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In centrifugal compressors, variable inlet guide vanes (IGVs) are used to control the mass flow rate with negligible change in pressure ratio and shaft speed. The efficient operation of IGVs is limited to the range of aerodynamic stability of their vane profiles. Thin symmetric profiles, which are usually used, tend to exhibit flow separation already at low setting angles, resulting in a rapid decrease in the global efficiency. Three different guide vane profiles, including a symmetric reference profile, a two-piece tandem profile with a variable angle, and an s-cambered profile, together with two auxiliary profiles, were considered. These variants were numerically and experimentally investigated at different setting angles, covering negative and positive inlet swirls, and at different operating points. The results show that tandem and s-cambered vanes have superior aerodynamic performance in comparison with the symmetric reference profile and extend the operating range of the compressor over which the efficiency remains high. The tandem profile shows better aerodynamic performance than the s-cambered profile at negative inlet swirl, while the asymmetric s-cambered profile offers an extended range of high efficiency at positive preswirl. Considering manufacturing requirements and cost, the s-cambered profile is preferred for applications with positive preswirl.

Abstract. Developments of the background oriented schlieren technique (BOS) towards a three-dimensional measurement technique for 3D density fields are presented. The projections of the density are tomographically reconstructed using filtered back-projection. Theoretical investigations show that the sensitivity of the BOS method depends on the geometric setup, the resolution of the camera, the focal length of the lens and the evaluation algorithm whereas the resolution of the method is described by its transfer function. Applications of the method at an under-expanded free jet of air and to the wake of wind tunnel cascade showed good results -qualitatively as well as quantitatively.

In centrifugal compressors, variable inlet guide vanes (IGVs) are used to control the mass flow rate with negligible change in pressure ratio and shaft speed. The efficient operation of IGVs is limited to the range of aerodynamic stability of their vane profiles. Thin symmetric profiles, which are usually used, tend to exhibit flow separation already at low setting angles, resulting in a rapid decrease of the global efficiency. Three different guide vane profiles including a symmetric reference, a two-piece tandem profile with variable angle, and an s-cambered, together with two auxiliary profiles were considered. These variants were numerically and experimentally investigated at different setting angles, covering negative and positive inlet swirl and at different operating points. The results show that tandem and s-cambered vanes have superior aerodynamic performance in comparison with the symmetric reference profile and extend the operating range of the compressor over which the efficiency remains high. The tandem profile shows better aerodynamic performance than the s-cambered profile at negative inlet swirl, while the asymmetric s-cambered profile offers an extended range of high efficiency at positive preswirl. Considering manufacturing requirements and cost, the s-cambered profile is preferred for applications with positive preswirl.