Experiments on air entrainment due to free falling- and wall-jets

In this article, air entrainment as a result of an impinging round water jet and a wall-jet was experimentally studied by means of videometry and image processing methods and also by means of a measuring technique based on a wire-mesh sensor. Therefore, two different experimental setups were utilized. For the first setup, a series of experiments at different conditions was performed and evaluated for both round jets and wall-jets. Jet lengths ranged between 0.01m and 0.2m and jet exit velocities between 0.9 m/s and 3.5 m/s. Image processing algorithms were applied to extract information about jet penetration depth, width of the bubble plume and bubble size distribution. The second facility was used to create a falling film in a square pipe (5 cm x 5 cm). Downstream of the impact point, a wire-mesh sensor was used to measure the gas entrainment characteristics at one axial location. Video image processing was also used in this experiment to gather more qualitative information about the gas entrainment process. Video images were compared with the images obtained by the wire-mesh sensor showing good agreement. The induction trumpet and a thin sheet of gas that is formed around the jet and penetrates into the pool causing the entrainment were clearly identified. Results indicate that the gas void fraction increases and the bubble size decreases as the superficial liquid velocity increases.