Hydrodynamics of the swirling steam-water two-phase flows over a surface with protrusions: Shape optimization and effect of rise in temperature on drag reduction

The current study studies the drag reduction offered by the plate with protrusions to the swirling steam-water two-phase flows. Protrusions of different types, having triangular, sinusoidal, and trapezoidal shapes, are investigated. The variations and modifications were made in the dimensions and orientations (facing along or across, flush-mounted or protruded into the swirling flows), to test the effectivity of these protrusions. At 5 bars of inlet gauge pressure and rpm varying from 360 to 1440, the trapezoidal shape provides the most effective drag reduction. It is also found from the PIV images of the fluid region near the protrusions that the drag reduction in the trapezoidal-shaped protrusions occurs mainly due to the secondary flow structures formation over those protrusions.

On the one hand, the lateral spacing between the adjacent protrusions created the region of strengthening and expansion against the higher opposite inertial forces from the main swirling core fluids. While on the other hand, these spacings justify the impact of the axial elongation of the vortices above the protrusion tips. Further trends in drag reduction have also been investigated by observing the effect of the rise in the temperature of the fluid medium. Reduction in drag from 1.3 to 1.6% is noted using the trapezoidal protrusions for a temperature rise of 25–60 °C. It is also observed that the drag reduction profiles show a rising trend under the influence of the rising Nusselt Number. This trend is attributed to the decrease in viscosity. Whereas under the influence of a rising Prandtl number, the drag reduction shows a decreasing trend, mainly attributed to the rise in the pressure drop at the boundary layers and the pressure drop due to the friction.