Numerical Simulations of CO2 Compressors at Near-Critical and Sub-Critical Inlet Conditions

An advanced real-fluid numerical framework for transcritical CO2 spanning both the supercritical and sub-critical regime near the critical point is presented. The numerical algorithm is specifically designed to faithfully model sharp variations of thermodynamic derivatives near the critical point. Numerical results for compressor performance in the Sandia test loop [1] at near critical inlet conditions have been presented over a range of flow rates from the choke point to the stall line. The analysis identifies the shift in the flow losses from the impeller to the diffuser as the flow rate increases from the stall to choke limit. The simulations have been compared with both the Sandia performance curves as well as raw data from Barber Nichols Inc. (BNI); the results compare well with the raw data and provide good qualitative comparisons with Sandia’s performance curves [1]. The numerical framework has also been extended to sub-critical conditions by solving for separate transport equations for each phase. Methodology for NIST table lookup at sub-critical conditions to identify liquid and vapor properties has been developed. Phase change is triggered when local conditions go sub-critical and do not require the phase to be specified a priori. Calculations at both near critical inlet temperatures as well as a two-phase inlet condition at lower temperatures were modeled. Significant difference in the phase change characteristics at these two conditions have been identified and discussed in the paper.