Transient flow induced by the change of fluid flow rate occurs frequently in two-phase pipelines. Many transient experiments are conducted on the authors’ big multiphase flow loop. Increasing gas flow rate causes pressure overshoots arising from gas inertia in stratified flow and from gas compressibility and liquid inertia in plug and slug flow. Decreasing the gas flow rate leads to pressure undershoot. In gas transients

flow patterns may appear which are not observed in corresponding quasi-steady state. By contrast

in liquid transients

pressures and flow pattern undergo smooth transitions and transient effects are small. In addition

the pressure waves and void fraction waves propagating downstream depend heavily on flow pattern and evolution of them determines the transient process. In gas transient flow

the speed of pressure change equals to sound velocity while in liquid transient flow it equals to the void fraction wave velocity.