Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The characteristic curves of the compressor stage resulting from the unsteady simulations and the experiments show a good agreement over the whole operating range. On the contrary, the total pressure ratio resulting from the steady simulations is clearly overestimated. A detailed analysis of the flow field at design operating point led to identify the physical mechanisms involved in the blade row interaction that underlie the observed shift in performance. Attention was focused on the deformation in shape of the vane bow shock wave due its interaction with the jet and wake flow structure emerging from the impeller. An analytical model is proposed to quantify the time-averaged effects of the associated entropy increase. The model is based on the calculation of the losses across a shock wave at various inlet Mach numbers corresponding to the moving of the jet and wake flow in front of the shock wave. The model was applied to the compressor stage performance calculated with the steady simulations. The resulting curve of the overall pressure ratio as a function of the mass flow is clearly shifted towards the unsteady results. The model in particular enhances the prediction of the choked mass flow.
Control devices based on casing treatments have already shown their capability to improve the flow stability in compressors. However their optimization remains complex due to a partial understanding of the related physical mechanisms. The present paper proposes to use a budget analysis of the Navier Stokes equations to support the understanding of such flow phenomena. Based on the original work of Shabbir and Adamczyk (2005), the strength of the present contribution is to generalize the flow analysis method to all Navier-Stokes equations, including unsteady terms. A high-pressure multistage compressor equipped with circumferential casing grooves is chosen to demonstrate the potential of this approach. Steady and unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are solved with a structured multi-blocks solver. Results are then briefly compared to experimental data to validate the numerical method. The analysis of the unsteady axial momentum equation for configurations with and without casing treatment points out some of the mechanisms responsible for the stability improvement. The analysis also indicates that the flow unsteadiness generated by upstream stator wakes (stator/rotor interaction) reduces viscous efforts and increases convective forces, significantly modifying the compressor stability. Finally, the proposed post processing method shows very interesting results for the understanding of circumferential grooves and it should be also used for non-axisymmetric casing treatment configurations.
The design of modern aircraft engines increasingly involves highly sophisticated methodologies to match the current development pace. International company relations affect the collaboration between design offices all around the world. An important part of academic mission of modern engineering education is to produce graduates with skills compatible with industrial needs. Education may readjust accordingly to meet the higher requirements. However, a realistic scenario of the design process of an aircraft engine cannot possibly be transferred one-to-one into the student education process. A unique attempt to overcome this discrepancy was the International Gas Turbine Project. Within this project, undergraduate students have designed the cooling system of the HPT blades for a 30,000 lb thrust two-spool turbofan aeroengine. This project was collaboration between the Jet Propulsion Laboratory of TU Berlin, the Turbomachinery Group of EC Lyon and the Turbomachinery Laboratory of ETH Zurich. It also involved mentoring industry professionals from Rolls-Royce Deutschland, MTU, SNECMA and Alstom Power. Similar to modern aeroengine company structures, the design tasks included multi-component, multi-disciplinary and international interfaces of different educational systems. The student teams considered various aerothermodynamic and mechanical integrity aspects of the design. Particular attention was paid to design of the compressor, the secondary air system and the HP turbine including blade cooling. The three Universities integrated the project differently into their education curriculum and approached the tasks with different levels of software involvement. In this paper, the technical details of the design process, and the different approaches adopted are presented. Besides the application of turbomachinery-related knowledge, the impact of student interactions on the technical aspects of the project is discussed. The interfaces, including information management and the involvement of industrial partners are also addressed. Team spirit developed between the students from an initial competitive behavior to a final feeling of sitting in the same boat. It was observed that increased effort was required from academic staff in comparison to the conventional academic instruction. Nevertheless, students greatly benefited from the social interaction and an early training-on-the-job tuned to current industrial needs.
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