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Whitepaper Draft V0_1_1

Starting: 14 Apr Ending

0 days left (ends 14 May)

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This discussion is open for a limited time. Please review, comment and vote for the individual sections. You can quote certain parts of a paragraph by marking the text and open a comment.

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P70

The most universally applicable solution of cable termination throughout different measuring technologies is having open-wire leads. This can be done for any measuring technology that relies on the measurement of electrical properties and is not limited to a certain number of leads.

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P71

For two-wired connections three connector types are in widespread use. 2 pole connectors similar to the LEMO®PCA.0S.302 are commonly used for lower to medium temperature applications. For connections in hotter environments transducer manufacturers offer 2 pole connectors with a 7/16”-27 UNS-2A thread. The third common option for two-wired connections is represented by the termination as open leads, which allows the customer to decide on a suitable connection method. The open leads option is generally rated for low to medium temperatures. The three mentioned connection types are presented in Figure 7.1 below [10]. Smaller transducer shapes sometimes offer coaxial 10-32 UNF Microdot connectors [17].This type of connector is usable for different surrounding temperatures, depending on the temperature rating. Generally speaking, it can be noted, that the temperature capability of all the mentioned two-wire connection types is assessed and given by the transducer manufacturer. This means, that the same type of connector may have different rated temperatures depending on the manufacturer.

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P72



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P73

For connections involving more than two wires, there are no commonly used cable terminations except for open leads. Most datasheets for commonly used piezoresistive transducers offer this type of termination and provide the according color code and a wiring diagram for the product [15][16]. Some transducer manufacturers do offer suitable connector solutions for their products with three or more wires but they are not as widespread as the mentioned two-wired connectors.

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P74

Outline:

This chapter shall present different transducer mounting best practices (flush mounting, standoff mounting, ...) with their respective advantages and disadvantages. Mounting interfaces (flanges, threads, ...) in general and considerations towards standardized port designs for transducer reference shapes can be included. The following content can be used as a basis for further work.

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P75

The following section describes common mounting configurations for measuring dynamic pressure in gas turbines. It also focuses on the advantages and disadvantages of those basic principles. There are two main mounting principles: Flush mounting and standoff mounting. The naming refers to the positioning of the transducer membrane or pressure port within the measuring setup.

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P76

A transducer is mounted flush when the membrane of the transducer is in plane with the inner side of the engine housing. Flush mounting allows for direct measurement of pressure fluctuations within the observed section of the engine. It also enables measuring the pressure at exact locations without disturbing the fluid flow around, which is relevant for various applications. Flush mounting enables using the available frequency range of the transducer without accounting for pipe oscillations or other reflection phenomena caused by additional space between the membrane and the pressure vessel wall. A major disadvantage of flush mounting is the direct exposure to the heat and heat fluctuations within the observed part. Susceptibility to measure local flow disturbances instead of the wider pressure behavior can also be a contributing factor to decide against flush mounting. The angle of incidence between the axis of the sensor’s diaphragm and the approaching pressure wave can lead to uneven deflections of the diaphragm, which can alter the measurement [18]. This mounting form is one of the most demanding and requires rugged transducers.

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P77

The depiction in Figure 8.1 shows a transducer mounted flush inside a threaded boss which is welded to the engine casing. This type of mounting is suitable when only a specific transducer shape will be used for the measurements.

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P78



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P79

Figure 8.2 shows the same transducer but mounted in a flange which is mounted to the boss. This type of mounting is preferable when transducers with different shapes and dimensions are to be used since different flanges for different transducer shapes can be designed to fit the same boss.

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P80



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P81

A common practice for measuring dynamic pressures at sections of the engine where the temperature exceeds the temperature capabilities of the transducer available is to mount the transducer to a standoff sensing tube. This mitigates the thermal stress on the transducer whilst still offering the possibility to measure at specific points within the engine. A recess mounted transducer at the end of a standoff tube is the simplest form of such mounting. Figure 8.3 shows an exemplary draft for such a setup.

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P82


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P83

This type of mounting reduces or removes the influences of the angle of incidence between the diaphragm and the pressure wave. A major disadvantage is its vulnerability to reflection phenomena like influences by pipe oscillations.

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P84

Pipe oscillations are phenomena where pressure waves get reflected within tubes at open and/or closed endings. The result is standing waves with frequencies of the resonant frequencies of the respective tube, which are formed naturally. These frequencies need to be taken into account when measuring dynamic pressure. The formula to compute these resonance frequencies is given in Eq. . In the formula L is the length of the tube, c is the speed of sound of the measured fluid and n is a factor for calculating the resonant frequencies. [19]

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P85


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P86

One way of mitigating the influence of pipe oscillations is by mounting the transducer to the side of a sensing tube which is continued into a semi-infinite tube at the end. Figure 8.4 depicts a schematic of such a mounting. The idea behind this method is to make the semi-infinite tube from a flexible material and long enough to dampen the pressure waves before they can be reflected. [20]

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