An Investigation of Real Gas Effects in Supercritical CO2 Centrifugal Compressors

Abstract

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.

The influence of real gas effects on the performance of supercritical CO 2 dry gas seals

“…For ideal gas, the compressibility factor is close to unity [22,27]. compared in term of opening force and outflow leakage rate.…”

Evaluation of Property Methods for Modeling Direct-Supercritical CO2 Power Cycles

“…(5) Given its superior performance in predicting saturated CO 2 properties and saturated water content in binary CO 2 :H 2 O mixtures, LK-PLOCK is the recommended property method if REFPROP cannot be used due to species compatibility or computational time constraints. LK-PLOCK has also been recommended for CO 2 compression studies [10,13]. PC-SAFT is the next best option after LK-PLOCK, though it tends to underpredict saturated water content in sCO 2 , whereas LK-PLOCK tends to overpredict this quantity.…”

Numerical Investigation on Aerodynamic Performance of SCO2 and Air Radial-Inflow Turbines with Different Solidity Structures

“…Additionally, the high density and low viscosity of SCO 2 can result in the high efficiency and compact mechanical structure of turbines and compressors in the power system [9][10][11]. The rapid changes of density near the critical point can reduce the compressor work and lead to a higher heat transfer coefficient in heat regenerators and precoolers [12]. Finally, there is little phase change in a SCO 2 power cycle.…”