Triggered transition in the pipe flow of dilute solutions of random-coiling macromolecules

Abstract

Laminar-to-turbulent transition was studied experimentally in the flow of dilute, distilled water solutions of two polethyleneoxide polymers through three pipes, each 200 diam long and equipped with a ’’trigger’’ orifice, of i.d. half the pipe i.d., to provide a strong inlet disturbance. Polymer molecular weights, 0.7×106 and 5.0×106, and pipe sizes, 0.292, 0.457, 0.945 cm i.d., were chosen to cause transition from the laminar regime (L) into each of three turbulent regimes associated with drag reduction,1 namely, Newtonian (N), polymeric (P), and maximum drag reduction (M). The corresponding three types of laminar-to-turbulent transitions observed, designated L→N, L→P, L→M, had the following characteristics: (1) L→N. In this case the transition occurred from laminar flow into a turbulent regime wherein the polymer solutions did not cause drag reduction, and the transition process appeared to be the same as in the usual Newtonian case.2,3 (2) L→P. In this case the polymer solutions exhibited drag reduction at the lowest fully turbulent flow rates. The critical Reynolds number, below which turbulent slugs were not observed, was essentially the same as Newtonian. At a given Re in the transitional regime, the intermittency factor at the pipe axis was distinctly greater than Newtonian, the more so with increasing drag reducing ability of the polymer solution. The greater intermittency factor seemed to result mainly from an increased turbulent slug formation frequency relative to Newtonian; turbulent slug lengths had mean values essentially the same as Newtonian, although the dispersion about the mean was somewhat greater in the polymer solutions. (3) L→M. In this case the transition occurred from laminar flow into the turbulent regime of maximum drag reduction. Velocity fluctuations appeared at Re≃1500, appreciably lower than the Newtonian critical Re≃2000, and the fluctuation amplitude increased by an order of magnitude over the transitional range, 1500<Re<4000. Intermittency could not be discerned during this type of transition.