Aerodynamic Losses in an Aero Engine Centrifugal Compressor with a Close-Coupled Pipe-Diffuser and a Radial-Axial Deswirler

摘要

In the work presented here, a detailed aerodynamic analysis of an aero engine centrifugal compressor is given. Steady and unsteady 3D-RANS simulations, as well as extensive experimental data have been used for the analysis. The compressor investigated contains a close-coupled pipe-diffuser. A radial-axial deswirler guides the air into the combustion chamber.The investigation presented gives a detailed insight in the loss mechanisms as well as their origin within the centrifugal compressor. In the past, the aerodynamic loss mechanisms within the centrifugal compressor were discussed on a phenomenological basis. In this work, the loss mechanisms are identified by analyzing the irreversible entropy production. The single loss mechanisms are quantified, enabling consolidated statements on the potential improvement, as well the approach on how to reduce the losses induced.An investigation of four different diffuser concepts is given, representing the time-wise evolution of the diffusion system towards a higher efficiency, increased stall margin and smaller outer diameter. Within the original diffusion system, a flow separation in the pipe-diffuser results in a thick shear layer with a high level of entropy production. By truncating the pipe diffuser, the large flow separation is prevented. The original downstream deswirler is not matched with the truncated pipe-diffuser's discharge flow. In order to increase the compressor’s efficiency, two 3D deswirler designs using a single row and tandem rows respectively, are compared. The lower efficiency found for the tandem blade design is not in agreement with the established design rules for two-dimensional compressor blades with a high diffusion factor. This outcome can be traced back to the high impact of the end-wall effects for the low-aspect-ratio deswirler designs investigated. The unsteady investigation demonstrates the change in loss production due to unsteadiness. The decrease in isentropic efficiency due to unsteadiness within the impeller is traced back mainly to the unsteady tip clearance flow, induced by the diffusor’s potential field. Within the diffuser, counteracting unsteady mechanisms are found and discussed in detail.