Visualization of oxygen transfer across the air–water interface using a fluorescence oxygen visualization method

Oxygen concentration fields in a water body were visualized by the fluorescence oxygen visualization (FOV) method. Pyrenebutyric acid (PBA) was used as a fluorescent indicator of oxygen, and an intensive charge coupled-device (ICCD) camera as an image detector. Sequential images (over 2000 images) of the oxygen concentration field around the surface water of the tank (1×1×0.75 m3) were produced during the 3 h experiment. From image processing, the accurate pathway of oxygen-rich, cold water at the water surface was also visualized. The amount of oxygen transferred through the air–water interface during the experiment was measured and the oxygen transfer coefficient (KL) was determined as 0.22 m/d, which was much higher than that is expected in molecular diffusion. Results suggest that vertical penetration of cold water was the main pathway of oxygen in the water body in the tank. The average velocity of cold water penetrating downward in water body was also measured from consecutive images and the value was 0.3–0.6 mm/s. The FOV method used in this research should have wide application in experimental fluid mechanics and can also provide a phenomenological description of oxygen transfer under physically realizable natural conditions in lakes and reservoirs.