Effective thermal conductivity simulations of suspensions containing non-spherical particles in shear flow

Suspensions of oblate spheroid particles can have different thermal characteristics from suspensions of spherical particles based on its orthotropic geometry. However, the effective thermal conductivity of non-spherical particle suspensions under shear flow conditions has not been reported yet. In a first step dilute suspensions are considered ignoring particle-particle interactions, and a representative volume is observed containing one single oblate spheroid rotating at the center coupled to the shear flow. The fluid-side convective heat and mass transfer is computed by the Lattice Boltzmann Method (LBM). And the Discrete Element Method (DEM) accounts for particle dynamics, while the Finite Element Method (FEM) for heat conduction inside particles. Three codes are coupled together. As predicted by Maxwell's model and the model of Nan et al., the effective thermal conductivity of a water suspension would saturate if the conductivity of the particle exceeds 20∼30W/(mK) at stagnant state, because the thermal resistance of the particles becomes negligible in comparison to the fluid thermal resistance. The simulation results in this study confirm such saturation also to a shear flow. Besides, another saturation mechanism exists for spherical particles if the shear rate is increased to a point where particle rotation induces convective transport that supersedes conductive transport within the particle. However, this is not the case for the oblate ellipsoidal particles due to different rotating states. And comparing different rotating states, if the particle's thermal conductivity is higher than the base fluid, the shear-induced rotation around the oblate minor axis (log-rolling) results in a higher effective thermal conductivity than around the oblate major axis (tumbling) or the rotation of a spherical particle of equivalent volume fraction. The results provide a design guideline for particle in suspensions with better heat transfer behaviors.