EP1552257A1

EP1552257A1 - Thermal flow comparator comprising an activating circuit

Info

Publication number: EP1552257A1
Application number: EP03775480A
Authority: EP
Inventors: Pierre Thery; Florian Raucoules
Original assignee: Captec
Current assignee: Captec
Priority date: 2002-10-18
Filing date: 2003-10-09
Publication date: 2005-07-13
Also published as: AU2003283506A1; WO2004038352A1; FR2846091A1; FR2846091B1

Abstract

The invention concerns a thermal flow comparator comprising a thermoelectric activating circuit consisting of a metal strip (3) partly covered with wafers (4). The activating circuit is arranged parallel to a detecting thermoelectric circuit consisting of another metal strip (1) and associated wafers (2). The detecting and activating circuits have respective superimposed circuit portions, such that each wafer in said detecting circuit portion is superimposed to a wafer of said activating circuit portion, or superimposed to a gap between two successive wafers of said activating circuit portion. The activating circuit is connected to an electric current generator (30) and the detecting circuit is connected to an electric voltage sensor.

Description

THERMAL FLOW COMPARATOR HAVING A CIRCUIT

ACTIVATION

The present invention relates to a heat flux comparator comprising an activation circuit, more particularly suitable for measuring variations in heat flux in various applications, such as the detection of flow of a fluid.

In contrast to devices for measuring the temperature of an external environment, known thermal detection devices allow the detection of thermal fluxes without any variation in temperature. Such a device is called a heat flux comparator. French patent application No. 01 15310 describes such a device which allows the comparison of instantaneous heat fluxes coming from two external environments with which the two faces of the device are respectively brought into contact.

Such a heat flux comparator nevertheless requires that heat fluxes be exchanged between one of the external media and the comparator. When the external environment (s) are permanently at the same temperature as the comparator, no heat flux is exchanged and the comparator does not provide any detection signal. Such conditions are for example achieved when one of the external media is a fluid in motion relative to the comparator, and the speed of the fluid varies without the temperature of the fluid being modified. The comparator of the French patent application n ° 01 15310 is then inoperative to detect a variation in the flow speed of the fluid medium. Furthermore, thermal detectors are also known, which include an activation circuit and make it possible to detect variations in an external medium such as a change in a flow speed. European patent application 0 446 546 describes such a detector, but its sensitivity is particularly low. In addition, its operation does not allow detection when the activation circuit is electrically supplied. Activation periods and measurement periods must then be alternated, this which generates a complicated operating mode and requires complex control means.

The object of the present invention is to provide a very sensitive and easy to use heat flux comparator. The invention relates to a heat flux comparator having at least a first substantially planar inlet face and capable of exchanging heat flows with a first external medium with which said first inlet face is brought into contact. The comparator includes:

a thermoelectric detection circuit having a determined longitudinal direction situated in a plane substantially parallel to the first input face, comprising at least a first strip of a first metallic material having a determined thermal conductivity, a determined electrical conductivity and a power thermoelectric determined, having at least two faces, and being partially covered on one of its faces by first disjointed metallic plates of a second metallic material which has a thermal conductivity and an electrical conductivity respectively higher than the thermal conductivity and the electrical conductivity of the first metallic material and which has a thermoelectric power different from the thermoelectric power of the first metallic material;

- a device for detecting an electric voltage having two terminals respectively connected to two longitudinal ends of said first strip;

- a thermoelectric activation circuit, comprising at least a second strip of a third metallic material having a determined thermal conductivity, a determined electrical conductivity and a determined thermoelectric power, having at least two faces, and being partially covered on one of its faces by second disjointed metal plates of a fourth metallic material which has a thermal conductivity and an electrical conductivity respectively higher than the thermal conductivity and the electrical conductivity of the third material metallic and which has a thermoelectric power different from the thermoelectric power of the third metallic material;

- An electric current generator having two terminals respectively connected to two longitudinal ends of said second strip so as to supply said second strip with a continuous supply electric current;

a first layer of insulating material disposed between the thermoelectric detection circuit and the therrήoelectric activation circuit, the insulating material of said first layer of insulating material having a thermal conductivity lower than the respective thermal conductivities of the first, second, third and fourth materials metallic; and

- A second layer of insulating material placed between the first input face on the one hand and the thermoelectric detection circuit and the thermoelectric activation circuit on the other hand. Respective portions of the first and second ribbons are parallel and superimposed in a direction perpendicular to the first entry face, and are respectively covered with first and second plates arranged substantially at right angles to each other in said perpendicular direction, or arranged in a manner that the first plates of said portion of the first ribbon are substantially in line with separation intervals between second plates of said portion of the second successive ribbon in the longitudinal direction.

