RADIATION PROBE AND METHOD OF USE

Abstract: A radiation probe comprises an optionally cooled tube 10 with a window 22 at one end, and can be traversed across a gas turbine engine combustion chamber 20 by drive means 26. A radiation detector 28 simultaneously measures radiation inten­ sity at a plurality of wavelengths, at each of a succession of po­ sitions during the traverse. From this data, local values of radia­ tion levels, grey and non grey absorption coefficients, local temperature and local gas composition and soot concentration are calculated, using a recurrence formula which relates inten­ sities at neighbouring positions.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to devices for measuring radiation, and may be used for example in analysing combustion processes e.g. heat transfer in the combustion chambers of gas turbine engines. Description of Prior Art The life and reliability of engine components and in particular combustor walls has always been a subject of a great deal of interest. The evaluation of combustor wall life is done by using various equations and empirical relations. A lot of this in turn depends on the temperature of the walls. These temperatures are a consequence of various convective, conductive and radiative heat fluxes within the combustion gas. Therefore it is important both to be able to measure and to be able to model radiative heat transfer accurately. The present modelling of radiative heat transfer needs input of absorption coefficients. An absorption coefficient is a function of wavelength, local gas temperature and gas composition, although the present modelling techniques use grey gas absorption coefficients (ones that do not depend on wavelength). A known technique (the Schmidt method) for making radiation measurements in a combustion chamber uses a detector located outside a window in one wall of the combustion chamber and a background radiation source (a furnace) located in a line of sight outside a window in an opposite wall. A chopper is positioned in front of the radiation source to enable alternate measurements along the line of sight with the radiation source, and without it (i.e. measurement of just the emission from the combustion process). Since the radiation emitted by the furnace is known, this gives the proportion of radiation that is absorbed by the flame in the combustion chamber. This quantity is often known as absorbtivity of the flame. It is also possible to deduce radially weighted values of temperatures. The temperatures so determined are sometimes called Schmidt temperatures or brightness temperatures because they are not strictly temperatures. Norgren and Claus (NASA technical notes TN6410,1971; TMX-3394,1976; and technical paper 1722,1981) have used this technique with measurements in specific wavelength bands to determine smoke concentrations. One major drawback of Schmidt type methods is that they give non-linearly weighted averages of unknown quantities across the width of the combustor, and local variations along the line of sight cannot be deduced. Moreover, on a test rig, there may not be two appropriately aligned holes in the combustor walls for the source and detector, so modification of the combustor may be required; and on a working gas turbine engine aligned holes are out of the question. Furthermore, the local gas composition cannot be deduced. Present gas analysis techniques involve taking gas samples, which is difficult to arrange without disturbing the combustion process. It has also been proposed to measure temperature and soot concentration by traversing across a flame (Beer and Claus, "The traversing method of radiation measurement in luminous flames", J. Inst. Fuel, vol.35 pp.437-443, 1962). A development of this technique in which spectral measurements (i.e. measurement of radiation at a plurality wavelengths) are made is described by Hammond and Beer, "Spatial Distribution of Spectral Radiant Energy in a Pressure Jet Oil Flame", Heat Transfer in Flames (Chapter 19), Scripta Book Co., 1974. In this technique, a sight tube from the source furnace is traversed across the flame, and averaged measurements are obtained alone a line of sight from the position of the end of the sight tube in the flame to the detector. Thus, this technique still has the problem noted above of requiring a line of sight, and of providing only averaged measurements (from the position of the end of the sight tube in the flame to the external detector). There is no correlation between measurements at different positions along the traverse of the sight tube. EP-A-143, 282 discloses a known method and apparatus for calculating the temperature of an object using measurements made at several different temperatures of the radiation emitted in several radiation ranges. A set of equations containing the measured values are solved by a given formula to yield values for the unknown temperatures. The measured radiation is still collected, however, by a fixed, non-intrusive optical system and therefore suffers from the drawback of a Schmidt type method by measuring an unknown quantity averaged along the line of sight of the optical system. Apparatus described in GB 2,109,472 for measuring the temperature of an object in a combustion region comprises a pyrometer and a moveable radiation collecting system which, capable of withstanding the temperatures encountered, is adapted for insertion into the combustion region. The pyrometer