Radial cOmpreSsor Surge INception Investigation

The ROSSINI project 'Analysis of centrifugal compressor instabilities occurring with vaneless diffusor at low mass flow momentum' was launched in Oct. 2016 in the framework of Horizon 2020 CleanSky2, topic H2020-CS2-CFP02-2015-01. It brings together world-leading researchers with expertise in radial compressor technology, unsteady turbomachinery aerodynamics, and advanced optical diagnostics to address an important area for the Systems ITD members in an integrated program of work.

Clean Sky 2 strives for key Technologies to achieve the H2020 goals based on the more stringent goals of Flightpath 2050. Continually increasing crude oil prices confront carriers and their customers with a serious economic challenge due to the associated high operating costs. This in turn drives the need to further reduce the specific fuel consumption of civil aircrafts. Since CO2 emissions are directly related to fuel burn, a reduction in fuel consumption directly leads to lower CO2 emissions. This research project is focused on the needs and requirements of the industrial Topic Leader.
 

Motivation:

Centrifugal compressor stages using vaneless diffuser are known to have a wide operating range. However, when operated outside of this range, instabilities have been reported to commonly develop in the impeller (rotor) or diffuser or both [1]. The unsteady flow phenomena are believed to start from a localized flow separation (stall) that frequently will orbit within the stage at sub-harmonic rotational speeds (rotating stall). If not damped sufficiently the stall cells will eventually develop into more hazardous self-excited pressure oscillations within the entire compressor stage leading to surge phenomena that can induce high aero-elastic loads on the rotor itself leading to premature aging (fatigue) or ultimate failure of the stage. Predicting of the onset of the instabilities is a prerequisite to maintain compressor operating within safe operating margins. In practice this is can be achieved through online pressure monitoring with active feedback (throttle valves) or through passive measures (e.g. casing treatments or bleed slots, [2][3]). Nonetheless the physical understanding and associated modeling of the stall and surge inception is rather limited and mostly restricted to cause-and-effect investigations. With improved computational models being available nowadays more in-depth investigations of the underlying flow physics have become possible (see e.g. [4][5]) but rely on high quality experimental data for validation.

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.

Surge phenomenon in centrifugal compressors has been studied for almost sixty years. Following the important research work related to axial compressors in the literature (e.g. [6][7][8]), most of the contributions are focused on vaned diffuser stages [9][10][5]. An effort would be necessary on vaneless diffuser cases, even if the scientific production is getting bigger this last decade (as visible through [11][12][13]). Moreover, due to the various configurations found in the literature, a large spectrum of stall and surge behavior is reported and up to now, no overall rule can be given a priori concerning the instability inception in a given machine.
 

Objectives:

The objectives of the ROSSINI project are to:

• determine a surge inception scenario for a given industrial test case of a centrifugal compressor using a vaneless diffuser and a non-symmetric volute at the outlet. 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.
• 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 (long term) 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.

Centrifugal compressor facility at DLR

Expected 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.
The ROSSINI project can make a decisive contribution to the future of aircraft environmental control systems. The knowledge of compressor instabilities and impeller/diffuser interaction effects will be integrated in the development program at the Topic Leader, greatly improving their competitiveness. Because the results are applicable to the design and development of centrifugal compressors in general, such as automotive, power generation and industrial applications, a significant segment of the European industrial base will benefit from the results of this project.
 

Work Flow:

• develop the design and instrumentation concept for the LTS test case
• determine and validate the numerical model necessary to capture the unsteady flow phenomena occurring near surge
• test case implementation to centrifugal compressor facility
• detailed numerical and experimental flow investigation at low mass-flow operation points for a selection of representative shaft speeds, to capture and analyze the inception of flow instabilities
• correlation analysis of simultaneously recorded velocity and pressure data (time-resolved measurements involving Kulite array technology and high-speed PIV)
• numerical investigation of flow path geometry variations on performance and surge inception
• adaptation and evaluation of a hypothesis (analytical model) to predict the critical operating conditions in compressors and associated rotating instabilities (RI) therein – as a long term objective based on the results obtained in the ROSSINI project