GB2358472A

GB2358472A - Heat flux sensor

Info

Publication number: GB2358472A
Application number: GB0000965A
Authority: GB
Inventors: Peter Robin Nicholas Childs; Joanne Rachel Greenwood; Simon John Davies
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2000-01-18
Filing date: 2000-01-18
Publication date: 2001-07-25
Also published as: GB0000965D0

Abstract

A heat flux sensor (1) for measuring the heat flux (Q) through a surface (3). The sensor comprising at least one pair of thermocouple junctions (A,B) together forming a thermocouple element (5), at least one thin film patch (20) of a first thermal insulator material and electrical connections (28) connecting the sensor (1) to a measurement device (18). The pair of thermocouple junctions (A,B) each comprising the junction of a thermoelectric thin film patch (22) of a first thermoelectric material electrically connected to and adjoining a thermoelectric thin film patch (24) of a second thermoelectric material. The pair of thermocouple junctions (A,B) and the thermoelectric thin film patches (22,24) are laterally spaced apart on the surface (3), and the thin film patch (20) of the thermal insulator material is disposed in the region of one of the pair of thermocouple junctions (A) to distort the heat flux in the region of the said junction (A). The thermocouple element 5 may be arranged in a circular pattern (Fig 6a). The thermal insulator may be disposed above or below the thermocouple elements.

Description

2358472 Heat Flux Sensor The present invention relates generally to a heat

flux sensor for measuring heat flux and in particular to improvemen T ts to heat flux s ensors or gauges used to measure the heat flux through a surface, and specifically through the surface of a gas turbine engine component.

Heat flux can be defined as the energy in transit through a surface or plane due to a temperature difference per unit cross sectional area of the surface or plane normal to the direction of the flux. There is a frequent requirement to measure the heat flux through a surface with applications ranging from energy audits in buildings to medical and engineering use. Within gas turbine engine development it is often required to measure the heat flux emitted from and flowing through t he surface of a component, for example a turbine casing, nozzle guide vane, turbine blade, in order to understand the environment within, and behaviour of, the engine and components thereof.

Unfortunately there are no devices that can measure the energy and hence heat or heat flux directly. Instead the effects of the energy in transit must be monitored and the heat flux inferred.

-. A conventional type of device for measuring a heat flux is a thermopile. As shown in figure 1 a thermopile comprises a sandwich of two thermocouple materials 4,6 forming a first thermocouple element 14, an insulator 8, and a further pair of thermocouple materials 10,12 forming a further thermocouple element 16. The thermocouple elements 14, 16 and insulator 8 comprise a series of layers 4,6,8,10,12 of different material deposited on top of each other and on the surface 3 of a component 2 through which the heat flux Q is to be measured. Suitable electrical connections 28,28' are made to the thermocouple elements 14,16 to interconnect 28' the thermocouple elements 14,16 to form a thermocouple 5a and connect 28 them to measuring equipment 18 to measure the 2 voltage generated by the thermocouple 5a. In operation the heat flux Q passing through the surface 3, and thermopile, in a direction normal to the surface 3 will cause local heating of the thermocouple elements 14,16. The insulator layer 8 between the thermocouple elements 14,16 causes a differential temperature to exist in each of the thermocouple elements 14,16 on either side of the insulator 8. The differential voltage produced by the thermocouple elements 14,16 indicates the differential temperature. This differential temperature is a function of heat flux Q passing through the thermopile, and so through the surface 3 upon which the thermopile is mounted. Consequently by measuring the temperature differential between the thermocouple elements 14,16 the heat flux Q can be determined by comparing the measured temperature differential against calibrated results.

Alternatively the heat flux Q can be directly calculated from the measured differential temperature using a theoretical understanding of the heat transfer through the thermopile and the known properties of the material forming the thermopile.

Examples of such heat flux sensors for various applications, including the application of such sensors to measuring the heat flux through gas turbine engine components are proposed in US Patents numbers US 5,288,147; US 5,033,866; US 5,087,312; and US 4,779,994.

