Natural gas pipelines have an excellent safety record but on rare occasions they rupture and decompress. When this happens their contents cool rapidly and form two phases. The decompression behaviors of multiphase fluid released from pipeline are not well understood. Pipeline decompression modeling is useful in characterizing the rapid transient flow that occurs when a pipeline ruptures. Numerical simulation can provide detailed data for analyzing the consequences of pipeline bursts and the mechanical performance of pipelines as they decompress. Decompression behavior of fluids is complicated by the formation of two-phase flow due to gas cooling or liquid flashing effects. Based on the time-space-ensemble composite averaging procedure, a two-fluid flow model is derived for simulating high-pressure natural gas pipeline decompression. The composite averaging operator is supported and demonstrated by simple experimental data. A set of constitutive equations is formulated for the closure of the system of equations. The conservation equations along with closure equations are examined for compliance with the second law of thermodynamics. Characteristics analysis is performed to ensure that the set of equations is well-posed mathematically.

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