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  • 1
    Electronic Resource
    Electronic Resource
    Instabilities in multi-hole converging flow of viscoelastic fluids (1991)
    Springer
    Rheologica acta 30 (1991), S. 511-522 
    Details
    ISSN: 1435-1528
    Keywords: Flow instabilities ; spinneret ; Boger fluid ; multi-hole contraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Studies of the onset of instabilities were conducted on single hole and multi-hole contractions using laser speckle visualization. A well characterized elastic fluid was used with constant viscosity of 13.1 Pa · s and elasticity characterized by a longest relaxation time constant of 2.233 s. The onset of instabilities was characterized in terms of the Deborah number and the contraction ratio. Three types of instabilities were observed: pulsing vortices, azimuthally rotating vortices, and swirling vortices. For the single hole contractions the critical Deborah number for instability increased from 4.4 to 5.07 to 5.25 as the contraction ratio increased from 4: 1 to 8: 1 to 12: 1. The magnitude of the instabilities was much greater for the 4: 1 contraction than for the other two contraction ratios. For the multi-hole contraction a square array of nine holes was used and the ratio of the hole diameter to hole spacing was varied. The height of the vortices is very similar for the single hole and multi-hole contractions at low Deborah numbers. At high Deborah numbers the effect of adjacent holes is to reduce the height of the vortices by a factor of three. For the 4: 1 spacing no secondary vortex was observed below a Deborah number of De = 3.7. Secondary vortices occurred for the 8:1 and 10:1 spacing at all Deborah numbers. Unstable pulsing vortices appeared for all spacings at a critical Deborah number around 5.5. Adjacent holes decreased the strength of the unsteady vortex motions. The centerline velocities were measured for the multi-hole contraction at shear rates of 5, 30, and 300 s−1. The elongational strain rates are similar at a low shear rate of 5 s−1. As shear rate is increased the onset of stretching occurs closer to the plane of the contraction for the smaller contraction ratios.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00444369
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  • 2
    Electronic Resource
    Electronic Resource
    Extensional rheology of concentrated poly(ethylene oxide) solutions (1997)
    Springer
    Rheologica acta 36 (1997), S. 555-567 
    Details
    ISSN: 1435-1528
    Keywords: Key words Extensional viscosity ; concentration dependence ; fiber spinning ; opposed nozzles ; Giesekus model ; PEO solutions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The shear and extensional rheology of three concentrated poly(ethylene oxide) solutions is examined. Shear rheology including steady shear viscosity, normal stress difference and linear viscoelastic material functions all collapse onto master curves independent of concentration and temperature. Extensional flow experiments are performed in fiber spinning and opposed nozzles geometries. The concentration dependence of extensional behavior measured using both techniques is presented. The zero-shear viscosity and apparent extensional viscosities measured with both extensional rheometers exhibit a power law dependence with polymer concentration. Strain hardening in the fiber spinning device is found to be of similar magnitude for all test fluids, irrespective of strain rate. The opposed nozzle device measures an apparent extensional viscosity which is one order of magnitude smaller than the value determined with the fiber spinline device. This could be attributed to errors caused by shear, dynamic pressure, and the relatively small strains developed in the opposed nozzle device. This instrument cannot measure local kinematics or stresses, but averages these values over the non-homogenous flow field. These results show that it is not possible to measure the extensional viscosity of non-Newtonian and shear thinning fluids with this device. Fiber spinline experiments are coupled with a momentum balance and constitutive model to predict stress growth and diameter profiles. A one-mode Giesekus model accurately captures the plateau values of steady and dynamic shear properties, but fails to capture the gradual shear thinning of viscosity. Giesekus model parameters determined from shear rheology are not capable of quantitatively predicting fiber spinline kinematics. However, model parameters fit to a single spinline experiment accurately predict stress growth behavior for different applied spinline tensions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003970050070
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
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