Motion of polymer liquids under unstable conditions and in channel terminals

Abstract

The present paper deals with the motion of elastic and Newtonian polymer liquids in channel terminals when the Reynolds numbersRe ≪ 1, as well as with the low-molecular polyisobutylene motion through capillaries in the unstable flow mode. The investigations were conducted on a set-up, including a constant pressure capillary viscometer and a laser anemometer. The 40 % butyl rubber solution withM w ≈ 3 ⋅ 104 in transformer oil was used as the elastic liquid. As a Newtonian one, the divinylnitrile rubber withM w ≈ 104 has been investigated. The longitudinal velocity distribution measurements were conducted along the channel axis at different cross-sections: in front of the channel inlet, in the inlet portion, in the developed flow region and in the outlet channel portion. It was found that the dimensionless axial velocity distribution along the channel length was invariant for Newtonian fluid with respect to the flow rate. The inlet and outlet portion lengths are approximately equal to half the channel hydraulic radius. It was shown, that for the elastic liquid the character of velocity profile formation in the inlet portion and the reverse process: of profile reconstruction at the channel outlet qualitatively differ from corresponding processes in a Newtonian fluid under conditions in which a substantially nonlinear fluid behaviour is realized. Axial velocity maxima were observed in the channel terminals. The inlet portion lengths were determined, as well as their dependence on the flow mode. It was shown, that in all cases investigated the inlet portion lengths are larger than for a Newtonian fluid under the same flow conditions (Re ≪ 1). Outlet portion length is approximately equal to double hydraulic radius, and does not, practically, depend on the flow mode. The longitudinal velocity distribution over the channel cross-section undergoes considerable changes in the outlet portion. The unstable flow of P-20 low-molecular polyisobutylene (with Flory molecular weight of 2.1 ⋅ 105) was realized in capillaries with different lengths and radii. In the unstable flow mode, a region was discovered in the flow rate vs. pressure relationships, where the flow rate decreases as the pressure drop increases at the capillary end portions. Correlation of velocity fluctuations in the channel central region with disturbances on the exit jet was discovered by the laser anemometry method. Polymer wall slip was discovered and measured. It was found that the radial distribution of the time-averaged longitudinal velocity had an inflection point near the capillary wall.