The study of two-phase fluid flow behavior in hydraulic structures such as pressurized flow tunnels, culverts, sewer pipes, junctions and other similar conduits is of great importance. A two-phase mixture flowing in a pipe can exhibit several interfacial geometries such as bubbles, slugs or films, depending on the fluid and hydrodynamic properties of flow. The main variables, giving rise to a variety of flow patterns, include relative discharge rate of fluids and the pipe slope. The
flow patterns mostly attainable with air and water include stratified include and slug patterns. In this paper, the experimental results of pressurized water tunnel model are presented. The results include pressure transient and its variations for different hydraulic and geometric properties. It is shown that trapped and released air can cause tremendous pressure surges in the system and, eventually, may cause failure in systems (e.g. the maximum pressure inside the pipe would reach up to 10 times of upstream hydrostatic pressure). Finally, relations for forecasting maximum and minimum pressure in these situations are presented as a function of mean pressure, flow characteristics and pipe geometry.


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