Transient behaviour of liquid CO2 decompression: CFD modelling and effects of initial state parameters

Abstract

A non-equilibrium phase transition computational fluid dynamics (CFD) model for liquid CO2 decompression with higher prediction accuracy is presented. In this model, Span-Wagner equation of state (EoS) is applied and compiled, the slip velocity between phases is considered, and the interphase mass transfer is controlled by introducing relaxation time coefficients. For validation, the numerical results are compared with the “shock tube” test results, showing good agreement. On this basis, the transient behaviour of the high-pressure liquid CO2 pipelines during decompression is investigated, and the influences of initial state parameters (density and temperature) on the transient behaviour are further studied. It is found that the pressure change amplitude, the pressure change rate, the temperature change amplitude and the temperature change rate, as well as the outlet massflow increase with an increase in initial density and temperature. The durations of the sudden drop process of pressure and temperature are about 100 microseconds (μs). The degree of superheat of liquid CO2 decreases with an increase in initial density during decompression. Besides, the pressure plateau value has a negative linear correlation with the initial density, while a positive linear correlation with the initial temperature.