A problem with these proposed designs and conventional thermopile heat flux sensors and methods is that a large number of layers are required to be deposited or mounted upon the surface of the component to be measured. Sensors utilising such a large number of individual layers are costly and. complex to manufacture. The use of a large number of individual layers spaced on top of each other, and in particular separated by an insulator, also introduces capacitance errors in to the electrical signals generated by the thermocouples. Furthermore even though the individual layers may be thin the large number of layers results in the sensor having a significant thickness or size in the 3 direction normal to the surface. Such thick sensors can adversely affect the heat flux through the surface, generating errors. In addition for gas turbine applications the sensors are often required to be used on components and surfaces which are subject to a gas or airflow. Such thick sensors on the surfaces of a component exposed to an air or gas flow can effect the flow and flow conditions over the surface. This may adversely alter the operation of the component and/or gas turbine engine. The engine and/or component may therefore not function correctly and/or in a way representative of the normal operating conditions under which the measurements are required to be taken. Thick sensors are also more susceptible to erosion and damage when exposed to the harsh environment and air or gas flow of a gas turbine engine.

It is therefore desirable to provide an improved heat flux sensor for measuring the heat flux through a surface, in particular for application to measuring the heat flux through the surfaces of gas turbine engine components, which is less intrusive than the conventional methods and devices, addresses the above mentioned problems and/or which offers improvements generally.

According to the present invention there is provided a.heat' flux sensor for measuring the heat flux through a surface the sensor which in use is attached to the surface comprising at least one pair of thermocouple junctions together forming a thermocouple element, at least one thin film patch of a first thermal insulator material and electrical connections connecting the sensor to a measurement device, the pair of thermocouple junctions each comprising the junction of a thermoelectric thin film patch of a first thermoelectric material electrically connected to and adjoining a thermoelectric thin film patch of a second thermoelectric material; characterised in that the pair of thermocouple junctions and the thermoelectric thin film patches are laterally spaced apart relative to the surface, 4 and the thin film patch of the thermal insulator material is disposed in the region of one of the pair of thermocouple junctions and in use is arranged to distort the heat flux in the region of the said junction.

Preferably the heat flux sensor further comprises a further thin film patch of a second thermal insulator material which is disposed in the region of the other of the pair of thermocouple junctions and in use is arranged to distort the heat flux in the region of the said junction, with the first and second thermal insulator materials having significantly different thermal properties.

The at least one thin film patch of thermal insulator material may be disposed on top of the thermocouple junction. Alternatively the at least one thin film patch of thermal insulator material is disposed underneath the thermocouple junction.

The thin film patch of the second thermoelectric material may overlap a portion of the thin film patch of the first thermoelectric material. The thermoelectric thin film patch of the second thermoelectric material may be thicker than a portion of the thermoelectric thin film patch of the first thermoelectric material.

An electrically insulating layer may be provided between the. surface and the thermoelectric thin film patches.

Preferably the electrically insulating layer comprises the thin film patch of the thermal insulating material and a further thin film patch of a second thermal material that has different thermal properties to the first thermal insulating material. 30 The thermoelectric thin film patches may adjoin with and may be electrical y connected to an adjacent thermoelectric thin film patch along substantially the whole length of a side of the thermoelectric thin film patch. Preferably the heat flux sensor comprises a series of pairs of thermocouple junctions. Furthermore the series of pairs of thermocouple junctions may comprise a series of the thermoelectric thin film patches of the first and second thermoelectric materials which adjoin each other and which are electrically connected in series. Preferably the series of thermoelectric thin film patches are arranged alternately over the surface. The series of thermoelectric thin film patches may be arranged in a circular pattern laterally over the surface.

The present invention will now be described by way of example only with reference to the following figures in which:

Figure 1 shows a schematic sectional representation of a conventional prior art thermopile heat flux sensor;

Figure 2 shows a schematic perspective representation of a thermopile heat flux sensor according to the present invention; Figure 3 shows a schematic perspective representation, similar to that of figure 2 of a second embodiment of a thermopile heat flux sensor according to the present invention; Figure 4 is a schematic cross section through further embodiments of thermopile heat flux sensor according to the present invention; Figure 5 shows a schematic representation, similar to that of figure 2 of a yet further embodiment of a thermopile heat flux sensor according to the present invention; Figure 6a and 6b are schematic plan views of further embodiments of thermopile heat flux sensors according to the present invention.

Figure 2 illustrates a thermopile heat flux sensor 1 3 )0 according to the present invention for measuring the heat flux Q flowing nomal to a surface 3 of a component 2. The sensor 1 is mounted, or deposited, on the surface 3 of the component 2 through which the heat flux Q is to be measured. The thermopile 1 comprises a series of discrete planar patches 22,24 of alternating dissimilar first and second thermoelectric materials extending laterally parallel to the 6 surface. The patches 22,24 are arranged such that laterally across the surface patches 24 of a second thermoelectric material separate alternate patches 22 of a first thermoelectric material. The patches 22,24 are disposed in generally the same plane substantially parallel to the surface 3 of the component 2. The patches 22,24 laterally abut each other and are arranged such that where they abut they are electrically connected to each other. At the interface 23a,23b of the laterally abutting patches a thermocouple junction A,B is formed by virtue of the electrical contact between the two dissimilar thermoelectric materials. Each pair of successive thermocouple junctions A,B forms a single thermocouple element 5. In the arrangement shown in figure 2, a planar series of four individual thermocouple elements 5 are formed along the surface 3 of the component 2. It will be appreciated though that by adding further patches 22,24, more thermocouple elements 5 could easily be produced.

