FR2942037A1 - Radiative and convective heat flow measuring device for wall of heating unit, has fluxmeters with upper surfaces covered with respective paint layers, where fluxmeters are placed side-by-side on wall so as to be located in same flux - Google Patents

Abstract

The device has fluxmeters (1, 2) comprising lower surfaces (3, 5) placed in thermal contact with a surface (S) i.e. wall, and upper surfaces (4, 6) covered with respective paint layers (14, 15) having different emissivities. The fluxmeters are identical and placed side-by-side on the wall so as to be located in a same convective flux. The difference between emissivities of the layers is greater than 0.1. A thermocouple determines the temperature of the wall. The thickness of each fluxmeter is less than 2 mm. An independent claim is also included for a method for measuring a radiative and convective heat flow exchanged between a surface and an external medium.

Description

Thermal flow measuring device and method for separating radiative and convective flux

TECHNICAL FIELD OF THE INVENTION The present invention relates to thermal flow measurement devices, commonly called thermal flux meters.

Background technology Traditional heat flow sensors, also known as heat flowmeters or heat flowmeters or heat flowmeters in the English language, are designed for the measurement of heat conduction exchanges in solid media. The measurement of a heat flow is generally done by introducing into said flow a material of known characteristic creating between the two faces of the material a thermal gradient proportional to the heat flux to be measured.

When bonded to a wall, the thermal flux meter measures a total heat flow between the lower face in contact with the wall and the upper face in contact with the surrounding environment. This total heat flux is the sum of a radiative flow and a convective flow which in this case are counted together, the flowmeter not allowing to separate these contributions.

Document FR2847037 discloses a device for measuring a non-stationary radiative and convective thermal flux generated within a gaseous fluid. However, the device in question is suitable for an extreme environment, that is to say for a highly corrosive gaseous fluid under high pressure and high temperature such as a gas resulting from the combustion of a propellant. Such a device is complex and economically unsuitable for measurements of the radiative and convective flows exchanged at the surface of a wall in industrial environments where the operating temperatures and pressures are clearly of lower order.

Also known from US7318671 is an emissivity measuring device comprising a thermal flux meter whose lower face is bonded to a wall of a heat source and whose upper face comprises an emitting surface whose emissivity is the physical property to determine. However, the device is essentially dedicated to the determination of this emissivity, and does not deal with the determination of the contributions of the radiative and convective flows. On the contrary, the document recommends using the device in a very low pressure environment so as to cancel convective heat transfer. The document also discloses an arrangement of four heat flux meters, however this is nothing more than to multiply the measurements to obtain a better statistical representativeness of the emissivity calculation. The object of the present invention is to overcome the drawbacks of the prior art by proposing a new device for measuring heat flow capable of enabling the radiative flow and the convective flow to be determined.

The invention thus relates to a device for measuring heat flux exchanged between a surface and an external medium comprising a first flowmeter having a first substantially planar and substantially parallel first and lower face, a second flowmeter having a second lower face and a second substantially planar and substantially parallel upper face, the first and second lower faces being disposed on the surface in thermal contact therewith, the first upper face being covered with a first layer having a first emissivity, the second face upper surface being covered with a second layer having a second emissivity different from the first emissivity, characterized in that the first and second fluxmeters are substantially identical, arranged on the surface side-by-side so as to be substantially in the same convective flow and in that the second emissivity is t different from the first emissivity of a difference greater than 0.1.

Such a device has the additional advantage of being simple and economical.

In addition, the invention may include one or more of the following features:

- The device includes a thermocouple to determine a temperature of the surface. Indeed, it is important to be able to determine the temperature of the surface and a thermocouple reported to the device of the invention, therefore more compact, makes it possible to have a determination of the surface temperature more representative because carried out at the same place as the measurement of flow. the difference between the first emissivity and the second emissivity is greater than 0.4. Indeed, the greater the difference between the first emissivity and the second emissivity is important, the more the device of the invention will allow a precise determination of radiative and convective flux. The first and second flowmeters are arranged on the surface with a spacing of less than 3 cm. the first and the second flowmeters have a thickness of less than 2 mm; the first and the second fluxmeter have a greater dimension smaller than 2 cm in a plane parallel to the surface. Indeed, remaining in the range of dimensions of the invention, one can consider valid the necessary assumption of a convective flow seen by the first and second flowmeter substantially identical. the first and second flowmeters are kept in thermal contact with the surface by a layer of thermo-conductive adhesive. Indeed, the maintenance of fluxmeters on the surface must be reliable while not constituting a thermal barrier such that it would disrupt the result of the measurement of heat flow by the device of the invention. - The device comprises an adhesive intermediate support and thermo-conductive for maintaining in thermal contact the first and the second fluxmeter with the surface. This provides a removable and reusable flow measurement device.

