Thermocouple probe

Abstract: A thermocouple probe (10) projects through an aperture (30) in the casing (32) of a gas chaser (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. The method of mounting a thermocouple probe in a gas stream can be of paramount importance to its survival. If the probe protrudes through the casing of a chamber, the gas flow will generate preferred fluctual vibrations in directions at 90° and parallel to the gas stream, but more specifically as defined by the structural details of the probe. If the mounting arrangement is not well defined, it is possible that there will be little damping of vibrations at this position. This is especially significant to the fundamental or cantilever mode of vibration. According to a first aspect of the present invention, there is provided a method of mounting a thermocouple probe comprising a thermocouple element having a sensing tip, mounted in a rigid cylindrical sheath having an outwardly extending flange which is remote from the sensing tip, which is for sensing the temperature of a gas flowing through a chamber; wherein, when the sensing tip is inserted into the chamber through an aperture in the casing thereof and the flange abuts against the outer surface of the casing, the longitudinal axis of the probe and the central axis of the aperture are angularly offset and the outer surface of the sheath is in contact with the inner periphery of the aperture in two locations which are diametrically opposite and axially offset from one another. In a preferred form, the angular offset is such that the longitudinal axis of the probe and the points of contact between the outer surface of the sheath and the inner periphery of the aperture are contained in a plane which is inclined at an angle to a principal direction of the flow of gas through the chamber. The plane is preferably inclined to the natural directions of vibration of the cantilever constituted by the thermocouple probe; namely in the direction of the gas flow and at 90° to the direction of the gas flow. Particularly preferably, the angle between the said plane and the principal direction of the flow of gas is 45°. Hence the two contact points are spaced at 45° with respect to the direction of the preferred fundamental modes of vibration and-at 45° with respect to the gas flow direction. This is desirable since although vibration in line with the contact points will not be well damped, the thermocouple probe itself will not tend to vibrate naturally at 45° to its preferred direction. 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 bending stresses tend to be more prevalent. In accordance with the present invention in a second aspect, there is provided an assemblage for sensing the temperature of a gas flowing through a chamber, comprising a thermocouple probe which includes a thermocouple element, having a sensing tip, mounted in a rigid cylindrical sheath having an outwardly extending flange which is remote from the sensing tip; wherein the thermocouple probe extends through an aperture in the casing of the chamber and the flange thereof abuts the outer surface of the casing such that the longitudinal axis of the thermocouple probe and the central axis of the aperture are angularly offset and the outer surface of the sheath is in contact with the inner periphery of the aperture in two locations which are diametrically opposite and axially offset from one another. Typically, the central axis of the aperture in the casing is substantially perpendicular to the inner and outer surfaces of the casing. According to a first embodiment of the second aspect of the invention, the angular offset between the two axes is produced by having the normal to the mating face of the flange inclined at an angle to the longitudinal axis of the probe. Alternatively, and according to a second embodiment, the desired effect may be achieved by having a stud-like protrusion on the mating face of the flange which is radially offset from the longitudinal axis of the probe. The different aspects of the invention, and embodiments thereof, will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows an assemblage according to the invention; Figure 2 is a schematic diagram illustrating the orientation relationship between the thermocouple probe and the principal direction of gas flow; Figure 3 shows the longitudinal cross-section of a thermocouple probe sheath according to a first embodiment of the invention; and Figure 4 shows the longitudinal cross-section of a thermocouple probe sheath according to a second embodiment of the invention. Figure 1 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 « 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 Figure 2, 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 Figure 4. The flange 60 has a stud-like protrusion 62 on the mating surface 64. The protrusion 62 is radially offset from the longitudinal 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: A method of mounting a thermocouple probe (10) comprising a thermocouple element having a sensing tip, mounted in a rigid cylindrical sheath (16) having an outwardly extending flange (40) which is remote from the sensing tip, which is, for sensing the temperature of a gas flowing through a chamber; wherein, when the sensing tip is inserted into the chamber through an aperture (30) in the casing (32) thereof and the flange abuts against the outer surface (38) of the casing, the longitudinal axis of the probe and the central axis of the aperture are angularly offset and the outer surface of the sheath is in contact with the inner periphery of the aperture in two locations which are diametrically opposite and axially offset from one another. A method according to claim 1, wherein the angular offset is such that the longitudinal axis of the probe (10) and the two points of contact between the outer surface of the sheath (16) and the inner periphery of the aperture (30) are contained in a plane which is inclined at an angle to a principal direction of flow of the gas through the chamber and which is a non-preferred plane of vibration for the probe. A method according to claim 2, wherein the angle between the said plane and the principal direction of the flow of gas is 45°. A method according to claim 2 or 3, wherein the thermocouple probe (10) is oriented such that at least a component of any force exerted thereon by the gas flowing through the chamber increases the contact pressure exerted by the sheath (16) on the inner periphery of the aperture (30). An assemblage, for sensing the temperature of a gas flowing through a chamber, comprising a thermocouple probe (10) which includes a thermocouple element (12), having a sensing tip, mounted in a rigid cylindrical sheath (16) having an outwardly extending flange (40) which is remote from the sensing tip; wherein the thermocouple probe extends through an aperture (30) in the casing (32) of the chamber and the flange thereof abuts the outer surface of the casing such that the longitudinal axis of the thermocouple probe and the central axis of the aperture are angularly offset and the outer surface of the sheath is in contact with the inner periphery of the aperture in two locations which are diametrically opposite and axially offset from one another. An assemblage according to claim 5, wherein the central axis of the aperture (30) is substantially perpendicular to the inner and the outer surfaces of the casing (32). A thermocouple probe (10) for use in the assemblage of claim 6, wherein the normal to the mating face of the flange (40) is inclined at an angle to the longitudinal axis of the probe. A thermocouple probe (10) for use in the assemblage of claim 6, wherein the mating face of the flange (40) has a stud-like protrusion (62) which is radially offset from 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