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Effects of Installation on Dynamic Pressure Measurements

Guoqiang Li* and Ephraim J. Gutmark.† University of Cincinnati, Cincinnati, OH 45220, USA

Combustion dynamics pose serious challenge for modern low NOx emission gas turbine combustors. Accurate detection of pressure oscillation signals associated with combustion dynamics is crucial for control of combustion dynamics and assurance of reliable operation and extended life time of the engines. However, the installation of the pressure transducer is often limited by the temperature the sensor can withstand. This paper addresses the possible signal distortion due to commonly used pressure sensor installation on a stand-off tube. The effects of the tube and extension “tails” lengths on the measured pressure signals using different types of piezoelectric pressure transducers are described. A two-dimensional wave tube is used to generate clean planar wave for this study. The stand-off tube was found to be responsible for pseudo peaks in the spectrum whose frequency depends on the tube length. The extension “tail” also modifies the spectrum of the detected signal by introducing low frequency modulation of the spectrum. For proper signal detection, the length of the tube has to be carefully selected to avoid artificial distortion of signals at the frequency range of interest.

Nomenclature Ls = length of the stand-off tube Lb = length of the installation tube Lt = length of the extension “tail” f = frequency

I. Introduction easurement of dynamic pressure inside combustion chambers is crucial for the control of combustion dynamics in modern lean burn gas turbine

engines. The extremely high temperature of the combustion chamber prevents the direct installation of any type of commercially available piezoelectric pressure transducer on the combustor liner. Therefore, a short length of a stand-off tube is often used for installation of the pressure transducer to provide temperature cushion and protect the sensors 1, 2. An example of this application is shown in Fig. 1. A 2” (5cm) long ¼” tube was installed between the sensor and the combustion chamber wall. With this short section tube, the sensor can survive even when the wall temperature reaches 1000ºC. However, the installation of a short tube can introduce artificial noise that can contaminate the dynamic pressure signal as seen in Fig. 2. The frequency peak detected near 1000 Hz remains even when the combustion was turned off and the system was driven only by white

* Research Assistant Professor, Department of Aerospace Engineering and Engineering Mechanics, University of Cincinnati, Cincinnati, OH, AIAA Member. † Professor, Ohio Eminent Scholar, Department of Aerospace Engineering and Engineering Mechanics, University of Cincinnati, Cincinnati, OH, AIAA Associate Fellow.

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Figure 1: Installation of pressure sensors on the wall of a combustion chamber.

44th AIAA Aerospace Sciences Meeting and Exhibit9 - 12 January 2006, Reno, Nevada

AIAA 2006-1387

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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noise. Therefore, the effect of the tube length has to be addressed in order to correct the distorted signal and avoid signal contamination in the frequency range of interest. It is often seen in dynamics pressure measurement of combustion dynamics investigation 1, 2 that a very long tube is attached at behind the sensor installation tube to prevent acoustic reflection. Identifying the noise source and evaluating the effects of installation on the dynamic pressure signal is critical for filtering the noise and extract the real signal when the installation tube has to be used for measurement. The primary objective of this study is to investigate the effects of sensor installation via signal testing using a specifically designed wave tube.

II. Experimental Setup The test of pressure transducers was conducted on a

two dimensional wave tube in the Gas Dynamics and Propulsion Laboratory. Figure 3 shows the picture of the wave tube with the sensors being tested mounted on the top end wall. The wave tube3 has a 0.0381-meter by 0.0381-meter square cross section and is 0.635 meter long. An acoustic driver is attached at one end of the tube and a mounting plate for the different pressure transducers is placed at the other end. Acoustic analysis of hard wall duct shows that the cut-off frequency of this wave tube is 4500Hz for the dominant transverse mode 3. The acoustic speaker (Model 24V-JBF from JBF) is driven by an amplifier (Model MPA-101 100W P.A. amplifier) which is driven by a function generator (HP 8904A Multifunction Synthesizer DC-600kHz). The driving frequency for this study is limited to 50 - 1000 Hz.

