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High Bandwidth Temperature and Pressure Sensors

Bill MacPherson, Jim Barton, Julian Jones

Introduction

High bandwidth temperature and pressure sensors are becoming increasingly important for challenging engineering applications.  In our research we have concentrated on airflow measurements for aeroengine and air-blast characterisation.

The drive to improve fuel efficiency and performance of modern aircraft engines requires optimum designs for engine compressors and turbines. Unsteady transonic gas flow through a turbine rotor stage is difficult to model numerically, therefore experimental trials of new blade shapes are made in transient flow wind tunnels that simulate engine conditions at scaled-down temperatures. Turbine blades run in combustion gases above the melting point of the blade alloy, so it is vital to understand heat transfer processes in some detail in order to ensure effective blade cooling.

This project developed fibre optic probes, rugged enough to use in test rigs, for measuring gas temperature, heat flux and gas pressure at high bandwidths. In this work we collaborated with the Osney Laboratory (Oxford University), Sheffield University and QinetiQ Pyestock on probe designs suitable for their applications.

Typical flow conditions:

  • transonic (Mach ~1)

  • 10 kHz rotor blade passing frequencies and/or microsecond event risetimes

  • transient flow temperature ~150K above ambient

  • particulates such as oil mist.

Potential advantages of fibre sensors:

  • small size probe head and sensing area

  • high bandwidth (10s kHz )

  • all-dielectric construction (fibre-only connection, no e/m pickup, inherent safety).

Principle of operation

Heat flux and gas temperature measurements are based on optical fibre interferometers operated in reflection. Heat flux sensors are a few mm of single-mode fibre reflectively spliced to a connecting fibre ‘downlead’, and embedded within the test object to act as a calorimeter. Gas temperature sensing is accomplished by a thin film optical coating on the fibre end face, exposed normally to the flow. A dual head probe has also been demonstrated capable of total gas temperature measurement based on a total temperature probe designed at Oxford University.

Pressure sensors are very small scale diaphragms attached to the end face of an optical fibre, with a small (10s of microns) air gap between the diaphragm and the fibre downlead end face. The sensor is interrogated by a stepped-wavelength technique to measure the interferometer’s optical phase. The small size maximises the frequency response and minimises temperature cross-sensitivity.

High bandwidth pressure sensing

One compelling advantage of micromachined sensors is their small (sub-mm) size. In collaboration with the University of Oxford and QinetiQ Pyestock, we embedded 5 sensors in the trailing edge of a nozzle guide vane (NGV), which is the stator blade ahead of the rotor in a gas turbine stage. The NGV was installed in the Pyestock Isentropic Light Piston Facility, which simulates closely the flow conditions inside the high-pressure turbine of an aircraft engine. The 60-blade rotor spins at 8000 rpm for transient flow tests between 500 and 1000 ms duration, during which time-dependent pressure signals were measured by the optical sensors. Three-wavelength interrogation was used to recover the phase signals from the Fabry-Perot cavity in each sensor.

The results show pressure fluctuations of about ~8-10 kPa at the blade passing frequency of 8 kHz with harmonics up to 180 kHz above the noise floor. Previously, the larger size of conventional high bandwidth pressure sensors limited the experiments to scale-model tests in less realistic flow regimes. Data on unsteady flow are required for a better understanding of turbine aerodynamics, to improve engine efficiency.

  

 (left) NGV with 5 embedded sensors.    (right) Rotor-averaged pressure signals.

References

  1. S. Watson, M J Gander, W N MacPherson et al, "Laser-machined fibres as ultra-miniature pressure sensors", European Workshop on Optical Fibre Sensors, Santander, Spain, 2004.

  2. M.J.Gander, W.N.MacPherson, J.S.Barton, R.L.Reuben, J.D.C.Jones, R.Stevens, K.S.Chana, S.J.Anderson and T.V.Jones ‘Embedded micromachined fiber-optic Fabry-Perot pressure sensors in aerodynamics applications’ IEEE Sensors Journal: Special Issue on Fibre Optic Sensors, 3, 102 – 107, 2003

  3. J M Kilpatrick, W.N.MacPherson, J.S. Barton, J.D.C. Jones, D.R. Buttsworth, T.V. Jones, K.S. Chana and S.J. Anderson, (2002) Measurement of unsteady gas temperature with optical fibre Fabry-Perot microsensors, Measurement Science & Technology 13, 706-712

  4. Miniature fiber optic pressure sensor for turbomachinery applications, WN MacPherson, JM Kilpatrick, JS Barton, JDC Jones, Review of Scientific Instruments, 1999, Vol.70, No.3, pp.1868- 1874

 

 

 

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