CFD modelling of flow-induced vibration under multiphase flow regimes.

Internal multiphase flow-induced vibration (MFIV) in pipe bends poses serious problems in oil and gas, nuclear and chemical flow systems. The problems include: high amplitude displacement of the pipe structure due to resonance; fatigue failure due to excessive cyclic stress, induced by fluctuating forces; and structural wear, due to the relative motion of the pipe and its support. Current industry guidelines are based on single phase flows, while the few existing MFIV models in literature are based on small scale laboratory experiments, which do not completely address the complexities in multiphase flows, or the differing multiphase flow mechanisms between small and large pipes. Therefore, numerical simulations of two-phase flow induced fluctuating forces, stresses, displacements and natural frequencies at 900 bends have been carried out, in order to investigate the characteristics of MFIV in pipes of 0.0525m, 0.1016m and 0.2032m internal diameters (I.D.). An integrated high-fidelity CFD and FEA-based numerical-analytical modelling framework was applied, to predict the defining characteristics of MFIV in the pipes. The CFD simulations of thirty-five cases of slug, cap bubbly and churn turbulent flow-induced fluctuations at the bends were carried out using the volume of fluid (VOF) model for the two-phase flows, and th

Investigation of the effects of pipe diameter of internal multiphase flow on pipe elbow vibration and resonance.

Computational fluid dynamics modelling of internal two-phase flow induced transient forces at 90° elbows have been carried out to evaluate the effect of pipe diameter on the characteristics of multiphase flow induced vibration. Simulations of two-phase flows of slug, cap bubbly and churn induced vibration at a pipe elbow were carried out using the volume of fluid model for the two-phase flows and the k – ε model for turbulence. Modal analysis has been carried out to evaluate the risk of resonance. Results were compared across three geometrically similar pipes of different diameters. Simulation results showed that the behaviours of the flow induced forces at the pipe elbow as a function of gas velocity for internal diameters of 0.0525 and 0.2032 m are similar. However, the multiphase flow induced force characteristics are different in the 0.1016 m diameter (intermediate) pipe. It can be attributed to the transition behaviour of gas–liquid two-phase flows caused by Taylor instability in an intermediate sized pipe. The predicted root-mean-square flow induced forces as a function of Weber number were correlated with an existing empirical correlation for a wider range of pipe sizes and gas volume fractions between 40% and 80%. Furthermore, the pipe natural frequencies increase with the increase of gas volume fraction in smaller pipes and the resonance risk increases with the increase of pipe diameter