Thanks to this arrangement of the first wafers relative to the second wafers in portions of the detection and activation circuits, the comparator of the invention makes it possible to detect variations in the external environment (such as a flow speed) when the latter keeps a constant temperature equal to that of the comparator. When the activation circuit is supplied by an electric current, each of the second wafers has a heated end and a cooled end by thermoelectric effect, creating local heat flows exchanged with the outside environment. Each of the first wafers of the detection circuit located above a second wafer thus activated or located between two seconds successive plates thus activated, then instantly detects possible variations in these local heat fluxes, due to variations in the external environment, even without appreciable variation in the temperature thereof.

In addition, the structure of the heat flux comparator allows excellent detection sensitivity.

Finally, its use is particularly simple, given the possibility of supplying the activation circuit for the duration of the measurements. This possibility thus provides good detection stability.

According to one embodiment, the comparator further comprises third plates arranged on a face of the second layer of insulating material opposite to the thermoelectric circuits. The third plates are each arranged at right, in the perpendicular direction, of a respective longitudinal end of the first plates. The third plates create an asymmetry between the two longitudinal ends of each first plate which makes it possible to detect thermal fluxes even in the absence of electric current in the activation circuit.

In another embodiment, the heat flux comparator further comprises a second inlet face, substantially planar and parallel to the first inlet face. The heat flux comparator can then exchange heat fluxes with a second external medium brought into contact with the second input face. The second input face is disposed on one side of the detection and activation circuits opposite to the first input face, and a third layer of insulating material is disposed between the second input face and the thermoelectric circuits.

Such a configuration of the thermal comparator reduces the influence of the thermal characteristics of the detection device itself on the detection signals obtained. These signals then essentially depend on the thermal characteristics of the two external media. The invention also relates to methods for comparing heat fluxes using a heat flux comparator as described above. In a first example, the activation thermoelectric circuit is supplied with a constant direct electric current, and the electric voltage detected by the detection thermoelectric circuit is identified.

In a second example, the value of the electric supply current of the thermoelectric activation circuit is adjusted so as to locate the value of said current which cancels the electric voltage detected by the thermoelectric detection circuit.

Other features and advantages of the present invention will appear in the description below of two nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

- Figure 1 is a section through an example of heat flux comparator according to the invention;

- Figure 2 is an exploded perspective view of part of the heat flux comparator of Figure 1; - Figure 3 illustrates an example of use of a heat flux comparator according to the invention;

- Figure 4 is a section through a heat flux comparator according to a preferred embodiment of the invention;

- Figure 5 is an exploded perspective view of the heat flux comparator of Figure 4;

- Figure 6 is a section through a heat flux comparator according to a variant of the preferred embodiment of the invention.

According to FIG. 1, the heat flux comparator comprises a thermoelectric detection circuit formed by the metal strip 1, partially covered with a series of metal plates 2 arranged in a longitudinal direction L of the strip 1. The strip 1 can be arranged in meanders juxtaposed in the same plane. The direction L is then the local longitudinal direction of the ribbon 1. The plates 2 are separated from each other and distributed in a substantially regular manner along the ribbon 1. The plates 2 are applied to the ribbon 1 so as to have with him a good electrical contact and a good thermal contact at least at the level of two longitudinal ends of the plates 2 in the direction L. The plates 2 may be rectangular in the plane of the tape 1, or of any other shape.

