Jie, Hongen (2013) CFD modelling of transient two-phase flows for high pressure pipeline decompression. (PhD thesis), Kingston University, .
Abstract
A CFD model has been developed with the aim to predict transient two-phase flows for pipeline decompression. The arbitrary Lagrangian-Eulerian method was introduced to solve separately the convection terms from the other terms in a sub-cycled explicit manner using a sub-time step that is only a fraction of the main computational timestep, which can significantly simplify solution procedures and improve computational efficiency. The homogeneous equilibrium model (HEM) and homogeneous relaxation model (HRM) were employed to treat multi-component two phases as a continuous mixture based on the basic assumption of homogeneous flow. HEM assumes that the two phases are not only in thermodynamic equilibrium but also in mechanical equilibrium, namely the two phases share identical velocity, temperature and pressure and the rate of phase change is rapidly enough so that equilibrium is reached. However, the rate at which the phase change took place depends on interphase heat transfer and non-equilibrium effects. For the rapid pipeline decompression, the rates of interphase heat transfer are a limiting factor for phase change. In order to examine the non-equilibrium effect, the rate equation is introduced to evaluate the non-equilibrium generation of vapour or liquid phase by an approach of relaxation. HRM is proposed to deal with two-phase flows involved during pipeline decompression, and is extended for the multi-component dense fluid. The use of CFD allows the effect of pipe wall heat transfer and friction to be quantified. The wall heat transfer is considered through the implementation of a conjugate heat transfer model while the wall friction is computed using established empirical correlations. The Peng-Robinson-Stryjek- Vera equation of state (EOS), which is capable of predicting the real gas thermodynamic behaviour of mixture, has been implemented in addition to the Peng-Robinson EOS and Span-Wagner EOS, and the latter is used as a reference specifically for pure CO2. GERG-2004 is also employed for C02-rich mixture. Additionally, the liquid-vapour phase equilibrium of a multi-component two-phase mixture is determined by flash calculation. The current code with HEM is validated for natural gas, rich gas, liquefied petroleum gas, gaseous and dense phase C02 decompression against the available data of shock tube test. The decompression curve, which describes the propagation of the expansion wave immediately following a rupture, is obtained to be treated as the key input to the Battelle two-curve method which often used to determine the toughness required to arrest a running ductile fracture in a pipeline. Furthermore, the predictions of pressure and temperature time traces are compared with the results of British Gas shock tube tests, Botros's rich gas experiments, Isle of Grain full-scale experiments. The predictions show reasonably good agreement with the experimental data. Finally, C02 shock tube decompression is examined with the current model. Gaseous and dense phase C02 and C02-rich mixture shock tube tests are predicted. Predictions are compared with the available experimental data. The results show good agreement for C02 tests. The decompression behaviours of high pressure CO2 pipeline are studied and discussed. The effect of initial conditions and impurities on the decompression behaviour is investigated. Additionally, the effects of friction and heat transfer are evaluated for the gaseous and dense tests. Lastly, the non-equilibrium effect on the decompression behaviour is also evaluated for dense tests by employing the approach of HRM.
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