Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical CO₂

This paper presents a design strategy for very low flow coefficient multistage compressors operating with supercritical CO₂ for carbon capture and sequestration (CCS) and enhanced oil recovery (EOR). At flow coefficients less than 0.01, the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance. The impeller exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale reinjection compressor currently in service. Three-dimensional, steady, supercritical CO₂ computational fluid dynamics (CFD) simulations of the full stage with leakage flows are carried out with the National Institute of Standards and Technology (NIST) real gas model. The design study suggests that a nondimensional impeller exit width parameter b₂* =(b₂ /R)φ of six yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the nondimensional impeller exit width parameter b₂* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at impeller exit. Combined with the twoorders- of-magnitude higher Reynolds number for supercritical CO₂ , the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin

An Investigation of Real Gas Effects in Supercritical CO₂ Centrifugal Compressors

This paper presents a comprehensive assessment of real gas effects on the performance and matching of centrifugal compressors operating in supercritical CO₂. The analytical framework combines first principles based modeling with targeted numerical simulations to characterize the internal flow behavior of supercritical fluids with implications for radial turbomachinery design and analysis. Trends in gas dynamic behavior, not observed for ideal fluids, are investigated using influence coefficients for compressible channel flow derived for real gas. The variation in the properties of CO₂ and the expansion through the vapor-pressure curve due to local flow acceleration are identified as possible mechanisms for performance and operability issues observed near the critical point. The performance of a centrifugal compressor stage is assessed at different thermodynamic conditions relative to the critical point using computational fluid dynamics (CFD) calculations. The results indicate a reduction of 9% in the choke margin of the stage compared to its performance at ideal gas conditions due to variations in real gas properties. Compressor stage matching is also impacted by real gas effects as the excursion in corrected mass flow per unit area from inlet to outlet increases by 5%. Investigation of the flow field near the impeller leading edge at high flow coefficients shows that local flow acceleration causes the thermodynamic conditions to reach the vapor-pressure curve. The significance of two-phase flow effects is determined through a nondimensional parameter that relates the time required for liquid droplet formation to the residence time of the flow under saturation conditions. Applying this criterion to the candidate compressor stage shows that condensation is not a concern at the investigated operating conditions. In the immediate vicinity of the critical point however, this effect is expected to become more prominent. While the focus of this analysis is on supercritical CO₂ compressors for carbon capture and sequestration (CCS), the methodology is directly applicable to other nonconventional fluids and applications

An Investigation of Nonlinear Flow Oscillations in a High-Pressure Centrifugal Pump

High-pressure multistage pumps and their coupled piping systems, typically used in the process and power generation industry, can experience dangerous system-level instabilities. This can occur at flow coefficients well away from the surge limit and in the absence of cavitation. Such a pumping system and a related new kind of instability are the focus of this paper. A system-wide instability was observed at 0.05 times rotor frequency for flow coefficients near maximum head rise but at negative slope, thus on the stable side of the head rise characteristic. A previous study based on system-level experiments concluded that this instability differs from classical surge, cavitation surge, rotating stall, and rotating cavitation, but the underlying mechanism and necessary flow conditions remain unknown. This paper investigates the root cause of the system-wide pump instability, employing a systematic analysis of the impact of geometry changes on pump stability and performance. It is found that the upstream influence of the unsteady flow separation in the return channel leads to a time-varying incidence angle change on the volute tongue which causes periodic ingestion of low-stagnation pressure fluid into the diffuser passages. This sets up a limit cycle, promoting the system-wide instability. With the instability mechanism determined, the pump is redesigned to remove the flow separation while maintaining performance at design conditions. Unsteady numerical simulations demonstrate improved efficiency and pressure recovery at low flow coefficients. A time accurate calculation also indicates stable operation at all relevant flow conditions. The paper resolves a long-standing pump stability problem and provides design guidelines for reliable and improved performance, important to the chemical processing and power generation industry

Aeronautical Engineering: A special bibliography with indexes, supplement 51

This bibliography lists 206 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System in November 1974

Centrifugal Compressor Surge in Closed Loop Systems: Initial Modelling and Comparison With Experiments

Heat pumps are expected to play a primary role in electrification of thermal users in the residential and industrial sectors. Dynamic compressors are widely used in large size heat pumps, thanks to their industrial replicability, compact size, affordable costs, and good performance in terms of efficiency and low acoustic emissions. The instability, which may occur in a compressor installed in closed loop cycle such as in a heat pump, is quite different from the classic open loop configuration involving dynamic compressors. This is mainly due to the complexity of the compressor instability mechanism coupled to a vapor compression system connected with two- phase heat exchangers, having different thermal and fluid dynamic capacitance properties, under a physical feedback loop. The aim of this paper is to investigate the behavior of a dynamic compressor installed in an innovative heat pump prototype, of laboratory scale, under stable and unstable conditions. A preliminary simple dynamic model of the compression system consisting of a radial compressor, condenser and evaporator, is developed to represent the heat pump compression system, aiming at its time-dependent representation during compressor instable behavior. The evaporator and condenser are modelled using empirical correlations representing the heat exchange and phase change phenomena and including the thermal capacitances due to refrigerant mass and heat exchanger pipes. The preliminary validation of the dynamic model results is done through a dedicated experimental campaign, under different operating conditions. Results show the complexity of the interaction between the centrifugal compressor and the heat pump loop, discerning the different contributions to the time-dependent response of the system. Future steps will encompass a more detailed modelling of the heat pump loop and the use of updated field measurements, including liquid level meters in the heat exchangers

Turbomachinery Overview for Supercritical CO2 Power Cycles

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