GB2075212A - Optical probe - Google Patents

Abstract

An optical probe 12, which is particularly suitable for continuous monitoring of combustion processes at close range, essentially comprises an image guide 14 and a pinhole 18 which projects an image onto end 17 of the image guide 14 for relaying to a remote location. A sharp image with large depth of focus is obtained without the need for lenses. Awkward lines of sight can be taken care of if necessary by incorporating a mirror 19 (or prism) between the pinhole 18 and the image guide 14 in the manner of a lenseless camera obscura. The image guide is an ordered array of optic fibers or a image forming optic fibre. <IMAGE>

Description

SPECIFICATION Image relay systems The present invention relates to image relay sys tems, and is particularly suitable for remote observa tion of hostile environments.

During the development and operation of gas turbine aeroengines it is desirable for analytical purposes to be able to view the combustion process at close range whilst the engine is running normally, and without significantly disturbing the normal gas 'flows and flame pattern within the combustion chamber.

It is known to utilise optical probes in the form of borescopes or flexible fibre-optic image guides in conjunction with focussing lenses and a source of illumination as a means of inspecting the condition of the components in engines without having to dismantle the engine first, the inspection being done whilst the engine is not working. However, these existing devices are not suited to withstand con tinuous duty in a combustion chamber whilst the engine is running, due to the effects of heat and combustion products on the lenses. These effects can be mitigated'by using sapphire lenses to with stand the heat and complicating the design of the probe so as to protect the objective lens from contamination with combustion products by causing large amounts of pressurised purging air or other gas to exit into the combustion chamber past the lens.However, as a result, the probe becomes much more expensive to manufacture, and the purging gas could affect the combustion process being observed. Furthermore such probes have a res tricted depth of focus and must either be moved in or out or provided with a focussing mechanism if it is desired to examine features at widely varying dis tances from the lens.

Accordingly, the object of the present invention is to facilitate a simple, inexpensive and easily manu factured optical probe with a large depth of focus which is capable for long periods of relaying an image from the inside of a gas turbine engine combustion chamber whilst combustion is pro ceeding.

According to the present invention in its broadest aspect, an image relay system includes image guide means and means defining a small aperture for lenseless projection of an image onto an image receiving end of said image guide means. The image guide means conveniently comprises a rigid cohe rent bundle of optical fibres.

Utilising the above principle, an optical probe for viewing combustion processes at close range com prises a probe body which has an objective portion provided with the small aperture, the probe body enclosing the image-receiving end of the image guide means and the portion of the image guide means adjacent to said end thereby to protect said end and said portion of the image guide means from the effects of the combustion process.

If required, the probe may have means allowing entry of gas to the interior of the probe body to pressurise it against entry of combustion products through the small aperture, whereby the gas acts to purge said aperture. The gas or another heat exchange medium may be utilised to cool the image guide means.

An optical element such as a plain mirror or a prism may be incorporated to intercept the image projected by the small aperture and direct it onto the image-receiving end of the image guide means.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of an embodiment of the invention in the form of a simple optical probe; Figure 2 illustrates an application of the invention to a specific problem and includes a diagrammatic cross-sectional view of a further optical probe; Figure 3 is an enlarged cross-section of the optical probe of Figure 2 taken on line Ill-Ill in Figure 2.

The drawings are not to scale.

Figure 1 shows how the principle of the invention can be utilized to produce an optical probe, 1, for relaying an image I of object 0 to a remote location The probe consists essentially of an image guide 2, and a cylindrical probe body 3 enclosing the plane image-receiving end 4 of the image guide 2 and the portion 8 adjacent the end 4. At what may be termed the objective end 5 of the probe body 3 there is provided a small aperture 6, the objective end 5 being othewise closed off. The assembly acts in the manner of a pinhole camera, the aperture 6 being small enough to project an image I of object 0 onto the image receiving end 4 of the image guide. The optimum diameter of the aperture is given by the well-known relationship V where 1 is the desired imaging distance and Xis the wavelength of the light being utilised.Where more than one wavelength is present, a mean value of X can be used.

The image guide 2 comprises a coherent bundle of many small-diameter optical fibres, which together are capable of relaying an image of acceptable quality to the remote location, each fibre transmitting that portion of the light comprising image I which falls on its end. A mere light guide, such as a solid glass rod or a non-coherent bundle of optical fibres, is not suitable, since it will not transmit a satisfactory image.

Means, such as ring glands 9, must be provided to maintain the end 4 of image guide 2 fixed in relation to aperture 6, and at least that portion of the image guide within probe body 3 is rigid to facilitate this.

Rigidity of the image guide is achieved by rigidly bonding the fibres together, either by fusing them together under heat and pressure, or by incorporating them in a rigid resin matrix.

It will be readily appreciated that the abovedescribed optical probe is simple in construction and readily manufactured. Given an adequate source of illumination, it readily lends itself for use in hostile environments, such as extreme heat, since it has no lenses which are susceptible to damage or obscuration. Assuming that at least those fibres in the bundle within the probe are bonded together by fusing, the optical fibres in image guide 2 may be made of glass to withstand temperature up to 450"C, or of quartz (silica) to withstand substantially higher temperatures. If addtional performance is required, the probe can be modified to allowthe image guide 2 within the probe body 3 to be cooled by a flow of air or other heat exchange medium in a cooling circuit around the image guide.In order to keep aperture 6 free of blockages and to prevent ingress of hot gases or contaminants through it, a gas purge through the aperture can be arranged by the simple expedient of pressurising the space 7 with the purge gas to a higher pressure than ambient. If air or other gas is used for cooling, the purging can conveniently be done by utilising some of the cooling gas.

