Kenny Krogh Nielsen

Experimental Results and Computational Fluid Dynamics Simulations of Labyrinth and Pocket Damper Seals for Wet Gas Compression

The most recent development in centrifugal compressor technology is toward wet gas operating conditions. This means the centrifugal compressor has to manage a liquid phase which is varying between 0% and 3% liquid volume fraction (LVF) according to the most widely agreed definition. The centrifugal compressor operation is challenged by the liquid presence with respect to all the main aspects (e.g., thermodynamics, material selection, thrust load) and especially from a rotordynamic viewpoint. The main test results of a centrifugal compressor tested in a special wet gas loop (Bertoneri et al., 2014, “Development of Test Stand for Measuring Aerodynamic, Erosion, and Rotordynamic Performance of a Centrifugal Compressor Under Wet Gas Conditions,” ASME Paper No. GT2014-25349) show that wet gas compression (without an upstream separation) is a viable technology. In wet gas conditions, the rotordynamic behavior could be impacted by the liquid presence both from a critical speed viewpoint and stability-wise. Moreover, the major rotordynamic results from the previously mentioned test campaign (Vannini et al., 2014, “Centrifugal Compressor Rotordynamics in Wet Gas Conditions,” 43rd Turbomachinery Symposium, Houston) show that both vibrations when crossing the rotor first critical speed and stability (tested through a magnetic exciter) are not critically affected by the liquid phase. Additionally, it was found that the liquid may affect the vibration behavior by partially flooding the internal annular seals and causing a sort of forced excitation phenomenon. In order to better understand the wet gas test outcomes, the authors performed an extensive computational fluid dynamics (CFD) analysis simulating all the different types of balance piston annular seals used (namely, a tooth on stator (TOS) labyrinth seal and a pocket damper seal (PDS)). They were simulated in both steady-state and transient conditions and finally compared in terms of liquid management capability. CFD simulation after a proper tuning (especially in terms of LVF level) showed interesting results which are mostly consistent with the experimental outcome. The results also provide a physical explanation of the behavior of both seals, which was observed during testing.

Friction Factor Jump in Honeycomb Seals Explained by Flow-Acoustic Interactions

In a series of experiments over flat plate honeycomb seal surfaces with deep cavities, Ha and Childs [1] have reported, under certain conditions, sudden “jumps” in the friction factor, which possibly could seriously impair honeycomb seal rotordynamic performance. This phenomenon was never fully explained before. In this study, the results have been investigated both analytically and computationally in the light of the Strouhal number dependency, and it was found that the friction factor jump phenomenon is nearly uniquely correlated with the Strouhal number of the flow. This lends strong credence to the hypothesis that the friction factor jump is due to a flow-acoustic interaction. Indeed, the measured friction factors were found to jump at the exact Strouhal Numbers where the energy transfer rate from the flow mode to the acoustic mode were at a maximum. CFD direct numerical simulations at the configurations of friction factor jump also demonstrate a lock-in between the acoustic and vortex shedding frequencies.Copyright

Stability Analysis and Assessment of Rotor Trains Using Operational Modal Analysis

Stability of lateral rotordynamic modes is an essential consideration during design and acceptance testing of rotating machinery. Experimental modal analysis (EMA) can be used for assessment but is in practice very difficult in actual operating conditions, principally due to the challenges of quantifying the excitation force. Operational modal analysis (OMA) does not require measurement of excitation forces and therefore presents significant logistical advantages over EMA. The background and theoretical concepts of OMA are presented and its use is demonstrated on a 500 kW centrifugal compressor. Measurements performed during commissioning at first raised suspicion that the stability was marginal. However OMA was used to confirm that the first forward mode of the compressor was actually stable and the measurements were reconciled with the predicted behavior. The results show that the assessment of stability margins of rotating machinery can be measured with OMA using only proximity probe data acquired during normal operation.

Characterization and Erosion Modeling of a Nozzle-Based Inflow-Control Device

In the petroleum industry, water-and-gas breakthrough in hydrocarbon reservoirs is a common issue that eventually leads to uneconomic production. To extend the economic production lifetime, inflow-control devices (ICDs) are designed to delay the water-and-gas breakthrough. Because the lifetime of a hydrocarbon reservoir commonly exceeds 20 years and it is a harsh environment, the reliability of the ICDs is vital. With computational fluid dynamics (CFD), an inclined nozzlebased ICD is characterized in terms of the Reynolds number, discharge coefficient, and geometric variations. The analysis shows that especially the nozzle edges affect the ICD flow characteristics. To apply the results, an equation for the discharge coefficient is proposed. The Lagrangian particle approach is used to further investigate the ICD. This allows for erosion modeling by injecting sand particles into the system. By altering the geometry and modeling several scenarios while analyzing the erosion in the nozzles and at the nozzle edges, an optimized design for incompressible media is found. With a filleted design and an erosion-resistant material, the mean erosion rate in the nozzles may be reduced by a factor of more than 2,500.

Experimental Results and CFD Simulations of Labyrinth and Pocket Damper Seals for Wet Gas Compression

The most recent development in centrifugal compressor technology is towards wet gas operating conditions. This means the centrifugal compressor has to manage a liquid phase which is varying between 0 to 3% Liquid Volume Fraction (LVF) according to the most widely agreed definition. The centrifugal compressor operation is challenged by the liquid presence with respect to all the main aspects (e.g. thermodynamics, material selection, thrust load) and especially from a rotordynamic viewpoint. The main test results of a centrifugal compressor tested in a special wet gas loop [1] show that wet gas compression (without an upstream separation) is a viable technology. In wet gas conditions the rotordynamic behavior could be impacted by the liquid presence both from a critical speed viewpoint and stability wise. Moreover the major rotordynamic results from the previous mentioned test campaign [2] show that both vibrations when crossing the rotor first critical speed and stability (tested through a magnetic exciter) are not critically affected by the liquid phase. Additionally it was found that the liquid may affect the vibration behavior by partially flooding the internal annular seals and causing a sort of forced excitation phenomenon.In order to better understand the wet gas test outcomes, the authors performed an extensive CFD analysis simulating all the different types of balance piston annular seals used (namely a Tooth on Stator Labyrinth Seal and a Pocket Damper Seal). They were simulated in both steady state and transient conditions and finally compared in terms of liquid management capability.CFD simulation after a proper tuning (especially in terms of LVF level) showed interesting results which are mostly consistent with the experimental outcome. The results also provide a physical explanation of the behavior of both seals, which was observed during testing.© 2015 ASME

Numerical Simulation of Pipeline VIV for Steady and Unsteady Flow

The ability to predict Vortex Induced Vibrations (VIV) is important for the design and assessment of subsea pipelines lying on the sea-bed and susceptible to spans. Recent advances in CFD methods and the availability of powerful computational resources has made numerical simulation a viable option for VIV prediction. In this paper, an advanced meshing technique which is capable of handling moving boundaries, is used to discretise the fluid domain. This approach, in conjunction with a structural model, is employed to simulate the complex case of VIV in a pipeline in close proximity to the sea-bed. Simulations are carried out using 2-D CFD models for various configurations of sea bed proximity for flow regimes ranging from moderate to high Reynolds numbers and KC numbers in the range 10 to 40. The results obtained are benchmarked with published experimental data and good agreement is obtained. This study demonstrates the feasibility of using CFD models to obtain accurate VIV predictions for subsea pipelines. Future studies will examine more complex flow regimes including random waves and also consider 3D modelling.Copyright