Highly efficient and concurrent flexible operation are heavy demands on today's centrifugal compressor units. Diffuser end wall contouring is a measure to delay the incipience of instability and therefore to extend the compressor's operating range while maintaining efficient performance. In the presented paper, a hub-side wall contouring, applied in the vaneless space upstream the diffuser vane's leading edge and within the diffuser passage of a state-of-the-art centrifugal compressor stage with an open impeller, is examined. Computational fluid dynamics computations are performed for both a baseline diffuser design with parallel channel walls and the hub-side wall contoured diffuser design. Comparisons of characteristic and diffuser stability decisive flow variables are made in perpendicular sections along an extrapolated camber line of the diffuser vane for full span, near shroud and near hub wall. In operating points near the stability limit at two different stage Mach numbers, the stabilizing effect of the hub-side wall contouring on the diffuser flow is clearly shown. In a scale-model test rig, experimental data including pneumatic five-hole probe data for a full diffuser blade-to-blade passage, static wall pressures at various planes as well as total temperatures were measured. The experimental data are utilized for the validation of the presented numerical calculations. The flow stabilizing effect of the hub-wall contouring is clearly visible in the measurements, which showed 8% range extension at highest stage Mach number Mu₂ = 1.16 and a range extension of 2% at design stage Mach number Mu₂ = 1.

State-of-the-art centrifugal compressor units utilised in pipeline and in energy storage applications face the challenge of flexible and highly efficient operation. Geometric contouring on the hub side near a vaned diffuser affects the flow in a way which increases operational flexibility by delaying the incipience of instability and thereby increases compressor operating range. In the present paper, a hub-side wall contouring is applied within the vaneless space and the vaned diffuser of an open impeller centrifugal compressor stage. The performance characteristic of the novel hub contouring is evaluated in a scale-model test rig and compared against a baseline design. A stable operating range increase of 8% is achieved for the contoured design at ${\mathrm{Mu}}_{\mathbf{2}}=$ 1.16. 5-hole probe measurements covering a complete diffuser blade-to-blade passage are performed upstream the diffuser and compared both against CFD simulations and against the measurements of the baseline design for an operating point near the stability limit.

Unshrouded industrial centrifugal compressor impellers operate at high rotational speeds and volume flow rates. Under such conditions, impeller blade excitation is dominated by high frequency interaction with stationary parts, i.e., vaned diffusers or inlet guide vanes. In a previous study conducted on two full compression units of the original equipment manufacturer (OEM), the authors identified, characterized, and quantified resonant blade vibration caused by the interaction of the impeller blades with rotating stall cells during severe off-design conditions. This caused significant dynamic stress in the blades. In a follow-up study, this phenomenon was reproduced successfully experimentally under representative off-design conditions in a downscaled test rig and numerically with unsteady computational fluid dynamics (CFD) and structural mechanical finite element method (FEM) analysis. The gained knowledge was translated into a new diffuser design philosophy, based on sectorwise circumferential variation of the leading edge angle. This paper presents the patented philosophy, which is experimentally verified on the same test rig configuration in terms of flow path geometry and measurement equipment that was used in the mentioned prior study to assess resonant blade interaction. The results confirm the design aims: rotating stall onset was delayed without affecting the aerodynamic performance of the stage. Resonant blade interaction with rotating stall observed in the baseline diffuser could not be avoided with the two new diffuser designs. However, with the two new diffusers, the induced mechanical stresses in the impeller and the excitability were reduced by up to 12%.

Highly efficient and concurrent flexible operation are heavy demands on state-of-the-art centrifugal compressor units. Diffuser end wall contouring is a measure to delay the incipience of instability and therefore to extend the compressor’s operating range while maintaining efficient performance. In the presented paper, a hub-side wall contouring, applied in the vaneless space