Method and apparatus for measurement of unsteady gas temperatures

Apparatus for the measurement of unsteady gas temperatures comprises (a) a temperature probe having a sensing element. The sensing element has an optical interferometer optically coupled to one end of a first, addressing optical fibre. The interferometer has a first partially reflective surface defined at the end of the addressing fibre and a second partially reflective surface spaced from the first partially reflective surface by an optical path length I. The apparatus further comprises (b) a light source optically coupled to a second end of the addressing fibre, (c) an interrogating optical path optically coupled to the addressing fibre by a beam splitter whereby a portion of an optical phase signal from the sensing element is directed to a first end of the interrogating path and a portion of the input light from the light source is directed to a second end of the interrogating path, (d) first photodetector coupled to the first end of the interrogating path; and (e) data acquisition and processing means connected to the photodetector means, the data acquisition and processing means being adapted to derive the temperature of the sensing element from the phase signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to sensors and associated apparatus and 
methods for high bandwidth, unsteady gas temperature measurement, based on 
interferometry using optical fibres. The invention is particularly 
concerned with the measurement of gas total temperature measurement in 
turbomachinery, most particularly in gas turbines and compressors as used, 
for example, in aeroengines. 
2. Discussion of Prior Art 
Aeroengine development continues to demand exacting improvements in 
compressor performance. Military aeroengines require increased 
thrust-to-weight ratio and decreased cost of ownership while maintaining 
adequate levels of stable operating range and efficiency. Improving 
efficiency is the primary aim of civil engine development. These 
requirements are responsible for generic trends in modern compressor 
design. Fewer rotor stages tend to be used, which increases the 
aerodynamic stage loading. Blade rows tend to be spaced closer together 
which, coupled with the increased aerodynamic loading, increases the 
influence that each row exerts on neighbouring rows. There is also a 
tendency for the aspect ratio of the blades to be decreased which 
increases the complexity of the boundary layer flows on the blades, and on 
the end walls. The consequence of these trends is that the unsteady flow 
field within the compressor becomes more significant and needs to be taken 
into account during the design and development of future compressors. 
In response to these trends, there is a growth in the attention being paid 
to unsteady blade row interaction in turbomachinery. However, because of 
the problems associated with taking unsteady aerodynamic measurements in 
high speed turbomachinery, there are few measurements in engine-relevant 
compressors and there is consequently a poor empirical understanding of 
the fundamental flow processes involved. Nevertheless, concerted effort 
since the mid-1980's has overcome many of the problems associated with 
taking wide bandwidth pressure measurements in high speed compressors and 
such sensors have been employed in engine-relevant machines. 
Unfortunately, the situation is less satisfactory when unsteady 
temperature measurements are considered. Both unsteady pressure and 
temperature measurements are required if the compressor efficiency and 
entropy flux are to be measured accurately. Further, it is recognised that 
the measurement systems used to derive steady state blade row performance 
do not respond properly to the highly pulsatile filed behind rotor rows. 
Therefore there is a need for pressure, and particularly temperature, 
measurement systems capable of resolving the fluctuating flow field so 
that more accurate steady-state measurements can be derived. 
SUMMARY OF THE INVENTION 
The invention thus relates to measurement apparatus and methods, including 
a measurement probe based on a fibre optics sensor, for use in measuring 
rapidly varying temperatures in such applications. The present disclosure 
includes the results of a demonstration of an embodiment of the probe in a 
continuous flow compressor test rig. Similar techniques may also be 
applied to the measurement of pressure at high bandwidth. 
It is an object of the invention to provide an optical fibre based sensor 
allowing the measurement of unsteady temperature fluctuations in high 
speed compressors. Unsteady pressure measurements in such a compressor 
have revealed periodic and random flow effects. A corresponding 
measurement of unsteady temperature was sought in the same compressor, 
operating under the same conditions. For this purpose it was required that 
the optical fibre sensor have: 
(i) a wide frequency bandwidth of up to 60 kHz to resolve the main flow 
features associated with blade passing frequencies of up to 12 kHz; 
(ii) small physical size (diameter 6 mm) to allow insertion of the probe 
between closely spaced compressor blade rows; and 
(iii) robustness to withstand the harsh physical environment (i.e. 
high-transonic Mach number flows laden with oil mist). 
A gas temperature resolution of less that 1 K is desirable, with a sensor 
operating range up to 600 K. 
Whilst the above mentioned performance parameters are desirable for typical 
turbomachinery applications of the invention, the invention is not 
restricted to apparatus or methods fulfilling these requirements. In 
particular, embodiments of the invention providing bandwidths less than 60 
kHz may be useful in other applications while still providing significant 
advances over known unsteady temperature sensing techniques. 
A variety of temperature sensors are known for unsteady measurements in 
turbomachinery, though none have a bandwidth as high as 60 kHz. For 
example, thermocouple response is limited to about 1 kHz; constant current 
hot-wire sensors are cross-sensitive to gas velocity fluctuations; 
thin-wire resistance thermometers are less sensitive to velocity but 
require compensation as a function of flow speed and show ageing effects 
in use. Another technique is the aspirating probe with a reported 
bandwidth of about 20 kHz. This probe consists of a pair of hot wires 
operating at different overheat ratios upstream of a choked orifice. While 
this configuration is more robust than an isolated hot wire (and is 
capable of providing pressure measurements as well), the wires are still 
prone to ageing and the calibration procedure required is complex. 
