HEAT AND MASS TRANSFER MECHANISM ACROSS THE AIR-SEA INTERFACE

根据风浪槽中界面区有组织运动的湍流结构，对无破碎气泡夹带的剪切气-水界面传质机理进行了实验研究。通过CO_2复氧实验测量了CO_2在水侧的传输速度，并利用热线风速仪和激光多普勒测速仪测量了空气和水流中的流体速度。结果表明，风切变对气水界面传质的促进作用比无切变时更强。然而，风切变对传质的影响趋于饱和，在高剪切区在现有的风浪池，在那里的质量传递速度的增加率与风切变迅速下降。根据气水界面附近的湍流结构可以很好地解释风切变对传质的影响。也就是说，通过界面上方的空气流中产生的强有组织运动，通过空气-水界面上的高风切变在水侧诱导表面更新涡流，并且更新涡流控制穿过剪切界面的质量传递。传质速度与表面更新涡流出现的频率相关，因为它是在具有未剪切界面的明渠流中，并且它近似与表面更新频率的根成比例地增加。表面更新频率随风切变的增加而增加，但对于高切变，增加的速率减慢。这导致了现有风浪池中高剪切区风切变对传质的饱和效应。基于表面更新时间分数概念的表面更新涡池模型能较好地估计传质速度。

The mass transfer mechanism across a sheared air-water interface without bubble entrainment due to wave breaking was experimentally investigated in terms of the turbulence structure of the organized motions in the interfacial region in a wind-wave tank. The transfer velocity of the carbon dioxide (CO_2) on the water side was measured through reaeration experiments of CO_2, and the fluid velocities in the air and water flows were measured using both a hot-wire anemometer and a laser-Doppler velocimeter. The results show that the mass transfer across a sheared air-water interface is more intensively promoted in wind shear, compared to an unsheared interface. However, the effect of the wind shear on the mass transfer tends to saturate in the high-shear region in the present wind-wave tank, where the increasing rate of mass transfer velocity with the wind shear decreases rapidly. The effect of the wind shear on the mass transfer can be well explained on the basis of the turbulence structure near the air-water interface. That is, surface-renewal eddies are induced on the water side through the high wind shear on the air-water interface by the strong organized motion generated in the air flow above the interface, and the renewal eddies control the mass transfer across a sheared interface. The mass transfer velocity is correlated with the frequency of the appearance of the surface-renewal eddies, as it is in open-channel flows with unsheared interfaces, and it increases approximately in proportion to the root of the surface-renewal frequency. The surface-renewal frequency increases with increasing the wind shear, but for high shear the rate of increase slows. This results in the saturated effect of the wind shear on the mass transfer in the high-shear region in the present wind-wave tank. The mass transfer velocity can be well estimated by the surface-renewal eddy-cell model based on the concept of the time fraction when the surface renewal occurs.