Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

Methodology to Correct the Magnetic Field Effect on Thin Film Measurements

[+] Author and Article Information
D. G. Cuadrado

Mechanical Engineering Department,
Purdue University,
West Lafayette, IN 47906
e-mail: gonza279@purdue.edu

S. Lavagnoli

Turbomachinery Department,
von Karman Institute for Fluid Dynamics,
Brussels 1640, Belgium
e-mail: lavagnoli@vki.ac.be

G. Paniagua

Mechanical Engineering Department,
Purdue University,
West Lafayette, IN 47906
e-mail: gpaniagua@purdue.edu

Contributed by the Controls, Diagnostics and Instrumentation Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 16, 2015; final manuscript received August 4, 2015; published online September 22, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(3), 031602 (Sep 22, 2015) (8 pages) Paper No: GTP-15-1336; doi: 10.1115/1.4031321 History: Received July 16, 2015; Revised August 04, 2015

Machined ferrous metal components may carry a magnetic field, which in rotation disturb the output of electrical sensors. To minimize the effect on the electrical instrumentation, the rotating components are usually demagnetized. However, even after the demagnetization process, a residual magnetism unavoidably remains. This paper presents a methodology to predict the effects of a rotating magnetic field induced on thin film measurements. In addition to the prediction of the magnetic effects, a procedure to correct the spurious variation in the readings of thin film gauges has been developed to enhance the fidelity of the measurements. An analytical model was developed to reproduce the bias on the electrical signal from sensors exposed to rotor airfoils with magnets. The model is based on the Biot–Savart law to generate the magnetic field, and the Faraday's law to calculate the electromotive force induced along the measurement circuit. The model was assessed by means of controlled experiments varying the rotor tip clearance and rotational speed. The presented methodologies allowed the correction of the magnetic field effects. The raw signal of the thin film sensors, in the absence of any correction, is prone to deliver errors in the heat flux amounting to about 8% of the mean overall value. Thanks to the developed corrective approach, the residual magnetic effect contribution to the heat flux error would be 2% at most.

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Fig. 1

Representation of the model with the different variables used during the calculation

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Fig. 2

Sketch of the experimental arrangement, composed of a 16-teeth wheel disk, a micro-adjustable table, and a pneumatic engine, without MACOR block

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Fig. 3

(a) Location of the measurement points used for the magnetic field verification. (b) Different orientations checked using a gauss meter (red line) attached to the MACOR block support (unfilled rectangles).

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Fig. 4

(a) Magnetic field at orientations 0 deg and 180 deg. (b)Magnetic field characterization at 90 deg.

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Fig. 5

Experimental versus modeled EMF at: (a) 0 deg, 2000 rpm, hlf  = 0.2 mm; (b) 90 deg, 1000 rpm, hlf  = 1.0 mm; (c) 0 deg, 1000 rpm, hlf  = 1.0 mm; and (d) 90 deg, 2000 rpm, hlf  = 0.2 mm

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Fig. 6

(a) X distance input. (b) Model fitting curve with nlinfit. (c) Model fitting curve at 500 rpm and 1.5 mm.

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Fig. 7

(a) C1 coefficient surface model result, (b) C2C3C4 coefficients model result at 1500 rpm and different distances, and (c) C2C3C4 coefficients model result at 1 mm and different rpm

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Fig. 8

(a) Model C1 coefficient fitting, (b) model C2 coefficient surface fitting, and (c) model C4 coefficient surface fitting

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Fig. 9

(a) Fitting result at 2500 rpm and 0.5 mm. (b) Fitting result at 2000 rpm and 0.75 mm.

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Fig. 10

(a) C1 coefficient surface experiment result, (b) C2C3C4 coefficients experiment result at 1500 rpm and different distances, and (c) C2C3C4 coefficients experiment result at 1.5 mm and different rpm

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Fig. 11

(a) C1 coefficient fitting for experiment, (b) C2 coefficient fitting for experiment, and (c) C4 coefficient fitting for experiment

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Fig. 12

(a) Voltage perturbations with 20 deg phase delay. (b)Output comparison. (c) Direct subtraction error.




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