Air–water gas transfer and near-surface motions
Air–water gas transfer and near-surface motions
复制标题
空气-水气体传递和近地表运动
DOI：
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发表时间：
2013
影响因子：
3.7
通讯作者：
Sanjoy Banerjee
中科院分区：
文献类型：
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作者：
D. Turney;Sanjoy Banerjee
摘要空气和水之间的气体传输速率仍然很难预测或模拟，由于广泛的长度和时间尺度和缺乏近地表流体速度和气体浓度的实验观测。表面更新模型（SR）和表面发散模型（SD）提供了两个主要的模型的过程中，但他们仍然不好的测试，通过观测，因为近地表速度是难以测量的。为了有助于这些模型的评估，我们采用了新的技术，称为界面粒子成像测速（IPIV）和三维IPIV（3D-IPIV）测量水的速度在一毫米的移动变形的空气-水界面。后一种技术（3D-IPIV）同时测量空气-水界面地形。我们将这些技术应用到湍流明渠水流和风剪切水流与微尺度破碎波。对每种流动条件进行的额外测量包括整体湍流长度尺度、整体湍流速度尺度、空气-水气体传输速率、摩擦速度和波动特性。我们分析这些数据来测试界面气体传输的表面发散模型。第一个测试是来自Banerjee的预测（第九届国际传热会议，主题演讲，第1卷，1990年，第110页）。395-418，半球出版社）表面发散模型的气体传输均匀各向同性湍流与平面自由表面相互作用。第二个测试是来自McCready，Vassiliadou和Hanratty的预测（AIChE J.，第32（7）卷，1986年，第32页。1108-1115）表面发散模型，适用于明渠流和风剪切波浪流。我们发现Banerjee和McCready等人模型的预测与明渠水流条件下的实验数据一致。另一方面，对于有风浪的风驱动流动，我们发现McCready等人的预测与我们直接测量的气体传递系数之间存在不一致。通过对地表水斑块的表面辐散的拉格朗日追踪，并通过分析相应的带有平流扩散概念的拉格朗日时间序列来研究不一致的原因。提出了一个基于表面发散强度和寿命的定量判据来区分每种近表面运动对引起界面气体传输的有效性。毛细波被发现有助于表面发散，但有太短的时间尺度，导致界面气体转移。随着风速的增加，毛细波和其他无效的近地表运动的空气-水界面上的存在和强度被减少的湍流尾流从微尺度破碎波的上升，从而导致表面发散模型的预测传输速率与测量的不一致。空气-水-气体传输模型，结合表面更新和表面发散模型的制定，发现同意从明渠流量和风驱动的流量，而不需要一个经验系数的数据。
Abstract Rates of gas transfer between air and water remain difficult to predict or simulate due to the wide range of length and time scales and lack of experimental observations of near-surface fluid velocity and gas concentrations. The surface renewal model (SR) and surface divergence model (SD) provide the two leading models of the process, yet they remain poorly tested by observation because near-surface velocity is difficult to measure. To contribute to evaluation of these models, we apply new techniques called interfacial particle imaging velocimetry (IPIV) and three-dimensional IPIV (3D-IPIV) for measuring water velocities within a millimetre of a moving deformable air–water interface. The latter technique (3D-IPIV) simultaneously measures the air–water interface topography. We apply these techniques to turbulent open-channel water flows and wind-sheared water flows with microscale breaking waves. Additional measurements made for each flow condition are bulk turbulent length scales, bulk turbulent velocity scales, air–water gas transfer rates, friction velocities, and wave characteristics. We analyse these data to test the surface divergence models for interfacial gas transfer. The first test is of predictions from the Banerjee (Ninth International Heat Transfer Conference, Keynote Lectures, vol. 1, 1990, pp. 395–418, Hemisphere Press) surface divergence model for gas transfer for homogeneous isotropic turbulence interacting with a planar free surface. The second test is of predictions from the McCready, Vassiliadou and Hanratty (AIChE J., vol. 32(7), 1986, pp. 1108–1115) surface divergence model, as applied in both open-channel flow and wind-sheared wavy flows. We find the predictions of the Banerjee and McCready et al. models to agree with the experimental data taken for open-channel flow conditions. On the other hand, for wind-driven flows with wind waves we find disagreement between the McCready et al. predictions and our direct measurements of the gas transfer coefficient. The cause of the disagreement is investigated by Lagrangian tracking of surface divergence of surface water patches, and by analysis of the corresponding Lagrangian time series with advection–diffusion concepts. A quantitative criterion based on surface divergence strength and lifetime is proposed to distinguish the effectiveness of each near-surface motion toward causing interfacial gas transfer. Capillary waves are found to contribute to surface divergence but to have too short a time scale to cause interfacial gas transfer. As wind speed increases, the presence and intensity on the air–water interface of capillary waves and other ineffective near-surface motions is diminished by the rise of turbulent wakes from microscale breaking waves thus causing the disagreement of the surface divergence model’s predicted transfer rates with measurements. A model of air–water gas transfer that combines the surface renewal and surface divergence models is formulated and found to agree with the data from both open-channel flows and wind-driven flows without requiring an empirical coefficient.