Thermocouple probe

Abstract: A thermocouple probe (10) suitable for use in a gas turbine engine comprises a thermocouple element (12) which is coaxially arranged inside a protective sheath (16). The thermocouple element (12) is resiliently supported by a helical member (18) which insulates the element (12) from deleterious vibrations induced in the sheath (16). In use, the thermocouple probe (10) projects through an aperture (30) in the casing (32) of a gas chamber (34). Vibrations in the probe (10) are further reduced by ensuring that the longitudinal axis AA of the probe is angularly offset from the central axis BB of the aperture (30), such that the outer periphery of the sheath (16) is in contact with the inner periphery of the aperture (30) in two diametrically opposite and axially offset locations.

Description: This invention relates to a thermocouple probe for sensing the temperature of a gas flowing through a chamber, and is more particularly but not exclusively concerned with a thermocouple probe suitable for use in gas turbine engines. Thermocouple probes for use in gas turbine engines typically have to withstand high temperatures and high levels of vibration. At the same time, there is frequently a requirement for a long reach thermocouple, i.e. one which projects a long way into the flow duct which conveys the gas whose temperature is to be sensed, as well as a requirement for a fast response. Thus when designing an appropriate support for the thermocouple element, and an appropriate mounting arrangement for the thermocouple probe, all these factors have to be taken into account. In gas turbine engine thermocouple probes, it is conventional to mount the thermocouple element within an outer protective sheath, to provide both strength and support (see e.g. US-A-4 499 330). It is of course then necessary to provide some support between the thermocouple element and the protective sheath. With such an arrangement, problems caused by vibration and differential thermal expansion between the protective sheath and the thermocouple element, as well as the problem of ensuring coaxial alignment of the element within the sheath, all need to be solved, without compromising the desired fast response. The manner in which a thermocouple probe is mounted on a gas turbine engine is critical to its survival. The vibration levels attained in such an engine can be very high. In addition, the flow of gas through the duct may activate natural resonances in the protuberant part of the probe. Either of these factors can produce fluctuating stresses in the probe which may contribute to premature fatigue failure. It is clearly desirable to minimize such effects in an attempt to prolong the life of the probe. According to the present invention in a first aspect, there is provided a thermocouple probe for sensing the temperature of a gas flowing through a chamber comprising an elongate thermocouple element disposed substantially coaxially within an elongate cylindrical sheath such that there exists a cavity around the thermocouple element, characterised in that the thermocouple element is resiliently supported within the sheath by a helical member, arranged coaxially with the thermocouple element and which has a plurality of turns of non-circular cross-section such that the helical member is alternately in contact with the inner periphery of the sheath and the outer periphery of the thermocouple element. According to a first embodiment of the first aspect of the invention, the helical member is corrugated such that the radial extremities of the corrugations produce the points of contact which are circumferentially distributed around the inner periphery of the sheath and the outer periphery of the thermocouple element. According to a second embodiment of the first aspect of the invention, the turns are substantially elliptical in cross-section and angularly offset such that the points of contact are circumferentially distributed around the inner periphery of the sheath and the outer periphery of the thermocouple element respectively. In a preferred form, the helical member consists of a filament having a rectangular cross-section which is employed such that the thickness of the filament in a radial direction is less than the width in the axial direction. This provides the necessary support in the high temperature, high vibration conditions and overcomes the oxidation problems associated with wire of small cross section. Typically, to ensure a fast response to temperature fluctuations, the sheath has a pair of apertures which permit the gas whose temperature is to be sensed to flow over the thermocouple element. The apertures are preferably elliptical in cross-section and aligned such that their major diameter is parallel with the longitudinal axis of the probe. This configuration is favourable since it is less detrimental to the integrity of the sheath in comparison to a circular aperture of identical cross-sectional area. The shape of the apertures is especially significant towards the root of the probe where tending stresses tend to be more prevalent. The different embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a partially cut away schematic view of a thermocouple probe with a helical member according to a first embodiment of the invention; Figure 2 is a cross-sectional view of a thermocouple probe showing a turn of the helical member according to a first embodiment of the invention; and Figure 3 is a cross-sectional view of a thermocouple probe showing a turn of the helical member according to a second embodiment of the invention. Figure 1 shows a device which may be used to sense the temperature of a gas flowing through a chamber. A thermocouple probe 10 has a longitudinal axis AA and comprises a thin elongate mineral-insulated thermocouple element 12 having a sensing tip 14. The thermocouple element 12 itself comprises a cylindrical outer jacket, typically of Inconel 600, containing at least one pair of thermocouple wires. The wires are joined together to form a thermocouple junction at the sensing tip 14, but are otherwise insulated from each other and from the outer sheath by magnesium oxide powder. The outer diameter of the thermocouple element 12 is typically 4.5mm, although smaller diameters are possible. For example, to enhance the speed of response, the diameter of the sensing tip 14 may be swaged down to about 2.0mm. The thermocouple element 12 is disposed coaxially within a cylindrical protective sheath 16, which defines an annular space around the element. The sheath 16 is made of a material resistant to high temperatures, such as Nimonic 80 or Hastelloy X. The thermocouple element 12 is resiliently supported within the sheath 16, and held in coaxial alignment with it, by means of a helical member 18, which consists of a filament having a rectangular cross-section. The radial thickness of the filament is less than its width in the axial direction; and the radial thickness is also less than the radial separation between the thermocouple element 12 and the sheath 16. The helical member 18 is adapted to make alternate contact with the inner periphery of the sheath 16 and the outer periphery of the thermocouple element 12. The circumferential spacing of the contact areas is selected to give the desired support between the thermocouple element and the sheath. The contact areas spiral around the longitudinal axis of the probe 10 along the axial length of the helical member 18. The helical member 18 may comprise rippled or corrugated turns, so that it makes contact with both the sheath 16 and the thermocouple element 12, the maximum amplitude of the corrugations in a radial direction being at least as great as the radial separation between the sheath 16 and the element 12 (Figure 2). A helical member 18 composed of a corrugated filament may be produced by wrapping a strip of material around a mandrel having the desired varying cross-sectional shape. The dimensions are chosen such that on assembly the thermocouple element 12 is an interference fit within the helical member 18, and the helical member is an interference fit within the sheath 16. Alternatively, the helical member 18 may comprise turns of elliptical cross-section (Figure 3) which obviates the need for a corrugated filament. The elliptical turns are dimensioned such that along a major diameter the helical member is in contact with the sheath 16, and that along a minor diameter the helical member is in contact with the thermocouple element 12. Adjacent elliptical turns are angularly offset so that the contact areas spiral around the longitudinal axis of the probe 10. A helical member 18 having offset elliptical cross-sections may be produced by firstly wrapping a strip of material around a mandrel having an elliptical cross-section, and then twisting the resulting helical structure about its longitudinal axis to progressively offset adjacent elliptical turns. In Figure 3, the successive contact areas with the sheath 16 are shown more than 180° apart, if the angle is measured following the sense of rotation around the helical member 18. However, the degree of twist imparted to the structure to offset adjacent elliptical turns may be such that successive contact points are less than 180° apart. The protective sheath 16 is provided with axially offset apertures 22, 24 which permit the gas flowing through the aforementioned chamber to pass over the sensing tip 14 of the thermocouple element. The apertures 22, 24 have elliptical cross-sections which are orientated such that the major diameters of the ellipses are aligned parallel to the longitudinal axis of the thermocouple probe 10. The extreme end of the sheath 16 adjacent the sensing tip 14 may be closed by a weld-on end cap (not shown). Alternatively, the thermocouple element 12 may comprise two independent thermocouples, the first located at the sensing tip 14 of the probe 10 and the second located at about half way along the length of the element 12. In this case, two helical members 18 are employed, one on each side of the second (half-way) thermocouple junction. The sensing tip 14 actually protrudes slightly from the sheath 16 into the gas flow, and three apertures (one inlet and two outlet) are provided near the second junction to promote the desired fast response. When the thermocouple probe 10 is in use, the inlet aperture faces into the gas flow, while the exit apertures are circumferentially and axially offset and positioned just over 90° from the inlet aperture, so as to occupy areas of low pressure. The materials used for the thermocouple element 12, the sheath 16 and the helical member 18 are selected to have optimum properties for their particular roles. If desired, different materials having slightly different thermal expansion coefficients could be selected for these components since any resulting differential expansion in a longitudinal direction may be accommodated in the helical member 18. It will be appreciated that various modifications can be made to the described embodiments of the invention. In particular, the various dimensions and materials mentioned in relation to the described embodiments are examples only.