is used to make spot measurements of the radiation emitted by the object. The collection system consists of a moveable tube, internally cooled for insertion into the combustion region close to the object. The end of this tube is fitted with a window to admit radiation to this interior where it is reflected by a mirror towards the pyrometer. The tube is moveable axially and rotationally to permit an object like a turbine blade to be scanned. The radiation collecting window has to be positioned close to the object in question to avoid the drawbacks of line of sight averaging previously referred to. The invention is intended to overcome these drawbacks in measurements of the local temperature of hot gases in a combustion chamber or furnace by, in broadest terms, obtaining two measurements at positions very close together and basing a temperature calculation on differential radiation intensity measurements. Furthermore, the invention enables full spatial resolution of the gas temperature from a whole series of such measurements made in incremental steps traversing the gas volume. SUMMARY OF THE INVENTION In accordance with the present invention, intensity is measured at at least two neighbouring local positions. Preferably spectral radiance (ie at a plurality of wavelengths) is measured at the neighbouring positions, and used to determine local gas composition and temperature. The spectral measurements at a given position are preferably made simultaneously. The invention provides a method of making measurements on combustion gases in a combustion process comprising the steps of:- making a plurality of first intensity measurements of radiation from the combustion process at a first position within the combustion gases, to provide a plurality of first intensity values, making a plurality of second intensity measurements of radiation from the combustion process at a second position within the combustion gases, adjacent the first position, to provide a plurality of second intensity values, characterised in that: a plurality of said first and second intensity measurements at different wavelengths are made at each said position, giving a plurality of intensity values at each position for the different wavelengths, generating a set of simultaneous equations relating to the local temperature of the combustion gases in the vicinity of the first and second positions, using a recurrence formula which relates said first and second intensity values for said local temperature in terms of a plurality of absorption coefficients, one for each component species in the gas at each said wavelength, said absorption coefficients each being pre-stored values, and solving said equations to obtain the local temperature. Each component (i) of the combustion gas has its own characteristic trace of absorption coefficient of the combustion gas (Kλ , i ) for the wavelength spectrum at a given pressure and temperature. For a small volume of gas the total absorption coefficient (Kλ) at wavelength λ can be related: K λ = Σ i C i K λ,i (1) where c i is a proportional fraction of species i. Let I n,λ be the intensity of radiation emitted and absorbed by gas at wavelength λ at a local node n. Then using a method of analysis that is similar to the Discrete Transfer method one can obtain a recurrence relation: where I n+1,λ is intensity at a neighbouring node distance s away from the previous node at wavelengthλ; C₁ and C₂ are First and Second Radiation constants; and T is the temperature. Hence by measuring intensities at two neighbouring nodes at various selected wavelengths, one can obtain T from the measurement at one of the wavelengths and the Kλ for the rest of the wavelengths from the above recurrence relation. One can also use this relation over the whole spectrum and obtain the grey gas absorption coefficients. Each of these K λ 's can then be used in equation 1 to obtain a system of simultaneous equations. Given known values of K λ , i for each species i, it is then possible to calculate the gas composition. Equation (2) above can be simplified to give the following relationship for "grey" raidation (ie over a broad band of wavelengths): where K is the "grey" absorption coefficient given by K = ∫ K λ dλ (4) and σ is Stefan-Boltzmann's constant. Using the simplified recurrence relation (3) therefore, even "grey" (ie non-spectral) measurements of radiation at neighbouring nodes n and n+1 (separated by distance s) can yield useful results. The present invention also provides a device for measuring raidation from combustion gases in a combustion process according to the above method, comprising: a probe made of a material capable of withstanding the temperatures and pressures of the combustion process and having at least one sensor which is capable of measuring the intensity of the radiation from the combustion process, simultaneously at a plurality of different wavelengths; means for moving the probe, whereby the probe can traverse a path within the combustion chamber, said path including a first position (n) and a second position (n+1) adjacent thereto; and a remote signal processor including data acquisition means for taking intensity measurements from the probe at each said position. