(108b) Scalar Transfer across the Air-Water Turbulent Interface

The phenomena of mass transfer across the air-water interface widely exist in various natural industrial processes. Because of its wide application, a general model capable of predicting the mass transfer rate across the air-water interface under various flow conditions is highly desirable.

In the past decade, a new kind of model named as ?surface divergence model' attracted many researchers' attention. It is generally accepted that the gradient of the vertical fluctuating velocity with respect to the interface plays an important role in mass transfer process. This new developed model tries to correlate this parameter to the mass transfer velocity across the air-water interface. This parameter is acquired from the turbulence structure in the very vicinity of the interface. This feature guarantees it can be used in various flow conditions. We have developed a reliable method to measure this key parameter in a circular wind wave channel. In this method, two cameras are use to observe the flow field from two different view angles. One from above to capture the clear view of the water interface, and the other one form below to get the detailed information under the water interface. These two different images from the two views are correlated through careful calibration before the experiment.

The mass transfer experiments were carried out under gas absorption and/or gas evasion conditions in a sealed circular channel, where the flow experiment is also taken. For gas evasion experiments, carbon dioxide was used to flush the headspace of the experimental set-up. It yields a known zero oxygen surface concentration. For gas absorption experiment, pure oxygen gas was introduced such that the total gaseous volume above the interface was considered to be saturated. Under such condition, the water surface is maintained at the saturation of oxygen in the water side at that temperature. By measuring the initial and final dissolved oxygen concentration, the mass transfer velocity can be obtained. Following figure gives out the comparison of the mass transfer results between gas absorption and gas evasion. It is noted that the oxygen sensor, used to measure the dissolved oxygen concentration in this work, use two known oxygen concentration standards for calibration. These two standards are a zero concentration standard and an environmental saturated concentration standard. So the oxygen sensor works in extrapolated region of calibration for the gas absorption experiments. Larger experimental measurement error exists for high level oxygen concentration. The pure gas was input just for 10minutes to make sure the work range of the instrument is not very far away from the calibration region.

The mass transfer velocities from gas absorption experiments are slightly higher than that from gas evasion experiments. Taking account of the limitation of the instrument and measurement uncertainty, this slight difference is acceptable. This phenomenon indicated that our presented relationship would be applicable for both gas absorption and evasion.

Based on the improved measurement method provided in this work, quantification of the vertical velocity with respect to the fluctuating interface and evaluation the associated velocity gradient in the vicinity of the interface were carried out. The critical parameter â was obtained from several distinct flow conditions. These distinct flow conditions can be regarded as the simplifications of all the other turbulence generation methods. Mass transfer experiments were also carried out. It was found that a general relationship can be proposed to correlate the interfacial hydrodynamics parameter and the mass transfer velocity.