An experimental investigation of the flow of dilute polymer solutions through corrugated channels
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Measurements of the velocity using Laser Doppler Velocimetry and normal stress are made for the flow of dilute polymer solutions through a channel with corrugated top and bottom plates. (Since we are dealing with non-Newtonian fluids, there can be significant contributions to the normal stress from non-linear terms in the constitutive expression, even when the flow is slow. The measurements being made are the normal stresses and not the "pressure".) The surfaces of the plates are sinusoidal. A Reynolds number based on half the average plate spacing as the length scale and the characteristic velocity as the velocity scale was used and the range of Reynolds numbers studied was 50 < Re < 1000. The centerline velocities indicate that the experiments were performed in the inertial regime, as confirmed by the asymmetry of the centerline velocities along the channel length. The velocity profiles at the trough near the wall, for a channel with wavelength of 2.54 cm, indicate the presence of secondary flow. Sinusoidal plates with nearly identical aspect ratios ( a ) allowed for dramatic changes in the way in which the friction factor varied with Reynolds number, in that, in one case the friction factor associated with the fluid without polymer was higher than the friction factor associated with the fluid with polymer, while in others it was just the opposite. This would call into question the use of aspect ratio as an appropriate parameter for studying such problems. Changes in plate wavelength either increased or decreased the friction factor depending on the Reynolds number. Increasing plate amplitude increased the friction factor of the fluid for the range of values for the Reynolds number that was considered. The amplitude associated with the dimensionless normal stress increased with decreasing wavelength, for particular Reynolds numbers, irrespective of the fluid studied. Increasing the polymer concentration in the fluid decreased the difference in the amplitude of the dimensionless normal stress, the Reynolds number being fixed. Increasing the plate amplitude increased the amplitude of the normal stress, while an increase in plate wavelength decreased the amplitude of the normal stress. 1995.