Two piezoelectric pressure transducers were tested in this study: 7061B from Kistler (with 5010 Charge amplifier gain at 0.1 and resolution 10000Pa/Volt), 102A from PCB (with resolution 68940PA/Volt), each with its corresponding charge amplifier, as seen in Fig. 3. Hereafter, we name 7061B as Sensor1, 102A as Sensor2. For the same sensor, the installation tube length varied from 3.81cm (1.5”) to 11.43cm (4.5”) with or without the “infinite” long “tail”. The “tail” consisted of a rolled tube with one end plugged. Figure 4 shows the different length of tubes, Lb, used in this study. Lb is 5.7, 8.26, and 13.3cm. The stand-off distance, however, includes the length of the fitting mounted to the plate and the fitting on which the sensor in mounted. Therefore, the stand-off length, Ls, is 7.6, 10.2, and 15.2cm, respectively. The length of the “tail”, Lt, is 23 and 609cm. The flush mounting configuration, L=0, is the baseline case in which no installation tube affects the measurement.

Two sets of tests were conducted on the wave tube: tests with different tube lengths and tests with different “tail” lengths. In order to compare the different sensors, two sensors were installed on the same mount using the same length of installation tube (Fig. 5) and the signals were sampled simultaneously. The data is sampled by a National Instrument DAQ board at 8000Hz for each channel. At each driven frequency, the sensor signals are recorded and analyzed with FFT to get the pressure magnitude of the driving frequency.

Figure 2: Noise signal introduced from the installation tube during atmospheric combustion

Figure 3: Wave tube with installation of pressure sensors at the end.

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III. Results and Discussion

A. Effects of installation tube length Lb

Figure 6 shows the comparison of detected pressure magnitude by Sensor1 installed with Lb =0, 5.7, 8.26, and 13.3cm at the driving frequency from 50 to 1000Hz. When the sensor was flush mounted on the end wall, the signal detected by the sensor was flat in the frequency range from 200 to 1000 Hz. It should be noted that the increase in the pressure level from 50 to 200Hz is due to the reduced low frequency response of the acoustic speaker. When Lb=5.7cm, the frequency response peaks at 850Hz. This peak frequency shifts to a lower frequency when the length increases: to 700Hz for L=8.26cm and to 500Hz for L=13.3cm. Similar results were observed for Sensor2. The signal distortion can be interpreted by comparison between the spectrum with the installation tube to the baseline case, in which the sensor is flush mounted and no signal distortion is expected. For Lb=5.7cm, the spectrum overlaps with the baseline case, i.e. real signal spectrum, up to 500Hz. This overlapping region reduced to about 300Hz when Lb elongated to 13.3cm. Therefore, if the interested frequency region is around 300Hz, the shorter tube of Lb=5.7cm can be used to detect the real signal if a proper low-pass filter (500Hz cutoff frequency) is inserted. For high frequency signal, a longer tube is required.

Figure 4: Different length installation tubes and one example of tail used for tests.

Figure 5: Installation of sensors on the end plate of the wave tube.

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Figure 6: Detected pressure magnitude for Sensor1 installed with Lb =0, 5.7, 8.26, and 13.3 cm.

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When plotting the peak frequency against the tube length as shown in Fig. 7, it was found that the frequency shift followed nearly the same trend for both Sensors 1 and 2. If the speed of sound is assumed to be 340m/s for 25ºC room temperature, a quarter wave mode frequency can be calculated for each stand-off distance, as shown by the calculated curve in Fig. 7. Comparing the calculated frequency with the measure values, it was determined that the peak frequency was very likely to be close to the quarter wave mode frequency that varies with the stand-off distance from the sound source. The disparity between the measured frequency and the calculated value can be contributed to the measuring error of the exact distance between the sound source and the sensor, and the irregularity of the interface between the ¼” tube and the fitting. This irregularity is also the possible reason for the difference between the peak frequency of the two Sensor 1 and 2.

