Some limitations on the detection of high elongational stress effects in dilute polymer solutions

Ting, Robert Y.; Hunston, Donald L.

doi:10.1002/app.1977.070210710

Cited by 6 publications

(1 citation statement)

References 26 publications

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“…Note that our experiments involve nonconstant stretch rates. However, the work by Ting and Hunston (1977) shows that the basic considerations in such cases remain the same as above.…”

Section: Possible Mechanistic Explanatlon For Anomalous Mass Transpormentioning

confidence: 92%

Anomalous convective diffusion in films of polymeric solutions

Mashelkar

1984

AIChE Journal

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The process of gas absorption in freely falling films of dilute polymer solutions under different kinematic conditions shows some hitherto unreported and dramatic differences. The absorption rates are reduced when films are generated under rapid stretch conditions over axisymmetric bodies of revolution, whereas they are enhanced when films are generated in a long, wetted wall column. Absorption rates are enhanced on polymer addition in freely falling turbulent films. Possible mechanistic explanations have been offered for these phenomena. R. A. MASHELKAR Chemlcal Engineering Division National Chemical LaboratoryPoona, IndiaAlong with the study of the aspects of momentum transport in dilute polymer solutions, lately the study of heat/mass transport processes appears to be attracting attention. The importance of such a study is threefold. First, whenever heat/mass transport rates are augmented due to polymer addition, one can derive obvious economic advantages by polymer addition. Secondly, the studies are helpful from the viewpoint of enabling us to discern the manner in which dilute polymer solutions may behave in complex flow situations. Finally, if we can accept that the kinematics will be changed in a known way, it is possible to test the validity of the well-known models of heat/mass transfer.With this view, we have studied here the process of gas absorption in films generated under a variety of flow conditions. These include films rapidly stretched over axisymmetric bodies of revolution and films formed in a long, vertical wetted wall column under laminar wavy and turbulent conditions. CONCLUSIONS AND SIGNIFICANCERapidly stretched films of dilute polymeric solutions over axisymmetric bodies of revolution show reduced rates of gas absorption, whereas the films formed in a vertical wetted wall column show enhanced rates of gas absorption. A mechanistic explanation for this phenomenon has been offered by focussing attention on the possible kinematic adjustments in rapid ac--celeratingdecelerating flows in elastic fluids. The observations reported place an upper limit on the range of operation of the wetted sphere or doublecone flow technique used for diffusivity measurements in macromolecular solutions. Additionally, a particularly significant point pertains to a caution which must be exercised when applying purely viscous considerations for heat/mass transport processes in external fields where extensional flow elements are dominant.The rates of gas absorption in a long, vertical wetted wall column are increased in the laminar wavy and turbulent region. This is qualitatively in agreement with a simple eddy diffusivity model. This observation, which is of pragmatic importance by itself, also supports the recent findings that small-scale Kolmogoroff eddies do not control mass transfer processes across free surfaces of falling turbulent films.

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“…Note that our experiments involve nonconstant stretch rates. However, the work by Ting and Hunston (1977) shows that the basic considerations in such cases remain the same as above.…”

Section: Possible Mechanistic Explanatlon For Anomalous Mass Transpormentioning

confidence: 92%

Anomalous convective diffusion in films of polymeric solutions

Mashelkar

1984

AIChE Journal

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Evidence for molecular interactions in drag reduction in turbulent pipe flows

Berman

1980

Polymer Engineering & Sci

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Experiments which test the concentration and molecular weight dependence of turbulent pipe flow drag reduction for random coiling polymers in dilute solutions show correlations with concentration to the one‐half power and molecular weight to the 0.8 power for good solvents. This result is not consistent with a model of extension of single1 molecules, but could be related to the increase in bulk viscosity of interacting molecules after some extension. In this work, measurements for very low amounts of drag reduction for rigid rod molecules arc reported, and the effect of tube diameter on the amount of drag reduction is examined for fiexible rod molecules. No diameter effect is observed for the rigid rods, but an increase in drag reduction with increase in pipe diameter is found for the flexible polyeleetrolytes. In all cases, the volume occupied by spheres which circumscribe the molecules is greater than the actual volume when drag reduction is found. The results indicate that combined effects of individual molecule stretching and molecular interactions are present in drag reduction for random coiling or flexible rod molecules.

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A visualization study of poly(ethylene terephthalate) flow using potential chain‐ordering die geometries

Crater

Cuculo

1983

J. Polym. Sci. Polym. Phys. Ed.

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Streak‐photographic and stress birefringence techniques were used to analyze the flow of poly(ethylene terephthalate) through potential chain‐ordering die geometries. The streak photographs were used to determine velocity distributions and streamlines in various convergent dies. The different contours were seen to have a significant effect on the polymer streamlines and velocity distributions. The measured velocities were used to develop empirical equations, specific for each geometry, which relate velocity to position within the die and to throughput rate. Flow birefringence was used to determine the extent of molecular ordering. The optimum chain‐ordering die geometry was determined to be one which included a rapid initial decrease in cross‐sectional area. Birefringence was also used to monitor polymer flow instability. An unusual mechanism for instability was observed at intermediate throughput rates.

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Some limitations on the detection of high elongational stress effects in dilute polymer solutions

Cited by 6 publications

References 26 publications

Anomalous convective diffusion in films of polymeric solutions

Anomalous convective diffusion in films of polymeric solutions

Evidence for molecular interactions in drag reduction in turbulent pipe flows

A visualization study of poly(ethylene terephthalate) flow using potential chain‐ordering die geometries

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