Drag reduction in aqueous cationic soap solutions

The pipe flow drag-reducing properties of mixtures of alkyltrimethylammonium halides with 1-naphthol in aqueous solution have been investigated. The effects of solution concentration, soap-naphthol ratio, soap molecular weight and solution temperature upon drag reduction and swirl decay time are reported. The critical wall shear stresses above which the drag-reducing properties cease correlate well with swirl decay time. At low soap concentrations greater than equimolar proportions of 1-naphthol with the soap are required for maximum drag reduction. The drag-reducing properties of these solutions are greatest at and around the Krafft point of the pure soap. A phenomenon similar to onset for polymer solution drag reduction is reported for these soap solutions.     

Secondary flows in turbulent surfactant solutions at maximum drag reduction

Secondary flows are always present when a turbulent Newtonian fluid flows through a square duct. We show that they are suppressed in surfactant solutions at maximum drag reduction. This result confirms the one-dimensional behaviour of such flows, which is caused by the alignment of the micelles.     

Comparison of drag reduction, rheology, microstructure and stress-induced precipitation of dilute cationic surfactant solutions with odd and even alkyl chains

Cationic surfactant systems of different alkyl chain lengths with counterion, C_nTAC(5 mmol/l)/3-Cl-Benzoate(12.5 mmol/l) (n=15, 16, 17, 18), were investigated for drag reduction, rheological behaviors, microstructure, and stress-induced precipitation. These are the first measurements of these characteristics for odd chain length (C₁₅ and C₁₇) quaternary ammonium surfactants. The lower and upper effective drag reduction temperature limits, viscoelasticity, and stress-induced precipitation temperature increased with alkyl chain length. Krafft temperature, critical turbidity temperature, and lower drag reduction effectiveness temperature limit showed a zigzag odd-even effect, while the stress-induced precipitation temperature did not. Light microscopy and cryo-TEM showed that cooling the C₁₅ solution below 20 °C produced crystals, while above that temperature threadlike micelles were present. The same was true for the solutions of C₁₈ that had threadlike micellar network microstructures when clear and crystals formed upon cooling. Micellar solutions can remain in a homogenous metastable state at a temperature below the Krafft temperature and above the critical turbidity temperature for days without external disturbance. Imposition of flow stress causes the systems to overcome the energy barrier and precipitate.     

The velocity field of dilute cationic surfactant solutions in a Couette-viscometer

With the aid of particle image velocimetry (PIV) momentary two-dimensional velocity fields of dilute cationic surfactant solutions are measured in an annular gap between coaxial cylinders. In the shear-induced state the velocity profiles of the surfactant solutions differ from a Couette profile drastically: Time dependent flows with high shear rates near the walls are observed. The flow is almost one-dimensional, velocity components in radial or axial direction cannot be detected. The velocity profile can exhibit a local maximum. A macroscopic heterogeneous structure is observed, domains with high viscosity alternate with low viscosity regions. Elastic phenomena are observed in a relaxation experiment. Existing rheometric data of cationic surfactant solutions in the shear-induced state give only apparent viscosities.     

Effect of variations in counterion to surfactant ratio on rheology and microstructures of drag reducing cationic surfactant systems

Rheology, drag reduction and cryo-TEM experiments were performed on Arquad 16–50/NaSal and Ethoquad O/12/NaSal surfactant systems at different counterion-to-surfactant ratios and at constant low surfactant concentrations, 5 mM, appropriate for drag reduction. The molar ratio of counterion-to-surface was varied from 0.6 to 2.5. All the surfactant systems described here are viscoelastic and drag reducing. The viscoelasticity and drag reducing effectiveness increase with increase in counterion/surfactant ratio. Network are present in the solutions with high ratio, and they are viscoelastic. However, shear is needed to induce network formation for solutions at low ratio. Cryo-TEM images confirm the existence of thread-like micelles which form entanglement networks, and show that the micellar network becomes denser with increasing counterion/surfactant ratio in one surfactant series. Both increase in the counterion/surfactant ratio and increase in the shear rate result in shorter relaxation times. For some of these systems, abrupt increase in viscosity is observed at certain shear rates which are time effects affecting microstructure rearrangements rather than formation of shear induced structures.     

