A One-Dimensional Design Model for High-Speed Centrifugal Compressors with Wet-Outlet Assessment

Abstract

: High-speed centrifugal compressors are promising candidates for compact and efficient compression in high-temperature thermal-fluid processes, but their feasibility depends strongly on the interaction between refrigerant thermodynamic properties, operating conditions, impeller aerodynamics, and outlet phase behavior. Conventional preliminary assessments often focus on cycle-level performance or simplified compressor sizing, while the transition between dry and wet outlet operation is not explicitly treated. This study develops a one-dimensional design model for high-speed centrifugal compressors with real-fluid thermodynamic properties and wet-outlet assessment. The objective is to provide an early-stage framework that can distinguish between intrinsic thermodynamic limitations and design-dependent aerodynamic constraints. The model combines real-fluid property evaluation, compressor state calculation, phase detection, velocity-triangle analysis, slip-factor-based blade-angle estimation, inlet and outlet geometry calculation, and design-space classification. The model is demonstrated for a brewery hot-water process using three hydrocarbon working fluids: R-601/n-pentane, R-601a/isopentane, and R-600/n-butane. For each refrigerant, 3300 combinations of inlet flow coefficient, outlet flow coefficient, work coefficient, and blade number are evaluated. The results show that all three refrigerants provide broad preliminary aerodynamic design spaces. The acceptable design fractions are 0.818 for R-601, 0.776 for R-601a, and 0.790 for R-600. A common representative design region is identified at an inlet flow coefficient of 0.16, outlet flow coefficient of 0.07, work coefficient of 0.85, and 20 blades. The corresponding impeller outlet diameters are 0.338 m, 0.330 m, and 0.266 m for R-601, R-601a, and R-600, respectively. The inlet relative Mach numbers remain below unity for all representative designs. However, R-601 and R-601a enter wet-outlet operation, with vapor qualities of 0.877 and 0.852, respectively, whereas R-600 remains single-phase at the outlet. The study shows that compressor feasibility cannot be assessed from thermodynamic performance or geometry alone. Wet-outlet operation is identified as an intrinsic thermodynamic constraint for the hydrocarbons within the present fixed thermodynamic-state model and under the specified boundary conditions, while inlet Mach number, inlet diameter ratio, blade angle, and blade-height ratio are design-dependent aerodynamic quantities. The proposed model therefore provides a structured preliminary tool for connecting real-fluid thermodynamics, compressor geometry, and wet-outlet risk before detailed loss modelling, diffuser design, and full compressor-unit co-design.