In accordance with a first aspect of the invention there is provided 
apparatus for the measurement of unsteady gas temperatures characterised 
in having (a) a temperature probe having a sensing element comprising 
optical interferometer means optically coupled to a first end of a first, 
addressing optical fibre, said interferometer means comprising a first 
partially reflective surface defined at said first end of said addressing 
fibre and a second partially reflective surface spaced from said first 
partially reflective surface; (b) a light source optically coupled to a 
second end of said addressing fibre; (c) an interrogating optical path 
optically coupled to said addressing fibre by beam splitting means whereby 
a portion of an optical phase signal from said sensing element is directed 
to a first end of said interrogating path and a portion of the input light 
from said light source is directed to a second end of said interrogating 
path; (d) first photodetector means coupled to said first end of said 
interrogating path; and (e) data acquisition and processing means 
connected to said first photodetector means, said data acquisition and 
processing means being adapted to derive the temperature of said sensing 
element from said phase signal. 
Preferably the interferometer means comprises a thin optical film deposited 
on the end face of said addressing fibre at the first end thereof, 
providing a first partially reflective surface at the interface between 
said film and said end face and a second partially reflective surface at 
the outer face of said film remote from said fibre end face. The thin 
optical film preferably has a thickness of up to 5 microns. The use of 
thin films tends to give the apparatus a better signal to noise ratio 
because the thin film has a higher thermo optic coefficient. Also, 
preferably the optical film comprises zinc selenide or titanium dioxide. 
In accordance with a second aspect of the invention there is provided a 
method of measuring unsteady gas temperatures characterised by a (a) 
locating a temperature probe in the required position, said temperature 
probe having a sensing element comprising optical interferometer means 
optically coupled to a first end of a first, addressing optical fibre, 
said interferometer means comprising a first partially reflective surface 
defined at said first end of said addressing fibre and a second partially 
reflective surface spaced from said first partially reflective surface; 
(b) illuminating said sensing element with light from a light source 
optically coupled to a second end of said addressing fibre; (c) 
interrogating said sensing element by means of an interrogating optical 
path optically coupled to said addressing fibre by beam splitting means 
whereby a portion of an optical signal from said sensing element is 
directed to a first end of said interrogating path and a portion of the 
input light from said light source is directed to a second end of said 
interrogating path; (d) monitoring the optical signal from said sensing 
element by means of first photodetector means coupled to said first end of 
said interrogating path; and (e) processing said optical signal to derive 
the temperature of said sensing element by means of data acquisition and 
processing means connected to said first photodetector means.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, FIG. 1 shows a first embodiment of a sensor 
in accordance with the invention, including a sensing element comprising a 
thin optical film 10 deposited on the end face of an optical fibre 12. The 
fibre 12 comprises a core 14 surrounded by cladding 16. Laser light 
launched into the input end of the fibre core 14 is partially reflected by 
both sides of the film; i.e. a first partial reflection occurs at the 
interface between the fibre 12 and the film 10 and a second partial 
reflection occurs at the outer face of the film 10 remote from the end of 
the fibre 12. The two reflected beams differ in phase by an amount 
proportional to the optical thickness of the film 10. Interference between 
the two beams results in a total reflected light intensity that is a 
periodic function of the optical phase difference between the beams. This 
phase difference is a linear function of the mean temperature of the film, 
since the film thickness and refractive index depend linearly on 
temperature, as is discussed in greater detail below. The reflected signal 
can therefore be used as a measure of the film temperature. As shown, the 
film thickness=1, film refractive index=n; and n.sub.c and n.sub.o are the 
refractive indices of the fibre core and the medium in which the fibre is 
immersed, respectively. In this example, the optical film 10 employed is 
formed from zinc selenide (ZnSe), which has a relatively high refractive 
index and a strong temperature coefficient. Other suitable materials might 
be employed, such as titanium dioxide (TiO.sub.2) which has a refractive 
index somewhat higher than zinc selenide but a weaker temperature 
coefficient. 
The basic optical arrangement of a temperature measurement system 
incorporating the sensor of FIG. 1 is shown in FIG. 2. The arrangement 
includes a first, addressing optical fibre 20 and a second, interrogating 
optical fibre 21, coupled together by a directional coupler 22. Laser 
diode light is launched from a laser diode 18 into a first arm 30 of the 
addressing fibre 20 via a first, collimating, lens 24, an optical isolator 
26 and a second, focusing, lens 28 (as is well known in the art). The 
directional coupler 22 splits the incoming light between a second arm 32 
of the addressing fibre 20 (corresponding to the fibre 12 of FIG. 1), 
leading to the measurement probe 34 incorporating a sensor as shown in 
FIG. 1, and a first arm 38 of the interrogating fibre 21, leading to an 
intensity reference detector 36, respectively. The intensity reference 
detector 36 allows the output from the laser diode 18 to be monitored for 
comparison with the signal from the probe 34 itself. 
The signal reflected from the sensor of the probe 34 returns to the coupler 
22, where it is split between the first arm 30 of the addressing fibre 20, 
leading back to the isolation optics 26 (which prevent the light of the 
reflected signal from reaching the laser diode 18), and a second arm 40 of 
the interrogating fibre 21, leading to a signal detector 42. Output 
signals from the intensity reference detector 36 and the signal detector 
42 are passed to data acquisition and processing means 44 for processing 
to provide the required temperature measurements. 
