A transonic centrifugal compressor was aerodynamically optimized by means of a numerical optimization process. The objectives were to increase the isentropic efficiency and to reduce the acoustic signature by decreasing the amplitude of pre-shock pressure waves at the inlet of the compressor. The optimization was performed at three operating points on the 100% speed line in order to maintain choke mass flow and surge margin. At the design point, the specific work input was kept equal. The baseline impeller was designed by using ruled surfaces due to requirements for flank milling. To investigate the benefits of arbitrary blade surfaces, the restrictions of ruled surfaces were abolished and fully three-dimensional (3D) blade profiles allowed. In total, therefore, 45 parameters were varied during the optimization. The combined geometric and aerodynamic analysis reveals that a forward swept leading edge (LE) and a concave suction side at the tip of the LE are effective design features for reducing the shock strength. Beyond that, the blade shape of the optimized compressor creates a favorable impeller outlet flow, which is the main reason why the performance of the vaneless diffuser improves. In total, a gain of 1.4% points in isentropic total-to-static efficiency, evaluated by computational fluid dynamics (CFD) at the exit plane of the vaneless diffuser, is achieved.
The subject of this paper is the investigation of unsteady flow inside a transonic centrifugal compressor stage with a pipe-diffuser by utilizing unsteady 3D Reynolds-averaged Navier–Stokes simulations (unsteady 3D URANS). The computational fluid dynamics (CFD) results obtained are compared with detailed experimental data gathered using various steady and unsteady measurement techniques. The basic phenomena and mechanisms of the complex and highly unsteady flow inside the compressor with a pipe-diffuser are presented and analyzed in detail.
The objective of the presented work is to investigate the influence of the tip clearance on the wake formation inside a radial impeller. The position and size of the wake region does not only depend on the clearance height, but also on the distribution of the clearance gap along the blade chord. In order to examine this influence, several calculations have been performed with a three dimensional Navier-Stokes flow solver on a centrifugal impeller, which was experimentally investigated in much detail at Aachen University. The original clearance gap was 0.7 mm at the leading edge and 0.48 mm at the trailing edge. These values were independently varied in the computations, such that different distributions of clearance heights could be chosen. The wake position of the smallest clearance height at the leading and trailing edge was closest to the pressure side. The calculations show, that a relatively large clearance height at the leading edge combined with a small height at the trailing edge move the wake further to the suction side, which corresponds very well with the experimental results. Reasons for that behavior are discussed in the paper.
Due to the increasing demand for higher efficiencies of centrifugal compressors, numerical optimization methods are becoming more and more relevant in the design process. To identify the beneficial features of a numerical optimized compressor design, this paper analyses the influence of arbitrary blade surfaces on the loss generation in a transonic centrifugal compressor. The paper therefore focuses on an analysis of the secondary flow development within the impeller blade passages. To do this, steady simulations were performed on both a baseline and an optimized blade design. Two distinct design features of the optimized compressor stage were identified, which lead to a more homogenous impeller exit flow and thus to an increase in total-to-static efficiency of 1.76% points: the positive lean in the near-tip region and the positive blade curvature in the rear part of the optimized impeller. Furthermore, through extensive experimental investigations conducted on a large scale test rig it has been possible to prove the particular impeller outflow characteristics of the baseline compressor stage.
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.
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