Fast Fourier Transform

A theory is presented to predict the occurrence of weak rotating waves in a centrifugal compression system with a vaneless diffuser. As in a previous study of axial systems, an undisturbed performance characteristic is assumed known. Following an inviscid analysis of the diffuser flow, conditions for a neutral rotating disturbance are found. The solution is shown to have two branches; one with fast rotation, the other with very slow rotation. The slow branch includes a dense set of resonant solutions. The resonance is a feature of the diffuser flow, and therefore such disturbances must be expected at the various resonant flow coefficients regardless of the compressor characteristic. Slow solutions seem limited to flow coefficients less than about 0.3, where third and fourth harmonics appear. Fast waves seem limited to a first harmonic. These fast and slow waves are described, and effects of diffuser-wall convergence, backward blade angles, and partial recovery of exit velocity head are assessed.

Weak rotating flow disturbances in a centrifugal compressor with a vaneless diffuser

Numerical and experimental analysis of automotive turbocharger compressor aeroacoustics at different operating conditions

Flow instability conditions, in particular during surge and stall phenomena, have always influenced the operational reliability of turbo-compressors and have attracted significant interest resulting in extensive literature. Nowadays, this subject is still one of the most investigated because of its high relevance on centrifugal and axial compressor operating flow range, performance and efficiency. Many researchers approach this important issue by developing numerical models, whereas others approach it through experimental studies specifically carried out in order to better comprehend this phenomenon. The aim of this paper is to experimentally analyze the stable and unstable operating conditions of an aeronautic turbo-shaft gas turbine axial-centrifugal compressor installed on a brand new test-rig properly designed for this purpose.The test facility is set up in order to obtain i) the compressor performance maps at rotational speeds up to 25,000 rpm and ii) the compressor transient behavior during surge. By using two different test rig layouts, instabilities occurring in the compressor, beyond the peak of the characteristic curve, are identified and investigated.These two types of analysis are carried out thanks to pressure, temperature and mass flow sensors located in strategic positions along the circuit. These measurement sensors are part of a proper control and acquisition system, characterized by an adjustable sampling frequency. Thus, the desired operating conditions of the compressor, in terms of mass flow and rotational speed and transient of these two parameters are regulated by this dedicated control system.© 2016 ASME

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Experimental Investigation of Stall and Surge in a Multistage Compressor

At present, off-design centrifugal compressor suffers from great instabilities, such as rotating stall and surge, which lead to work failure, strong pressure fluctuations, and violent vibrations of the entire system. One cause of these instabilities is the rotating stall occurring in vaneless diffuser. In this review, recent researches on rotating stall of vaneless diffuser in centrifugal compressor are reviewed, and its inductive mechanism and characteristic description are emphatically introduced. In vaneless diffuser, the rotating stall starts up and fully develops under non-design conditions with the characteristic of increasing pressure fluctuation amplitude. The negative effects of rotating stall include reverse flow and vortex, both of which have significant effects on the instability of centrifugal compressors. Some improvement methods of rotating stall are also introduced, and a lot of examples are given. Finally, some suggestions and discussions are put forward for the future work.

A Review of Rotating Stall in Vaneless Diffuser of Centrifugal Compressor

Identification of phenomena preceding blower surge by means of pressure spectral maps

Time-frequency analysis of the surge onset in the centrifugal blower

Singular spectrum analysis as a tool for early detection of centrifugal compressor flow instability

The influence of plenum volume on surge phenomenon in a centrifugal blower was studied by means of quasidynamic analysis. In this procedure, signals were gathered at 5 pressure tappings at 146 different positions of the throttling valve controlling the mass flow rate. Frequency spectra obtained by means of Fourier analysis are combined together in the form of colour maps with frequency as the abscissa and valve position as the ordinate. Such a map provides high-resolution information about spectral structures of pressure signals attained at different mass flow rates. Analysis was conducted at two system configurations characterized by different volumes between the blower and the valve i.e. plenum volume. Research confirmed that in both cases the first disturbances appear in the vicinity of the impeller leading edge in the same position of a throttling valve before the surge. Arising flow structure is characterized by strong and random pressure jumps and does not have any dominating frequency. At further valve closure pressure disturbances propagate towards the volute and at deep surge the strongest peaks are observed at the outlet. The moment of deep surge onset is also independent of the plenum volume, however, a difference is observed in the frequency and amplitude of the main modes. With the higher outlet volume the observed oscillations fit well to the frequency of a Helmholtz resonator while, in the case of the smaller volume, the frequency is higher than the frequency of a corresponding Helmholtz resonator.

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Use of Pressure Spectral Maps for Analysis of Influence of the Plenum Volume on the Surge in Centrifugal Blower

Methods of dynamical system analysis were employed to analyze unsteady phenomena in a centrifugal blower. Pressure signals gathered at different control points were decomposed into their Principal Components (PCs) by means of Singular Spectrum Analysis (SSA). Certain number of PCs was considered in the analysis based on their statistical correlation. Projection of the original signal onto its PCs allowed to draw the phase trajectory that clearly separated non-stable blower working conditions from its regular operation.

Grzegorz Liśkiewicz

Papers

Identification of phenomena preceding blower surge by means of pressure spectral maps

Time-frequency analysis of the surge onset in the centrifugal blower

Singular spectrum analysis as a tool for early detection of centrifugal compressor flow instability

Use of Pressure Spectral Maps for Analysis of Influence of the Plenum Volume on the Surge in Centrifugal Blower

Dynamical system analysis of unstable flow phenomena in centrifugal blower