The patches 22,24 must be electrically isolated from the component 2. If the component 2 is non-electrically conducting then the patches 22,24 can be directly applied to the surface 3, as shown. However if the component 2 is electrically conducting then an electrically insulating layer (not shown) is interposed between the patches 22,24 and the surface 3 of the component 2. Such an electrically insulating layer could comprise a thin layer of ceramic, for example alumina, aluminium nitride or silicon monoxide or a polymer layer such as a polyimide.

On top of, and over, alternate thermocouple junctions A, and a small region either side of the junction A, there are thermal patches 2q of a non-electrically conducting thermal insulating material. The thermal patches 20 comprise a thermal insulating material which would normally be understood to refer to a material which has a low thermal conductivity relative to the operating environment, thermoelectric material, and material of the component 2. The 7 thermal patches 20 distort the local thermal conditions in the region where they are applied. Consequently the thermal conditions at each of the junctions A,B of each of the thermocouple elements 5, and of successive thermocouple junctions A,B are different with the thermocouple junctions A subject to different thermal conditions to the thermal conditions at thermocouple junctions B. The thermal patches 20 could alternatively comprise a material, which has a high thermal conductivity relative to the operating environment, thermoelectric material, and material of the component 2. Accordingly in the context of this patent the term thermal insulator material refers to any material which affects the thermal heat flux and so covers both material with a low thermal conductivity and a high thermal conductivity.

Electrical connections 26 at each lateral end of the series of thermoelectric patches 22,24 connect the series of thermoelectric material patches 22,24 and so series of thermocouple elements 5, formed by the patches 22,24, to voltage measuring equipment 18. The voltage measuring equipment is of a conventional type and measures a combined voltage Vtotal across the series of thermocouple elements 5.

In order to protect the thermopile 1, in particular when used'on components 2 exposed to harsh environments typical of for example the conditions within a gas turbine engine, a protective layer or coating (not shown) may be applied over the entire thermopile 1.

In operation the heat flux Q flowing normally to the component 2 surface 3 flows through the thermopile sensor 1.

The- flux Q causes local heating of the thermopile 1 and patches 22,24 of hermoelectric material 22,24. The thermal patches 20 modify the flow of heat through the portion of the thermopile covered by the patches 20. This affects the local heating in that region caused by the heat flux Q, with the result that the temperature of the thermopile 1 in the region of the patches 20 is different to the temperature of the 8 thermopile not covered by the patches 20. This temperature difference between the two regions caused by the presence of the patches 20 is dependent upon the magnitude of the heat flux Q, with a larger heat flux Q producing a larger temperature difference. The discrete thermal patches 20 therefore generate a local temperature difference between the successive thermocouple junctions A,B which is proportional to the heat flux Q.

The temperature difference between the pairs of 10 thermocouple junctions A,B of each thermocouple element 5 generates an emf (electromotive force) or voltage V across each thermocouple element 5. This voltage V is proportional to the temperature difference between the adjacent thermocouple junctions A,B of the thermocouple element 5 and so is proportional to the heat flux Q normal to the surface 3 of the component 2. With the individual thermocouple elements 5 arranged in series the total voltage Vtota, across the thermopile 1 is the sum of the individual voltages V generated by each of the thermocouple elements 5. By comparing the total voltage Vtota, across the thermopile 1, measured by the measurement equipment 18, against previous calibrated readings for known heat fluxes Q through the thermopile 1 the heat flux Q through the thermopile 1 and so through the surface 3 of the component 2 can be determined.

Alternatively the heat flux Q can be determined by direct calculation based upon the theoretical model of the thermocouple elements 5, the number of such elements 5, the thermal and thermoelectric properties and sizes of the thermal patches 20 and thermoelectric patches 22,24. The provision of a number of thermocouple elements 5 in a series increases the voltage Vt,,t,,l signal produced which makes the measurement of the voltage Vt,t,l and so of the heat f lux Q easier and more accurate. It will be appreciated that any number of thermocouple elements 5 could be provided depending upon the application of the sensor 1.

