Verlustmechanismen eines beschaufelten und eines Röhren-Diffusors für Triebwerksradialverdichter

The quantification of the loss mechanisms in diffusers for radial compressors is the major topic of this work. The common foundation form a pipe- and a vaned-diffuser, which have been investigated numerically for one single aero-engine centrifugal compressor. To separate the individual loss mechanism from each other, in addition to the 3D-CFD analysis, a one-dimensional remodeling of the CFD solution is conducted. For the quantification of loss mechanisms, a wall friction coefficient is needed. Many different friction coefficients can be found in literature. This work proposes a possible way out to use a constant wall friction coefficient. Considering the simplification on an effective cross-section area in the derivation of the conservation equations for a one-dimensional modeling of a vaneless diffuser, a blockage correction of the static pressure gradient is introduced. The first comprehensive application of this modified post-processing method takes place in the vaneless space downstream of the thick trailing edge of the pipe diffuser. All loss mechanisms of the 3D-CFD solution can be quantified one-dimensionally with only a small error. This comprises the geometrically fixed losses due to wall friction and due to the blockage correction, as well as the mixing losses spatially separated in production and dissipation location. The mixing losses are further distinguished in the two causes blockage and inhomogeneity. It is shown that the mass averaged analysis accounts the losses at their actual dissipation location. While the flux averaging accounts the mixing losses at their production location. Since the production and dissipation location of the mixing losses is spatially separated, mass and flux averaging should be look at always together. The second part of the results focuses on the complete radial compressor stage with the different diffusion systems pipe and vaned diffuser. The comparison based on the characteristic maps identifies, although the diffuser leading edges have very different shapes, an almost identical upstream effect of both diffusers. And it identifies a worse deceleration inside the vaned diffuser as the major cause for its worse stage efficiency compared to the pipe. This leads to a 50% higher wall friction loss inside the vane compared to the pipe. In the last part the causes for a drop of the total to total stage efficiency of up to 0.5%-points, when a hub-side cavity is applied, are analyzed. Because of the interaction between cavity and core flow an upstream effect and an exhaust along the trailing edge are observed. The quantification of loss mechanisms shows how the interaction at the trailing edge and how the bleeds cause, without a work input, a rise in total pressure downstream of the impeller