Computational fluid-dynamic investigation of a centrifugal compressor with inlet guide vanes for supercritical carbon dioxide power systems

A centrifugal compressor for large-scale supercritical carbon dioxide applications (∼50 MW) is analyzed by means of RANS simulations. The purposely designed compressor includes adjustable inlet guide vanes, an open impeller, and a wedge-shaped vaned diffuser. The upstream total state is located at 78.70 bar and 305.15 K, close to the thermodynamic critical point, whereby non-ideal effects and two-phase flows are significant. To account for these effects, the computational model implements a homogeneous equilibrium model (using pressure and specific enthalpy as state variables) complemented with a state-of-the-art equation of state explicit in the Helmholtz free energy and specific correlations for transport properties. It is found that the compressor can provide the entire pressure ratio required by the cycle with high efficiency (87.9%, excluding parasitic losses). Three inlet vane rotations are considered: −10deg, 20deg, and 40deg. The preswirl has a limited effect on efficiency, which is reduced by 0.3% points away from choked conditions with vane rotation of 40deg. Nonetheless, the preswirl affects the extent of the two-phase region at the impeller intake, which has a detrimental impact on the compressor flexibility by setting an early choking.