High-dimensional CFD optimization of a low-flow coefficient S–CO2 centrifugal compressor for enhanced oil recovery systems

Abstract

The design of low-flow coefficient (∼0.01) centrifugal compressors with supercritical CO2 as working fluid is still a challenge for engineers due to its increased friction losses at the impeller. However, the reinjection pressure required for Enhanced Oil Recovery (EOR) systems is achieved by compression trains with stages of high-Pressure Ratios (PR > 3) which can only be obtained by lowering the flow coefficient of the equipment. The carbon dioxide mitigation, due to the reinjection process, also increases oil productivity and extraction lifetime. A four-staged compression system was considered and the preliminary geometry of its last stage was considered herein after a 1D optimization that decreased the total required power of the system. In order to further increase the systems' performance, a CFD model was developed and submitted to Sensitivity Analysis (SA) and parametric optimization procedure, considering polar angles, meridional profile and vaneless diffuser passage (25 variables). The assessment of the sequential SA using, Morris' screening method Design of Experiment (DoE) and SS-ANOVA for variable ranking and response surface training, has exposed the method's limitation in recognizing interaction between variables since low-quality Response Surfaces (RS) were trained. However, the Incremental Space Filler (ISF) sampling has complemented the sample space screening, guarantying adequate RS at a low computational cost. This indirect optimization strategy that increased the equipment's polytropic efficiency by 1.19%, diminishing total entropy generation by 8.5% can deliver important cost reductions to the operation of EOR compression systems. The ‘entropy-guided’ phenomenology analysis strategy combined with SA results, identified that the narrowing of the vaneless diffuser has extinguished the recirculation present in the original geometry's impeller/diffuser interface region, which was the largest difference in the entropy histogram. Moreover, the enlargement of the impeller's meridional profile has smoothed the fluid flow change of direction (from axial to radial) and displaced the swirl structures that restricted the fluid flow in the main passage.