Numerical analysis of laminar forced convection heat transfer with nanofluids in circular channels

In this study, forced convection heat transfer characteristics of nanofluids are investigated by numerical analysis of transient laminar flow in a circular duct under step change in wall temperature and wall heat flux. The thermal responses of the system are obtained by solving energy equation under both transient and steady-state conditions for hydro-dynamically fully developed flow. In the numerical analysis, Al2O3/water nanofluid is assumed as a homogenous single-phase fluid. For the effective thermal conductivity of nanofluids, Koo-Kleinstreuer model in the analysis which takes the effects of temperature and the particle diameter into account. Variations of thermal conductivity in turn, heat transfer enhancement is obtained at various times as a function of nanoparticle volume fractions, at a given nanoparticle diameter and Peclet number. Steady-state conditions are obtained by reaching larger times steps. In the second part of this study, Computational Fluid Dynamics (CFD) simulations for initially hydro-dynamically fully developed laminar flow with nanofluids in a circular duct under constant wall temperature and constant wall flux conditions are performed with two-phase mixture model by using Fluent software. Thermal behaviors of the system are investigated for Al2O3/water nanofluid by using different thermal conductivity and viscosity models. The effects of thermal conductivity and viscosity models on the heat transfer enhancements are calculated for different nanoparticle volume fractions and diameters. The results are compared with a previous experimental study in the literature for the same geometry and boundary conditions. Also single-phase model Fluent simulations are performed for Koo-Kleinstreuer thermal conductivity model and compared with two-phase mixture model results.