Turbulence measurements in polymer solutions using hot-film anemometry

Hot-film anemometry was used to study the detailed structure of turbulence (intensities, energy spectra, and auto-correlations) in Newtonian solvents, non-drag reducing polymer solutions, and drag reducing polymer solutions. This was done in two smooth wall tubes with diameters of 1.0 inch and 2.0 inches. A probe traversing mechanism was used for measurements at radial positions from the center to as near the wall as possible for both the film probes (r/a=0.85 in the 2-inch tube) and the impact tubes (r/ a=0.98). The impact tubes were used to measure velocities for film probe calibration. The solvents used in this investigation were toluene, cyclohexane, and benzene. Three concentrations of a medium molecular weight polyisobutylene (Vistanex L-80, molecular weight about 720,000) in cyclohexane, two concentrations of the same polymer in benzene, two concentrations of a high molecular weight polymethyl methacrylate (Plexiglas, molecular weight about 1,500,000) in toluene, one concentration of a low molecular weight polymethyl methacrylate (V-100 molding powder, molecular weight about 110,000) in toluene, three concentrations of a high molecular weight polyisobutylene (Vistanex 1-200, molecular weight about 4,700,000) in toluene, and one concentration of the same polymer in cyclohexane were used. In the liquids not showing drag reduction a viscous and/ or elastic effect was found for both turbulence intensities and energy spectra. Turbulence intensities were higher and energy spectrum frequencies were lower for the polymer solutions of high viscosity. Unfortunately the most viscous solutions were also elastic. So purely viscous liquid studies will be necessary to distinguish between elastic and viscous effects. During drag reduction it was found that the energy spectra changed little from purely viscous solvents. The turbulence intensities, however, showed very unusual effects. The intensities relative to friction velocity increased at low drag ratio values (high drag reduction), rather than remain constant as expected from mixing length considerations. This behavior was dependent upon the degree of mechanical polymer degradation, lower intensities occurring for fresh than for degraded solutions during drag reduction. Normal stress differences (P₁₁ - P₂₂) were measured for two of the solutions used in this investigation, one showing drag reduction at attainable flow rates in the l-inch tube, the other showing drag reduction only in 0.5-inch and smaller tubes. Both solutions yielded normal stress differences of about the same level. A quantitative viscoelastic mechanism of drag reduction was tested using the viscosity and normal stress data for the two solutions discussed above. The drag reduction mechanism demonstrated the relative effects of elasticity and viscosity on drag reduction. The adequate prediction of drag ratios for two solutions at two flow rates in each of two tube sizes demonstrated the validity of the mechanism and the reasonableness of the assumptions made --Abstract