9 As mentioned above the thermal patches 20 can be of a high or low thermal conductivity. If the thermal patches 20 are of a low thermal conductivity (i.e. a thermal insulator) then the heat flow through the region of the thermopile 1 covered by such a patch 20 will be restricted. In such a case the resultant temperature beneath such a patch 20 would be higher than that in a region not covered by the patch 20. On the other hand if the thermal patches 20 are of a high thermal conductivity material then heat will more easily be radiated from the region of the patch 20 and so the temperature of the thermopile 1 in the region of the patch 20 will be lower as compared to the region not covered by the patch 20. In both cases however the temperature difference caused by the patches 20 will still be proportional to the heat flux Q through the thermopile 1. All that is required is that the thermal patches 20 effect the thermal characteristics of the regions of the thermopile 1 to which they are applied. Examples of suitable materials for the thermal patches and their respective thermal conductivity's k at 200C include aluminium oxide (k=-25W/mK), aluminium nitride (k=-180W/mK), silicon monoxide (k-=1. 3W/mK), polyimide (k=-0.14W/mK), silicon nitride (k=-30W/mK) and titanium dioxide (k_=3w/mK).

The patches of thermoelectric material 22,24 and the thermal patches 20 are preferably applied to the surface 3 of the component 2 by conventional deposition processes to produce thin films of material, typically between lgm and 50im thick. Conventional deposition processes, well known in the art, suitable for depositing the patches 20,22,24 include vacuum sputtering, evaporation, chemical deposition, electroplating and plasma spraying but other known techniques may also be suitable dependent upon the materials used. Each of the different materials forming the thermopile 1 is typically deposited separately in successive deposition operatJLons in order to build up the complete thermopile on the surface of the component. To control the specific geometry of the patches 22,24 forming the thermocouple elements 5 and the thermal patches 5 masks can be used to protect those areas of to the surface 3 where the material being deposited is not required. Alternatively a complete layer of the material could be deposited and portions on the surface 3 where it is not required removed using chemical etching or laser cutting techniques. A combination of these processes could also be used.

It will also be appreciated that although depositing the sensor on the surface is the preferred method since it minimises the thickness of the sensor, it is also possible to form the sensor and patches 20,22,24 from thin plate elements that are attached and mounted upon the surface 3 by suitable means. The sensor as a whole could also be formed as a separate unit, which is then mounted upon the surface of the component.

Any conventional thermoelectric materials conventionally used to produce thermocouples could be used for the patches 22,24. It is desirable though that the thermoelectric materials chosen for the patches 22,24 have as high an output per degree of temperature as possible in order to improve the accuracy of the measurement and also that the material operate over the desired temperature range. A further consideration is the possible effect of the material on the component and the effect that the operating environment may have on the thermoelectric materials and their properties. The exact materials and suitable combinations are therefore selected for the particular temperature ranges, application and operating conditions. A particularly suitable combination of thermoelectric materials for the patches 22,24 is Chromel (a nickel chromium alloy) and Alumel (a nickel aluminium t alloy) which in combination is known in the art as a K type thermocouple. Such a thermocouple produces 41gV per 'C and has an operating range of - 250 to 1100'C. Other known combinations which are suitable include platinum 10% rhodium alloy and platinum to produce an S type thermocouple, platinum 13% 11 rhodium and platinum to produce an R type thermocouple, iron and constantan (a copper nickel alloy) to produce a i type thermocouple, chromel and constantan to produce an E type thermocouple, and copper and constantan to produce a type T thermocouple. In addition to these generally standard combinations other thermocouple combinations are also known and are suitable. It should also be noted that, as is known in the art, the thermoelectric properties of thin films of thermoelectric materials differs from those of the bulk material due to the small thickness, large surf ace-to-volume ratio bad structure of the thin films With this arrangement the thermoelectric patches 22,24 forming the thermocouples 5 of the thermopile 1 are arranged in a generally planar manner with the patches generally arranged in a lateral side by side manner parallel to the surface 3 of the component 2. An advantage of this arrangement is that the overall thickness t of the thermopile 1 is reduced as compared to the conventional arrangement. In addition the number of layers of material which are deposited on the surface 3 to form the thermopile 1 is reduced. This reduces the complexity of the thermopile 1 and makes its fabrication simpler and so cheaper. Furthermore the arrangement, and in particular geometric arrangement of the patches 20,22,24 is also considerably simpler and easier to manufacture than some of the previous conventional proposals and this also reduces costs. The number of deposition steps required to fabricate the thermopile sensor is also reduced.