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be better understood, preferred embodiments will now be described with reference to the accompanying schematic drawings, wherein:- Figure 1 shows a probe arrangement, Figures 2 and 3 show two alternative detector arrangements for use in the probe of figure 1, Figures 4 and 5 are a longitudinal section and an end view of a probe with a further alternative detector arrangement, Figure 6 shows a modification of figures 4 and 5, Figure 7 shows in graphical form characteristic wavelengths of various typical combustion gas constituents, and Figure 8 is a flowchart of a computer program for use with the probe. DESCRIPTION OF PREFERRED EMBODIMENTS The example probe shown in figure 1 comprises an elongate sight tube 10, made of a material capable of withstanding the temperature and pressures of a gas turbine engine combustion chamber. A suitable material is SiC sold under the trade name Refel. The tube 10 passes slidingly through a seal 12 in an outer combustor casing 14, and through an air bleed hole 16 in an inner combustor wall 18, so that an inner end 10a of the tube 10 is transversable across a combustion chamber 20, and an outer end 10b is located outside the engine. The tube 10 has a calcium fluoride window 22 at its inner end 10a and also possibly at its outer end 10b. The tube may be evacuated or may contain nitrogen gas (which has satisfactory radiation transmitting properties). The tube may have an outer sleeve 24 for circulation of a cooling fluid, or the outer end 10b may be connected to a cooling nitrogen gas supply, so that the nitrogen contents are purged into the interior of the combustion chamber. In the latter case, the window 22 may not be necessary, but there could be a risk of the nitrogen purge disturbing the combustion process to be studied. In order that the inner end 10a of the tube 10 can traverse the combustion chamber, drive means 26 are provided. Schematically this is illustrated in Fig. 1 as a rack and motorised pinion, and a stepper motor is preferred since this gives accurate control of position during the traverse. The speed of the traverse depends on factors such as flame temperature. Alternative drive means are of course possible, for example an hydraulic piston and cylinder, preferably with a transducer to indicate position during the traverse. It will be appreciated that an accurate knowledge of the position of the tube end 10a is desirable if one is to measure localised conditions within the combustion chamber. The traverse path would normally be radial, but any other path which it is desired to study can be chosen. While the entire tube 10 is illustrated as being movable in Fig. 1, it would be possible to devise a telescopic arrangement whereby the inner end 10a traversed the combustion chamber while the outer end 10b remained stationary. The outer end 10b of the tube 10 is provided with a radiation detector 28 which is capable of measuring intensities of radiation transmitted along the tube 10, simultaneously at a plurality of different wavelengths. This is done at a plurality of different positions along the traverse path of the end 10a. The wavelengths detected are typically in the range 0.3 to 12 µm, though wavelengths outside this range are not excluded. Fig. 2 illustrates a simple detector arrangement for achieving this. The incoming radiation from the tube 10 is split into different wavelengths by a diffraction grating or prism at 38, and directed at an array of photo-sensors 40. In this way, each sensor 40 receives radiation of a unique wavelength and produces a corresponding output. A reading is taken simultaneously from each sensor 40 at each of a plurality of positions during a single traverse. The same effect is achieved by the arrangement of Fig. 3. Here, the incoming radiation passes to an array of sensors 42, each of which has an individual filter 44 making it sensitive to a specific wavelength. The sensors 42 and filters 44 could be individual miniature detectors, or could all be integrated on one semiconductor chip. The arrangement of Fig. 2 has the disadvantage that the detectors need to be spaced to suit the wavelengths to be detected, and so a compact arrangement may not be possible. On the other hand, the arrangement of Fig. 3 requires the use of very small detectors, which is difficult to achieve at present. Figs. 4 and 5 show an arrangement which overcomes these problems. Thermopile detectors 46, each made sensitive to a chosen wavelength by an individual filter 48 in front of it, are arranged circumferentially around a circular cross-section prism 50 at the end 10b of the tube 10. The prism 50 shown is cylindrical, but the outer face of the prism 50 has an internally conical surface 52, and the angle of the cone is chosen so that incoming radiation from the tube 10 suffers internal reflection at the surface 52 and is reflected equally on to each of the surrounding detectors. In this way, it is easy to arrange, say, twelve detectors in the minimum possible space. As an alternative, Fig. 6 shows a cone 54 with a reflecting outer conical surface 56, which reflects the incoming radiation to the detectors 46. Instead of a reflecting cone 54, a reflecting hemisphere or pyramid (with as many sides as there are detectors) could be used; and the surface 52 in Fig. 4 could likewise be hemispherical or pyramidal. The arrays of detectors and filters in Figs. 4 to 6 are cylindrical and the angle of the reflecting conical surface to the incoming radiation is 45°. This ensures that the radiation is incident on the filters in the direction normal to their surface. The importance of this is that the calibration of