GT2010-22753

Detection of Stall Regions in a Low-Speed Axial Fan. Part I – Azimuthal Acoustic Mesurements GT2010-22753 Anthony. G. SHEARD Stefano BIANCHI, Alessandro CORSINI Flakt Woods Ltd FMGroup @ DMA-URLS Turbo Expo Turbine Congress and Exhibition June 14 – 18, 2010, Glasgow, Scotland UK

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Introduction • A stall is a situation of abnormal airflow which is found in dynamic operating machines, particularly axial compressors and fans. • The stall results in a loss of compressor performance, which can vary in severity from a temporary engine power drop (occurring so quickly it is barely registered on engine instruments) to a complete loss of compression (compressor surge) necessitating a reduction in the fuel flow to the engine. • Surge may become sufficiently pronounce to cause loud bangs and engine vibration. In most case , this condition is of short duration , and will either correct itself or can be corrected by retarding the throttle or power lever to Idle and advancing it again , slowly. Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Images courtesy ANSV Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Images courtesy AAIB Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Introduction • Modern compressors are carefully designed and controlled to avoid or limit stall within an engine's operating range. Stall was a common problem on early jet engines with simple aerodynamics and manual or mechanical fuel control units, but has been virtually eliminated by better design and the use of hydro-mechanical and electronic control systems such as Full Authority Digital Engine Controls. • Stall is also a concern for low speed heavy duty fans because the aerodynamic stall conditions can be a mechanical risk for the machine endurance which may result in a mechanical failure. This happens usually when the fan operates under a pressure pulse. Pressure rate Volume flow rate Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Aims • Thepresent work is a two part paper and itwasaimedtoprovide the background for the developmentof a stall detection tool in low-speedaxialfans. •In thispaper (Part I), the stallcharacteristicsof a low-speed fan are establishedusingflush-mountedmicrophonesplaced at four azimuthal positionsaround the casing. • Spatial and temporal correlations between rotating instabilities are established, which facilitates a full analysis of stall inception. ,[object Object],Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Summary • Background ofstallphenomena •Quickreviewof the stall detection tecniqueswhich best fitwith the needsofthis work. • Description of the tested fan. ,[object Object]

Descriptionof the spectralanalysisusedfor the detection ofrotating flow disturbances

ConcludingremarksTurbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Background • The complexity of the flow in the blade-tip region is recognized as one of the key factor in fixing rotor operating margin in tip-stalling rotors tip aerodynamics affects the overall blade span-wise loading and losses, similar physical mechanisms govern the tip-region flow around blades in turbomachinery compressors apply in low-speed & low-solidity heavy duty fans (Ganz et al., 1998) • The diminished performance associated with the presence of these flow mechanisms has prompted the designers of fans and compressors to devise new design features which minimisethe adverse aerodynamic effects of tip gap Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Techniques and concepts for the study of stall in turbomachinery, a review • Techniques appeared to date are generally designed to accomplish the goal of studying the effect of tip leakage flow, affecting the primary-secondary flow momentum transfer and its role in the stall generation and evolution. Day and Cumpsty (1978) studied the flow within rotating stall cells in axial compressors and provided physical interpretations. Camp and Day (1998) studied the modal behavior of the stall cells in low speed axial rotors Turbo Expo 2010, Turbine Congress and Exhibition

Detection of stall regions in a low-speed axial fan by visualisation of sound signals Techniques and concepts for the study of stall in turbomachinery, a review •The use ofspectralanalysisallowed a deepthoughtabout the presenceofanyperiodicphenomenonoccurring in the rotor: rotatingstallcells. Kameier and Neise (1997) detected the flow instabilities which rotates in the rotor annulus by the mean of phase shift spectral analysis and a cross correlation applied to the near-field noise emitted by a fan rotor. Inoue and co-authors (GT2004-5335) studied the effect of tip clearance in the stall evolution problems for low speed rotors. Hah and co-authors (GT2010-22101) studied in detail the unsteady flow phenomena due totipclearance flow instability in a transonicaxialcompressorrotormeasuring the wallpressurespectrum. “The phase angle of the Cross Spectrum of the casing wall pressure fluctuation has a linear variation with frequency and a positive gradient in the range of a rotating instability” (Neise, 1997 in JSV 203(5))