The ribbon 1 can be made of a resistive alloy known as constantan, or another resistive alloy such as chromel. The plates 2 are made of a metallic material such as copper, gold, silver, nickel, aluminum, or an alloy comprising at least some of these metals. The materials of the tape 1 and the plates 2 are chosen so as to have different respective thermoelectric powers, and so that the plates 2 have an electrical conductivity and a thermal conductivity greater than those of the tape 1. For example, the tape 1 is in constantan and the plates 2 are made of copper.

A second thermoelectric circuit consisting of a metal strip 3 and wafers 4 is identical to the detection circuit formed by the strip 1 and the wafers 2. This second thermoelectric circuit is the activation circuit. It is arranged parallel to the detection circuit, in a position offset with respect to the detection circuit in a direction N perpendicular to the plane of the strip 1, so that the plates 2 and the plates 4 are superimposed two by two in the direction N. FIG. 2 shows, in a perspective view, the superposition of the detection and activation circuits for a meandering configuration of the strips 1 and 3.

A layer 5 (Figure 1) of insulating material is interposed between the detection and activation circuits. Another layer 6 of insulating material covers the detection circuit, on a side opposite to the activation circuit. Layers 5 and 6 can be made of Kapton® or of alumina fiber fabric, especially for high temperature applications. The whole of layer 6, of the detection circuit, of layer 5 and of the activation circuit is tightened in the direction N in order to present good thermal contacts along planes perpendicular to the direction N.

The free face of layer 6 constitutes an input face E1 of the heat flux comparator. It can be brought into contact with an external medium 100, which can be of any kind, gaseous, liquid or solid.

Optionally, when the medium 100 is solid, the face of the layer 6 constituting the input face E1 can be covered with a thin metallic layer, preferably with a soft metal, so as to improve the thermal contact between the external medium 100 and the comparator. Furthermore, the comparator is placed on a support S by its face opposite to the input face E1.

Preferably, the layer 6 has a thermal conductivity in the direction N less than the thermal conductivity of the layer 5 in the direction N. Thus the detection and activation circuits can be considered to be almost confused with respect to the flows heat exchanged between the comparator and the medium 100, through the input face E1.

According to Figure 3, the strip 1 is connected by its two ends to two respective terminals of an electric voltage detector 20, such as, for example, a voltmeter. The ribbon 3 is connected by its two ends to two respective terminals of an electric current generator 30, capable of delivering a direct current of adjustable intensity I. The current I thus delivered then flows through the ribbon 3. In FIG. 3, E2 represents the contact surface between the comparator and the support S (FIG. 1).

The thermoelectric activation circuit being supplied with a non-zero continuous electric current by the generator 20, it creates local heat fluxes around the longitudinal ends of the plates 4. The two longitudinal ends of each plate 4 are respectively heated and cooled by the Peltier effect. , known to a person skilled in the art, produced by the supply current of the activation circuit. These local heat flows diffuse in the direction N through the input face E1 and reach the external medium 100. The external medium 100 then influences these local thermal fluxes according to its intrinsic thermal characteristics. In particular, an external fluid medium 100 in movement, by its renewal in contact with the inlet face E1, promotes the local heat fluxes generated by the activation circuit. We then identify, by means of the voltmeter 30, an electrical voltage representative of all of the local heat fluxes and of the thermal characteristics of the external medium 100. When the current I is kept constant, the variations of this voltage reveal variations of the medium 100 , such as change in its flow velocity, for example. Thus, the heat flux comparator of the invention makes it possible to detect changes in the thermal characteristics of an external medium, even when the external medium and the comparator have strictly equal temperatures. The activation circuit, powered by the current I delivered by the current source, in fact makes it possible to overcome the need for spontaneous heat fluxes through the input face of the comparator.

For a relative arrangement of plates 2 and 4 as shown in FIGS. 1 and 2, the ends of the strip 1 connected to the electrical voltage detector 20 each have an electrical polarity identical to that of the end of the strip 3 to which it is superimposed in direction N.

For a heat flux comparator with a structure similar to that of FIGS. 1 and 2, but in which the plates 2 are each arranged in line with a separation interval between two successive plates 4, the superimposed ends of the strips 1 and 3 have opposite electrical polarities when using this detector as described above.

