conductivity. The aggregation is simulated by a diffusion-limit-cluster-aggregation model, where the effects of nanoparticle size, concentration, and number in a cluster on the aggregation structure are studied. Results show that nanoparticle aggregation can dramatically increase the thermal conductivity of nanofluids, and aggregation puts up stronger effects for the case of smaller nanoparticles and lower concentrations. The main mechanism is that the aggregation raises the phonon MFP in the solid phase, which further results in the thermal conductivity enhancement of nanoparticles.