GB2313187A - Surface temperature profile detection - Google Patents

Abstract

A pyrometer arrangement for monitoring the temperature profile of the surface of a hot object eg a turbine blade B includes digital signal processing circuitry 3 which corrects for excess radiation caused by flaring particles by repeatedly detecting the surface temperature distribution over a period in which the true temperature distribution does not change substantially, and comparing detected successive temperature values of each point in the distributions and deleting those too far above a minimum found for that point among a group of successive measurements. The resulting processed temperature distributions are combined and a corrected surface temperature distribution is obtained by fitting a function of predetermined form (e.g. a fourth order polynomial) to a digitised representation of the combined temperature distribution.

Description

SURFACE TEMPERATURE DETECTION The present invention relates to an apparatus and method of obtaining a corrected surface temperature distribution of an object's surface. The invention relates particularly but not exclusively to a method and apparatus suitable for measuring the surface temperature of a turbine blade in use.

It is known to monitor the surface temperature distribution of turbine blades in a turbine by scanning the surface of the blades with an optical imaging arrangement provided with a suitable photosensitive detector at the conjugate focal point At the high temperatures normally found in gas turbines, turbine blades and other components behave almost as black bodies and hence the temperature of any given point can be found by measuring the detected radiation intensity over a given wavelength range and applying a known correction factor to compensate for the known departure of the turbine surface from black body behaviour.

A major problem which arises in such arrangements is caused by flaring of carbon particles and the like in the buming gas stream, which emit substantial radiation which is collected by the pyrometer optics. As a result, the detected temperature of a given point is higher than its true temperature if a flare occurs whilst that point is being scanned by the optical system of the pyrometer. Clearly, similar additive errors occur from primary and secondary reflections of radiation arising from surrounding hot engine components.

In one aspect the invention provides a method of obtaining a corrected surface temperature distribution of an object's surface, the method comprising the steps of: i) repeatedly detecting the temperature distribution of the object's surface over a period in which the true temperature distribution is substantially unchanged; ii) applying a selection criterion to the resulting temperature distributions to eliminate at least some spurious data; iii) subsequently combining the resulting processed temperature distributions, and iv) obtaining the corrected surface temperature distribution by fitting a function of predetermined form to a digitised representation of the combined temperature distribution.

Preferably the surface temperature distribution is detected by a pyrometer and said selection criterion involves the deletion from each temperature distribution of step i) of any spot temperature value which deviates by more than a predetermined extent from the lowest value of that spot's temperature found in the temperature distributions of step i).

The above function is preferably a polynomial of predetermined order, e.g. fourth order.

In another aspect the invention provides pyrometer apparatus for monitoring the surface temperature distribution of an object, comprising optical means arranged to direct radiation from said surface onto a photosensitive detector, said detector being arranged to generate an output signal representative of the surface temperature distribution, an analogue-tdigital converter arranged repeatedly to generate a digitised version of said surface temperature distribution, processing means for applying a selection criterion to a group of such surface temperature distributions obtained from said converter or said processing means, means for combining the digitised processed temperature distributions and further processing means for obtaining the corrected surface temperature distribution, the further processing means being arranged to fit a function of predetermined form to the combined temperature distribution.

A further problem which arises in pyrometer systems, particularly when used to monitor a surface such as the surface of a turbine blade in a combustion chamber, is the tendency of the exposed optical surface(s) of the pyrometer optics to become contaminated by carbon particles and other combustion residues. This reduces the detected radiation intensity and hence results in a false low temperature reading.

In a preferred embodiment of the invention, the pyrometer is provided with a light source of known or constant intensity arranged to transmit a light beam through said optical means to a monitoring photosensitive detector, the monitoring photosensitive detector being arranged to generate a correction or calibration signal which can be utilised to compensate for variations in the transmissivity of said optical means.

A preferred embodiment of the invention is described below by way of example only with reference to Figures 1 to 4 of the accompanying drawings, wherein: Figure 1 is a diagrammatic representation of one embodiment of the invention; Figure 2 is a sketch perspective view of two surface temperature distributions of a turbine blade in the arrangement of Figure 1; Figure 3 is a sketch perspective view of a combined temperature distribution obtained from a group of temperature distributions such as those shown in Figure 2; and Figure 4 is a block diagram of the electronic circuitry 3 shown in Figure 1, the circuitry being shown connected to a personal computer.