Figures 4 and 5 show a second embodiment of a heat flux comparator. Elements 1 to 6 described above are repeated identically. In particular, the plates 2 and 4 are arranged in line with each other in the direction N. A layer 7 of insulating material, identical to the layer 6 is placed against the activation circuit, on a side opposite the circuit of detection. The free face of layer 7 constitutes a second input face E2 of the heat flux comparator. The input face E2 is brought into contact with a second external medium 200.

According to this second embodiment of the comparator, plates 8 and 10 are respectively arranged on the faces of the layers 6 and 7 of insulating material facing outwards from the comparator. Each plate 8 is positioned centrally relative to the first longitudinal end, in the direction L, of a plate 2 of the detection circuit, itself located in line, in the direction L, of a plate 4 of the activation circuit. Likewise, each plate 10 is positioned centrally relative to the second longitudinal end of a plate 2. The comparator comprises as many plates 8 and plates 10 as plates 2 or 4, so that it appears to consist of thermal cells juxtaposed in the direction L, each thermal cell comprising a plate 2, a plate 4, a plate 8 and a plate 10 (see FIG. 5).

The plates 8 and 10 are advantageously identical to each other and can be of different types, in particular:

the plates 8 and 10 can be radiation reflectors, acting respectively to reflect a part of the incident radiation on the input faces E1 and E2, coming from the external media 100 and 200. They then consist of a material to high electrical conductivity such as aluminum, silver, gold, chromium, copper and alloys comprising at least one of these metals. In this case, the comparator makes it possible to compare radiative fluxes emitted by flux sources disposed substantially opposite, between which the comparator is placed;

- The plates 8 and 10 can be radiation absorbers, acting respectively to absorb part of the radiation incident on the input faces E1 and E2. In this case, they can be made of a material with high absorption power, such as a black film, a ceramic or a chromium oxide, possibly combined with another metal. The use of such a comparator is then identical to that of a comparator with reflecting plates 8 and 10;

- The plates 8 and 10 can constitute thermal bridges between layers of insulating material forming the entry faces E1 and E2 and the layers 6 and 7. FIG. 6 represents an example of a heat flux comparator having such a structure. Two additional layers of insulating material 9 and 11 are arranged on either side of layers 6 and 7, parallel to the latter and on the side of each opposite to the thermoelectric circuits. The layers 9 and 11 are respectively separated from the layers 6 and 7 by the plates 8 and 10. The plates 8 and 10 can be made of an epoxy type material or of a metallic material such as copper, aluminum, silver, chromium, nickel and an alloy comprising at least one of these metals. Advantageously, a volume of thermal insulation 12 can be arranged between neighboring plates 8 or between neighboring plates 10, in a separation interval between layers 6 and 9 for plates 8, and between layers 7 and 11 for plates 10. To obtain a comparator with increased sensitivity, the volumes 12 can be empty or filled with gas.

As before, the ends of the ribbon 1 are connected to the terminals of an electric voltage detector 20, and the ribbon 3 is connected to the terminals of a current generator 30 (FIG. 3), which delivers a direct current I in the ribbon 3.

In the absence of electric current delivered by the generator 30 (that is to say for l = 0), the voltmeter 20 measures an electric voltage representative of a balance of the thermal fluxes incident on the input faces El and E2 of the comparator, according to an operation identical to that of any known thermal detector.

The use of a comparator according to the second embodiment with a non-zero supply current I; which will now be described, makes it possible to measure in a particularly precise manner heat fluxes coming from external environments 100 and 200, respectively through the input faces E1 and E2. For this, the value of the electric supply current I delivered by the generator 30 is adjusted so as to cancel the electric voltage detected by the voltmeter 20. Such a measurement method, qualified as a "compensation" method, makes it possible in particular to carry out a calibration of the comparator before the measurement. The value of the current delivered by the generator 30 which cancels the voltage across the ribbon 1 is thus identified. Next, a variation in the thermal fluxes incident on the input faces E1 and E2 is detected in the form of a variation in the intensity of the supply current to the activation circuit which is necessary to compensate for the variation in the electric voltage generated in the comparator by said variation in heat fluxes.