In the prior art examples, for example as shown in figure 1, the thermoelectric plates 4,6,10,12 which are conductors are separated by an insulator material 8. In effect this undesrably acts like a capacitor which in use introduces capacitive errors into the measurement. With the proposed arrangement no such capacitor is formed, or at least not to the same extent, and the capacitive errors are reduced. This improves the accuracy of the sensor.

12 Further embodiments of the invention are shown in figures 3 to 6. These embodiments are generally similar to the embodiment shown in figure 2 and described above.

Consequently only the differences between these embodiments and the embodiment of figure 2 will be described and like reference numerals have been used for like features.

In the thermopile la shown in figure 3 the patches 24 of the second thermoelectric material have a 'T' shaped cross section and are thicker than the patches 22 of the first thermoelectric material so that the arms of the patches 24 overlap the patches 22 of the adjacent first thermoelectric material. Whiist this increases the thickness of the thermopile la it ensures that the patches 22,24 of the dissimilar thermoelectric material abut each other and that there is a good electrical contact between the individual patches 22,24. It is also simpler to fabricate since the accuracy of depositing and/or positioning of the patches 22,24 does not need to be as accurately controlled in order to provide a sufficient electrical contact between the patches 22,24. The thermal patches 20 also have an L shaped cross section which is arranged to encase one of the arms of each of the patches 24 of the second thermoelectric material and the junction A between the two patches 22,24 of thermoelectric material. By encasing the arm, and thermocouple junction A, the thermal effect of the thermal patches 20 on the junction A and the region of the thermopile la encased is improved. This enhances the temperature difference produced when the heat flux Q flows through the thermopile la and so the size of the voltage V produced and therefore the accuracy of the measurement. Furthermore such L shap ed thermal patVhes 20 are also easier to deposit and the accuracy required during the deposition is less.

In the arrangement shown in figure 4 a series of alternating thermal patches 32,34 of two different first and second thermal materials are provided on the surface 3 of the component 2. The two different thermal materials have significantly different thermal properties with typically the first series of thermal patches 34 comprising a material with a high thermal conductivity whilst the second series of thermal patches 34 comprise a material with a low thermal conductivity. On top of the series of thermal patches 32, 34 there is a series of alternating abutting electrically connected thermoelectric patches 38,40 similar to the patches 22,24 of figure 2 and 3. As with the previous embodiments thermoelectric junctions C,D are created at the interfaces between successive patches 38,40 and successive pairs of these junctions C,D define a series of individual thermocouples 5. As shown the thermoelectric patches 38,40 are laterally staggered with respect to the thermal patches 32,34 such that a first thermocouple junction C of each thermocouple element 5 is disposed on top of a thermal patch 32 of the first thermal material and the second thermocouple junction D is disposed on top of a thermal patch of the second thermal material. In operation since the thermal patches 32,34 have different thermal properties the different thermal patches 32,34 will distort the heat flux Q differently in a similar way to in which the thermal patches 20 on top of the thermoelectric elements 22,24 distorted the temperature in the thermopile of figure 2 and 3. As a result the-temperature at the first thermocouple junctions C will be different to the temperature at the second thermocouple junction D with the temperature difference indicative of the heat flux Q. Consequently in a similar way to that described in relation to the embodiment of figure 2 the heat flux Q can be determined by measuring the resultant voltage Vtotal produced across the thermopile lb.

An advantage of this arrangement is that if non electrically conducting materials are used for the thermal patches 32,34 then the thermal patches 32,34 will in themselves form an electrically insulating layer 35 which will electrically insulate and isolate the thermoelectric patches 38,40 from the component 2. Furthermore the 14 thermopile 1b also has a smoother surface than that of the previous embodiments. Consequently in gas turbine engine applications where the component 2 may be a gas or airwashed component 2 the aerodynamic effect of the thermopile lb will be less and the thermopile 1b will be less susceptible to erosion damage during operation.

As shown in figure 5 the patches 42,44 of the thermoelectric material may be arranged in pairs 50,52 with each pairs 50 arranged at an angle to the adjacent pair 52 of patches 42,44. In such an arrangement first thermocouple junction F is formed between the abutting of the patches within each pair of patches whilst the second thermocouple junction E is formed at the apex between the pairs of patches. By separating pairs of patches 50,52 and making the surface area of one junction E considerably smaller than the other insulated junction F electrical capacitance is minimised. Advantageously therefore this arrangement minimises the capacitive effects generated by the electrically conducting thermoelectric patches 42,44 particularly when an initial insulating layer is required between the thermoelectric patches 42,44 and the component 2. This improves the accuracy of the sensor.