Refening to Figure 1, a turbine disc D is shown in the x-y plane rotating anticlockwise, its shaft being aligned with the z axis. Blades B are mounted on the periphery of disc D and are each inclined relative to the x-y plane. Accordingly the focal point P of an imaging optical system of a pyrometer sweeps a forward surface of each blade in the x direction as that blade moves through point P. The optical system comprises a mirror M1 arranged (by conventional means, not shown) to scan point P in the y direction at a sufficiently rapid rate to generate a raster scan over the entire front surface of each passing blade B and a lens system L which focuses radiation from the mirror onto a photosensitive detector D1. The entire optical system including its tubular casing C can be moved parallel to its optical axis (i.e. from left to right as shown in Figure 1) within an outer casing (not shown). A convex mirror M2 is mounted within the outer casing and is used in conjunction with a constant intensity light source S for calibration purposes, as will subsequently be described in detail.

Electronic circuitry 3 in accordance with one aspect of the invention processes the output of detector D1 so as to obtain a corrected surface temperature distribution of each (or a typical) blade B and will be described in more detail subsequently.

Referring now to Figures 2 and 3, circuitry 3 is arranged to sample the scans from photodetector D1 at a rapid rate to generate a group of successive temperature profiles as shown in plots a) and b) of Figure 2. Each group of profiles relates to the temperature distribution of a given blade. Preferably these temperature profiles are in digitised form.

The time interval between the first and last profiles in any given group is typically 5s, i.e.

much smaller than the time needed for a significant change in true temperature profile.

Accordingly, any difference between the detected profiles indicates an error in at least one of them.

For example, the profile of Figure 2a) exhibits peaks X, Y and Z whereas the profile of Figure 2b) exhibits only peaks X and Y. Peak Z is almost certainly a spurious peak caused by radiation from a buming soot particle or the like rather than radiation from a blade surface, and hence a selection criterion is applied to all the profiles in the group, according to which any spot temperature value on any profile which is nominally at a temperature 50C or greater above the lowest value of that spot's temperature found in any of the temperature profiles of that group is eliminated.

The resulting processed temperature distributions are then combined to give a combined temperature distribution featuring peaks X' and Y as shown in Figure 3. Peak X' omits a subsidiary peak appearing in the profiles of Figures 2a) and 2b) because one or more further profiles (not shown) of the same group omit this subsidiary peak. Thus it is only necessary for one profile in the group to give the correct temperature of a given spot on the blade surface in order to ensure that that spot's temperature is correctly represented in the combined profile of Figure 3.

In order to ensure that spot temperature values in the initial group of profiles are not unreasonably excluded, e.g. as a result of electrical noise in the output of photodetector D1 or digitising errors, only those spot temperature values which deviate by more than a predetermined extent from the lowest in the group for the corresponding spot are excluded.

Other selection criteria may be employed in addition to or instead of that referred to above. For example, each pair of points of each profile could be compared and that point of higher temperature eliminated if its temperature deviates from that of its neighbour by more than a predetermined amount, e.g. an amount determined from the known thermal conductivity of the blade.

Having obtained a combined temperature profile as shown in Figure 3, this profile is then fitted to a two-dimensional polynomial function of predetermined form, e.g.

T(x,y) = a+bx+c+dx3+ex4 + fy +gy2 + hy3 + iy4 + jxy + ky + Ix + mx3y + ny3x + where a to o are constants which are adjustable to achieve the best fit to the input data.

The resulting best fit polynomial is the output.

The circuitry 3 used for the above processing is shown in Figure 4. The analogue signal from photodetector Dl is fed via a low-pass filter 13 to a high speed analogue-to-digital converter 14 which samples the signal, e.g. at a rate of 4 Mhz. The analogue-to-digital converter 14 may, for example, be a MAX 176 converter which produces a multiplicity of successive 12-bit output signals each representing the temperature of a given point in a profile such as the profile shown in Figure 2a) for example.

Typically a complete raster scan of focusing point P (Figure 1) over the entire front surface of a given blade will result in 1000 such 12 bit signals, each corrected by the output of ADC 14' which together define one temperature profile as shown in Figure 2a) or 2b) for example. A group of, e.g. 50 such corrected profiles are transferred to a digital signal processor 15 which stores them in RAM (not shown).

The selection criterion referred to above, involving the deletion from each stored profile of any temperature value which exceeds by more than a predetermined amount the lowest temperature value found in any profile of that group at the same point (i.e. defined by the same x,y coordinates) is then applied by a microprocessor 17. DSP 15 may be an AD 2181 processor, for example, and microprocessor 17 may be a type 68020 for example.

The required program is stored in RAM 18.

The resulting combined profile (such as that shown in Figure 3, for example), also in digital form, is fitted to the fourth order polynomial referred to above, under the control of a microprocessor 17 which in tum executes a program stored in RAM 18. The required polynomial and curve-fitting sub-routine are suitably stored in ROM 16.

