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Radial cOmpresSor Surge INception Investigation

Periodic Reporting for period 3 - ROSSINI (Radial cOmpresSor Surge INception Investigation)

Reporting period: 2019-10-01 to 2021-06-30

Centrifugal compressor stages are used in a variety of applications across an aircraft – like in the environmental control system (ECS). At low mass flow rates, their efficiency declines rapidly be-cause of flow separation and unsteady behavior leading to high pressure fluctuations on the blade. To anticipate it, or even try to avoid it, a detailed knowledge of the flow occurring at the limit of the operating range is necessary.
The objectives of the H2020 CleanSky2 project ROSSINI are to:
• investigate the efficiency issues within centrifugal compressor stages by creating a “surge inception scenario” that could predict how the centrifugal compressor stages would react in various situations. Detailed numerical and experimental investigations are planned provid-ing an improved understanding of the complex flow phenomena leading to flow instabilities and surge.
• analyze the numerical and experimental results in order to establish a theoretical approach supporting the prediction of instabilities leading to rotating stall and surge.
Experimental and theoretical Investigations:
A new rig was designed and manufactured at DLR Cologne for detailed experimental investigations of the specific aerodynamic phenomena near the stability limit. Profound knowledge was gained regarding the development of flow instabilities for off-design operation: At reduced mass flow, the onset of instabilities was visible as a broad, low-frequency peak below BPF in the power spectra of unsteady pressure signals. Rotating instability signatures were detected in terms of distinct pro-nounced frequencies in the hump region of the spectra.
Laser-based diagnostic were performed at selected operating conditions with and without flow instabilities: The L2F technique upstream of the impeller inlet plane provided flow angles, velocity and turbulence that were used to validate the domain inlet parameters of the numerical calcula-tions. Time-resolved PIV measurements were performed at the same operating conditions provid-ing statistical flow quantities, velocities and spectra at measurement positions upstream and down-stream of the impeller.
Based on the subsequent spectral analysis of the unsteady pressure data a theoretical approach was developed to support the prediction of instabilities.
Numerical Investigations:
URANS calculations were conducted at 100% rotating speed, for peak efficiency and near stall, both starting from converged steady state solutions. Flow separation regions and near stall mass flow oscillations were identified. The results reveal a more complex flow than expected, with a high lev-el of unsteadiness at both peak efficiency and near stall.
URANS/X-LES calculations have been performed at the same operating conditions as chosen for Laser-based flow diagnostic. The performance results of the URANS/X-LES calculations are in per-fect agreement with the experimental results of the ROSSINI test rig.
A numerical re-design of the volute has been undertaken aimed at removing the flow separation observed in the numerical results. The final configuration has a drastically reduced flow area (cross sections). The flow is completely free of flow separation, and the overall compressor efficiency has increased with 0.7%. The performance map was re-calculated with this volute geometry, resulting in improved efficiency of app. 0.6% over the entire operating range.
Communication, Dissemination and Exploitation activities:
The proof of concept and technical realization of a new instrumentation concept and miniaturized sensors for centrifugal compressor testing was demonstrated through a conference presentation and technical article (peer-reviewed) at the 14th ETC in Gdansk, Poland (Apr. 2021, DOI: 10.5281/zenodo.4911546).
Both beneficiaries, NLR and DLR, have been able to validate and improve the numerical models for turbomachinery flow simulation, especially with regard to unsteady investigations of stall inception and surge, which will be a great benefit for the Topic Manager as well as for all future project part-ners.
Based on the ROSSINI results the development of a correlation analysis method for the identifica-tion of flow phenomena leading to stall is planned. Such a method - that would enable the predic-tion of stall inception - would be of high interest for industry partners. The method (once estab-lished and validated) will be available for future projects addressing similar tasks. Possible licensing of the method will be considered. The results of the combined analysis of TR-PIV and unsteady pressure measurements have been published through a conference presentation and technical article (peer-reviewed) at the 14th ISPIV in Chicago, Illinois (Aug. 2021, DOI: 10.5281/zenodo.5599508).
At annual Status Seminars related to the Energy and Aeronautics national research programs the ROSSINI activities have been reported on. Here, almost all national DLR stakeholders in terms of industrial partners, government representatives (national funding), universities and other research institutions are present.
In the context of the ROSSINI project a Master Thesis was written, dealing with the numerical set-up, mesh generation and simulation of the aerodynamic performance of the ROSSINI compressor stage. The work was presented on a conference (ODAS 2017, France).
ROSSINI Public website: https://www.dlr.de/at/rossini

Conclusion
In the ROSSINI project the onset of flow instabilities could be experimentally determined in the power spectra of unsteady pressure instrumentation. Rotating instabilities have been detected at all analyzed rotating speeds near the stability limit. In the numerical simulations a separation bubble was detected in the volute that might be a driving mechanism for flow instabilities or mass flow oscillations. With a numerical re-design of the volute with reduced cross-sections, the separation could be completely suppressed.
One of the findings of the ROSSINI project showed that the individual design and rig dimensions themselves do have a significant impact on the performance near the stability limit, hence it will always be trade-off to be made between numerical (costly) effort during the design phase of new compressors on the one hand and the setup and instrumentation of prototype testing facilities at the end of a qualification phase on the other hand.
The ultimate objective would be to develop an analytical model that could predict the critical oper-ating conditions in compressors. Such a method would be applicable early in the design process of industrial turbomachinery components.
The results of this project form an extensive database that will feed into Clean Sky’s work on the aircraft Environmental Control System. The new technologies being developed under Clean Sky for the ECS are increasingly electrical, which means increased heat release into the aircraft and requires more cooling. The introduction of new bleedless engines also puts increased demands on the ECS, as it also needs to provide cabin pressurization. A better understanding of the centrifugal compres-sor stages and how they work will contribute to solving these problems and will lead to greater effi-ciency gains for aircraft of the future as well as to the design and development of centrifugal com-pressors in general, i.e. also those used e.g. in automotive, power generation and industrial applica-tions. This greatly leverages the impact of this project.
ROSSINI test rig at DLR facility - detail
First run of ROSSINI test rig - performance maps compared to CFD results
ROSSINI compressor map comparing WP2 and WP3 results of RANS and uRANS simulations
ROSSINI test rig including TR-PIV setup
ROSSINI compressor map comparing WP3 numerical and experimental results
ROSSINI test rig at DLR facility