FIELD OF INVENTION
The present invention generally relates to a measurement system
which uses optical media with Bragg gratings. More particularly, the
present invention relates to a non-instrusinic,non-contact hi-fidality
method to determine the gas temperature in a typical furnace, in
particular a combustion chamber of a steam generator. The invention
further discloses a multipoint optical device adaptable to carry-out
the innovative method.
BACKGROUND OF THE INVENTION
Steam generating units are employed universally by Power Plants,
Industries and Refineries to produce steam for their operation. The
steam generators are designed so as to be capable to fire fuel oil,
fuel gas or coal. The fuels are admitted in the combustion chamber
of the steam generator only after determining that the combustion
chamber has attained the requisite ignition temperature. The
products of combustion that is to say, the high temperature flue gas-
proceed through various heat absorbing elements located in the first
pass and second pass of the steam generator.

Since the steam generators in a plant requires to be operated at a
relatively high temperature and pressure, conditions within the
combustion chamber must be continuously monitored. One of the
monitorable conditions constitutes flue gas temperature which indeed
is critical parameter to be consistently monitored through sensing
and acquiring characteristic data.
Flue gas temperature sensing in a steam generator is normally
accomplished with one or more temperature monitoring devices
operated by electric or mechanical means.
One such monitoring device is the thermocouple- a device based on
thermoelectric effect, a plurality of which is disposed throughout the
combustion chamber and heat transfer surface. The thermocouples
are placed in the steam generator in such a way that they are
separated by protective sheaths. The sheaths are provided to protect
the relatively fragile thermocouple junctions from the hostile
environment inside the steam generator. Consequently, the
thermocouples do not really sense the process temperature directly,
but instead respond to the heat transmitted, and thus, the sensing of
temperature is influenced by the time lag inherent in conductive heat
transfer . Accordingly, a substantial delay in thermocouple response,
generally occurs corresponding to the changes in temperature within
the combustion chamber. This phenomenon is particularly true during
system startup when combustion reaction initially results in a rapid

temperature rise which must be detected in order to confirm
combustion establishment. In addition, heat transfer time lag affect
thermocouple response to the changes in operating conditions
during normal system operation. Moreover, the thermocouples are
basically effective for spot measurement.
As an alternative to thermocouples, mechanical devices like acoustic
pyrometers are occasionally used to measure the gas temperature.
The acoustic pyrometer is externally mounted on the combustion
chamber. The pyrometer is connected to the combustion chamber via
a purgeable lance tube which normally extends from the pyrometer
into the chamber internals. A major limitation of monitoring devices
like acoustic pyrometer, arises from the difficulty encountered in
keeping the lance tube free of obstructions. Further, the acoustic
signals generated from the auxiliary components of the steam
generator for example, soot blowers, and those from the steam leaks
in the combustion chamber, also influence the measured data to a
considerable extent. Thus, the acoustic pyrometer can only provide
an averaged value of the measurand, particularly, in the path
between the receiver and the transmitter.
There are already known various constructions of sensing
arrangements, for example, U.S. Pat. No. 4,806,012, entitled
"Distributed, Spatially Resolving Optical Fiber Strain Gauge", discloses

a sensing device capable of sensing stresses within a structure which
comprises an optical fiber containing a plurality of periodic Bragg
gratings of different original periodicities. Thus each reflecting light in
a narrow range around a central wavelength can be determined by
the respective periodicity. Such bragg gratings are disposed at
different regions of the structure so as to be subjected to different
stresses, temperatures and strains depending on their locations in
the structure, and undergoing strain-related changes in their
periodicities including in their central wavelengths of reflection.
During the use of this known sensing arrangement, light is launched
into the optical fiber in a wavelength range, such that, the
wavelengths of interest with respect to all of the Bragg gratings
under all conditions can be embraced. Then, the light returned back
to the launching end of the fiber is examined , or the light reaching
the other end is examined, respectively for the presence or the
absence or diminishing or otherwise of intensity, in respect of the
altered light around each central wavelengths of the gratings, the
alteration being taking place due to the stresses existing at the
respective locations of the structure, thereby the magnitude of such
stresses is determined.
Prior to installation of the optical fibre core in the structure,the
individual Bragg gratings are provided in the optical fiber core, by
exposing the optical fiber core through the cladding to an

