Hydrodynamics and Mixing Characteristics in Different-Size Aspirator Bottles for Water-Accommodated Fraction Tests

The preparation of the water-accommodated fraction (WAF) is important for evaluating the toxicity of oil. The Chemical Response to Oil Spills: Ecological Research Forum method, which uses a magnetic stirrer in aspirator bottles, is commonly used. Thus, it is investigated herein focusing on the hydrodynamics. The particle image velocimetry technique was applied to measure the fluid velocity and mixing characteristics of three bottles 2, 4, and 20 L in volume, respectively. The hydrodynamics and mixing performance of cases without an air tube and cases in which the air tube occupied approximately 22.5% of the water height were considered. The energy dissipation rates were highest near the center of the bottles, and the average values ranged from 5×10-5 W/kg in the no-air-tube bottles to approximately 1.1 W/kg in the air-tube bottles. The latter value is comparable to non-breaking waves or small breakers (0.4 m in height). Knowing the rotation speed, one can use the results and the computed power number, KT, to obtain the energy dissipation rates in comparable bottles. A new concept, vessel efficiency (VE), was introduced and represents the product of the energy dissipation rate in the impeller region and the residence time of water in this region. The VE of cases without and with air tube vessels showed that VE significantly increased with an air tube in each bottle. The VE for cases with an air tube also increased with bottle size due to the high residence time in the impeller region. The hydrodynamics investigation herein provides a foundation for interpreting the droplet size distribution in actual WAF and chemically enhanced water-accommodated fraction tests.