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

Development of an Innovative Multisensor Waveguide Probe With Improved Measurement Capabilities

[+] Author and Article Information
Giulio Lenzi

Department of Industrial Engineering,
University of Florence,
Via S. Marta, 3,
Florence 50139, Italy
e-mail: giulio.lenzi@unifi.it

Andrea Fioravanti

Department of Industrial Engineering,
University of Florence,
Via S. Marta, 3,
Florence 50139, Italy
e-mail: andrea.fiovaravanti@unifi.it

Giovanni Ferrara

Department of Industrial Engineering,
University of Florence,
Via S. Marta, 3,
Florence 50139, Italy
e-mail: giovanni.ferrara@unifi.it

Lorenzo Ferrari

CNR-ICCOM,
National Research Council of Italy,
Via Madonna del Piano, 10,
Sesto Fiorentino (FI) 50139, Italy
e-mail: lorenzo.ferrari@iccom.cnr.it

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 21, 2014; final manuscript received September 12, 2014; published online November 25, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(5), 051601 (May 01, 2015) (12 pages) Paper No: GTP-14-1420; doi: 10.1115/1.4028682 History: Received July 21, 2014; Revised September 12, 2014; Online November 25, 2014

Currently, waveguide probes are widely used in several turbomachinery applications ranging from the analysis of flow instabilities to the investigation of thermoacoustic phenomena. There are many advantages to using a waveguide probe. For example, the same sensor can be adopted for different measurement points, thus reducing the total number of sensors or a cheaper sensor with a lower operating temperature capability can be used instead of a more expensive one in case of high temperature applications. Typically, a waveguide probe is made up of a transmitting duct which connects the measurement point with a sensor housing and a damping duct which attenuates the pressure fluctuations reflected by the duct end. If properly designed (i.e., with a very long damping duct), the theoretical response of a waveguide has a monotone trend with an attenuation factor that increases with the frequency and the length of the transmitting duct. Unfortunately, the real geometry of the waveguide components and the type of connection between them have a strong influence on the behavior of the system. Even the smallest discontinuity in the duct connections can lead to a very complex frequency response and a reduced operating range. The geometry of the sensor housing itself is another element which contributes to increasing the differences between the expected and real frequency responses of a waveguide, since its impedance is generally unknown. Previous studies developed by the authors have demonstrated that the replacement of the damping duct with a properly designed termination could be a good solution to increase the waveguide operating range and center it on the frequencies of interest. In detail, the termination could be used to balance the detrimental effects of discontinuities and sensor presence. In this paper, an innovative waveguide system leading to a further increase of the operating range is proposed and tested. The system is based on the measurement of the pressure oscillations propagating in the transmitting duct by means of three sensors placed at different distances from the pressure tap. The pressures measured by the three sensors are then combined and processed to calculate the pressure at the transmitting duct inlet. The arrangement of the sensing elements and the geometry of the termination are designed to minimize the error of this estimation. The frequency response achieved with the proposed arrangement turns out to be very flat over a wide range of frequencies. Thanks to the minor errors in the estimation of pressure modulus and phase, the probe is also suitable for the signal reconstruction both in frequency and time domain.

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References

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Bergh, H., and Tijdeman, H., 1965, “Theoretical and Experimental Results for the Dynamic Response of Pressure Measuring Systems,” National Aero- and Astronautical Research Institute, Amsterdam, The Netherlands, Technical Report No. NLR-TR F.238.
Tijdeman, H., 1975, “On the Propagation of Sound Waves in Cylindrical Tubes,” J. Sound Vib., 39(1), pp. 1–33. [CrossRef]
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Figures

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Fig. 1

Scheme of a waveguide probe, d is the radius of the ducts, L1 and L2 are the length of transmitting duct and damping duct, respectively

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Fig. 2

Scheme of the analyzed system, Vd1, Vv1, and Vd2 are, respectively, the volume of transmitting duct, sensor housing, and damping duct. R and x are the duct radius and axial coordinate.

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Fig. 3

Transfer function for a waveguide probe with d=2 mm, L1=1, 3, 5 m, and L2=20 m

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Fig. 4

Transfer function for a waveguide probe with d = 2 mm, L1 = 1 m, and L2 = 5, 10, 15 m.

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Fig. 5

Transfer function of waveguide probe with L1 = 1 m, L2 = 20 m, and varying the sensor housing volume Vv1

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Fig. 6

Scheme of the sensor section in side-branch resonator configuration, d is the diameter of the duct and neck, LN is the length of the neck, LC is the length of camber resonator, and dS is the diameter of sensor head

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Fig. 7

Comparison between the response of a single sensor waveguide probe with an infinite long damping duct or with a muffler optimized for low frequencies

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Fig. 8

Scheme of a multisensor waveguide probe with a muffler as the damping termination

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Fig. 9

Scheme of the two sensors waveguide probe, Di duct of length Li, Ji junction, Dsi side branch sensors housing, pi vi state variables at the considered section. The sections are highlighted as dotted lines.

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Fig. 10

Scheme of the junction at second sensor housing J2, V3=V2=0 as boundary conditions

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Fig. 11

Scheme of the junction J1, V1=0 as boundary condition

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Fig. 12

Influence of δp0 modulus and phase on the estimation of p0. The smaller the modulus δp0, the smaller the error on the estimation of the modulus (Em) and phase (Eϕ) of p0.

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Fig. 13

Two sensor waveguide geometry (dimensions in mm)

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Fig. 14

Error amplification for the two sensors waveguide

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Fig. 15

Configuration chosen to calculate the pressure at the measurement section as a function of frequency

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Fig 16

Absolute error modulus estimation for different pressure oscillation amplitude at measurement section

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Fig. 17

Absolute error phase estimation for different pressure oscillation amplitude at measurement section

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Fig. 18

Comparison between probes with different number of sensors in terms of error amplification modulus

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Fig. 19

Effect of the spacing of the sensors on the modulus of the absolute error (δp0) with a three sensor probe in terms of average error and standards deviation over the range of 0–4000 Hz

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Fig. 20

Picture of the three sensor probe

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Fig. 21

Design of probe with infinite duct as damping termination

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Fig. 22

Calibration-test rig

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Fig. 23

Comparison between the ideal muffler an the real one

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Fig. 24

Picture of the waveguide equipped with the muffler

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Fig. 25

Plot of the configuration used as a function of the frequency

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Fig. 26

Absolute error modulus estimation for different pressure oscillation amplitudes at measurement section

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Fig. 27

Absolute error phase estimation for different pressure oscillation amplitudes at measurement section

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Fig. 28

FRF and phase shift of a single sensor waveguide with infinite long damping duct

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Fig. 29

Schematic of the blower and probe positioning

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Fig. 30

Pressure oscillations measured by the reference sensor and estimated by the multisensor waveguide

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Fig. 31

Pressure oscillations measured by the reference sensor and estimated by the single sensor waveguide

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Fig. 32

FFT of the pressure oscillation measured by the reference sensor and estimated by the multisensor waveguide

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Fig. 33

FFT of the pressure oscillations measured by the reference sensor and estimated by the single sensor waveguide

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Fig. 34

Differences between the FFT of the reference sensor and those estimated by the two probes

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Fig. 35

Map of the static pressure oscillation at the outlet of three blower blades

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