interference pattern of two ultraviolet light beams , the light
frequency and/or orientation of the light beams relative to the optical
fiber longitudinal axis for each of the gratings, being such that, the
interference pattern maxima and minima extend through the fiber in
directions normal to the longitudinal axis, and that the periodicity
(e.g. the distance between two consecutive maxima) being that
desired for the particular grating.
However, this approach is particularly advantageous, in the
applications in which the number of locations along the optical fiber
to be monitored for stresses in the structure is relatively small,
because the device entails an important drawback in that each of the
gratings has to have assigned to it a considerable amount of the
available spectrum (i.e. not only its relatively narrow wavelength or
frequency band but also the separation from the adjacent
wavelength or frequency band assigned to another grating by an
amount sufficient to avoid overlapping or crosstalk between the
adjacent channels under all circumstances, that is, even when the
central wavelengths of the adjacent channels have moved, as a result
of the stresses applied at the locations of the associated gratings,
toward one another to the maximum extent). Such allocation of
larger spectrum to each of the gratings, severely limit the number of
grating sensors that can be employed within each sensing optical

fiber. For example, if a solid state device, such as an edge-emitting
diode or a laser diode is used as the light source, which would be
highly desirable because of the relatively low cost and reliability of
such device, it would only cover a wavelength range of few tens of
nm. On the other hand, each of the optical sensors of the type
disclosed in the above-cited reference, would require up to 5 nm of
bandwidth to cover the entire measurement bandwidth. Thus, only a
limited number of sensors can be associated with each optical source
disclosed in the US-patent under discussion.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a non-
instrusinic, non-contact, hi-fidility method to determine the gas
temperature in a typical furnace, in particular, a combustion chamber
of a steam generator.
Another object of the invention is to propose a multipoint optical
sensing device to determine the gas temperature in any typical
furnace, in particular, a combustion chamber of a steam generator.
A still another object of the invention is to propose a non-instrusinic,
non-contact, hi-fidility method to determine the gas temperature in a

typical furnace, in particular, a combustion chamber of a steam generator which
makes use of the proportional change in a physical property of the optical
sensing device.
A further object of the invention is to propose a multipoint optical sensing device
which consists of an array of multipoint sensors arranged in a single optical
device.
A still further object of the invention is to propose a non-instrusinic, non-contact,
hi-fidility method to determine the gas temperature in a furnace, in particular, a
combustion chamber of a steam generator, which makes use of the proportional
change in a physical property of the optical sensing device, which generates a
planar gas temperature profile from the measure and information derived from
the multipoint optical sensing device.
SUMMARY OF THE INVENTION
The present invention provides an optical fiber-based sensor device, for
measuring a gas temperature in a high-temperature environment like a steam
generator combustion chamber, and a method of using the optical fiber-based
apparatus for such measurement. More particularly and in accordance with
exemplary embodiments, the optical fiber-based sensor device comprises:

an optical fiber-based sensor including an optical fiber which has a
plurality of fiber gratings disposed thereon at appropriate locations
along the longitudinal axis of the optical fiber. The gratings exhibit
exceptional thermal stability and do not degrade at conditions
prevailing inside the steam generator combustion chamber, thereby
enabling the optical fiber-based sensor of the present invention to be
used for measuring flue gas temperature in high temperature
environment of steam generator combustion chamber.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
Figure: 1 shows a schematic diagram of an optical fibre-based
apparatus according to the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION:
As shown in fig 1, the method determines the temperature of flue
gas in the combustion chamber by adapting an optical sensing device
having atleast four quasi-distributed optical sensing array means (1).
The array means (1) is disposed within the combustion chamber (2)
of the steam generator. Light originating from a light source when
launched into the optical sensing array means (1), undergoes

