On the drag effect of one fluid driven by another in a vertical channel

The idea of dragging a viscous fluid by another kind of fluid via the shear stress has fascinated the scientists and engineers. The dependence of the drag effect on the physical parameters of the two immiscible fluids is very much desired but still challenging. In this research, three different kinds of fluids are employed to drag a pure fluid between vertical parallel channel walls, that is, the viscous fluid, the non-Newtonian power-law fluid, and the nanofluid. The drag effects of two-layer fluids are investigated by comparing the velocity fields and the mean velocity curve. Essential parameters determining the dragging efficiencies of the driven fluid are studied systematically: the drag effects of the density ratio p, the thermal conductivity ratio k, the thermal expansion coefficient ratio b, and the viscosity ratio m of the two-layer fluids are focused. Both dilatant flows and pseudo-plastic fluids are considered in driving the viscous fluid. When the pure fluid is driven by the nanofluid, the single-phase model is adopted. The example of 47 nm-Al2O3 nanoparticles suspended in water is analyzed for demonstration: the thermal expansion, the effective viscosity, and the effective thermal conductivity are dependent of the concentration of nanofluid, which makes the nanoparticle volume fraction ϕ a major concern in the drag effects. The findings in the paper reveal several potential strategies to promise high effectiveness on fluid driving via interface shear, which we hope will inspire engineers and researchers in relative working fields.