Gas cooled quenching and many other applications require high-speed uniform-velocity flows, with minimal pressure drop. The flow ducting geometry is often rather complex, with flow splitting, 90-180 • bends, and circular-to-rectangular cross-section transition ducts (the latter are used, for example, between the circular blower duct and the rectangular quenching baskets). Similar situations exist in forced convection furnaces. To provide design guidance in the choice of such ducts, and focusing primarily on circular-to-rectangular transition ducts, the flow was modelled and computed, and the results were successfully validated. Sensitivity of the velocity uniformity and pressure drop with respect to the primary geometric parameters, pressure, and Reynolds numbers was examined in the range (1.3)10 5 # Re # (7.8)10 5 , with an ultimate objective to produce optimal designs. For a length-to-diameter ratio AL = L/D < 1.0, flow nonuniformity at the exit plane and pressure drop are increased by 33 and 83%, respectively, as the aspect ratio (rectangular duct width-to-height) AR decreases from 4 to 1. Increasing AR beyond 1.5 leads to linearly increasing nonuniformity and pressure drops. A diverging-contracting duct has proven to lead to lesser nonuniformity, while it did not influence the pressure drop. Increasing the inlet pressure from 1 to 20 bar led to a decrease in flow distortion by 11% at the duct exit planes. At atmospheric pressure, increasing the Reynolds number from (1.3)10 5 to (7.8)10 5 increased distortion by 8%. Some preliminary design recommendations for circular-to-rectangular duct transitions are to try and keep AL > 1 and AR < 1.5.
CFD analysis of a high pressure 2” pipe test loop with water-gas flow was undertaken using three different solvers. Multiphase flow induced forces were predicted on the bends at a range of operating pressures between 10 and 80 barg and compared with forces reconstructed from vibration measurements.
Overall the three different CFD solvers predicted consistent results. The fluid forces predicted on the bends of the double U-loop test rig have a good range of values compared to the test reconstructed forces. The forces predicted at low pressure were in line with the experimental reconstructed values, whilst at high pressure all three CFD solvers predicted higher forces. The trend of the forces reducing with increased operating pressure, evident in test, was matched by one of the CFD methods, but less well by the other two. At low operating pressure the forces are dominated by the momentum of the liquid in the multiphase flow, whilst at high pressure the pressure fluctuations and turbulent effects will be more important. All three CFD solvers use VOF methods and above about 40 barg it is possible that they struggle to fully resolve the flow behaviour, which will be more influenced by bubble and droplet entrainment and turbulence.
Multiphase flow can induce high amplitude vibrations in piping systems, potentially leading to fatigue failures. CFD modelling offers a potentially powerful tool to provide the flow induced forces required for assessing and diagnosing multiphase flow induced pipework vibration problems in hydrocarbon production systems.
The scale of large finite element (FE) models may nowadays easily exceed 10 7 or even 10 8 degrees of freedom (DOF), leading to excessive calculation times when performing transient simulations. Such long simulation times hinder effective structural or thermal design and optimization and make any engineering insight into a problem difficult. The Krylov subspace-based model order reduction (MOR) is a reduction method based on projection of a discretized model onto a lower dimension subspace. The paper presents a methodology based on this method in the context of thermal transient simulation of a large scale subsea equipment FE model. The finite element model mesh size exceeds 30 × 10 6 DOFs. The problem has nonzero initial conditions (ICs) and has to be transformed into a problem with zero ICs in order to apply the Krylov based MOR. Coupling the Krylov based MOR models employs a novel technique involving coupling through their surface interfaces. The approach is compared with the solution obtained using a full FE simulation which takes about 7 days to solve with a fine time step. The results are compared using an error norm which computes maximum absolute difference of temperature fields over time taking the full FE simulation with the fine time step as a reference. The study shows that applying the proposed method using Krylov MOR for performing thermal transient simulations is valid and leads to substantial reduction of the computational time.
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