B. Effects of the extension “tail”

The attachment of a long “tail” tube can also affect the signal detection. To evaluate this possibility, a 23cm or a 609cm long “tail” rolled from ¼” tube was attached to the sensor mounting fitting. The result is shown in Fig. 8 for Sensor 1. Attachment of long “tails” eliminated the peak resonance frequency that was identified in Fig. 6. However, the frequency response caused signal amplitude waviness for varying frequencies. Compared with the baseline case, the signal peaks at 200, 375, 550, 700, 850, and 1000Hz for Lt=23cm, and 150, 450, and 700 Hz for Lt=609cm, indicating a low frequency modulation of the spectrum. This low frequency can be estimated as 6 Hz for Lt=23cm and 3 Hz for Lt=609cm. The longer “tail” results in lower modulation frequency. This observation suggests that the attached “tail” is effective in reducing the resonance of the installation tube but can generate low frequency modulation of the signal that is dependant on the “tail” length. Tests with Sensor 2 confirmed the effect of the “tail” on the signal modulation as seen in Fig. 9.

Although long “tail” attachment has

been used in the dynamic pressure signal monitoring of real engines1,2, the reliability of the results is questionable, especially when the sensor is used for detecting low frequency pressure oscillations near the lean flame off. The unexpected low

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Figure 7: Dependence of peak frequency on the stand-off distance for Sensor 1 and 2.

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Figure 8: Detected pressure magnitude for Sensor1 installed with Lb =5.7cm and Lt=23cm and 609cm.

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Figure 9: Detected pressure magnitude for Sensor2 installed with Lb =5.7cm and Lt=23cm and 609cm.

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frequency modulation on the frequency spectrum is more difficult to filter because it may fall in the range of the frequency of interest. It is also noticed that for industrial applications, a smaller diameter 1/8” tube is used rather than ¼”. The effects of tube diameter on signal distortion require further study if a possible volumetric acoustic resonance is present in the installation tubing in addition to the quarter wave mode discussed in this study. Some signal conditioning technology suggested by Harrison et al.4 may be needed to reconstruct the real signal from possible distortion.

IV. Conclusions Installation effects on the accuracy of dynamic pressure measurements are often overlooked because of the

relatively high signal to noise ratio during strong combustion dynamics. However, this issue can become significant when the signal to noise ratio becomes low for flames close to lean blow off, for example. It was shown in the present study that the installation tube length, or the stand-off distance between the sensor and the combustion chamber wall, affected the detected pressure signal by forming a resonance frequency signal that is close to the quarter wave mode of the tube. The attachment of long “tail” complicated and contaminated the pressure signal by introducing low frequency modulation that depended on the “tail” length. Signal distortion due to installation is particularly important when the frequency range of interest overlaps the resonance frequency caused by the installation tubes. Further acoustic modeling and analysis is needed to extend the understanding of this phenomenon and for using the knowledge in practical applications.

V. References 1Rea, S., James, S, Goy, C, and Colechin, M., “On-line Combustion Monitoring on Low NOx Industrial Gas

Turbines”, Measurement Science and Technology, Vol. 14, 2003, pp.1123-1130. 2Lubarsky, E., Shcherbik, D., Scarorough, D., Bibik, A., and Zinn, B. T., “Onset of Severe Combustion

Instability during Transition to Supercritical Liquid Fuel Injection in High Pressure Combustors”, AIAA-2004-4031, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida 11-14, July, 2004.

3Hillereau, N., “Suppression Characteristics of Acoustic Liners with Porous Honeycomb”, Master Thesis, 2004, University of Cincinnati.

4Harrison, M.F., Stanev, P.T., “Measuring wave dynamics in IC engine intake systems, Journal of Sound and Vibration, Vol. 269, 2004, pp. 389–408.