The analysis of the expansion wave propagation generated by full-bore ruptures of pressurized pipelines containing compressible fluids must be carried out during the assessment of the possible use of crack arrestors. If the internal fluid is two-phase, the sound velocity dependence from the local void fraction and flow regime has to be taken into account, by considering that it may be much lower than for single phase gases, thus promoting crack propagation. In this paper a model for the simulation of an expansion wave in a two-phase fluid pipeline is presented; this model includes several possible descriptions of the thermodynamics and flow regimes, ranging from the simpler homogeneous equilibrium approach to the non-equilibrium slip flow evaluation. The sound velocity trend inside a rarefaction wave can give rise to particular phenomena as curve inversions and jumps. The impact of different formulations on the expansion wave calculation is discussed, giving hints for the design of the pipelines under consideration.

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