such filters is only accurate for radiation incident normally. If the angle of the conical surface is other than 45° (e.g. to ensure total internal reflection) the array of detectors and filters (and the adjacent external surface of the prism) should be in the form of a cone, of an angle to ensure normal incidence. Where a hemispherical or pyramidal reflecting surface is used, the external surface of the prism, and the receiving surfaces of the filters taken together, are likewise hemispherical or pyramidal (with faces at an appropriate angle to the tube 10 to ensure normal incidence). It will be seen that the advantage of a pyramidal arrangement is that the surfaces of the filters do not need to be concave in the circumferential direction to ensure adherence to their calibration. The detector 28 in any of the above arrangements is connected by cables 60 to a remote signal processor 58 (Fig.1) which includes an amplifying circuit 62 and an analogue to digital converter and sampling circuit 64. This automatically takes a batch of readings from the detector 28 at a succession of adjacent positions throughout the traverse, synchronised with the motion of the tube 10 by a circuit 66 so that the positions at which the readings are taken are known. At each position, there is a simultaneous reading for each wavelength of interest. The readings can be stored, e.g., on magnetic tape, for subsequent processing by a computer 68 or such processing can take place in real time if adequate computing power is available. If desired, the amplifying circuit 62, the analogue-to-digital converter and sampling circuit 64 and the synchronising circuit 66 could be located with the detector 28. The readings obtained for a given pair of neighbouring locations are processed by the computer in accordance with the discussion above relating to equations (1) and (2). Soot emits and absorbs throughout the spectrum. But the gaseous species have specific wavelengths that are predominant. This is illustrated in graphical form for various typical species in Fig. 7, but these values are not exhaustive, and the required data includes the absorption coefficients K λ,i at each wavelength for the various pure species at the pressures and temperatures of the combustion gas. More complete data is given in a NASA handbook by C. B. Ludwig et al, "Handbook of Infra-red Radiation from Combustion Gases", NASA SP3080, 1973 or may be specifically measured in advance for each pure species encountered in the combustion gases. However obtained, these values of K λ,i are loaded into a database held in computer storage 70. A simple illustration of the processing performed is as follows. One can find two 'clear' wavelengths where only soot effects are important. Applying equation (2) and then (1), one can solve for C soot the local proportional fraction of soot in the combustion gases and local temperature T simultaneously. Other wavelengths can then be analysed taking C soot , and T as known quantities. Hence C₀₂ C H2O and C CO are obtained. The local concentrations of each species is a useful by-product of this approach. Hence one can get local absorption coefficients for each wavelength. This processing is repeated for other locations. The practical use of this procedure is as follows. In use, the probe 10 is caused to traverse the combustion chamber, and the computer 68 is programmed as shown in Fig. 8 (which assumes the results are to be calculated in real time). Firstly, at step 72, values of I n,λ are simultaneously measured for the various wavelengths λ at a first, known position n of the probe 10, using the various sensors in the detector 28. Next, the detector is allowed to continue its traverse to the adjacent known position n+1 (step 74). Here, corresponding values of I n+1,λ are acquired (step 76). The computer now uses these acquired intensity values, the known distance s from n to n+1 and the stored values of K λ , i from the storage 70 to generate a system of simultaneous equations from formulae (1) and (2) above (step 78). It then proceeds to solve these equations for the local temperature T and the local concentrations C i of each species present in the vicinity of positions n and n+1 (step 80). These results can be output or stored for later use (step 82). Meanwhile, the probe 10 is moving to the next adjacent position n+2 (step 84) where the process of acquiring intensity values I n+2,λ is repeated (step 86). Steps 78, 80 and 82 are now repeated on the values I n+1,λ and I n+2,λ thus obtaining values of T and C i in the vicinity of positions n+1 and n+2. This process is repeated at successive probe positions throughout the traverse of the combustion chambers. It will be appreciated that, should insufficient computing power be available for this real time approach, the acquired intensity values at each position of the traverse may simply be recorded on magnetic tape or some other medium. Steps 78, 80 and 82 are then performed subsequently on the recorded data. If desired, a simplified probe and processing procedure may be used. The detector 28 contains only one sensor, sensitive to a band of wavelengths (i.e., "grey" radiation). At steps 72 and 76 in Fig. 8, only grey intensities I n and I n+1 are acquired and at step 78, formulae (3) and (4) are used instead of formulae (1) and (2). It will be appreciated that one can only solve for T at step 80 if a theoretical estimation of the value of K has previously been made.