Unlike conventional sensors, the fibre optic probe 34 has no electrical 
connections to the measurement area, thus eliminating electrical 
interference. Connecting fibre lengths of 200 m are feasible, allowing the 
launch and detection optics and signal processing to be situated remote 
from the extreme noise and vibration of an operating compressor rig. 
The fibre sensor possesses several features implying its potential for high 
bandwidth temperature measurement. The optical power required to 
interrogate the sensor is too small to produce a significant heating 
effect; thus, cross-sensitivity to velocity is negligible. The film 
thickness is only a few .mu.m, such that the thermal mass is low, leading 
to small thermal time constants. Interferometry is capable of resolving 
very small changes in optical path length, thus ensuring high temperature 
sensitivity. The dielectric nature of the sensor avoids several noise 
sources, thus allowing the intrinsically high temperature resolution of 
the technique to be exploited. 
The theoretical basis underlying the operation of fibre optic 
interferometric temperature sensors of the type with which the present 
invention is concerned will now be discussed in greater detail. The 
following nomenclature will be used: 
C=stagnation point velocity gradient 
D=diameter 
I=optical intensity 
N=number of rotor revolutions 
a,b,c=constants in optical transfer function 
h=heat transfer coefficient 
k=thermal spatial frequency 
l=sensor length or thickness 
n=sensing film refractive index 
t=time 
u=mean flow velocity 
x=position 
.alpha.=thermal diffusivity 
.phi.=optical phase 
.kappa.=thermal conductivity 
.lambda.=optical wavelength 
.nu.=kinematic viscosity 
.omega.=optical angular frequency 
A.sub.o =wave amplitude 
P.sub.r =Prandtl number 
P.sub.(t) =ensemble averaged signal 
P'.sub.(t) =random unsteadiness 
n.sub.c =fibre core refractive index 
n.sub.o =gas refractive index 
T.sub.m =mean temperature of sensing element 
T.sub.g =gas total temperature 
.phi..sub.o =phase constant 
.kappa..sub.f =fluid thermal conductivity 
The sensor is a thin film interferometer used in reflection, deposited on 
the face of a single mode optical fibre. The optical phase difference 
between the front and back surface reflections is 
EQU .lambda.=4.times.nl/2 (1) 
where n is the refractive index of the film l the film thickness and 
.lambda. is the illumination wavelength. A mean temperature change 
.DELTA.T.sub.m of the film therefore results in a phase change 
EQU .DELTA..phi.=4.pi.(l/.lambda.)(dn/dt+(n/l)(dl/dt)! .DELTA.T.sub.m(2) 
Where dn/dT represents the thermo-optic coefficient of the film, and dl/dT 
is its thermal expansivity. If the film's absorption is negligible, the 
optical intensity reflected at normal incidence takes the form 
EQU I(.phi.)=I.sub.o (a-b+c cos .phi.)/(a+b+c cos .phi.)! (3) 
where a, b, c are defined in terms of the refractive indices: 
EQU a=(n.sub.c.sup.2 +n.sup.2) (n.sup.2 +n.sub.o.sup.2) 
EQU b=4n.sub.c n.sup.2 n.sub.o 
EQU c=(n.sub.c.sup.2 -n.sup.2) (n.sup.2 -n.sub.o.sup.2) (4) 
in which n.sub.c and n.sub.o are the refractive indices of the optical 
fibre core and of the medium in contact with the film. Thus the 
temperature dependence of phase .phi. results in a temperature-dependent 
optical intensity at the signal photodetector, which follows the periodic 
function of equation (3). The response to a temperature change T.sub.m 
small enough to give rise to a phase change .DELTA..phi.&lt;&lt;1 radian will be 
approximately linear, with a sensitivity depending on the slope of the 
consine function at the operating point. For a given laser wavelength and 
ambient temperature, the operating point is determined by the film 
thickness l, which can be chosen to avoid operation near the turning 
points of the cosine where the sensitivity approaches zero. 
The optical coating employed was zinc selenide. This material has a 
relatively large thermo-optic coefficient dn/dT and is suitable for 
deposition in a thin film by vacuum evaporation, as is discussed below. 
The laser wavelength was approximately 830 nm, at which n was 
approximately 2.6 and dn/dT approximately 1.0.times.10.sup.-4, with low 
optical absorption. For fused silica and air respectively, n.sub.c =1.46 
and n.sub.o =1.00. Using equation (2) and noting that the thermal 
expansion term is small compared with the thermo-optic coefficient, we 
find that a mean sensor temperature change of approximately 1700 K 
corresponds to an optical phase change of 2.pi.. Therefore in the 
application considered the sensor is always operating in the small signal 
regime. 
The thermal response of the fibre end face exposed to a gas flow (FIG. 1) 
can be considered in the simplest case as a one-dimensional problem with 
axial heat conduction into the sensor fibre. If the gas total temperature 
is time-varying, T.sub.g (t), a thermal disturbance will propagate through 
the film giving rise to a time-varying mean temperature T.sub.m (t) 
averaged along the sensor length, which can be measured according to 
equation (2). The frequency response to thermal oscillations can be 
calculated from the analytic solution for heat conduction into a 
semi-infinite rod exposed at its end to a harmonically oscillating ambient 
temperature. If the gas total temperature varies with unit amplitude as 
T.sub.g (t)=cos .omega.t, the temperature at a distance x into the film is 
given by 
EQU T(x,t)=A.sub.o e.sup.-loc cos(et-loc-.phi..sub.o) (5) 
where 
##EQU1## 
in which .alpha. and .kappa. are the thermal diffusivity and thermal 
conductivity of the thin film, h is the heat transfer coefficient at the 
sensor surface, and .phi..sub.o is a frequency-dependent phase constant. 
