Turbulent hydrodynamic behavior of a drag-reducing viscoelastic fluid in a sudden-expansion pipe

Abstract

The turbulent hydrodynamic behavior of a drag-reducing viscoelastic fluid moving across a sudden-expansion step in a circular pipe is investigated with expansion ratios, D/d, of 1.39 and 1.90. Aqueous solutions of polyacrylamide AP-273 (200, 500, and 1000 ppm) are used to represent the viscoelastic fluid. Pressure distributions before and after the sudden-expansion step are measured, from which the loss coefficient for the sudden-expansion step is calculated for water and the polyacrylamide solutions. In general, the loss coefficients for the polyacrylamide solutions are consistently smaller than those for water over a range of Reynolds number ( Re D = 4840–107,000), which may be attributed to their elasticity. With the expansion ratio ( D/d) increasing from 1.39 to 1.90, the loss coefficient also increases by 100% for water and by approximately 150–200% for the polyacrylamide solution. While the effect of the Reynolds number on the loss coefficient is negligibly small for water, the loss coefficient for the polyacrylamide solution is found to increase with increasing Reynolds numbers. From an instantaneous pressure fluctuation, recorded at each pressure tap location, it is discovered that the flow instability produced at a reattachment point is quickly attenuated for water but amplified for the polyacrylamide solution in the downstream of the test section.