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An experimental investigation is conducted to study the effect of middle length and inclination angle of an S-shaped channel on adiabatic two-phase flow patterns. To study this effect, three channel lengths of 0.25, 0.50, and 1.00m, and four bends at the angles of 22.5°, 45°, 67.5° and 90° were used along the test section which is 35.83m long. The channel cross section dimensions were 0.05×0.10m and the test fluids were air and water. The observed flow patterns in 12 cases were studied. Each flow pattern was identified by visual observation and recorded with video camera for further analysis of the collected data points. The momentum transfer phenomenon between the phases for passing the upward middle section along the S-shaped channel was closely investigated. Five flow patterns of vortex plug, vortex slug, vortex wavy-annular, churn and mist packet annular were observed. The vortex type flow patterns were recorded with high speed video camera. The physical mechanisms and momentum transfer related to formation of these flow patterns were explained in detail. Flow pattern maps were obtained and then compared for different middle lengths and inclination angles to perform a parametric study on the S-shaped channel. Moreover, effect of upstream/downstream of the S-shaped channel on the maps and transition lines were discussed. It was revealed that upstream effects reflect the interactions and momentum transfer between the phases. Therefore, flow pattern types mainly depend on upstream behavior. On the other hand, the amount of backward flow resulting in the establishment of countercurrent flow and creation of vortexes along middle section is determined by downstream effects. Increasing the middle length or inclination angle causes the transition to vortex plug flow pattern to occur at higher liquid velocities. It also causes the area with churn flow pattern on the map to expand ending in smaller vortex slug flow pattern area. However, increasing middle length or decreasing inclination angle, ends in smaller area of mist packet annular area. It is also observed that for higher middle lengths, increasing the inclination angle shifts the transition lines more remarkably. A dimensionless analysis of obtained flow pattern maps is also carried out. It is shown that effect of geometry changes along S-shaped pipeline on two-phase flow pattern transitions is determined by either inclination angle or elevation difference between two horizontal lines. The explained flow mechanisms, obtained flow pattern maps and the transition lines can be used to predict the particular flow pattern that will be established for a given mass flux, inclined length and angle along pipelines. [ABSTRACT FROM AUTHOR]