Rotating stall is an unsteady flow phenomenon that appears in both axial and centrifugal compressors. It is detrimental to the performance of the compressor, significantly narrowing its operating range. Numerical modeling of this phenomenon has been a major area of investigation for axial compressors with some success. While stall occurs less often in centrifugal compressors than axial ones, it can be much harder to predict. Some preventive measures are known but are mostly rules of thumb developed through experimental experience. This work focuses on the detection of impeller rotating stall using computational fluid dynamics. A compressor was chosen that has demonstrated rotating stall instabilities with different diffuser lengths and return vanes. Unsteady numerical simulations were performed on full 360 models of this geometry. The transient simulations were conducted using distributed processing on high-performance servers. The objective was to determine the accuracy in which rotating stall can be captured in simulations. For this purpose, simulation results were compared to experimental results for the same compressor and show a good correlation between the experimental and numerical tests.

The objective of this paper is to investigate the performance of composite material axial water turbines in a free stream using Fluent, a Finite Volume based commercial CFD package. Based on three dimensional numerical flow analysis and fluid-structure interaction, the flow characteristics of water turbines including a nozzle, impeller and diffuser are predicted. The extracted power coefficient is calculated for water turbines of different tip speed ratios in a free stream of water with inlet velocity of 2.5m/s. The extracted powers of one single turbine unit and an array with ten turbine units are analyzed and compared for different rotating speeds and water inlet velocities. The calculated results will provide a fundamental understanding of the impeller as a water turbine, and this design method is used to shorten the design process and improve the water turbine’s performance.

Rotating stall is an unsteady flow phenomenon that appears in both axial and centrifugal compressors. It is detrimental to the performance of the compressor, significantly narrowing its operating range. Numerical modeling of this phenomenon has been a major area of investigation for axial compressors with some success. While stall occurs less often in centrifugal compressors than axial ones, it can be much harder to predict. Some preventive measures are known but are mostly rules of thumb developed through experimental experience. This work focuses on the prediction of the onset flow rate of impeller rotating stall using computational fluid dynamics. Two different types of unsteady numerical simulations were performed on full 360 models of a compressor that exhibited rotating stall. The transient simulations were conducted using distributed processing on high-performance servers. Fast Fourier transform analysis was used to determine the mass-flow fluctuations in the impeller that can be associated to the phenomenon. The frequency analysis results were then used to determine the onset flow rate for impeller rotating stall. The simulation results and fast Fourier transform analysis were compared to experimental results for the same compressor and show a good correlation between the experimental and numerical tests.

As centrifugal compressors are pushed to operate at higher pressures and higher power levels, destabilizing gas forces often increase the challenge of designing a stable rotordynamic system. While technical innovations like damper seals, swirl brakes, and damper bearings that help stabilize compressors are numerous, predicting the impact that these improvements will have on a specific system is somewhat of an art form. To this end, researchers are constantly improving the depth of knowledge on these features so that the impact of these improvements is well defined. In the current work, the authors experimentally measured the impact of different swirl brake/vane concepts on the flow characteristics of a centrifugal compressor shroud cavity. The eye seal configuration studied here is a tooth on rotor labyrinth eye seal. The different shroud swirl vane geometries considered consist of various castellated features, each having the intent to reduce swirl velocity in the shroud cavity prior to entering the seal. The purpose of the testing was to determine whether a significant reduction in swirl velocity entering or exiting the seal could be measured with the different anti-swirl vane profiles over a conventional shroud cavity that was tested with the same setup. The metrics that determine the effectiveness of the swirl brake were based on upstream and downstream measurements of swirl velocity using pitot-probes at different depths in the shroud cavity, and measurements of seal exit angle and velocity using a traversing cobra-probe. The test data presented herein show definitively that the different swirl brake designs, including a slotted seal, a long vane, and a short vane, have a major impact on swirl velocities relative to the conventional shroud design. The most effective at reducing swirl entering/leaving the seal is the slotted seal, while the concepts employing shroud vanes were more effective at reducing swirl in the shroud cavity.

As centrifugal compressors are pushed to operate at higher pressures and higher power levels, destabilizing gas forces often increase the challenge of designing a stable rotordynamic system. While technical innovations like damper seals, swirl brakes, and damper bearings that help stabilize compressors are numerous, predicting the impact that these improvements will have on a specific system is somewhat of an art form. To this end, researchers are constantly improving the depth of knowledge on these features so that the impact of these improvements is well defined. In the current work, the authors experimentally measured the impact of different swirl brake/vane concepts on the flow characteristics of a centrifugal compressor shroud cavity. The eye seal configuration studied here is a tooth on rotor labyrinth eye seal.