Transient numerical analysis of thermophoresis and particle dynamics in a nanofluid – pool boiling conditions

Abstract

This paper attempts to apply two different equations of thermophoresis (applicable to micro-fluids) to nanofluids. Three significant parameters namely thermophoretic diffusion coefficient, thermophoretic velocity and thermophoretic mobility are used to analyse thermophoresis in a nanofluid. Both experiments and CFD simulations (ANSYS Fluent) are performed for water-alumina nanofluid under transient pool boiling conditions. A hollow stainless steel/glass chamber with a brass heater is used to boil water-alumina nanofluid under sub-cooled conditions. Three different heater surface temperatures (60, 70 and 80°C) are investigated to realize thermophoresis phenomena of the nanofluid. Random variation of temperature measurements in experiments is justified using an uncertainty analysis. Although, volume fraction of nanoparticles (diameter 46 nm) dispersed in the fluid is only 2% by volume, they are featured to exist in every frame of the numerical model for better understanding. 2-D transient implicit pressure based Navier-Stokes solver with SIMPLE (Semi-implicit Method for Pressure Linked Equation) pressure correction technique is used to discretize the numerical grid. Gauss-Seidel iterative procedure is used to solve the discretized equations. Simulation results are validated using a sequence of investigation (thermal property variation of water-alumina nanofluid with respect to pure water under steady-state/transient conditions, thumb rule from literature and experiments). A thin line of differentiation is found to exist between viscosity induced particle migration and thermophoresis induced particle migration for the nanofluid. Implications of equations and parameters used for estimating thermophoresis of nanofluid are discussed.