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 is a divisional of applicationInconel 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 4.5 mm, 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 2.0 mm.

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 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 (FIG. 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 (FIG. 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 FIG. 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.

FIG. 4 shows an assemblage in which a thermocouple probe 10 is mounted through an aperture 30 in the casing 32 of a chamber 34. The central axis BB of the aperture is perpendicular to the inner surface 36 and outer surface 38 of the casing 32. The thermocouple probe 10 comprises a thermocouple element, having a sensing tip, which is protected inside a rigid sheath 16. The sheath 16 has an outwardly extending flange 40 which is remote from the sensing tip of the thermocouple element. The aperture 30 is larger than the cross-section of the sheath 16, but smaller than that of the flange 40.

The thermocouple probe 10 protrudes through the aperture 30 such that the sensing tip of the thermocouple element is located in the chamber 34. The mating surface 42 of the flange 40 abuts, and is biased against the outer surface 38 of the casing 32.

The mating surface 42 of the flange 40 is not perpendicular to the longitudinal axis of the probe 10. Instead, the mating surface normal is inclined at an acute angle to the longitudinal axis of the probe 10. The region 41 of the flange 40 diametrically opposite from the mating surface 42 is shaped so as not to interfere with the desired fit of the thermocouple probe 10 in the aperture 30. Therefore, since the mating surface 42 is positively biased flush against the outer surface 38 of the casing 32, there is an angular offset α between the longitudinal axis of the probe 10 and the central axis of the aperture 30. Furthermore, the angular offset is such that the outer surface of the sheath 16 makes contact with the inner periphery of the aperture 30 in two locations, diametrically opposite and axially offset from one another. The first point of contact is adjacent the outer surface 38 of the casing; and the second point of contact is adjacent the inner surface 36. Hence in total, three points of contact are established in the assemblage. The first and second are between the outer surface of the sheath 16 and the inner periphery of the aperture 30; and the third is the high point between the mating surface 42 and the outer surface 38 of the casing 32.

Furthermore, the points of contact are positioned so that a component of any force exerted on the probe by the flow of gas through the chamber tends to increase the contact pressure exerted by the sheath on the inner periphery of the aperture.

The longitudinal axis of the probe 10 and the two points of contact between the inner, periphery Of the aperture 30 and the outer periphery of the sheath 16 define a plane. In FIG. 5, the plane is perpendicular to the page and intersects the page along the trace line 50. The central axis of the aperture may also lie in the plane. The gas which flows through the chamber 34 flows in a principal direction 52, viz. a gas stream in the flow duct of a turbine engine. There exists a preferred orientation relationship between the trace line 50 and the principal direction 52. The trace line 50 is inclined substantially at 45° to the principal direction 52.

The angular offset between the longitudinal axis of the probe 10 and the central axis of the aperture 30 may be achieved by using the alternative flange construction shown in FIG. 7. The flange 60 has a stud-like protrusion 62 on the mating surface 64. The protrusion 62 is radially offset from the longitidinal axis of the probe. When the flange 60 is biased against the outer surface 38 of the casing 32, the protrusion 62 prevents the longitudinal axis of the probe from being parallel to the central axis of the aperture.

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

1. A method for mounting a sensing probe in a fluid chamber, said sensing probe being exposed to fluid flowing through said chamber, comprising the steps of:

providing said sensing probe, said sensing probe comprising a rigid cylindrical body and a flange extending outwardly from said rigid cylindrical body;

providing an aperture in the casing of said chamber, said aperture being such that said flange is too big to pass therethrough;

inserting said sensing probe into said aperture such that said rigid cylindrical body projects into said chamber and such that said flange abuts against the outer surface of the casing of said chamber; and

positioning said sensing probe such that the longitudinal axis of said sensing probe and the central axis of said aperture are angularly offset, with the outer surface of said rigid cylindrical body in contact with the inner periphery of said aperture in only two locations which are diametrically opposite and axially offset from one another.

2. A method according to claim 1 wherein the angular offset is such that the longitudinal axis of said sensing probe and the two points of contact between the outer surface of said rigid cylindrical body and the inner periphery of said aperture are contained in a plane which is inclined at an angle to a principal direction of flow of fluid through said chamber and which is a non-preferred plane of vibration for said sensing probe.

3. A method according to claim 2 wherein said angle between said plane and said principal direction of flow of fluid is 45°.

4. A method according to claim 2 or 3 wherein said sensing probe is positioned such that at least a component of any force exerted thereon by the fluid flowing through said chamber increases the contact pressure exerted by said rigid cylindrical body on the inner periphery of said aperture.

5. An assemblage consisting of a sensing probe mounted in a chamber, said chamber having a casing with an aperture therein for receiving said sensing probe, said sensing probe comprising a rigid cylindrical body and a flange extending outwardly from said rigid cylindrical body, said flange being too big to pass through said aperture, wherein said rigid cylindrical body projects into said chamber through said aperture with said flange having a mating face which abuts the outer surface of said casing, such that the longitudinal axis of said sensing probe and the central axis of said aperture are angularly offset, with the outer surface of said rigid cylindrical body in contact with the inner periphery of said aperture in only two locations which are diametrically opposite and axially offset from one another.

6. An assemblage according to claim 5 wherein the central axis of said aperture is substantially perpendicular to the inner and outer surfaces of the casing of said chamber adjacent said aperture.

7. An assemblage according to claim 6 Wherein the normal to the mating face of the flange is inclined at an angle to the longitudinal axis of said sensing probe.

8. An assemblage according to claim 6 wherein the mating face of the flange has a stud-like protrusion, said stud-like protrusion being radially offset from the longitudinal axis of said sensing probe.