Nonlinearity dynamics of isothermal and non-isothermal viscoelastic flows through an axisymmetric sudden contraction

Nonlinear dynamics of isothermal and non-isothermal flows of a viscoelastic fluid (PIB/PB/C14 polymer solution referred to as PIB Boger fluid) in an axisymmetric sudden contraction geometry (4:1) are studied. Such flows are of great importance in commercial polymer processes such as extrusion, injection molding, and fiber spinning, and also in polymer rheology using capillary rheometers. This problem serves as a benchmark problem for the numerical simulation of viscoelastic flows in complex geometries as well.; Instantaneous measurements of pressure and temperature along with flow images acquired by streakline photography are performed to examine the spatial and temporal structure of the flow. The measurements indicate that the steady-axisymmetric flow at low flow rates (or low Deborah numbers) bifurcates to time-periodic three-dimensional flow in the vicinity of the contraction lip followed by subsequent global transitions to a time-periodic and finally time-irregular flow. Each transition is accompanied with changes in vortex dynamics such as the decay of the Newtonian comer vortex prior to the first transition followed by the formation, and then periodic and aperiodic growth of an elastic vortex.; It is shown that thermal effects have profound consequences for the onset, development, and structure of the vortical flows in these viscoelastic entry flows. For relatively moderate values of surface to inlet fluid temperature difference &parl0;-1.56≤Fs<2, where Fs=103Ts -T0/DH/R u&parr0; , although the overall nature of the local and global flow transitions are similar to those observed in the isothermal flow, the critical conditions for the onset of the elastic flow instabilities are altered. Heating of the walls Fs>0 delays the critical Deborah number (Decrit) for both local transition (lip instability) and global transition (periodic vortex growth) whereas cooling of the walls Fs<0 destabilizes the local transition but stabilizes the time-periodic global transition. For larger temperature differences Fs≥2 , however, not only the critical conditions are shifted but also the elastic flow transitions are completely different in nature.; Disturbances from the elastic vortices near the contraction extend upstream at least to 32d and are convected downstream to 6 d and beyond (d is the diameter of the downstream tube). A new instability downstream of the contraction plane is observed following the onset of flow instabilities occurring near the lip. This instability exists even for high flow rates (De). The flow far downstream of the contraction plane is significantly affected by this instability. The intensity of these convected fluctuations is smaller for both cooling and heating boundary conditions.