Three-dimensional shape optimization of a centrifugal compressor stage for supercritical carbon dioxide power systems

The design of a centrifugal compressor for supercritical carbon dioxide power cycle must consider non-ideal gas effects and the possible occurrence of two-phase flows. Shape optimization techniques, combined with computational fluid-dynamic (CFD) simulations, can produce optimized designs while inherently coping with such peculiar flow characteristics. This study presents a three-dimensional shape optimization of a compressor stage composed of the impeller and the vaneless diffuser, whose compression starts close to the critical point. Impeller blade angle distributions and meridional channel are parameterized with Bezier control points, enabling local shape control within the optimization routine. The pinch of the vaneless diffuser is also optimized. The experimentally validated CFD solver considers both non-ideal effects and two-phase homogeneous flows, assuming thermodynamic equilibrium and a barotropic fluid. The constrained optimization problem is addressed using genetic algorithms. To mitigate computational costs, Kriging surrogates for the objective function and constraints are trained using a limited number of CFD results. The optimized geometry shows an appreciable efficiency increase (1.1 percentage points) while delivering the design pressure ratio. Although performing better at the design condition, the operating range of the compressor is altered by the optimization. Future optimizations that include both design and off-design operating points in the definition of the objective function and constraints may mitigate this problem.