Flow pattern, pressure drop and void fraction of two-phase gas–liquid flow in an inclined narrow annular channel

Two-phase flow pattern, pressure drop and void fraction in horizontal and inclined upward air–water two-phase flow in a mini-gap annular channel are experimentally studied. A concentric annular test section at the length of 880 mm with an outer diameter of 12.5 mm and inner diameter of 8 mm is used in the experiments. The flow phenomena, which are plug flow, slug flow, annular flow, annular/slug flow, bubbly/plug flow, bubbly/slug–plug flow, churn flow, dispersed bubbly flow and slug/bubbly flow, are observed and recorded by high-speed camera. A slug flow pattern is found only in the horizontal channel while slug/bubbly flow patterns are observed only in inclined channels. When the inclination angle is increased, the onset of transition from the plug flow region to the slug flow region (for the horizontal channel) and from the plug flow region to slug/bubbly flow region (for inclined channels) shift to a lower value of superficial air velocity. Small shifts are found for the transition line between the dispersed bubbly flow and the bubbly/plug flow, the bubbly/plug flow and the bubbly/slug–plug flow, and the bubbly/plug flow and the plug flow. The rest of the transition lines shift to a higher value of superficial air velocity. Considering the effect of flow pattern on the pressure drop in the horizontal tube at low liquid velocity, the occurrence of slug flow stops the rise of pressure drop for a short while, before rising again after the air velocity has increased. However, the pressure does not rise abruptly in the tubes with θ = 30° and 60° when the slug/bubbly flow occurs. At low gas and liquid velocity, the pressure drop increases, when the inclination angles changes from horizontal to 30° and 60°. Void fraction increases with increasing gas velocity and decreases with increasing liquid velocity. After increasing the inclination angle from horizontal to θ = 30° and 60°, the void fraction appears to be similar, with a decreasing trend when the inclination angle increases.