When using the comparator, the electrical polarities of the ends of the ribbons 1 and 3 superimposed in the direction N can have identical or opposite electrical polarities, depending on the type of plates 8 and 10.

For a heat flux comparator with a structure similar to that of FIGS. 4 to 6, but in which the plates 2 are each arranged in line with a separation interval of two successive plates 4, the electrical polarities of the ends of the ribbon 3 can be reversed compared to the previous case, when using this detector according to the compensation method. According to a third embodiment, which corresponds to a simplification of the second embodiment, the comparator has the structure shown in FIG. 1, supplemented by plates 8, and possibly a layer of insulating material 9, arranged on the layer 6 of as described above. Depending on the nature of the wafers 8 and the presence of the layer 9, such a comparator may also have an operation by reflection or absorption of radiation, or by thermal bridges, with respect to the external medium 100 in contact with the face of input E1. The comparator can also possibly be placed on a support (S) by its face opposite to the input face E1. The use of the comparator according to the “compensation” measurement method is always possible. When the compensation is reached, the activation circuit traversed by the adjusted electric current generates an overall thermal flux opposite to the incident flux on the input face E1 coming from the external medium 100. It thus appears that different embodiments can implementing the present invention, in particular as a function of the number of heat flux inlet faces, of the nature of the plates placed at the level of the inlet faces and of the materials chosen. These embodiments all correspond to the present invention insofar as they comprise in particular a thermoelectric detection circuit and a thermoelectric activation circuit associated in one of the ways described above.

Claims

1.. Heat flux comparator having at least a first substantially flat inlet face (E1) and capable of exchanging heat fluxes with a first external medium (100) with which said first inlet face is brought into contact, the comparator comprising:

- a thermoelectric detection circuit having a determined longitudinal direction (L) situated in a plane substantially parallel to the first input face (E1), comprising at least a first strip (1) of a first metallic material having a thermal conductivity determined, a determined electrical conductivity and a determined thermoelectric power, having at least two faces, partially covered on one of its faces by first metallic plates (2) separated from a second metallic material having a thermal conductivity and an electrical conductivity respectively greater than the thermal conductivity and the electrical conductivity of the first metallic material and having a thermoelectric power different from the thermoelectric power of the first metallic material;

- a device for detecting an electric voltage having two - terminals respectively connected to two longitudinal ends of said first strip (1);

- a thermoelectric activation circuit, comprising at least a second strip (3) of a third metallic material having a determined thermal conductivity, a determined electrical conductivity and a determined thermoelectric power, having at least two faces, partially covered on the one of its faces by second metal plates (4) separated from a fourth metallic material having a thermal conductivity and an electrical conductivity respectively greater than the thermal conductivity and the electrical conductivity of the third metallic material and having a thermoelectric power different from the power thermoelectric of the third metallic material; - an electric current generator having two terminals respectively connected to two longitudinal ends of said second strip (3) so as to supply said second strip (3) with a continuous supply electric current (I); - A first layer of insulating material (5) disposed between the thermoelectric detection circuit and the thermoelectric activation circuit, the insulating material of said first layer of insulating material having a thermal conductivity lower than the respective thermal conductivities of the first, second , third and fourth metallic materials; and - a second layer of an insulating material (6) disposed between the first input face (El) on the one hand and the thermoelectric detection circuit and the thermoelectric activation circuit on the other hand; in which respective portions of the first ribbon (1) and the second ribbon (3) are parallel and superimposed in a direction (N) perpendicular to the first input face (E1), and are covered respectively with first (2) and second (4) plates arranged substantially at right angles to each other in said perpendicular direction (N), or arranged so that the first plates (2) of said portion of the first ribbon are substantially at the right of separation intervals between second plates (4) of said portion of the second successive strip in the longitudinal direction (L).

2. A heat flux comparator according to claim 1, in which the first (1) and / or the third (3) metallic materials are based on constantan or copper.

3. Heat flux comparator according to any one of the preceding claims, in which the second (2) and / or the fourth (4) metallic materials are based on copper, gold, silver, nickel, d aluminum, or an alloy comprising at least some of these metals.