Claims: A thermocouple probe (10) for sensing the temperature of a gas flowing through a chamber comprising an elongate thermocouple element (12) disposed substantially coaxially within an elongate cylindrical sheath (16) such that there exists a cavity around the thermocouple element, characterised in that the thermocouple element is resiliently supported within the sheath by a helical member (18), arranged coaxially with the thermocouple element and which has a plurality of turns of non-circular cross-section such that the helical member is alternately in contact with the inner periphery of the sheath and the outer periphery of the thermocouple element. A thermocouple probe (10) according to claim 1, wherein the helical member (18) has turns of substantially elliptical cross- section which are angularly offset such that the points of contact are circumferentially distributed around the inner periphery of the sheath (16) and the outer periphery of the thermocouple element (12) respectively. A thermocouple probe (10) according to claim 1, wherein the helical member (18) is corrugated such that the radial extremities of the corrugations produce the points of contact which are circumferentially distributed around the inner periphery of the sheath (16) and the outer periphery of the thermocouple element (12) respectively. A thermocouple probe (10) according to any of the preceding claims wherein the helical member (18) consists of a filament having a rectangular cross-section which is employed such that the thickness of the filament in a radial direction is less than the width in the axial direction. A thermocouple probe (10) according to any of the preceding claims wherein the sheath (16) is made from a high temperature alloy selected from nimonic and hastelloy. A thermocouple probe (10) according to any of the preceding claims wherein the sheath (16) has at least two apertures (22,24) which permit the gas whose temperature is to be sensed to flow over the thermocouple element (12). A thermocouple probe (10) according to claim 6, wherein at least one of the apertures has an elliptical cross-section which is orientated such that the major diameter is parallel with the longitudinal axis of the probe.

Inventors: METCALF ERIC, STANSFELD JAMES WOLRYCHE, METCALF, ERIC, STANSFELD, JAMES WOLRYCHE
Applicants: SOLARTRON GROUP LTD, SOLARTRON GROUP LIMITED
Application year: 1993
Main classification: G01K13
IPC codes: G01K13