It is well understood that a volute, depending on its operating point, acts either as a nozzle or diffuser. However, the resulting static pressure distortion at diffuser exit is rather considered as side effect and has rarely been investigated systematically in detail. Evidence whether this distortion is amplified or alleviated towards impeller exit is even contradictory in literature. In this work a thorough investigation of aerodynamic volute-impeller interaction is presented. First, a 1D-analysis is carried out in order to understand the fundamental relation between volute matching and pressure distortion. Then, one large external volute, coupled with different impellers, is investigated by means of CFD. These configurations feature different diffuser ratios and blade exit angles, while identical matching ensures comparable conditions. The results reveal that pressure distortion is directly related to the volute’s pressure recovery coefficient. Consequently, at diffuser exit, a local pressure recovery coefficient around circumference can be defined, which is widely independent from the impeller but rather depends on volute geometry and matching. It is this local pressure recovery factor that defines the static pressure distortion at diffuser exit, together with dynamic pressure. In a next step it is analysed, how the resulting pressure distortion evolves towards the impeller. Therefore, stand-alone CFD calculations of impellers around the entire circumference are performed, applying defined pressure distortions at diffuser exit and varying operational conditions carefully. In doing so, fundamental understanding of when distortion is amplified or alleviated towards the impeller is gained.

This paper deals with the numerical and theoretical investigations of the effect of geometrical dimensions and one-dimensional (1D)-design parameters on the impeller pressure slope of a transonic centrifugal compressor stage for industrial process application. A database being generated during the multi-objective and multipoint design process of a high flow coefficient impeller, comprising 545 computational fluid dynamics (CFD) designs is investigated in off-design and design conditions by means of Reynolds-averaged Navier–Stokes (RANS) simulation of an impeller with vaneless diffuser. For high flow coefficients of 0.16 < ϕdes < 0.18, the CFD-setup has been validated against measurement data regarding stage and impeller performance taken from MAN test rig experimental data for a centrifugal compressor stage of similar flow coefficient. This paper aims at answering the question how classical design parameter, such as the impeller blade angle distribution, impeller suction diameter, and camber line length affect the local and total relative diffusion and pressure slope toward impeller stall operation. A second-order analysis of the CFD database is performed by cross-correlating the CFD data with results from impeller two-zone 1D modeling and a rapid loading calculation process by Stanitz and Prian. The statistical covariance of first-order 1D-analysis parameters such as the mixing loss of the impeller secondary flow, the slip factor, impeller flow incidence is analyzed, thereby showing strong correlation with the design and off-design point efficiency and pressure slope. Finally, guide lines are derived in order to achieve either optimized design point efficiency or maximum negative pressure slope characteristics toward impeller stall operation.

Increases on mass flow rates of modern radial process compressors result on larger machine components. In particular, the dimensions of the outlet volutes increase strongly, resulting in disproportionately large machines whose technical feasibility is restricted due to technological and economical reasons. A resulting aim is to design modern radial compressors much more compact, while improving the efficiency and the pressure ratio. Therefore, the present experimental investigation addresses the compressor behaviour for reduced dimensions of rectangular volutes. Furthermore, the experimental setups are numerically modelled and different operating points are simulated with a commercial CFD-Code. A rectangular, external reference volute is equipped with differently shaped blockage-inlays and the global compressor parameters are measured for all variants. Additionally, the pressure and velocity distributions of the local flow field are determined experimentally for varying mass flow ratios at different circumferentially distributed volute layers. The decrease of the volute cross-section results in a reduction of the compressor map width especially at high mass flow rates. Recommendations are given for designing compact volutes of large radial compressors.

The present paper deals with the numerical and theoretical investigations of the effect of geometrical dimensions and 1D-design parameters on the impeller pressure slope of a transonic centrifugal compressor stage for industrial process application. A database being generated during the multi-objective and multi-point design process of a high flow coefficient impeller, comprising 545 CFD (Computational Fluid Dynamics) designs is investigated in off-design and design conditions by means of RANS (Reynolds Averaged Navier Stokes) simulation of an impeller with vaneless diffuser. For high flow coefficients of 0.16 < phi < 0.18, the CFD-setup has been validated against measurement data regarding stage and impeller performance taken from MAN test rig experimental data for a centrifugal compressor stage of similar flow coefficient. The paper aims at answering the question how classical design parameter, such as the impeller blade angle distribution, impeller suction diameter and camber line length affect the local and total relative diffusion and pressure slope towards impeller stall operation. A second order analysis of the CFD database is performed by cross-correlating the CFD data with results from impeller two-zone 1D modelling and a rapid loading calculation process by Stanitz and Prian. The statistical covariance of first order 1D-analysis parameters such as the mixing loss of the impeller secondary flow, the slip factor, impeller flow incidence is analyzed, thereby showing strong correlation with the design and off-design point efficiency and pressure slope. Finally, guide lines are derived in order to achieve either optimized design point efficiency or maximum negative pressure slope characteristics towards impeller stall operation.

Volute induced static pressure distortion may degrade impeller performance. In highpressure applications, it can also severely affect mechanical operation. It is therefore worthwhile, to know the magnitude of pressure distortion at impeller exit. The authors present a detailed study of aerodynamic impeller-volute interaction. Firstly, CFD simulations of a volute stage are analysed: The distortion of radial velocity is a consequence of the volute's pressure field and remains almost constant throughout the vaneless diffuser. In contrast, at impeller exit, local discharge pressure determines tangential velocity, whereas towards diffuser exit, it is affected by the volute's pressure field. Aerodynamic blockage within the diffuser reduces static pressure at diffuser exit and hence mitigates absolute distortion. With these findings, a new 1D method for predicting the static pressure distortion at impeller exit is derived. Testing the model against CFD-results of further volute stages yields very good agreement at various operating conditions.