Integration of T(x,t) through the film thickness l gives a mean 
temperature oscillation at frequency .omega., amplitude A.sub.m given by 
EQU A.sub.m (.omega.)=(A.sub.o /2kl) 2(1+e.sup.-2kl)-4e.sup.-k1 cos 
kl!.sup.1/2(7) 
The expected sensor response to an oscillating gas temperature can be 
calculated, provided the heat transfer coefficient h from the gas to the 
sensor is known. This can be estimated by assuming that the sensor is 
located at the stagnation point of the oncoming flow, which is a good 
approximation as the fibre core is small compared to its diameter. The 
heat transfer coefficient at the stagnation point of a body with 
axisymmetric geometry can be written as 
EQU h=.kappa..sub.f 0.762 Pr.sup.0.4 (C/.nu.).sup.1/2 (8) 
where .kappa..sub.f is the thermal conductivity, Pr the Prandtl number, 
.nu. the kinematic viscosity of the fluid and C is a stagnation point 
velocity gradient. For a flat-nosed body of diameter D in a mean flow 
velocity u for a Mach number below 1, White estimates that 
EQU C=1.35 u/D (9) 
Equations (8) and (9) can be used to calculate a heat transfer coefficient 
to the end face of a cylindrical fibre in specified mean flow conditions, 
and the response of the sensor is then found from equation (7). 
The calculated frequency response to a unit amplitude thermal oscillation 
is shown in FIG. 3 for a 2.4 .mu.m length sensor. This response can be 
compared with a shot noise of 1 .mu.radian/.sqroot.Hz for optical power 
levels in practical applications (this noise source being the fundamental 
limit associated with the random Poisson statistics of the photon flux 
from which an intensity measurement is made). A 1 .mu.radian/.sqroot.Hz 
noise level is an achievable noise level in the present signal detection 
system. 
The transfer function in equation (3) is periodic, and its slope, the 
small-signal sensitivity to phase fluctuations, is similarly periodic. The 
output signal in single wavelength operation (as in the system illustrated 
in FIG. 2) would be dependent on the operating point. FIG. 4 shows a 
modified system in which the output is made independent of operating point 
by illuminating the interferometer with light from two separate sources 46 
and 48 having different wavelengths .lambda..sub.1 and .lambda..sub.2 
chosen to give a phase shift of .pi./2 between the resulting transfer 
functions, so that two signals in phase quadrature are recorded. Light 
from the two separate laser diodes 46, 48 is combined by a directional 
coupler 50 before launch into the addressing fibre 52, and the 
.lambda..sub.1 and .lambda..sub.2 outputs I.sub.1 and I.sub.2 are 
separated spatially by a diffraction grating 54 to two photodetectors 56 
and 58. 
As an alternative to spatial separation, temporal demodulation may be 
employed, requiring a single photodetector and amplitude modulation of the 
two laser sources at different frequencies. Electronic demodulation then 
yields the return signal at each wavelength. The modulating frequencies 
must be chose to avoid: (i) cross-talk between the demodulated signals, 
and (ii) compromising the bandwidth of the overall sensor system. 
The sensor temperature change .DELTA.T is given by the amplitude 
(.DELTA.I.sub.1.sup.2 +.DELTA.I.sub.3.sup.2).sup.1/2 which can be 
computed in software from data acquired from the sensor. 
In practice, the phase difference between the .lambda..sub.1 and 
.lambda..sub.2 outputs is not exactly .pi./2. However, the optical phase 
and hence .DELTA.T remain analytic functions of .DELTA.I.sub.1 and 
.DELTA.I.sub.2. Furthermore, .DELTA.T may be obtained more precisely if 
more than two wavelengths are used to illuminate the sensor, so as to 
provide three or more outputs separated in phase, .DELTA.I.sub.1 . . . 
.DELTA.I.sub.n ; .DELTA.T is then an analytic function of .DELTA.I.sub.1 . 
. . .DELTA.I.sub.n. The phase difference between the respective signals is 
preferably as close as possible to .pi./2 but different values other than 
0 or .pi. (or multiples thereof) can be employed. 
In order to form the sensing element 10 of the sensor of FIG. 1, a ZnSe 
film is deposited on the cleaved end of the optical fibre 12 by a vacuum 
vapour deposition technique. This may be carried out, for example, in a 
fully automated Balzers 550 box coater, in which a molybdenum boat 
containing high purity (99.99%) ZnSe powder is resistively heated to 
approximately 900.degree. C. Base pressures of approximately 
2.times.10.sup.-6 Torr are maintained during evaporation, which is 
controlled at a deposition rate of 0.5 nm s.sup.-1. Prior to coating, the 
cleaved fibre ends are heat soaked by a radiant heater inside the coating 
unit, to improve coating adhesion and the optical quality of the deposited 
film. 
During evaporation, film thickness is monitored by quartz crystal monitor. 
Film thicknesses of up to 2.4 .mu.m have been successfully deposited. 
Fused silica substrates (approximately 25 nm in diameter) were coated 
simultaneously as a coating witness, to enable measurement of the 
refractive index and thickness of the coating by spectrophotometric 
analysis. 
