We investigated effects of solution concentration C, solution temperature T, and degradation on drag reduction (DR) of nonionic surfactant aqueous solutions, which were non-toxic and biodegradable, by measuring the pressure drop and flow rate in the circuit pipe flow with the inner diameter of 25.1 mm. The maximum drag reduction ratio DR max increased from about 70 % to 80 % with increasing from C = 50 to 1000 ppm at T ≈ 20 ºC. The DR max at C = 100 ppm was independent of the solution

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... e T from 10 to 40 ºC, while the region of Reynolds number at the large DR (> 60 %) was dependent on T. The degradation property strongly depended on whether the Reynolds number was smaller or larger than the critical Reynolds number, at which the DR was maximum. The drag-reducing effects obtained here were discussed as taking into account the strength of the shear thickening observed in the plot of shear viscosity vs. shear rate, which was measured by the homemade capillary viscometer.