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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ROSSINI (Radial cOmpresSor Surge INception Investigation)

Teaser

Summary

ROSSINI
Centrifugal compressor stages using vaneless diffuser are known to have a wide operating range. However, at low mass flow rates, their efficiency decreases because of flow separation and unsteady behavior. The occurrence of rotating stall or surge is a key problem for achieving a good working range. The surge phenomenon constitutes a limit of the operating range and can strongly damage the stage or even destroy it. Moreover, even if the compressor stage can operate with a stable rotating stall at near surge operating points, the strongly unsteady features of the flow field lead to high pressure fluctuations on the blade. Consequences are a fatigue of the compressor and a probable reduction of its lifetime. 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 this project are to:
â€¢ determine a surge inception scenario for a given industrial centrifugal compressor using a vaneless diffuser and a non-symmetric volute. Detailed numerical and experimental investigations are planned providing an improved understanding of the complex flow phenomena leading to flow instabilities and surge. Based on the results obtained, the numerical models necessary to faithfully capture the unsteady flow features at near stall operation will be determined.
â€¢ analyze the numerical and experimental results in order to establish a theoretical approach supporting the prediction of instabilities leading to rotating stall and surge. The ultimate objective is to develop an analytical model to predict the critical operating conditions in compressors and associated rotating instabilities (RI) therein. The model could be based on a linear stability analysis of laminar 2D flow profiles to determine the unstable modes and the associated frequencies. Such a method would be applicable early in the design process of industrial turbomachinery components. As a first step, an evaluation of a theoretical approach is required concerning its applicability to a centrifugal compressor with a vaneless diffuser and a non-symmetric volute at the outlet.

Work performed

WP1
A public website for the ROSSINI project has been established. The focus of WP1 activities was on the operational planning and work preparation. Team re-organization was necessary to mitigate personnel shortage and to react on the increased need for personnel resources during rig assembly phase.
Semi-annual progress meetings have been organized and a summarizing report was provided to the CS2 Project Officer. The project plan and schedule have been updated according to WP progress.
Controlling of the grant budget has been performed on a monthly basis (personal months and cost for traveling). All achieved milestones, deliverables and publications have been reported through ECAS portal.

WP2
Task 2.1: A preliminary analysis of the industrial compressor was performed to determine the numerical performance map and identify if unstable operation occurs as expected. The results were compared and validated with the CFD results of WP3, task 3.2. The geometry was subsequently scaled to the determined size of the geometry to match the dimensions of the rig environment.
Task 2.2: URANS calculations were conducted of the complete compressor at 100% wheel 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 level of unsteadiness at both peak efficiency and near stall.
The computational grid has been adjusted based on the realized experimental geometry. The performance map has been recacluated with the updated geometry. The URANS computation for the peak efficiency point has been redone as well. The various probe locations used in the test rig are being inlcuded in the numerical set-up. Remaining URANS/X-LES calculations have been put on hold until commissioning test results become available.

WP3
Task 3.1: The LTS test case was scaled to match the DLR testing facility. Based on preliminary CFD analysis a design and instrumentation concept for the LTS test case was prepared. Clearance of the concept was given at the PDR. A structural analysis (FEM calculation) was performed for the wheel to ensure safe operation. The final design and instrumentation plan was released for manufacturing with the CDR. All parts have been finalized at DLR Workshop (SHT) and released for rig assembly by end of April 2018. During rig assembly, utmost care was taken adjusting tip clearance and diffuser passage height using shims in order to meet LTS specifications. The rig assembly was finalized and all open Actions could be closed, including a check of tightness and functionality or calibration of all sensors and intrumentation.
The Test Readiness Review (TRR) was successfully passed on 20th March 2019. Clearance was given for the subsequent test campaign, hence the most important milestone of WP3 was achieved.
Task 3.2: The scaled LTS stage geometry was shared between NLR and DLR to establish the numerical mesh based on the same set up. Preliminary RANS calculations of the two different computational domains have been compared and validated. The RANS simulation of the LTS stage was summarized in a Master Thesis.
An optimized set up for the URANS simulations was achieved (grid structure, turbulence model). Performance maps have been calculated based on the realized settings at the compressor rig. They will be used as reference during the test campaign to identify safe rig operation regimes and for online monitoring.
Task 3.3: The first run of the ROSSINI compressor rig was performed on 9th July 2019. The rig could be operated covering the full performance range between idle mode and 120% rotational speed. A good agreement between the experimental and predicted numerical results was achieved.

WP4
A redesign 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

Final results

The ROSSINI project will result in a thorough quantitative knowledge of the flow instabilities near surge and the interactional aerodynamic effects between the impeller and the volute diffuser. This knowledge is essential for designing a compressor with high efficiency and wide operating range and will be integrated in the development program of the Topic Leader.
The proven numerical models and analytical prediction models for near-surge instabilities to be generated in this project will also improve the innovation capacity of the European Union. Numerical simulation models required for reliable surge onset prediction will be developed and validated against high-fidelity experiments. The numerical models will be applicable to the design and development of centrifugal compressors in general, i.e. also those used e.g. in automotive, power generation and industrial applications. This greatly leverages the impact of this project.