Denton, G.S.; (2009) CFD simulation of highly transient flows. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes the fundamental extension and extensive testing of a robust CFD model for predicting outflow following the failure of pressurised hydrocarbon pipelines. The main thrust of the study involves the extension of the basic outflow model to account for complex pipeline systems, improvements of the theoretical basis and numerical stability. The basic model, based on the numerical solution of conservation equations using the method of characteristics, incorporates a suitable equation of state to deal with pipelines containing pressurised multi-component hydrocarbon mixtures. It utilises the homogeneous equilibrium flow (HEM) assumption, where the constituent phases in a two-phase mixture are assumed to be at thermal and mechanical equilibrium. The first part of the study focuses on the development of an outflow model to simulate the failure of multi-segment pipelines incorporating valves and fittings passing through terrains of different inclinations. In the absence of real data, the numerical accuracy of the model is assessed based on the calculation of a mass conservation index. The results of a case study involving the comparison of the simulated outflow data based on the failure of a multi-segment pipeline as opposed to an equivalent single segment pipeline containing gas, liquid or two-phase inventories are used to highlight the impact of pipeline complexity on the simulated data. The development and extensive testing of two models, namely the Hybrid Model and the Modified Homogeneous Equilibrium Model (MHEM) each addressing a principal limitation of the HEM are presented next. The Hybrid Model deals with the failure of the HEM in predicting post-depressurisation outflow for inclined pipelines containing two-phase mixtures or liquids through its coupling with a hydraulic flow model. The MHEM on the other hand addresses the failure of the HEM to accurately predict the discharge rates of flashing/ two-phase fluids discharging through an orifice. Finally, the dilemma of the appropriate choice of the size and duration of the numerical discritisation steps expressed in terms of the Courant, Friedrichs and Lewy (CFL) criterion on the stability and computational workload of the pipeline failure model is investigated for different classes of hydrocarbon inventories. These include gas, liquid (flashing and incompressible) and two-phase mixtures.

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