Bianchi GT2011

Stall Inception, Evolution and Control in a Low-Speed Axial Fan
with Variable Pitch in Motion !

GT2011-45725

Stefano BIANCHI, Alessandro CORSINI, Anthony G. SHEARD

Luca MAZZUCCO, Lucilla MONTELEONE

Flakt Woods Ltd

FMGroup @ DIMA-URLS

Turbo Expo Turbine Congress and Exhibition
June 7 – 11, 2011, Vancouver, BC, Canada.

Stall inception, evolution and control in a low speed axial fan fitted with Variable Pitch in Motion

Background motivations (i)

• Huge amount of research works have been dedicated in the last decades to the goal of
finding reliable methods to monitor the approach of fans or compressors to the stability
limits while running

literature review spans from ’70s&’80s (Day and Cumpsty, 1978) (Greitzer, 1980) to stall
detection, management & control studies (Paduano et al., 2001) (Christensen et al., 2006)

• Two questions are still under scrutiny
alert methods are proposed for individual test-beds

techniques for the detection of stall initiation, based on experimental
observation of pre-stall behaviours, have sought to identify such behaviour as
early as possible

i.e. early detection period lasts less than two rotor revolutions for axial
compressors and it could be ten times longer in low-speed single stage industrial fans

Turbo Expo 2011, Turbine Congress and Exhibition
FMGroup @ DMA-Sapienza


Background motivations (ii)

• Several studies suggested the presence of tip flow phenomena (in low-speed and high-
speed rotors) directly responsible for the generation of those disturbances (also called
‘spikes’ or ‘pips’) causing the inception of part-span stall cells

• This inception mechanism could be correlated also with a modification of the noise
signal in the casing region

• A number of scholars have utilised azimuthal measurements in an attempt to link the
rotating unsteady patterns of flow instabilities to their acoustic signatures

to detect in centrifugal turbomachines low frequency noise due to rotating stall in (Okada,
1987) or aerodynamic sound sources (Mongeau et al., 1995)

In a similar vein (Kameier and Neise, 1997) established a link between tip-clearance
noise and associated blade-tip flow instabilities in axial turbo-machinery by correlating
rotating source and vortex mechanisms with rotating stall cells



Aims

• The present work was aimed to provide the background for the development of a stall
detection tool in low-speed axial fans with VPIM.

• In this paper, the stall characteristics of a low-speed fan are established using flush-
mounted microphones placed at two azimuthal positions around the casing.

• Spatial and temporal correlations between rotating instabilities are established, which
facilitates a full analysis of stall inception.

•  Energetic analysis on the stall cells and recovery behaviour is estabilished.

•  The study is finalised to characterise the stall and the recovery of a VPIM fan, for the
use of the previously patented ‘SDP stall detection tool’ (Sheard et al., J. Eng. Gas Turb and Power
GTP-10-1168).



Outline

• Methodology
Test fan and apparatus

• Stall visualization technique
Rationale and background information
Sensitivity analysis

• Discussion of the stall warning @ 700 rpm-speed
Fourier and energetic analysis
Visualization of the stall cell

• Conclusions



Methodology. Fan geometry & design

• Low-speed axial fans for heavy-duty applications intended for high-temperature
operations in the boilers of coal power plants.

blade profiles modified ARA-D geometry type

arbitrary vortex radial work distribution



Methodology. Fan Variable Pitch in Motion (VPIM) System

• Remotely controlled oil-dynamic actuators VPIM system @ 1.5°/s.



Methodology. Fan test section BS 848 P I – AMCA certified



Methodology. Test rig



Methodology. Fan performance @ steady conditions

• Total pressure-to-volume performance map @ different pitch angles



Methodology. Test apparatus

10 kHz

• The microphones were mounted flush with
the inner wall of the casing

• The microphones were mounted above the
blade vane



Signal processing technique
y
• Cross Correlation Analysis between the pressure measured
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): x

Analogous to the convolution theorem, the cross-correlation
satisfies:

denotes the Fourier transform, and ’*’ indicates the complex conjugate.

•  Coherence

The coherence between two signals x(t) and y(t) is a real-valued function that is defined as:



Measurements Uncertainty

• The overall uncertainty on unsteady pressure measurements was estimated as:
ΔV = 1000 mV ± 12 mV (20:3) on the voltage; and ΔG = 200 dB ± 2.4 dB (20:3) on the
raw signal gain in the range of frequencies considered.

•  The system frequency limitation of the pressure probes was calculated.
•  The system response for the phase angle was carefully checked.

α (deg)

f (Hz)


Measurements uncertainty

• The overall uncertainty on unsteady pressure measurements was estimated as:
ΔV = 1000 mV ± 12 mV (20:3) on the voltage; and ΔG = 200 dB ± 2.4 dB (20:3) on the
raw signal gain in the range of frequencies considered.

•  The system frequency limitation of the pressure probes was calculated.
•  The system response for the phase angle was carefully checked.

α (deg)

f (Hz)


Test Conditions
dynamic test

Machtip = 0.25 A
Re = 106
B

Pitch variation

C

Φstart = 0.200

Φend = 0.120


Visual inspection of wall pressure low-pass filtered @ 150 Hz

Stall inception


Detection of stall regions in a low-speed axial fan by visualisation of sound signals

Details at stall inception

I1: inception spike Instabilities with similar wave structure RS1: new-born stall cell


Details of the stall evolution

Cell leading edge Cell trailing edge

Partial recovery Stall cell passage Partial recovery Stall cell passage



Average velocity of the stall cells



Average velocity and angular estension of the stall cell

Probably: part-span stall cell



Cross-Correlation (Ch1-Ch2) during three different operatiing conditions



Cross-Correlation (Ch1-Ch2) during three different operating conditions

New-born stall cell
Instabilities with similar wave structure



Cross-Correlation (Ch1-Ch2) during three different operatiing conditions

New-born stall cell
Instabilities with similar wave structure




Stall recovery via VPIM




Stall recovery via VPIM - details



Stall recovery via VPIM – details of stall cell evolution @ recovery



Spectral Density of Energy (SDE) – Stall and recovery



Spectral Energy Density – details of the recovery



Concluding Remarks

•  The present study has established the stall characteristic of a subsonic axial fan by visual inspection of the
pressure traces and acoustic correlations from two different pressure probes mounted on the casing.

•  The spectral analysis detected the presence of low frequency tone components related to rotating stall. The
existence of these tone-band modulations could be inferred as being the signature of rotating stall in this type
of fans.

•  The potential of the VPIM system, operated to extend the classical stable operating range of the fan, could be
extended in a ‘joint control system’ of the fan unstable operations, because the VPIM shows an immediate
recovery of the stall, when the proper pitch angle is reached.

•  As a consequence, a sufficiently quick ‘stall prediction method’ should be developed. This is an ongoing work
of this research group, after the stall warning system presented in the last Turbo Expo and recently patented.



Roma – Foro Romano view
Vaxjio, Sweeden– The Lake view

Acknowledgement

The present research was done in the context of the contract FW-DMA10-11, between
Flakt Woods Ltd and Fluid Machinery Research Team @ Dipartimento di Ing.
Meccanica e Aerospaziale “Sapienza” University of Rome


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