The probe for use in the compressor measurements is shown in FIG. 5. The 
primary requirement of the probe is to provide a rugged mounting for the 
optical sensor and protection for the fibre optic feedout. The probe body 
60 may be adapted from a conventional pneumatic wedge probe fitted with an 
adjacent shielded thermocouple sensor 62. As such, the configuration is 
typical of that used routinely in high speed compressor testing. The 
design of such existing probes may be modified so as to be more suited to 
wide bandwidth temperature sensors. 
The principal problem of the probe design relates to supporting the optical 
fibre 64 through a 90.degree. turn having a bend radius of about 3 mm in 
such a way as to secure the end of the sensing optical fibre 64. This may 
be accomplished by the fibre 64 being supported in a preformed bend in a 
capillary tube 66 of either glass or metal. A schematic representation of 
a metal capillary tube-based probe is shown in FIG. 5. 
The metal capillary tube-based probe as shown may be constructed as 
follows: (i) a 5 m length of single mode optical fibre is cleaved at one 
end; (ii) this end is vacuum coated with a 2.4 .mu.m thick ZnSe film 68; 
(iii) a length of metal capillary tube 66 is tempered and a bend of radius 
3 mm and length 90.degree. formed; (iv) the capillary tube is trimmed to 
length to provide a supporting stem of 40 mm and 3 mm length projecting 
forward; (v) the fibre 64 is drawn through the capillary tube 66 and 
positioned with the ZnSe film 68 retracted from the flush end of the tube 
66 by 20 .mu.m; (vi) the tube 66 is lightly crimped to clamp the fibre 64 
in position; and (vii) the capillary tube 66 is secured inside the 
stainless steel wedge probe 60 with epoxy adhesive. 
For the purposes of aerodynamic testing, a method of generating thermal 
oscillations at kHz frequencies is required in a test experiment. Some 
previously reported techniques for generating such thermal oscillations 
are measurement of the spectrum of thermal fluctuations in a heated 
turbulent jet and both d.c. and a.c. electrical heating of a wire in an 
airstream. In the present case, vortex shedding from a heated bluff body 
was employed, specifically a metal wire carrying direct current and 
exposed transversely to an air flow. Vortices are shed form the wire at a 
frequency determined by the flow velocity, generating thermal fluctuation 
in the wake where warm air and ambient air are mixing at the vortex 
shedding frequency. This arrangement will also reveal any 
cross-sensitivity to air velocity, as a velocity fluctuation will still be 
present at the vortex shedding frequency when the heating current is 
removed. 
The vortex shedding wire was situated 20 nm downstream of the exit of a 
small open jet wind tunnel. The working section at the outlet was 80 mm 
square, with a flow velocity range from 5 to 12 ms.sup.-1 and a measured 
turbulence intensity of 0.4%. The sensor was positioned approximately 1 mm 
behind the shedding wire facing upstream with the fibre axis horizontal, 
so that the mean flow was incident normally on the fibre end face. To 
monitor the vortices, a conventional hot wire anemometer probe was mounted 
with its sensing wire coplanar with and at the same height as the sensor 
end face, approximately 5 mm to one side. Both the fibre and the hot wire 
probe could be translated together vertically relative to the shedding 
wire. 
The shedding wire wa 0.15 mm diameter nichrome alloy, and for the flow 
speeds available, the frequencies range from 4 to 13 kHz without applied 
heating. If the shedding wire is heated above ambient temperature, the 
Reynolds number of the air close to the wire is decreased, and the vortex 
shedding frequency is reduced for a flow regime similar to that used here. 
At an air velocity of 10.5 ms.sup.-1 (Re=104), the vortex shedding 
frequency was 11.3 kHz with no heating current applied. With 9 W d.c. 
heating power, the vortex frequency decreased to 9.6 kHz. The output 
signal was monitored by a spectrum analyser with a linewidth setting of 
125 Hz and the results are shown in FIG. 6. A clear spectral peak 10 dB 
above the noise floor appears at the shedding frequency with heating 
applied. There was no signal at 11.3 kHz when the heating was removed, 
returning the shedding wire to ambient temperature, which implies that the 
thin film sensor has no significant cross-sensitivity to air velocity 
fluctuations, and is responding to air temperature fluctuations only. 
Trials were carried out to expose the temperature sensor to the flow field 
behind the first stage rotor of a highly loaded 5-stage core compressor. 
The measurements were taken at several span-wise stations while the 
machine was operating close to peak efficiency on the design speed 
characteristic. Typical aerodynamic parameters for the first stage at this 
condition are given in Table 1 below. Unsteady pressure measurements were 
also taken. This was the first time unsteady temperature measurements had 
been attempted in this machine. 
TABLE 1 
______________________________________ 
Typical flow parameters behind first compressor stage 
Parameter Value 
______________________________________ 
Flow speed (ms-1) 225 
Mean total temperature (K.) 
339.5 
Mean total pressure (kPa) 
112 
______________________________________ 
During both pressure and temperature measurements, the data were recorded 
in two modes: as continuously sampled data, and as multiple data recorded 
phase-locked to a once-per-revolution trigger pulse. The latter were 
subsequently processed to reveal the ensemble-averaged temperature 
variations and the random unsteadiness found in the data. In both modes, 
the signals were sampled at 500 kHz. 
The discontinuous phase-locked data were processed on-line to determine the 
following parameters. 
a) Ensemble averaged signal, i.e. 
##EQU2## 
b) Random unsteadiness, i.e. 
