Equilibrium Conditions and Sound Velocities in Two-Phase Flows

We consider a hierarchy of hyperbolic models describing single-component two-phase flows in pipelines, with applications to CO₂ capture and storage. The hierarchy is characterized by the number of equilibrium assumptions made. We present a formal proof that every additional level of enforced equilibrium lowers the propagation velocity of pressure waves. This subcharacteristic condition holds for arbitrary thermodynamic state equations. We present numerical examples relevant for CO₂ transport, and argue the importance for pipeline integrity simulations.