In the previous arrangement the thermopile lc has been shown as laterally extending across the surface 3 of the component 2 in a generally linear direction. It will be appreciated however that in alternative arrangement that the series of patches 42,44 forming the thermopile 1 could be arranged laterally adjacent to each other but extending in a circular arrangement as shown in figure 6a and 6b.

Figure 6a depicts a thermopile ld arrangement along similar principles k to those shown in figures 2 and 3 but arranged in a circular fashion. An advantage of the circular arrangement is that the density of junctions A, B over a small area is increased. This increases the thermoelectric 3 )5 output and voltage produced by the thermopile lc and also allows more localised heat flux measurements to be taken.

In the arrangement of figure 6b alternate patches 58,60 of thermoelectric material are arranged on the surface 3 in a continuously connected 'zigzag' fashion in an overall circular arrangement with junctions G and H formed at the radially inner and outer ends of the patches 58,60. A further patch 56 of thermal material is deposited on top of the radiallyinner junctions H in order to distort the heat flux Q through the thermopile le and so generate a temperature differential in a similar way to the other embodiments.

Other arrangements of the patches of the thermopile are also possible and the preferred arrangement will depend upon the shape of the component to be measured and the area of the surface through which the heat flux Q is to be measured.

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Claims

1. A heat flux sensor for measuring the heat flux through a surface, the sensor which in use is attached to the surface comprising at least one pair of thermocouple junctions together forming a thermocouple element, at least one thin film patch of a first thermal insulator material and electrical connections connecting the sensor to a measurement device, the pair of thermocouple junctions each comprising the junction of a thermoelectric thin film patch of a first thermoelectric material electrically connected to and adjoining a thermoelectric thin film patch of a second thermoelectric material; characterised in that the pair of thermocouple junctions and the thermoelectric thin film patches are laterally spaced apart relative to the surface, and the thin film patch of the thermal insulator material is disposed in the region of one of the pair of thermocouple junctions and in use is arranged to distort the heat flux in the region of the said junction.

2. A heat flux sensor as claimed in claim 1 comprising a further thin film patch of a second thermal insulator material which is disposed in the region of the other of the pair of. thermocouple junctions and in use is arranged to distort the heat flux in the region of the said junction, with the first and second thermal insulator materials having significantly different thermal properties.

3. A heat flux sensor as claimed in claim 1 or 2 in which the at least one thin film patch of thermal insulator material is disposed on top of the thermocouple junction.

4. A heat flux sensor as claimed in claim 1 or 2 in which the at least one, thin film patch of thermal insulator material is disposed underneath the thermocouple junction.

5. A heat flux sensor as claimed in any preceding claim in which the thin film patch of the second thermoelectric material overlaps a portion of the thin film patch of the first thermoelectric material.

17

6. A heat flux sensor as claimed in any preceding claim in which the thermoelectric thin film patch of the second thermoelectric material is thicker than a portion of the thermoelectric thin film patch of the first thermoelectric 5 material.

7. A heat flux sensor as claimed in any preceding claim in which an electrically insulating layer is provided between the surface and the thermoelectric thin film patches.

8. A heat flux sensor as claimed in claim 7 in which the 10 electrically insulating layer comprises the thin film patch of the thermal insulating material and a further thin film patch of a second thermal material that has different thermal properties to the first thermal insulating material.

9. A heat flux sensor as claimed in any preceding claim 15 which the thermoelectric thin film patches adjoin with and are electrically connected to an adjacent thermoelectric thin film patch along substantially the whole length of a side of the thermoelectric thin film patch.

10. A heat flux sensor as claimed in any preceding claim comprising a series of pairs of thermocouple junctions.

11. A heat flux sensor as claimed in claim 10 in which the series of pairs of thermocouple junctions comprise a series of the thermoelectric thin film patches of the first and second thermoelectric materials which adjoin each other and which are electrically connected in series.

12. A heat flux sensor as claimed in claim 10 or 11 in which the series of thermoelectric thin film patches are arranged alternately over the surface.

13. A heat flux sensor as claimed in claim 11 or 12 in which 30 theseries of thermoelectric thin film patches are arranged in a circular pattern laterally over the surface.

14. A heat flux sensor as hereinbefore described with reference to figures 2 to 6b.