##EQU3## 
Where P(n,t) is an instantaneous AC coupled signal; N is the number of 
consecutive rotor revolutions during which phase-locked data capture was 
carried out in response to a once-per-revolution pulse; and t is the 
temporal duration of each of the segmented data records (typically 512 or 
2048 samples, depending on the recorder module capacity). 
Such processing is an established technique which accentuates the periodic 
unsteadiness correlated with the rotor. As the data are captured in 
response to a once-per-revolution signal, the rotor is in the same 
position each time the recording cycle is initiated and differences in the 
flow field associated with individual rotor passages are retained. 
Ensemble-averaged stagnation temperature measurements taken close to the 
hub (at 10% span) are shown in FIG. 7(a). These can be compared with 
corresponding stagnation pressure measurements, FIG. 8(a), taken at the 
same span-wise position, although during a different run. Also shown are 
the corresponding random temperature and pressure unsteadiness (FIGS. 7(b) 
and 8(b)). There is significant qualitative agreement between both 
temperature and pressure data, particularly regarding the increased random 
unsteadiness associated with the blade wakes. 
The power spectrum of a section of continuously sampled data computed via 
its Fourier transform is shown in FIG. 9. The dominant component of the 
spectrum is at the blade passing frequency of 9.2 kHz. However, components 
at two, three and four times the blade passing frequency are clearly 
observed above the noise floor, indicating a lower limit on the sensor 
bandwidth of approximately 36 kHz. Further signal processing reveals 
response components up to the eighth harmonic, indicating a potential 
bandwidth up to 74 kHz. 
Data were recorded for about one hour's total exposure to the flow, until 
the signal was lost. The probe was later examined, showing that part of 
the ZnSe coating had been damaged, presumably by particulates or oil 
droplets in the flow. 
The compressor trials described above were undertaken to demonstrate the 
feasibility of using an optical fibre sensor in a realistic aerodynamic 
test facility, rather than as a detailed investigation of the unsteady 
temperature field in the compressor. Calibration of the data obtained in 
these trials was not a critical issue, and was performed by comparing the 
unsteady temperature compressor data with the heated vortex shedding data. 
The 9.6. kHz vortex shedding signal was close to the blade passing 
frequency of 9.2 kHz in the radial traverse, therefore the frequency 
dependence of the sensor's response does not affect the comparison. The 
sensing film was 2.4 .mu.m of ZnSe in both cases. The heating power of 9W 
applied to the vortex shedding wire, if dissipated by convection, would 
result in a mean air temperature rise in the wake of approximately 10 K, 
or an amplitude of 5 K in mixing ambient and heated air. The ZnSe film 
optical response to this amplitude of gas temperature oscillation was 
determined in the vortex shedding experiment. Thus the sensor response 
observed in the compressor can be scaled to gas temperature amplitude, 
assuming the same gas-to-sensor heat transfer coefficient in the two 
experiments. The data show clear structures at the blade passing 
frequency, but there is also structure at frequencies above blade passing 
frequency. Significant components at twice and three times the blade 
passing frequency were observed. 
The sensor coating was damaged after about one hour's exposure to the flow 
in the compressor. However, the ZnSe coating in this trial sensor was 
unprotected, and technology exists to apply suitable protective coatings 
that are sufficiently thin to prevent an adverse effect on the high 
frequency thermal response. This new optical technique therefore provides 
the basis for high bandwidth unsteady temperature measurement in 
continuous flow. 
Fibre optic sensors based on interferometry have potential for other 
applications in aerodynamics test facilities. For example, it has been 
shown proviously that fibre Fabry Perot interferometers are suitable for 
the measurement of heat flux in transient flow wind tunnels. Furthermore, 
other transduction principles, such as the strain optic effect in special 
optical coatings, or miniature air-spaced interferometers, may be 
applicable for high bandwidth pressure measurement. 
The above described embodiment of the invention provides an all-optical 
temperature sensor that has demonstrated a response to air temperature 
fluctuations, estimated to be 5 K amplitude, at approximately 10 kHz in a 
low speed vortex shedding experiment. The sensor is not sensitive to 
velocity fluctuations. The optical sensor has been incorporated into a 
probe and run in a test compressor in mean flows of Mach 0.7 No electrical 
connection is required between the sensor and the associated signal 
processing means, which may be located remotely. Signals well above noise 
were obtained in ensemble averaged data showing a strong component at the 
9.2 kHz blade passing frequency estimated to range from 1 to 6 K 
temperature amplitude in a radial traverse from hub to tip. 
A second embodiment of a fibre optic interderometric sensor will now be 
described, with reference to FIGS. 10 to 15. 
In the first embodiment of the invention, a Fabry-Perot type interferometer 
was formed by an optical coating on the end face of the fibre, with the 
thickness of the coating providing the optical path length between the 
partially reflective faces of the coating. In the second embodiment, as 
shown in FIG. 10, the optical path of the interferometer is provided by a 
short length of fibre 90 spliced to the end of the addressing fibre 92, 
with a partially reflective coating 94 interposed between the adjacent 
ends of the addressing fibre 92 and the short sensing fibre 90. A second 
partially reflective coating may also be applied to the opposite end face 
96 of the sensing fibre remote from the splice. A spliced fibre 
interferometer of this type has previously been used as a heat flux 
sensor, in which case it is desirable that the sensing fibre be 
sufficiently long (about 1-2 mm) to prevent heat loss from the spliced end 
of the sensing fibre. Where a sensor of this type is to be employed as a 
temperature sensor, rather than as a heat flux sensor, it is desirable 
that the path length of the interferometer be made relatively shorter so 
that the sensor body reaches thermal equilibrium within a relatively short 
time span. The first embodiment, using an optical coating as the 
interferometer body, provides the shortest practical path length. However, 
a spliced fibre sensor can also be formed with a sensing fibre which is 
sufficiently short to provide a useful temperature sensor. 
