In order to explore the effect of drag reducer on resistance characteristics flowing in a variable cross-section pipe, the resistance for three mass concentrations of drag-reducing solution flows in a sudden expanded pipe with a diameter ratio of 1∶1.49 was measured by using quaternary ammonium salt cetyltrimethyl ammonium bromide as drag reducer. The results show that the critical Reynolds numbers for upstream DN10 tube flow at three mass concentrations which are 8×10-5、1.4×10-4 and 2×10-4 are about 20 000, 30 000 and 34 000 respectively, and the corresponding maximum drag reduction rates are 70%, 72% and 74% respectively. For downstream DN15 tube flow at 8×10-5, the corresponding critical Reynolds numbers are about 28 000, and the corresponding maximum drag reduction rate are 70%. For each mass concentration, as the Reynolds number of the upstream pipe flow is less than the critical Reynolds number, the sudden expansion drag coefficient is smaller than the theoretically predicted value of Newtonian fluids and increases with Reynolds number. While as the Reynolds number proceeds critical one, it is larger than that of Newton fluids and increases with increasing Reynolds numbers.
