Abstract:
In this thesis, the experimental results of flow pattern and pressure gradient of air-water flowing through the 180-degree vertical return bends are presented. The test sections are made of plexiglass which inner diameters of 4, 5 and 8 mrn and curvature ratios of 3, 5 and 7. The influences of tube inner diameter, tube curvature ratio and flow direction on flow pattern and pressure gradient are investigated. Bubbly, plug, slug, annular and gurgle flow are found in the present experiment. For upward flow at a specific liquid superficial velocity (U,,), the transition line between slug flow and annular flow shifts to a higher gas superficial velocity (U,,). The pipe diameter affects significantly the flow patterns. By increasing the pipe diameters, the transitions from slug flow to annular flow occur at lower U,,. It should be noted that the bubbly flow pattern is not found in the pipe of 4 mrn diameter. At the same U,, of 4 and 5 mm pipe diameters, as the curvature ratios increase, the transitions from slug flow to annular flow tend to occur at lower U,,. For 8 mrn pipe diameter, all flow pattern transitions in pipe with curvature ratio of 5 occur at lower U,, than that of 3 and 7 respectively. The effect of flow direction on pressure gradient is found that; the pressure gradient of fluid with upward flow is higher than that with downward flow. For the pressure gradient in different sizes of pipes, smaller pipes are found to have higher pressure gradient than larger ones. The curvature ratio also affects pressure gradient. It is found that, the pressure gradient in the pipe with higher curvature ratio is lower than that with lower curvature ratio.The experimental pressure gradient data are compared with those obtained from three prediction methods: the equation depending on each particular flow patterns, the Chisholm-B equation and the Geary correlation. The pressure gradient calculated from the Geary correlation provided the best agreement with experimental data.