Experimental study on drag reduction in a duct

The present study investigates drag forces on liquids pumped through a duct coated with a magnetic liquid of kinematic viscosity of 1.69 cSt. Due to a suitable arrangement of a set of permanent magnets, the magnetic liquid formed a fluid layer on the bottom of a square duct through which a silicone oil with a kinematic viscosity of 50 cst was pumped. Applied magnetic flux densities ranged from less than 0.01 T to about 0.15 T at the surface of the magnets. The influences of the strength and gradient of the magnetic field, and of the fluid flow rate, on the shape of the magnetic fluid have been investigated. It was found that it is not practical to form a uniform magnetic fluid layer under our experimental conditions, and consequently the drag was increased compared with the uncoated case because the flow of the silicone fluid over the magnetic fluid layer deformed this layer dramatically. For a viscosity ratio between the main flow and the ferrofluid less than about 30, it is therefore not feasible to use a magnetic fluid layer to reduce the drag in a duct. However, the generally high thermal conductivity of magnetic liquids, its convection, and interface deformation, might be exploited for heat transfer enhancement. © 2003 Society of Chemical Industry.