Shear-induced phase separation in cationic surfactant solutions around a rotating sphere

The results of an experimental investigation of the flow of a highly dilute cationic surfactant solution (tetradecyl trimethyl ammonium bromide with added sodium salicylate as counterion, equimolar 2.4mM) around a rotating sphere are presented. The flow and the shear-induced phase transition are visualized by means of a Toepler Schlieren optics. The buildup of the shear-induced structures occurs only above a critical shear rate. Once this critical value is exceeded the shear-induced phase separation starts after a characteristic deformation with the shear rate reduced by the critical one. Further analysis of the obtained data is performed on basis of an analytical calculation of the flow around a rotating sphere in a second order fluid (Thomas and Walters, 1964; Giesekus, 1965). From some characteristic features of the shear-induced structures as induction time and position the parameter describing the elastic properties of the fluid is estimated. Received: 6 January 1998 Accepted: 1 May 1998     

Rheological and drag reduction characteristics of xanthan gum solutions

The rheological and turbulent drag reducing characteristics of commercial and purified xanthan gum solutions of concentrations 50–500 ppm have been studied with and without addition of 100 ppm NaCl. The purification by soxlet extraction of xanthan gum using 95% ethanol is effective in removing low-molecular-weight impurities from xanthan. The increased content of higher molecular-weight xanthan in purified xanthan is evident from rheological and drag reduction behavior. The addition of 100 ppm salt to dilute solutions introduces semi-flexibility in xanthan gum solution without occurrence of self-association. The change in molecular behavior in the presence of salt is evident from rheological normal-stress and turbulent drag reduction behaviors.     

Formation of the shear-induced state in dilute cationic surfactant solutions

In cationic surfactant solutions a change of state occurs due to mechanical stresses. In the dilute regime of rodlike micelles the formation of a so-called Shear-Induced State (SIS) occurs above a critical shear rate. In this context dilute means that there is no sterical interaction between rodlike micelles, the solution is below the overlap concentration. Employing a mathematical model, it is shown that aggregation forces are weak compared to hydrodynamic forces. The mathematical formulation is based on a model of Israelachvili which describes the chemical potential of micelles. Hydrodynamic forces are calculated with a rigid-dumbbell model. SIS formation can be explained by the destruction of rodlike micelles.     

On the mechanism of drag reduction in dilute polymer solutions

Experimental evidence is given that drag reducing polymer molecules are preferentially collected by strained vortices. This can explain why extremely small amounts of additives can be so effective. They become concentrated in areas of a turbulent flow where they are most efficient.     

Unusual temperature gap in turbulent drag reduction of cationic surfactants with mixed counterions

Unusual temperature gaps have previously been observed in the turbulent drag reduction effectiveness of Dobon and Habon cationic surfactant systems containing two anionic counterions of different binding strengths. Here, we report drag reduction data for a cationic surfactant with a mixture of dodecyl sulfate and tosylate counterions showing a temperature gap. Cryo-TEM images of nanostructures at different temperatures for this system support an explanation for this gap, based on the counterions’ relative binding strengths.     

Rheo-optics of cationic surfactant micellar solutions with mixed aromatic counterions

The effects of different ratios of mixed aromatic counterions of sodium salicylate (NaSal) and sodium 4-ethylbenzenesulfonate (NaEBS) at a total counterion concentration of 0.125 M on the rheo-optical behaviors of 0.05 M cetyltrimethylammonium chloride (CTAC) cationic surfactant solutions were studied. The introduction of NaEBS into CTAC–NaSal solution led to a significant deviation from the Cox–Merz rule. Mixing of the two aromatic counterions lowered the CTAC solutions’ zero shear viscosities, increased their plateau shear modulus, and decreased their relaxation time. In addition, the persistence lengths of the threadlike micelles and the mesh size of the micellar network were also significantly reduced. These effects are believed to be due to increased degree of branching and denser meshes of the micellar networks in the mixed counterion systems.     

Drag reduction and heat transfer in surfactant solutions with excess counterion

We present the results of a study of turbulent drag reduction in a small circulating loop using surfactant solutions with excess counterion. In addition, these solutions were used in measurements of heat transfer, both in pipe flow and in an impinging jet. Both frictional drag and heat transfer were reduced in the pipe flow experiments. Measurements of heat transfer in the impinging jet revealed a dependence on the molar concentration ratio of the counterion. When the counterion was added at a molar concentration 30 times higher than that of the surfactant, the resulting surfactant solution did not reduce the rate of heat transfer in the impinging jet. By using this surfactant system in an impinging jet, we show both a reduction in pipe friction and normal heat transfer potential in a circulating heat exchange system. In order to investigate this difference in heat transfer between pipe flows and impinging jet flows, a comparison was made of the wall shear stress between these two flow regimes. The estimated wall shear stress was of the same order in both flows, and thus was not considered to be the primary cause of the difference in heat transfer. It is instead suggested that the micellar structure of the surfactant is influenced by a compressive deformation of the impinging flow in a manner that is different from the shear deformation observed in pipe flow.     