Spliced fibre sensors in accordance with this second embodiment can be made 
by fusion splicing two fused silica single mode fibres, one of which has a 
coating of, for example, titanium dioxide on its end face to act as an 
internal mirror when the splice is formed. One fibre forms the sensing 
element; the other, longer fibre is the downlead (addressing fibre) 
connecting to the rest of the optical system. The coatings may be 
deposited by electron beam evaporation in vacuum, with thicknesses in the 
range 40 to 80 nm for optimum reflectivity and strength of splice. After 
fusion splicing, the fibre is mounted on a translation stage and cleaved 
at the required distance from the splice. In the present example, the 
outer mirror of the interferometer was formed by the cleaved end face 96 
of the sensing element 90. If a higher reflectivity were required it would 
be possible to coat the outer face 96. With care, sensor lengths less than 
the fibre cladding diameter (125 .mu.m) were achievable, the shortest 
being 14 .mu.m, with several tens of microns as typical lengths. 
Previously fabricated FFP sensors for use as calorimeter (heat flux) 
gauges rather than fast response thermometers, had sensor lengths in the 
range 200 .mu.m to 2 mm. 
The optical arrangement is shown in FIG. 13 and is similar to that employed 
in the first embodiment. The source 100 is a laser diode (Sharp LTO24 with 
.lambda.=780 nm, 20 mW optical power) with launch optics 102 including 
collimating optics and isolation optics in the form of a Faraday isolator, 
as in the first embodiment. Light from the laser diode 100 is launched via 
a microscope objective into one arm 104 of a fibre directional coupler 106 
with a 50/50 split ratio. The sensor 107 is located at the end of coupler 
arm 108, and the signal detected by photodetector 109 at the end of arm 
110, with a reference detector 112 at the end of arm 114. The fibre ends 
are index matched to the detectors to minimise reflections. 
A single laser wavelength was employed in the present example, in which the 
objective was to demonstrate the sensor's ability to detect gas 
temperature oscillations in the frequency domain. As described below, the 
temperature oscillating was applied at a known narrow band of frequencies 
so that its presence could be revealed by examining the spectrum of the 
sensor's output signal. Temperature calibration was not required in this 
case; it was sufficient to ensure that the sensor was not operating at a 
turning point on its transfer function (equation (2)), so that small 
temperature oscillations gave a finite output signal. The response was 
checked by applying a hot air stream (approximately 200.degree. C.) to the 
sensor. 
This embodiment was also tested in a vortex shedding test rig in a manner 
similar to the first embodiment. The test rig, described in detail below, 
was the same as that used in the first embodiment. 
The vortex shedding wire 115 was again situated at the exit of a small open 
jet wind tunnel 116 (FIG. 13). The tunnel body was constructed from 
polystyrene with a rectangular cross-section and a 10:1 area contraction. 
Air was supplied by a 40 W centrifugal blower (not shown) at the tunnel 
inlet. The working section at the tunnel outlet 117 was 80 mm square, with 
a flow velocity range from 5 to 12 ms.sup.-1 and a measured turbulence 
intensity of 0.4%. The vortex shedding wire 115 was 20 mm downstream of 
the tunnel exit 117 with its axis horizontal, exposed to the flow over the 
central 70 mm of the 105 mm wire length. 
The fibre sensor 120 was positioned approximately 1 mm behind the shedding 
wire facing upstream with the fibre axis horizontal, so that the mean flow 
was incident normally on the fibre end face, as shown in FIG. 13. The 
fibre was attached parallel to a stiff wire support mounted on a 
translation stage 122 with the final 2 mm of fibre projecting unsupported 
into the air flow. To monitor the presence of the vortices, a conventional 
hot wire anemometer probe (DISA type 55, 5 .mu.m wire diameter) (not 
shown) was mounted with its sensing wire coplanar with and at the same 
height as the sensor end face, about 5 mm to one side. Both the fibre 
sensor 120 and the hot wire probe could be translated together vertically 
relative to the shedding wire 115. 
The shedding 115 wire was 0.37 mm diameter nichrome alloy. The expected 
vortex shedding frequency f from a circular cylinder diameter d in a flow 
velocity u i 
EQU f=0.198 1-(19.7/Re)! u/d (10) 
where Re is the Reynolds number of the flow with respect to the cylinder. 
Vortex shedding occurs provided Re is greater than 40, and for the flow 
speeds available, the frequencies for d=0.37 mm range from 2.2 to 6.0 kHz 
without heating applied to the shedding wire. If the shedding wire is 
heated above ambient temperature, the Reynolds number of the air close to 
the wire is decreased, and the vortex shedding frequency is reduced, as 
previously noted above. However, the reduction in frequency is not large 
provided Re is greater than or equal to about 100, a condition met in 
these experiments. 
