Residence time distribution of passive scalars in magnetic nanofluid Poiseuille flow under uniform rotating magnetic fields

Abstract

The diffusive and convective transport mechanisms that govern the residence time distribution (RTD) of magnetic nanofluid (ferrofluid) capillary Poiseuille flow under an external uniform rotating magnetic field (RMF) were experimentally and numerically investigated. The measured RTDs are reduced and approach plug flow under RMF excitation by shortening the elution time, stiffening the breakthrough rise, and delaying the breakthrough time as the RMF frequency and/or magnetic nanoparticle concentration increase(s). Ferrohydrodynamic (FHD) simulations predict the inception of secondary convective azimuthal flow but fail to explain the impact of the magnetic field on RTD evolution under the assumption of isotropic diffusion transport. The origin of anisotropy in scalar diffusive transport was elucidated by analyzing the rotational properties of magnetic nanoparticles with respect to ferrofluid vorticity and RMF arrangement. Therefore, independent measurement of an anisotropic effective diffusion tensor for RMF-excited quiescent ferrofluids facilitates improved FHD simulations of RTD without compromising the model’s predictive power.