Variable inlet guide vanes (VIGVs) are most commonly used as the major control unit of integrally geared centrifugal compressors (IGCCs). In order to enhance the efficient operating range of the compressor, the loss mechanisms and utilization limits of state-of-the-art VIGVs need to be better understood. Field measurements in the wake of a typical, commercially used configuration were therefore conducted at the VIGV test facility of the Bundeswehr University Munich. The investigations were carried out at application oriented subsonic flow conditions and stagger angles from 50${}^{\circ }$ to 90${}^{\circ }$ covering the full low-loss operating range, including the limits of efficient operation. For a precise local loss characterization, an inflow correlation was developed and applied to consider total pressure inhomogeneities caused by the radial inflow velocity profile and minor circumferential velocity deviations. Contrary to previous research efforts, not only the profile losses, but also the secondary flow losses induced by the open blade tips and wall-blade interactions were resolved in full detail. For this reason, a more precise and comprehensive loss assessment of realistic VIGV cascades is acquired.

Various experimental and numerical studies of advanced blade configurations for variable inlet guide vanes (VIGVs) were conducted during the past years. Nevertheless, experimental loss characteri-zations of VIGV blade designs vastly focused on the primary profile losses. Secondary flow losses as they occur in a real engine application were often neglected. For this reason, a new wind tunnel facility was erected and commissioned at the Bundeswehr University Munich to enable extensive investigations of application-oriented annular VIGV configurations at typical low subsonic Mach numbers. Five-hole probe field traverses are established by a probe traversing unit in the in-and outflow region of the VIGV. The requirements for a further extension of the applicable measuring techniques by optical methods is moreover provided. Thus, not only profile losses caused by the blades, but also secondary flow losses generated by blade-wall flow interactions and the open blade tips can be resolved in detail.

Throughout the past decades, remarkable efforts have been made to improve the part load operation of compressors by a broadened, more efficient working range of the variable inlet guide vane (VIGV). The main focus has been put on the development of advanced blade geometries. Especially adjustable blades with an independent front and tail blade segment proved to be particularly promising. Potential benefits of an implemented centrepiece, which is not self-evident in many industrial compressor applications, has often been neglected, in turn. Avoided blade tip vortices and a lower tendency of open flow separation by higher, minimum local Reynolds numbers nevertheless promise a significant advantage of the hub. To prove whether these advantages outweigh detriments such as losses induced by the hub struts and additional wall-blade interactions, the performance of two VIGV configurations with and without centrepiece are juxtaposed in the following. The experimental comparison was carried out by five-hole probe wake field traverses and oil film visualizations. The influence of recent blade developments is furthermore considered by the investigation of a customary, symmetric blade and a modern split blade geometry. The latter configuration was proposed by Händel et al. [1] and firstly integrated in an annular cascade by Frank et al. [2]. It is based on a variable front and rear blade segment, which enables an independent adjustment of the camber line at varied stagger angles. The measurements of the configurations with and without integrated hub and the two blade geometries were performed along the full low loss working range of the VIGV between stagger angles of 40 and 90_. The experiments were conducted at the VIGV test facility of the University of the Bundeswehr Munich [3]. Details regarding the benefits of an integrated centrepiece and its potential synergies with a split blade will be outlined in the proposed paper. Bibliography [1] Händel, D., Niehuis, R., and Klausmann, J., 2015. “Aerodynamic Investigation of an Advanced VIGV Design of Adjustable Geometry for Very High Flow Turning”. GT2015-42166, ASME Turbo Expo. [2] Frank, R. G., Wacker, C., and Niehuis, R., 2021. “Loss Characterization of a Conventional Variable Inlet Guide Vane”. Int. J. Turbomach. Propuls. Power, 6(30). [3] Frank, R. G., Wacker, C., and Niehuis, R., 2020. “A New Test Facility for Advanced Testing of Variable Inlet Guide Vanes”. MTT2520A18, MTT Measuring Techniques in Turbomachinery.