Bluff-body drag reduction using a deflector

A passive flow control on a generic car model was experimentally studied. This control consists of a deflector placed on the upper edge of the model rear window. The study was carried out in a wind tunnel at Reynolds numbers based on the model height of 3.1 × 10⁵ and 7.7 × 10⁵. The flow was investigated via standard and stereoscopic particle image velocimetry, Kiel pressure probes and surface flow visualization. The aerodynamic drag was measured using an external balance and calculated using a wake survey method. Drag reductions up to 9% were obtained depending on the deflector angle. The deflector increases the separated region on the rear window. The results show that when this separated region is wide enough, it disrupts the development of the counter-rotating longitudinal vortices appearing on the lateral edges of the rear window. The current study suggests that flow control on such geometries should consider all the flow structures that contribute to the model wake flow.     

Degradation effects of dilute polymer solutions on turbulent drag reduction in pipe flows

An important practical problem in the application and study of drag reduction by polymer additives is the degradation of the polymer, for instance due to intense shearing, especially in recirculatory flow systems. Such degradation leads to a marked loss of the drag-reducing capability of the polymer.Three different polymer types were tested on degradation effects in a closed pipe flow system. The polymers used were Polyox WSR-301, Separan AP-273 and Superfloc A-110, dissolved in water in concentrations of 20 wppm each. The flow system consisted of a 16.3 mm pipe of 4.25 m length. Two different pumps were used: a centrifugal pump and a disc pump. Different solution-preparation procedures were tried and the experiments were performed at different flow rates.Superfloc A-110 proved to be both the most effective drag reducer and most resistant to degradation. Because of very fast degradation, Polyox WSR-301 was found to be unsuitable for being used as a drag reducer in re-circulatory systems. The disc pump proved to be much better suited for pumping the polymer solutions than the centrifugal pump. The degradation curve of the combination Superfloc/disc pump showed a plateau-like region with reasonable drag reduction, which makes it possible to perform (laser Doppler) measurements under nearly constant circumstances during a sufficient time.     

Experimental studies on drag reduction and rheology of mixed cationic surfactants with different alkyl chain lengths

Experimental studies of the effects of mixtures of cationic surfactants on their drag reduction and rheological behaviors are reported. Cationic alkyl trimethyl quaternary ammonium surfactants with alkyl chain lengths of C₁₂ and C₂₂ were mixed at different molar ratios (total surfactant concentrations were kept at 5 mM with 12.5 mM sodium salicylate (NaSal) as counterion). Drag reduction tests showed that by adding 10% (mol) of C₁₂, the effective drag reduction range expanded to 4–120 °C, compared with 80–130 °C with only the C₂₂ surfactant. Thus mixing cationic surfactants with different alkyl chain lengths is an effective way of tuning the drag reduction temperature range. Cryo-TEM micrographs revealed thread-like micellar networks for surfactant solutions in the drag reducing temperature range, while vesicles were the dominant microstructures at non-drag reducing temperatures. High extensional viscosity was the main rheological feature for all solutions except 50% C₁₂ (mol) solution, which also does not show strong viscoelasticity. It is not clear why this low extensional viscosity solution with relatively weak viscoelasticity is a good drag reducer. Received: 3 November 1999/Accepted: 5 January 2000     

The effect of solvent solubility parameter on turbulent flow drag reduction in polyisobutylene solutions

Drag reduction is the effective reduction of the fluid flow friction brought about by the addition of small amounts of dissolved polymer, suspended particles, or emulsions. This study has focused on the turbulent-flow drag reduction effected by small amounts (10 ^-6–10 ^-3 g/ml) of polyisobutylene dissolved in organic solvents of varying solubility parameters. The data show that a maximum drag reduction (up to 70% for Reynolds numbers of 20,000) occurs in solvents with a solubility parameter near that of the polymer.     

The effect of a zwitterionic and cationic surfactant in turbulent flows

A zwitterionic and a cationic surfactant were compared concerning their effectiveness in drag reduction. Both reach even lower friction factors than can be determined by the known maximum drag reduction asymptote in polymers. It is shown that both the maximum effectiveness and the range of surfactant activity depend not only on the temperature and concentration of the solution, but also on the length of the direct entering section and on the quality of water. The influence of the particular locality of the flow rate regulation is also notified. Received: 1 December 1999/Accepted: 30 April 2000     

Turbulence measurements of drag reducing surfactant systems

LDA measurements were made of mean velocity and of turbulence intensity in a 39.4mm diameter tube, the first measurements in three directions on drag reducing surfactant solutions (0.05% and 0.1% Habon G). Drag reduction exceeded the predictions of the Virk maximum drag reduction asymptote and elastic sublayer mean velocity profiles are steeper than the profile proposed by Virk for maximum drag reducing asymptote solutions. Axial turbulence intensities for Habon G solutions are higher than those for water near the wall, lower in most of the outer region and about the same at the center. Tangential and radial turbulence intensities are lower than those for water.