Significant disturbance bythe purging gas of the environment being monitored is avoided, if necessary, by keeping the amount of gas escaping through the aperture 6 to a minimum. This can be assured firstly by the small size of the aperture and secondly by maintaining only a small pressure difference across the aperture.

As an example of the invention applied to a specific task, Figures 2 and 3 will now be considered.

The task is continuous observation of the pintle 10 of a fuel injector 11 in a gas turbine engine combustion chamber (not shown) under all combustion conditions in order to assess unwanted build-up of carbon on the pintle, the carbon being a by-productofthe combustion process.

The fuel from injector 11 is ignited electricaliy, and for this purpose an igniter plug is located close to the fuel injector 11. The igniter plug is contained in a tube, open at its free end. In Figures 2 and 3, an optical probe 12, which is part of an image relay system for observing the pintle 10 whilst the engine is running, conveniently comprises a dummy igniter tube 13 housing a rigid fused glass fibre-optic bundle 14. The fibre-optic bundle is contained within a support tube 15, which is located within and fixed to the outer tube 13 by means of iongitudinal support strips 16.In order to obtain a image of the pintle lOon the image-receiving plane end 17 of the fibre optic bundle 14, a small aperture 18 of 100 um (about 0.004 inches) diameter is provided in the appropriate place at the side of tu be 13, and the image is intercepted and reflected onto the end 17 of fibre-optic bundle 14 by a small metal mirror 19. The optical probe thus functions in the manner of a small lenseless camera obscura. The combustion process provides sufficient light to obtain a good image of the pintle 10, the small diameter of the hole ensuring that the image has a good definition.

As an alternative to mirror 19, a small prism could be used.

The fuel from injector 11 is ignited by propagation of the flame from the other adjacent injectors in the combustion chamber, these being provided with another igniter or igniters.

The temperature in the region of the ignitertube is typicaliy in the range 500-600OC whilst the engine is running, and therefore the fibre-optic bundle 14 requires some cooling. It is simplest to use air for this. To facilitate circulation of the cooling air around the support tube 15, the support strips 16 are used as partitions to divide the flow of input cooling air in semi-annular space 20 from the flow of output cooling air in semi-annular space 21, the air being fed in from a pressurised source (not shown) at the other end (not shown) ofthetube 13.

The rates of inflow and outflow of the cooling air are controlled to give adequate cooling of the fibre-optic bundle 14 and to maintain a pressure in the interior of tube 13 which is greater than the total pressure (static and dynamic) at the entrance to the aperture 18. The aperture 18 is thus continuously purged by a flow of air through it into the combustion chamber, which keeps the aperture free of debris and the interior of tube 13 free of contamination by combustion products. Because the aperture is so small, the amount of air coming out of it is too small to have a significant effect on the combustion process.

The optical probe 12 relays the image of the pintle 10 to the outside of the engine casing containing the combustion chamber. The rigid fibre-optic bundle 14 may be optically coupled to a longer flexible image guide, or the rigid bundle 14 may be merely a fused end portion of a long flexible image guide, the flexible image guide in either case being used to relay the image to a more remote location. Alternatively, the image may be viewed at the outside of the casing. In any case, a video camera is conveniently coupled to the end of the image guide so that the image can be displayed on a video screen and recorded photographically or on videotape.

Although in the above description relating to Figures 1 to 3, a bundle of conventional optical fibres is used as the image guide in the respective probes, such a bundle could be replaced by a single large diameter graded index glass fibre or rod (such as those sold under the trade name SELFOC), or a bundle of smaller diameter graded index fibres. The image would be projected onto one end of the graded index fibre or fibre bundle, and focussed by the graded index fibre or fibre bundle onto its own opposite end, where the image can be observed or relayed as necessary. Use of graded index fibres instead of conventional optical fibres allows transmission of an image with higher definition than is possible with conventional optical fibres.

Although only one simple method of cooling the image guide within an optical probe of minimum diameter has been discussed, many other coolant circuit arrangements are of course possible, and other coolants can be used instead of air. It is particularly desirable for efficient cooling to arrange that the input leg of the coolang circuit is a fully annular chamber surrounding the image guide, the annular chamber being defined between the image guide and a coolant jacket around the image guide.

Claims (11)

1. An image relay system including image guide means ane means defining a small aperture for lenseless projection of an image onto an imagereceiving end of said image guide means.

2. An image relay system according to claim 1 in which the image guide means comprises a coherent bundle of optical fibres.

3. An image relay system according to claim 1 in which the image guide means comprises at least one graded index fibre or rod.

4. An image relay system according to any one of claims 1 to 3 in which the output of the image guide means is coupled to a video camera for recordal and/or display of the image.

5. An image relay system according to any one of claims 1 to 4, in which an optical probe for viewing combustion processes at close range comprises a probe body which has an objective portion provided with the small aperture, the probe body enclosing the image-receiving end of the image guide means end the portion of the image guide means adjacent to said end thereby to protect said end and said portion of the image guide means from the effects of the combustion process.

6. An image relay system according to claim 5, the optical probe having means allowing entry of gas to the interior of the probe body to pressurise it against entry of combustion products through the small aperture, whereby the gas acts to purge said aperture.

7. An image relay system according to claim 5, the optical probe having means facilitating circulation of a heat exchange medium around the image guide means to cool it.

8. An image relay system according to claim 6, the optical probe having means facilitating circulation of the gas around the image guide means to cool it.

9. An image relay system according to any one of claims 1 to 8 incorporating a optical element which intercepts the image projected by the small aperture and directs it onto the image-receiving end of the image guide means.

10. An image relay system substantially as described in this specification with reference to and as illustrated by Figure 1 of the accompanying drawings.

11. An image relay system substantially as described in this specification with reference to and as illustrated by Figures 2 and 3 of the accompanying drawings.