Design Considerations for Remote High-Speed Pressure Measurements of Dynamic Combustion Phenomena Page: 3 of 11
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150 Hertz, it is important to understand the pressure measurement implications as a result of uncertainties in the
In the ideal situation, the pressure transducer could be placed directly into the combustion environment and the
errors introduced by the waveguide, or semi-infinite coil, could be avoided. However, this approach is limited by
the maximum service temperature of the pressure transducer. Therefore, it is sometimes necessary to attach the
pressure transducer to a remote sensing unit, such as a semi-infinite coil. Based on information available in the
literature, a continuous gas purge is not a common practice for remote sensing units used on most gas turbine
systems, and as a result the attenuation in these systems is primarily due to visco-thermal effects.
This paper will focus on the semi-infinite coil approach with no purge gas, or mean flow, through the remote
pressure sensing unit. Utilizing experimental data obtained through this study, theoretically derived transfer
functions relating the pressure measured in the remote unit to the desired pressure in the main body of a simulated
gas turbine combustor are compared to the actual measured response. Since viscous attenuation is sensitive to
surface imperfections and several other apparatus specific conditions, the data presented in this paper is specific to
this experimental setup. However, these results may provide an order of magnitude estimate of the errors for remote
pressure sensor configurations that incorporate a semi-infinite coil arrangement.
II. Experimental Setup
Figure 1 shows a sketch of Speaker
the experimental closed-opened Enclosure
tube arrangement used to Pressure Transducers
simulate a gas turbine combustor. P (Kistler Model 206)
A 43 cm (17 in) long, 7.6 cm (3
in) diameter commercial plastic
(PVC) pipe is attached to a
speaker enclosure on one end
while the other end is open to the
room. The remaining system Spea P1 Tubing Coils
components consist of a speaker
(Boston Model G212), an
amplifier (Realistic Model 32-
2027A), and a spectrum
analyzer/frequency generator Figure 1: Sc of pri t
(Agilent Model 35670A). Pressure r
(Po) and in the remote sensing unit (P1). The sensor at station Po is flush-mounted in the walls of the pipe while the
sensor at station P1 is inserted into a block that orients the sensor normal to the tube wall and minimizes the volume
of the transducer cavity. The volume of the transducer cavity is approximately 0.1 cm3 which is roughly three
orders of magnitude smaller than the total volume of the tubing. The inside diameter of the interconnecting tubing
for the semi-infinite coil is 3.8 mm (0.15-in) and the overall length of the remote sensing unit is approximately 12.93
meters (509 in). The coil arrangement allows for the distance between Po and P1 to be varied.
The amplifier/speaker is excited using a random noise generator from the dynamic signal analyzer (Agilent
35670A). This analyzer is also used to compute the complex frequency response functions and the respective
coherence functions. These functions are obtained from 150 spectral averages and the results are stored for post-
processing in a MathCad program. Utilizing the coherence as a filter, data with values less than 0.98 have been
III. Data Analysis
A. Segmented Line Equation Model (Bergh and Tijdeman13)
This model is an extension of the work of Iberall"7 (1950) and is considered by others12,s to be the best approach
for this type of problem. Samuelson16 provides a good overview of the assumptions required to simplify Bergh and
Tijdeman's generalized segmented-line equation into the expression shown in Eq. 1. The key assumption is that the
tubing is homogeneous everywhere, except at the transducer tap.
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Straub, D.L.; Ferguson, D.H.; Rohrssen, Robert (West Virginia University, Morgantown, WV) & Perez, Eduardo (West Virginia University, Morgantown, WV). Design Considerations for Remote High-Speed Pressure Measurements of Dynamic Combustion Phenomena, article, January 1, 2007; (digital.library.unt.edu/ark:/67531/metadc881575/m1/3/: accessed October 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.