reflection at each of the sensor locations. As the sensor array means (1) of the
device undergoes deformation in its geometric structure due to the effect of the
gas temperature in contact with the array means (1), the extent of light
reflection from the device undergoes a relative change. As the light passes
through the optical sensor array means, characteristic of the reflected spectrum
undergoes a change. A heat sensor (3) is disposed at the end of the optical array
to determine the gas temperatures at each of the sensor points as a function of
the shift in the reflected peaks of the received interrogating light signal. A signal
processor (4) is arranged to deduce a planar profile of the temperature of flue
gas in the combustion chamber, based on the point temperatures determined
using the optical sensing array.

We Claim:
1. A non-contact method to determine the gas temperature in a combustion
chamber of steam generators, the method comprising the steps of:
- providing a multipoint optical sensing array interposed in the combustion
chamber (2);
- providing a light source and launching light from the light-source to the
optical sensing array (1) having a plurality of quasi-distributed optical
sensors;
- acquiring data in respect of a change in the characteristic of the reflected
spectrum of light corresponding to a deformation in the original geometric
structure of the sensing array (1) upon fluctuations, in flue-gas
temperature inside the combustion chamber (2); characterized by
comprising:
- providing a heat sensor (3) downstream of the sensing array (1) which
capture the data relating to interrogating light signals representing the
gas temperatures at each sensor point as a function of the shift in the
reflected peaks of the received interrogating light signals; and

- providing a signal processor (4) operably connected to the heat sensor
(3), to generate a planar profile of the temperature of flue gas in the
combustion chamber (2) based on the captured data respecting tothe
gas-temperatures at each sensor point.
2. A multipoint optical sensor device to determine the gas temperature in a
typical furnace, in particular a combustion chamber of a steam generator,
the device comprising:
- a light source for launching light beams at different directions;
- at least four quasi-distributed optical sensors forming an sensing array (1)
interposed in the combustion chamber (2), the array (1) being susceptible
to geometric deformation in structure corresponding to changes in
temperature of flue-gas in the combustion chamber (2), the deformed of
the sensing array (1) emitting a reflected spectrum of light with changed
characteristic, the changes being in registration with the geometrical
deformation in the structure of the sensing array (1);.
- a heat sensor (3) disposed downstream of the optical sensing array (1) for
acquiring data in respect of the gas temperature at each sensor point of
the array (1),

the acquired data representing a shift in the reflected peaks of the interrogating
light signals, and
- a signal processor (4) operably connected to the heat sensor (3) to generate a
planar profile of the temperature of flue gas based on the data respecting to the
point temperatures captured at each sensor means.

ABSTRACT

TITLE: "A non-contact method and device to determine the gas temperature in a
combustion chamber of steam generators"
The invention relates to a non-contact method to determine the gas temperature
in a combustion chamber of steam generators , the method comprising the steps
of providing a multipoint optical sensing array interposed in the combustion
chamber (2); providing a light source and launching light from the light-source to
the optical sensing array (1) having a plurality of quasi-distributed optical
sensors; acquiring data in respect of a change in the characteristic of the
reflected spectrum of light corresponding to a deformation in the original
geometric structure of the sensing array (1) upon fluctuations, in flue-gas
temperature inside the combustion chamber (2); providing a heat sensor (3)
downstream of the sensing array (1) which capture the data relating to
interrogating light signals representing the gas temperatures at each sensor
point as a function of the shift in the reflected peaks of the received
interrogating light signals; and providing a signal processor (4) operably
connected to the heat sensor (3), to generate a planar profile of the temperature
of flue gas in the combustion chamber (2) based on the captured data respecting
to the gas-temperatures at each sensor point.