Numerical Analysis of Effects of Aspect Ratio of Rod-like Nanoparticles on Thermal and Flow Behavior of Nanofluids in Backward-facing Step Flows

The thermal and flow behavior of nanofluids with rod-like particles in flows in a backward-step channel heated at its bottom wall was numerically analyzed. In the numerical simulations, the distribution of the volume fraction of nanoparticles was computed and both the viscosity and thermal conductivity were described as a function of the volume fraction. The viscosity and thermal conductivity of nanofluids were described by a model for suspensions of prolate spheroid particles and the Hamilton-Crosser model, respectively, and the translational diffusivity of nanoparticles was expressed by a model derived for rod-like macromolecules. The numerical simulations revealed that the average heat transfer coefficient was increased more by the addition of nanoparticles having a higher aspect ratio, whereas the power necessary for inducing nanofluid flow increased further. In addition, the increase in the average heat transfer coefficient saturated above a certain particle aspect ratio. It is therefore necessary to adequately select the nanoparticle shape and the volume fraction of particles in consideration of the trade-off between the improvement of thermal characteristics and loss of mechanical energy.