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Fan geometry & design • Low-speed axial fans for high-temperature operations BPF = 220 Hz blade profiles modified ARA-D geometry type arbitrary vortex radial work distribution Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Test rig set up • BS 848 PI ,[object Object]

flow straightenertoallow steady and undisturbed flow

back throttletoincrease the pressureTurbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Blade and Casing instrumentations ,[object Object]

strain gauges on both sides of three bladesMicrophone position Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Casing instrumentations Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Test Conditions Fstart = 0.200 Fend = 0.120 Machtip = 0.25 Re = 106 Machtip = 0.12 Re = 5.105 Machtip = 0.06 Re = 2.5.105 Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Test Conditions Fstart = 0.200 Fend = 0.120 Machtip = 0.25 Re = 106 Fstart = 0.100 Fend= 0.0602 Machtip = 0.12 Re = 5.105 Machtip = 0.06 Re = 2.5.105 Fstart = 0.053 Fend = 0.021 Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Signal processing technique • Cross CorrelationAnalysisbetween the pressuremeasured in two azimuthal locations The cross-correlation of functions f(t) and g(t) is equivalent to the convolution of f *(−t) and g(t): Analogousto the convolutiontheorem, the cross-correlationsatisfies: y x denotes the Fourier transform, and ’*’ indicates the complexconjugate. ,[object Object], The coherencebetweentwosignalsx(t) and y(t) is a real-valuedfunctionthatisdefinedas: Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Measurements Uncertainty • The overall uncertainty on unsteady pressure measurements was estimated as: DV = 1000 mV ± 12 mV (20:3) on the voltage; and DG = 200 dB ± 2.4 dB (20:3) on the raw signal gain in the range of frequencies considered. ,[object Object]

The system response for the phase angle was carefully checked.(deg) f (Hz)

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Measurements uncertainty • The overall uncertainty on unsteady pressure measurements was estimated as: DV = 1000 mV ± 12 mV (20:3) on the voltage; and DG = 200 dB ± 2.4 dB (20:3) on the raw signal gain in the range of frequencies considered. ,[object Object]

FMGroup @ DMA-Sapienza throttle open closed Detection of stall regions in a low-speed axial fan by visualisation of sound signals Visual inspection of wall acoustic pressure at different azimuthal positions @100% rotationalspeed •pressuresignallow-passfiltered at 100 Hz Probe 1 Probe 2 Probe 3 Probe 4 rotation 0 +10 +5 t (rev) Fstart = 0.157 Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza throttle open closed Probe 1 slow emerging cell Probe 2 Probe 3 fast emerging cell Probe 4 rotation 0 +10 +5 t (rev) Detection of stall regions in a low-speed axial fan by visualisation of sound signals Visual inspection of wall acoustic pressure at different azimuthal positions @100% rotationalspeed •threedifferenttypeofunsteadyphenomenamayexist - Fast emergingpressureperturbation - Slow emergingpressureperturbation (at 40% of the shaft rate) - They interfere developinganIncipienceofrotatingcell. Fstart = 0.157 Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza Detection of stall regions in a low-speed axial fan by visualisation of sound signals Test Conditions @ 100% speed Finst = 0.157 Fend = 0.148 Machtip = 0.25 Re = 106 Machtip = 0.12 Re = 5.105 Machtip = 0.06 Re = 2.5.105 Turbo Expo 2010, Turbine Congress and Exhibition

FMGroup @ DMA-Sapienza BPF SN Lp (dB) 2nd BPF a)  (deg) b) f (Hz) Detection of stall regions in a low-speed axial fan by visualisation of sound signals Spectral analysis @ 100% rotational speed F = 0.200 •cross spectrumamplitude P1 vs P3 (a) ,[object Object]

GT2010-22753

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GT2010-22753