Claims: A method of making measurements on combustion gases in a combustion process comprising the steps of:- making a plurality of first intensity measurements of radiation from the combustion process at a first position within the combustion gases, to provide a plurality of first intensity values, making a plurality of second intensity measurements of radiation from the combustion process at a second position within the combustion gases, adjacent the first position, to provide a plurality of second intensity values, characterised in that: a plurality of said first and second intensity measurements at different wavelengths are made at each said position, giving a plurality of intensity values at each position for the different wavelengths, generating a set of simultaneous equations relating to the local temperature of the combustion gases in the vicinity of the first and second positions, using a recurrence formula which relates said first and second intensity values for said local temperature in terms of a plurality of absorption coefficients, one for each component species in the gas at each said wavelength, said absorption coefficients each being pre-stored values, and solving said equations to obtain the local temperature. A method according to claim 1, wherein said recurrence formula is wherein: I n,λ and I n+1,λ are said first and second intensity values at a wavelength λ , s is the distance between said first and second positions, K λ is the total local absorption coefficient of the gas at the wavelength λ , T is the said local temperature, and C₁ and C₂ are the Frist and Second Radiation constants; and wherein K λ is given by K λ = Σ i C i K λ,i in which C i is the local proportional fraction of a component species i in the gas, and K λ,i is the absorption coefficient of that species at the wavelength λ. A method according to claim 2 wherein the set of simultaneous equations is solved to give values of c i as well as T. A device for measuring radiation from combustion gases in a combustion process according to the method of any one of claims 1 to 3, comprising: a probe (10) made of a material capable of withstanding the temperatures and pressures of the combustion process and having at least one sensor (28) which is capable of measuring the intensity of the radiation from the combustion process, simultaneously at a plurality of different wavelengths; means (26) for moving the probe (10), whereby the probe can traverse a path within the combustion chamber (20), said path including a first position (n) and a second position (n + 1) adjacent thereto; and a remote signal processor (58) including data acquisition means (64, 66) for taking intensity measurements from the probe at each said position. A device according to claim 4 wherein the probe (10) is configured as an elongate radiation transmitting member (10), at least one end (10a) of which is movable by the means (26) for moving the probe, and the opposite end (10b) thereof has said at least one sensor (28). A device according to claim 5 wherein the probe includes a plurality of said sensors (28, 40, 42, 46), sensitive to different wavelengths, said data acquisition means (64, 66) taking intensity measurements from each sensor substantially simultaneously. A device according to claim 6 including a reflecting surface (52, 56) at said opposite end (10b) of the elongate member (10), arranged to reflect radiation received along the member in a plurality of generally radial directions, the sensors (46) being arranged circumferentially around the reflecting surface. A device according to claim 7 including a prism (50, 54) providing said reflecting surface.

Inventors: SHAH, NIPULKUMAR GULABCHAND
Applicants: ROLLS-ROYCE PLC
Application year: 1985
Main classification: G01J5
IPC codes: G01J5, F23N5, F01D17, F02P17, G01J1