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

Effects of Flow Velocity on Transient Behaviour of Liquid CO2 Decompression during Pipeline Transportation

“…The Lee model is commonly used to study the evaporation-condensation process [32], which originally takes the temperature T as the independent variable. In this case, the temperature variable "T" was replaced by the pressure variable "P" due to the fact that the driving force of phase transition came from pressure drop [29,30]. No mass transfer exists between CO 2 phases and air; therefore, the mass transfer between the CO 2 phases can be expressed as follows:…”

“…ρ l and ρ v are the corresponding density of liquid CO 2 phase and gas CO 2 phase. τ vl represents the condensation coefficient, and τ lv is the evaporation coefficient, which was recommended to be 15 s −1 [30]. Notably, the saturation pressure P sat is related to the local temperature T and can be defined as follows [33]:…”

“…In order to improve the prediction accuracy of the numerical model, the Span-Wagner equation of state (EoS) was used to calculate the thermodynamic parameters of CO 2 phases, which has been proved to be of high precision in predicting the physical properties of pure CO 2 , especially in the region up to pressures of 30 MPa and up to temperatures of 523 K. The estimated uncertainty of the equation ranges from ±0.03% to ±0.05% in the density, ±0.03% to ±1% in the speed of sound and ±0.15% to ±1.5% in the isobaric heat capacity [33]. Here, the Span-Wagner EoS was embedded into the CFD model to calculate the thermodynamic parameters of liquid CO 2 phase and gas CO 2 phase by the user-defined real gas model (UDRGM) and UDFs, respectively [30]. The air phase, however, was defined as ideal gas.…”

“…The length of the CO 2 domain of the computational model was 10 m and the diameter D was 146.36 mm; while the length of the air domain was set as 30 D, and the radius was 10 D. Since the duration time (tens of milliseconds) of the decompression process was short, the effect of gravity was ignored. A two-dimensional axial symmetric model, which was suitable for current calculations, was used to speed up the calculations [29,30]. For the axisymmetric boundary, the fluxes of all quantities across the boundary were 0.…”

“…That is, the CO 2 fluid within the pipeline is always in a flowing state and has an initial flow velocity when decompressing. In this paper, a CFD decompression model based on the non-equilibrium phase transition that we have established previously [30] was further developed and used to investigate the effects of the initial flow velocity of CO 2 on the decompression behaviour within the pipe and the near-field jet out of the rupture opening.…”