Research Papers

Experimental Determination of Mechanical Stress Induced by Rotating Stall in Unshrouded Impellers of Centrifugal Compressors

[+] Author and Article Information
Philipp Jenny

MAN Diesel & Turbo Schweiz AG,
Hardstrasse 319,
Zürich 8005, Switzerland
e-mail: philipp.jenny@man.eu

Yves Bidaut

MAN Diesel & Turbo Schweiz AG,
Hardstrasse 319,
Zürich 8005, Switzerland
e-mail: yves.bidaut@man.eu

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 15, 2016; final manuscript received October 12, 2016; published online December 1, 2016. Editor: Kenneth Hall.

J. Turbomach 139(3), 031011 (Dec 01, 2016) (10 pages) Paper No: TURBO-16-1241; doi: 10.1115/1.4034984 History: Received September 15, 2016; Revised October 12, 2016

Unshrouded centrifugal compressor impellers typically operate at high rotational speeds and volume flow rates. The resulting high mean stress levels leave little margin for dynamic excitations that can cause high-cycle fatigue. In addition to the well-established high-frequency impeller blade excitations of centrifugal compressors caused by the stationary parts, such as vaned diffusers or inlet guide vanes (IGVs), the presented study addresses an unsteady rotating flow feature (rotating stall) which should be taken into account when addressing the high-cycle fatigue during the design phase. The unsteady fluid–structure interaction between rotating stall and unshrouded impellers was experimentally described and quantified during two different measurement campaigns with two full-size compression units operating under real conditions. In both campaigns, dynamic strain gauges and pressure transducers were mounted at various locations on the impeller of the first compression stage. The casing was also equipped with a set of dynamic pressure transducers to complement the study. Rotating pressure fluctuations were found to form an additional impeller excitation at a frequency that is not a multiple of the shaft speed. The measurements show that the excitation amplitude and frequency caused by the rotating pressure fluctuations depend on the operating conditions and are therefore challenging to predict and consider during the design phase. Furthermore, the excitation mechanism presented was found to cause resonant impeller blade response under specific operating conditions. For the experimentally investigated impeller geometries, a rotating pressure fluctuation caused approximately 1.5 MPa of additional dynamic stress in the structure per 1 mbar of dynamic pressure amplitude when exciting the first bending mode of the impeller. The induced dynamic mechanical stresses due to rotating stall are in the order of 10% of the endurance limit of the material for the tested impeller geometries; therefore, they are not critical and confirm a robust and reliable design.

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Fig. 1

Schematic of first stage of compression units

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Fig. 2

Instrumentation on impeller

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Fig. 3

Instrumented impeller unit I

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Fig. 4

Numerically predicted stress for strain gauges C+ and A+ at 112% Ωnom. The plotted parameter is the strain in the direction of the strain gauges (direction x).

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Fig. 5

Maximum and minimum measured dynamic strain (gauge C1) during a speed ramp at IGV = 30 deg (left) and the corresponding spectrogram (right)

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Fig. 6

Measured maximum dynamic stress for EF7 ND5 during the ramps for different IGV positions

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Fig. 7

Spectrogram of pressure measurement on the impeller suction side during a speed ramp (IGV = 50 deg, PI3). The dashed lines indicate rotating cells.

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Fig. 8

Spectrogram of pressure transducer PC5 at 60% impeller chord length during the speed ramp shown in Fig. 7 (IGV = 50 deg)

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Fig. 9

Illustration of the estimation process of the pressure amplitude of the rotating cells

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Fig. 10

Spectrograms of pressure transducer PI3 (right) and strain gauge A3 at the blade leading edge (left) during a speed ramp at IGV = 50 deg. The dashed lines indicate locations where rotating cells occur.

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Fig. 11

Maximum dynamic strain during two IGV sweeps at 91.5% Ωnom measured with strain gauge A3 at the impeller leading edge

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Fig. 12

Cells detected during the two IGV sweeps at 91.5% Ωnom shown in Fig. 11 (left-hand side) and their characterization (right-hand side, negative frequency indicates counter-rotating movement)

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Fig. 13

Spectrogram of strain gauge A3 at the impeller leading edge during IGV sweep at 91.5% Ωnom shown in Fig. 11

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Fig. 14

Interference diagram of the impeller with the speed lines for Ωnom and 91.5% Ωnom and the prestress corrected natural frequencies

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Fig. 15

Spectrogram of strain gauge A2 at the blade leading edge (left) during a speed ramp at IGV = 50 deg

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Fig. 16

Interference diagram of the impeller with the speed lines for 71% Ωnom and the superimposed exciting rotating cells

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Fig. 17

Measured pressure amplitudes of the five- and seven-cell configurations during the IGV sweep

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Fig. 18

Sensitivity of the induced stresses to the pressure amplitude of the rotating seven-cell configuration for all the operating points with resonant blade response during the IGV sweep at 91.5% Ωnom




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