A bi-directional serial interface 19 is coupled to microprocessor 17 which enables control signals to be received from a terminal 4 and enables the results of the curve-fitting, namely the corrected temperature profiles of the surfaces of the blades B, to be displayed on screen.

In accordance with a preferred feature of the invention, the pyrometer arrangement shown in Figure 1 (which is the subject of a co-pending patent application) is provided with a light source S and a mirror M2 which can be used to correct the output of detector D1 for losses caused by deposition of combustion products on e.g. mirror Ml and other components of the optical system. Source S (which is suitably an LED energised by a constant current power supply) is so arranged that when the pyrometer optics are retracted (with casing C), its light is focused by lens L, and is intercepted by a convex mirror M2. Mirror M2 reflects the light back through the above optical system, which focuses the light onto detector D1. Hence the detected light from source S is doubly attenuated by any deposits on the optical system, and the percentage attenuation can be determined by comparing the detected intensity with the detected intensity under clean conditions, i.e. no deposits. If the percentage attenuation as determined from this comparison is 2x, then it can be assumed that in use of the pyrometer, the percentage attenuation of the detected radiation by deposits on the optical system will be x, and the output of D1 can be corrected accordingly, e.g. in circuitry 3 by appropriate software.

In order to ensure accuracy, the pyrometer optics are preferably withdrawn into the outer casing at regular intervals to altemately determine the attenuation by deposits on the optical system and the radiation intensity from blade B, so that each determination of the latter can be corrected. Also, the spectrum from source S is preferably similar to that of the radiation from blade B.

Claims (14)

1. A method of obtaining a corrected surface temperature distribution of an object surface, the method comprising the steps of: i) repeatedly detecting the temperature distribution of the object's surface over a period in which the true temperature distribution is substantially unchanged; ii) applying a selection criterion to the resulting temperature distributions to eliminate at least some spurious data; iii) subsequently combining the resulting processed temperature distributions, and iv) obtaining the corrected surface temperature distribution by fitting a function of predetermined form to a digitised representation of the combined temperature distribution.

2. A method as claimed in Claim 1, wherein the surface temperature distribution is detected by a pyrometer and said selection criterion involves the deletion from each temperature distribution of step i) of any spot temperature value which deviates by more than a predetermined extent from the lowest value of that spors temperature found in the temperature distributions of step i).

3. A method as claimed in Claim 1 or Claim 2, wherein said function is a polynomial of predetermined order.

4. A method as claimed in Claim 3, wherein said polynomial is fourth order.

5. A method as claimed in any preceding Claim wherein the surface temperature distribution is obtained by a pyrometer and a correction signal is obtained by monitoring the transmission through the pyrometer optics of light of known or constant intensity.

6. A method according to any preceding Claim in which the object is a turbine blade.

7. Pyrometer apparatus for monitoring the surface temperature distribution of an object, comprising optical means ananged to direct radiation from said surface onto a photosensitive detector, said detector being arranged to generate an output signal representative of the surface temperature distribution, an analogue to-digital converter arranged repeatedly to generate a digitised version of said surface temperature distribution, processing means for applying a selection criterion to a group of such surface temperature distributions obtained from said converter or said processing means, means for combining the digitised processed temperature distributions and further processing means for obtaining the corrected surface temperature distribution, the further processing means being arranged to fit a function of predetermined form to the combined temperature distribution.

8. Apparatus as claimed in Claim 6, wherein said first-mentioned processing means is arranged to delete from each received temperature distribution any spot temperature value which deviates by more than a predetermined extent from the lowest value of that spot's temperature found in a group of such received temperature distributions.

9. Apparatus as claimed in Claim 6 or Claim 7, wherein said further processing means is arranged to fit a polynomial of predetermined order to said combined temperature distribution.

10. Apparatus as claimed in any of Claims 6 to 8 wherein a light source of known or constant intensity is arranged to transmit a light beam through said optical means of said pyrometer and a monitoring photosensitive detector is arranged to detect the transmitted light beam, and to generate a correction signal.

11. Apparatus as claimed in any of Claims 6 to 10, wherein said optical means is arranged to direct optical radiation from a source of known or constant intensity onto a photosensitive detector, the photosensitive detector being arranged to generate a correction or calibration signal which can be used to compensate for variations in the transmissivity of said optical means.

12. Apparatus as claimed in Claim 11, wherein said optical means is arranged to transmit said optical radiation through an exposed optical surface thereof.

13. A method of obtaining a corrected surface temperature distribution of an object's surface, substantially as described hereinabove with reference to Figures 1 to 4 of the accompanying drawings.

14. Pyrometer apparatus substantially as described hereinabove with reference to Figures 1 to 4 of the accompanying drawings.