A 40 .mu.m length sensor 120 was exposed in the wake of the vortex shedding 
wire 115. The hot-wire probe monitored the vortex shedding frequency. At a 
fixed tunnel air velocity of 7.28 ms.sup.-1 (Re=178) the vortex shedding 
frequency was 3.46 kHz with no heating current applied. When 28 W DC 
heating power was applied to the shedding wire 115, the vortex shedding 
frequency decreased to 3.10 kHz. The electrical signal from the 
photodetector 109 (FIG. 13) monitoring the reflected signal was recorded 
by an FFT spectrum analyser (not shown) with an effective line width of 8 
Hz and the results are shown in FIG. 14. A clear spectral peak appeared at 
the shedding frequency in FIG. 14(a) when the heating power was applied to 
the shedding wire. There was no signal at 3.46 kHz when the power was 
removed (FIG. 14(b)), returning the shedding wire 115 to ambient 
temperature, which implies that the fibre optic sensor has no significant 
cross-sensitivity to fluctuations in air velocity, and is responding to 
air temperature fluctuations only. For comparison, the spectra of the 
velocity signals from the hot-wire anemometer are shown in FIG. 15. 
The spectral peak corresponding to the thermal signal detected by the fibre 
optic sensor varied with the vortex shedding frequency in the expected 
manner for air flow velocities up to about 7.5 ms.sup.-1. However, at 
higher air velocities, up to the tunnel maximum of 12 ms.sup.-1, any 
sensor response was masked by increased turbulence. The primary purpose of 
the vortex shedding experiment was to demonstrate that the short length 
fibre sensor could respond to temperature fluctuations in an air flow 
without observable cross-sensitivity to velocity when deployed as a 
stagnation probe. In the absence of suitable comparison sensor, the 
amplitude of the temperature fluctuations in the vortex wake was not known 
accurately, so the temperature calibration of the sensor could not be 
determined. However a rough estimate of the air temperature fluctuations 
can be made from the heat convected from the shedding wire in the 
experimental flow conditions described. An empirical expression relates 
the heat transfer coefficient to the Reynolds and Prandtl numbers for a 
cylinder in cross-flow, from which the convective heat loss is estimated 
to be 160 Wm.sup.-1. If this power input were uniformly applied to the 
mass flow in the vortex wake, then the resulting air temperature rise is 
approximately 11 K. The sensor is thus exploded to temperature oscillation 
amplitudes of about half this magnitude, or about 5K. The amplitude of the 
sensor's phase signal was estimated from observation of the turning points 
of the interferometric output as the sensor was heated from ambient to 
approximately 500 K by a hot air gun. The peak of the voltage signal in 
FIG. 15 corresponds to an amplitude of 7.1 .mu.V or 37 .mu.radian phase 
amplitude. The expected response to a unit amplitude air temperature 
oscillation at 3 kHz, from FIG. 11, is about 8 .mu.radian, giving an 
estimated observed temperature amplitude of about 5 K. Thus the observed 
signal amplitude is in good agreement with that expected from the 
calculated response at 3 kHz. 
The noise floor in FIG. 14 is 7.5 .mu.rad Hz.sup.-1/2, the main 
contribution being from laser intensity noise, which was not compensated 
in this experiment. Laser frequency noise contributes a phase noise 
component proportional to the path imbalance of the interferometer. A 
phase noise of &lt;0.1 .mu.rad Hz.sup.-1/2 is expected for a 40 .mu.m sensor 
from the known frequency noise characteristics of the laser diode 
employed. Shot noise, determined by the optical power at the detector, 
(about 8 .mu.W in these experiments) will set a practical lower limit of 
the noise floor at about 1 .mu.rad Hz.sup.-1/2. With this value of noise 
floor, the 40 .mu.m sensor bandwidth for a 5 K gas temperature amplitude 
is 2.5 kHz. 
The experiment described above demonstrated the sensor's response by narrow 
bandwidth signal processing using an FFT spectrum analyser. To increase 
the sensor's bandwidth to the 10 to 60 kHz range, the noise must be 
minimised and the sensor response maximised. The relative shot noise may 
be reduced by increasing the optical power at the detector, which may be 
achieved conveniently through the use of a higher reflectivity coating. 
The effect of laser phase noise may be reduced by using a shorter sensor, 
but forming the fibre Fabry-Perot cavity by precision cleaving becomes 
impractical for lengths as short as 10 .mu.m. 
Alternatively, the sensor may be formed from an optical coating, such as a 
zinc selenide film of several microns thickness as in the first 
embodiment. Such a thin film sensor allows the choice of a coating 
material with a thermo-optic coefficient higher than that of fused silica, 
thus offsetting the decrease in sensitivity associated with a shorter 
sensor length. 
The desired application to total temperature measurement in turbomachinery 
tests requires the fibre sensor to be packaged robustly yet exposed with 
minimum interference to the flow. As in the first embodiment, the fibre 
can be held rigidly in a glass or metal capillary tubing and the tip 
exposed at the end of a metal probe stem in a similar way to a 
thermocouple bead or pressure transducer, so that the aerodynamic 
characteristic of the probe is identical to a conventional design. 
A very short length fibre Fabry-Perot interferometer in accordance with the 
second sensor embodiment has demonstrated its capability to respond to 
oscillating air temperatures of approximately 5 K amplitude at a frequency 
of 3.1 kHz in a vortex wake. The sensor did not show an observable 
cross-sensitivity to air velocity. The observed response was consistent 
with that expected from a one-dimensional model of the thermal wave 
propagating axially in the fibre, using a heat transfer coefficient 
applicable to a stagnation point. 
The invention thus provides apparatus and methods for the measurement of 
unsteady gas temperatures at relatively high bandwidths, as encountered, 
for example, in turbomachinery. Improvements and modifications may be 
incorporated without departing from the scope of the invention.