In this work we study the effect of polymeric additives on the transition to turbulence in opposed-jets flow. In this type of flow, the transition to turbulence for Newtonian fluids is characterized by a decrease in the rate of change of pressure drop with flow rate. We have used various polymers whose equilibrium molecular conformation in aqueous solution is different: poly (ethylene oxide), which exists in a conformation close to a random coil, hydroxypropyl guar, which adopts an expanded coil conformation, and hydrolyzed polyacrylamide, whose conformation is close to a random coil in the presence of an electrolyte (sodium chloride) but it changes to an expanded coil in distilled water. The results show that small amounts of either flexible or semi-rigid polymers induce a delay in the critical Reynolds number at which turbulence sets in. This delay seems to be a result of the suppression of flow instabilities in a region close to the stagnation point, which is linked to macromolecular orientation. Since, for Newtonian fluids, this flow has an increase of drag with flow rate that is slower in the turbulent flow regime than in the laminar regime, the addition of polymer causes a substantially higher pressure drop in turbulent flow with respect to that of the pure solvent. Therefore, polymer addition causes, in this particular case, a drag increase in turbulent flow, as opposed to the commonly observed drag reduction in turbulent flow through pipes.
ASJC Scopus subject areas
- Mechanical Engineering