Two-phase flow is often considered in system hydraulics as well as the evaluation and design of pressure relief and effluent handling systems. A variety of scenarios can lead to two-phase flow under relief conditions.
In general, two-phase flow during relief can occur because of flow hydrodynamics and poor vapor/ liquid disengagement where (a) the liquid swells due to generation of vapor bubbles in the liquid 1, (b) fluid expansion occurs due to heating, and/or (c) the superficial vapor velocity is high enough through the pressure relief device. Oversized relief devices can induce two-phase flow because a large relief flow area yields a higher superficial vapor velocity. Runaway chemical reactions and/or chemical systems that are viscous and/or foamy almost always lead to homogeneous two-phase flow.
Two-phase flow can also occur by entrainment, for example, where gas is sparged at a high enough rate in the liquid. In some systems, condensation leading to two-phase flow in the discharge piping can also occur due to expansion cooling caused by pressure reduction through a control valve or a pressure relief device.
Numerous two-phase flow models have appeared in the literature. These models represent broad ranges of theory. Some are based on single-phase critical flow, others on homogeneous equilibrium flow, frozen flow, separated flow, slip flow, and/or non-equilibrium flow.
Homogeneous equilibrium flow models assume equal vapor and liquid velocities and calculate the change of quality with pressure using an isenthalpic or isentropic thermodynamic path. Homogeneous frozen models assume equal vapor and liquid flow velocities and that the quality is frozen along the flow path, i.e., no change with respect to pressure or temperature. The separated flow models assume different vapor and liquid flow velocities and account for mass, momentum and heat transfer between the separate phases.
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