Effect of Obstacle Shape on the Deposition Rate in Annular Two-Phase Flow

In annular two-phase flow, it is known that the deposition rate of droplets markedly increases if an obstacle is placed in a flow channel. In most of the available models to evaluate the obstacle effect on deposition rate, turbulence augmentation at the downstream of obstacle is considered as the key mechanism of the deposition enhancement. In this study, it was examined where the increase of droplet deposition occurred around the obstacle to reveal the primary cause of deposition enhancement. The test section was a round tube of 5 mm in inside diameter and air and water were used as test fluids; the double film extraction technique was adopted to measure the deposition rate. To specify the position where the deposition enhancement occurred, the liquid film was extracted at the upstream, middle and downstream of the flow obstacle. As a result, it was indicated that the significant amount of droplets was deposited not only at the downstream of flow obstacle but also at the upstream of obstacle. Using five tubular obstacles of different inside and outside diameters, the influence of obstacle geometry on the deposition rate was investigated. It was shown that the position of deposition enhancement was influenced significantly by the obstacle geometry. (authors)