Laser-Doppler anemometer measurements of turbulent structure in non-Newtonian fluids

Abstract

A laser-Doppler anemometer was used to measure the radial variation of axial turbulent velocity in the flow of water and aqueous polymer solution through a round pipe. Results were obtained for the mean velocity the turbulent intensity, and the one-dimensional energy spectrum. The polymers used were polyacrylamide (Separan AP 273; in a concentration range 25-200 WPPM). Both polymers are well known as effective additives to reduce turbulent drag, when dissolved in water to give a solution concentration of a few PPM. The mean velocity and turbulent intensity were both found to be increased by the polymer additives, with the magnitude of the effect depending on the polymer concentration This was in agreement with the results of other investigations. At low additive concentrations, the energy spectra were found to be identical to those for water flows; this also was in agreement with previous results. At higher concentrations, energy spectra showed noticeable attenuation in the largest (dissipation) range of wavenumbers. There was a corresponding enhancement of the small-wavenumber (energy-containing) range. This seemed to be a threshold effect with onset at a concentration of between 50 PPM and 100 PPM for Polyox WSR 301 and between 25 PPM and 50 PPM for Separan AP 273.

Turbulent pipe flow of a drag-reducing rigid “rod-like” polymer solution

“…11, the peak values of the turbulent fluctuation components, normalized with the bulk velocity, have been plotted against drag reduction together with selected data from the literature for comparison (Presti [22], Ptasinski et al [33], McComb and Chan [35], Allan et al [36], Chung and Graebel [37], Mizushina and Usui [38], Schummer and Thielen [39], McComb and Rabie [40] and den Toonder et al [41]). It can be seen that the data for the rigid-rod-like polymer from the current study agree well with the trend of decreasing normalized peaks with increasing drag reduction, similar to what is observed for two-dimensional channel flow [8].…”

Observations of asymmetrical flow behaviour in transitional pipe flow of yield-stress and other shear-thinning liquids

“…Apart from these rather special situations, so far as Newtonian fluids are concerned we are unaware of any evidence to suggest that the expectation of mean-flow axisymmetry in fully developed pipe flow is unfounded for any of the three flow regimes. As a consequence of the interest in drag reduction, a number of papers have been published concerned primarily with the fully developed turbulent pipe flow of very low concentrations of high molecular weight polymers [1,2,9,10,[16][17][18]. The velocity profile data for these turbulent flows appear to be axisymmetric as do data for the flow of non-Newtonian liquids (for the most part higher concentrations of polymers) [11,12,14].…”

Flow characteristics of Newtonian and non-Newtonian fluids in a vessel stirred by a 60° pitched blade impeller

“…Therefore, for the mixing system where the micro-mixing is important the impeller operates less effectively in the CMC flow compared to a Newtonian flow at a similar power number. Polymer additives, and CMC in particular, are known to cause drag reduction in the turbulent pipe and annular flows compared to the flows of a Newtonian solvent [20,25,26,27,28] who showed a dampening of the turbulence, especially in the cross-stream components, and also a change in the turbulence spectrum [29]. It has been suggested by [21] that this is caused by an increase in elongational viscosity of the fluid when the polymer molecules stretch under the influence of normal strain.…”