4. A heat flux comparator according to claim 1, in which the insulating materials of the first (5) and / or second (6) layers of insulating material are based on Kapton®.

5. Heat flux comparator according to any one of the preceding claims, in which the second layer of insulating material (6) has a thermal conductivity in the perpendicular direction (N) less than a thermal conductivity of the first layer of insulating material ( 5) in the perpendicular direction (N).

6. Heat flow comparator according to any one of the preceding claims, further comprising:

- a second inlet face (E2) substantially planar and parallel to the first inlet face (E1), and capable of exchanging heat fluxes with a second external medium (200) with which said second inlet face is placed contact, said second input face being disposed on one side of the thermoelectric detection circuit and of the thermoelectric activation circuit opposite to the first input face, and

- A third layer of insulating material (7) disposed between the second input face (E2) on the one hand and the thermoelectric detection circuit and the thermoelectric activation circuit on the other hand.

7. A heat flux comparator according to claim 6, in which the second (6) and third (7) layers of insulating material are identical.

8. A heat flux comparator according to any one of claims 1 to 5, further comprising third plates (8) disposed on one face of the second layer of insulating material (6) opposite the thermoelectric detection circuit and the thermoelectric circuit activation, and which are each arranged at right, in the perpendicular direction (N), of a respective longitudinal end of the first plates (2).

9. A heat flux comparator according to claim 8, in which the third plates (8) are radiation reflectors.

10. Heat flow comparator according to claim 9, in which the third plates (8) are made of a selected material. among aluminum, silver, gold, chromium, copper and alloys comprising at least one of these metals.

11. A heat flux comparator according to claim 8, in which the third plates (8) are radiation absorbers.

12. A heat flux comparator according to claim 8, further comprising a fourth layer of insulating material (9), disposed parallel to the second layer of insulating material (6), on a side thereof opposite the thermoelectric circuit of detection and to the thermoelectric activation circuit, in which the fourth layer of insulating material (9) is in contact with the third plates (8).

13. A heat flux comparator according to claim 12, in which a material constituting the third plates (8) comprises a metallic material.

14. A heat flux comparator according to claim 13, in which the material constituting the third plates (8) is selected from the list comprising copper, aluminum, silver, chromium, nickel and alloys comprising at least one of these metals.

15. A heat flux comparator according to any one of claims 12 to 14, wherein, between the second layer of insulating material (6) and the fourth layer of insulating material (9), a volume of thermal insulation (12) is arranged between the third plates (8).

16. A heat flux comparator according to claim 6 and any one of claims 8 to 15, further comprising fourth plates (10) disposed on one face of the third layer of insulating material (7) opposite the thermoelectric detection circuit. and to the thermoelectric activation circuit, and which are arranged relative to the first plates (2) to form with them and with the third plates (8) cells each comprising a first, a third and a fourth associated plates, said third plate ( 8) of a determined cell being disposed substantially at right, in the perpendicular direction (N), of a first longitudinal end of said first plate (2) of said determined cell and said fourth plate (10) of said determined cell being disposed substantially in line with a second longitudinal end of said first plate (2) of said determined cell different from said first longitudinal end.

17. A heat flux comparator according to claim 16, in which first plates (2) belonging to respective cells cover the first ribbon (1) in a portion of the first ribbon parallel and superimposed on a portion of the second ribbon (3), said portion of the first ribbon and said portion of the second ribbon being covered respectively with first (2) and second (4) plates arranged in line with one another in the perpendicular direction (N), or arranged so that the first plates (2) are substantially in line with separation intervals between second successive plates (4) in the longitudinal direction (L).

18. A heat flux comparator according to claim 16 or 17, in which the fourth plates (10) are identical to the third plates (8).

19. A method of using a heat flux comparator according to any one of claims 1 to 18, according to which the activation thermoelectric circuit is supplied with a constant direct electric current, and the electric voltage detected by the thermoelectric detection circuit.

20. A method of using a heat flux comparator according to any one of claims 8 to 18, according to which the value of the electric supply current of the thernioelectric activation circuit is adjusted so as to locate the value of said current which cancels the electric voltage detected by the thermoelectric detection circuit.