Furthermore, the invention also relates to a method for determining the radiative and convective heat fluxes exchanged between a surface and an external medium, the surface comprising a measuring device according to the invention, characterized in that it comprises the following steps: determining a first total heat flux passing through the first flux meter, determining a second total heat flux passing through the second flux meter, determining a convective flux exchanged between the surface and the external medium from the first and the second total heat flux, - the determination of a temperature of the surface, - the determination of a radiative average temperature from the temperature of the surface, the convective flow, the first total heat flow and the first emissivity the determination of an emissivity of the surface; the determination of a radiative flux exchanged between the surface and the external environment; from the average radiative temperature, the emissivity of the surface and the surface temperature.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages will appear on reading the following description of a particular embodiment, not limiting of the invention, with reference to the figures in which: - Figure 1 is a representation schematic of the device for measuring radiative and convective heat fluxes according to the invention. FIG. 2 is a schematic representation of a normal gradient flowmeter. FIG. 3 is a graphic illustration of the determination of the convective flow from the total heat fluxes measured by the two heat flow sensors.

DETAILED DESCRIPTION Figure 1 shows a first embodiment of the invention. In Figure 1, the reference S designates a heat source surface, such as the wall of a heating member, and the reference E an external medium such as air.

Due to a temperature difference between the surface S and the external environment, the surface S heat transfer. This heat transfer takes place in the form of radiation and convection. The intensity of these transfers is characterized by a radiative flux cpr and a convective flow cl), the sum of which corresponds to the global heat flux cpg exchanged between the surface S and the external medium E. We thus have: Pg ù (Pc + (Pr (1) The surface S is also characterized by an emissivity Es which proportionally depends on the radiative flux (pr.

For the purposes of the description we take the case of a surface S which emits heat towards a colder external environment E, however the device of the invention is equally apt to determine the radiative flux cpr and convective c) in the case where the surface S, colder than the external medium E, absorbs heat.

The device for measuring the radiative and convective thermal flows according to the invention comprises a first heat flux sensor or flux meter 1 and a second heat flow sensor or flux meter 2. The first and the second flux meter 1, 2 are arranged on the surface S in thermal contact, that is to say such that the heat transfer between the flux meters 1, 2 and the surface S is the least disturbed possible.

The first flowmeter 1 comprises a first lower face 3 and a first upper face 4. The first lower face 3 and the first upper face 4 are substantially flat and parallel. The first upper face 4 is covered with a first layer 14 having a first emissivity E1.

The second fluxmeter 2 comprises a second lower face 5 and a second upper face 6. The second lower face 5 and the second upper face 6 are substantially flat and parallel. The second upper face 6 is covered with a second layer 15 having a second emissivity different from the first emissivity E1.

According to the invention, the second emissivity E2 is different from the first emissivity E1 by a difference greater than 0.1. This difference in emissivity allows us here to determine the distinct contributions of the radiative flux cpr and convective cl), exchanged between the surface S and the external medium E, however the greater the emissivity difference between the two fluxmeters 1, 2 is important the more accurate the measurement method. Thus, preferably, the difference between the emissivities E1 and E2 of the two flowmeters 1 and 2 is greater than 0.4.

In practice, the first and second layers 14, 15 may be obtained a specific coating such as a paint, a film of a material deposited by CVD, etc., whose emissivities E1, E2 are known.

As a realistic example, the first upper face 4 may be coated with a first layer 14 of black paint which will confer a first emissivity El of 0.9 while the second upper face 6 may be coated with a second layer 15 of an aluminum-based paint which will confer a second emissivity E2 of 0.5. The difference obtained between the two emissivities El and E2 is here 0.4.

For information, the following table shows the emissivity of some materials that can be used as a coating: Material Emissivity Polished aluminum 0.05 Polished chrome 0.1 Matt black lacquer 0.97 Silver finish paint 0.31 Email 0.9 For example, the difference in emissivity between a matte black lacquer coating and polished aluminum is 0.92.

The first and second fluxmeters 1, 2 preferably have a substantially identical thickness ep. In addition, the thickness ep is preferably less than 2 mm. Indeed, the thickness ep of the two fluxmeters 1, 2 must be sufficiently small to be able to consider valid the hypothesis of a convective flow cl), received by the two fluxmeters 1, 2 identical.

In this first embodiment, the two fluxmeters 1, 2 are flowmeters with a normal gradient, that is to say flowmeters whose heat flow they absorb crosses perpendicular to their upper face 4, 6, ie are flowmeters that measure an overall heat flux from a temperature gradient generated along their ep thickness.

FIG. 2 is a cross section of the first flowmeter sensor 1 in a plane orthogonal to the upper face 4. The first fluxmeter 1 comprises a layer of a material which acts as a thermal barrier, whose thermal resistance is known, and of a set of temperature sensors for accurately measuring a temperature gradient within the flowmeter.

The first flowmeter 1 is formed of a layer of plastic 9 such as kapton, a polyimide developed by Dupont and used in the production of thermal flowmeters, which can remain stable in a temperature range up to 400 ° C. and comprises a plurality of thermocouples 10 connected in series within the plastic layer 9 for measuring the temperature gradient extending within the flowmeter 1 between the surface S and the external medium E. The thermocouples may be of type K ( chromelalumel) or be chosen from another type depending on the temperature ranges to be measured. The flowmeter 1 comprises a temperature measuring means such as a thermocouple 12 disposed on the side of the first lower face 3 to also measure a surface temperature S Ts. The voltage output of the thermocouples 10 for measuring the temperature gradient and thermocouple 12 for the measurement of the surface temperature S S is via the electrical connection 8 (Figure 1).

The flowmeter 1 is held in place on the surface S by a layer of adhesive 11. It is necessary to use a heat-conducting adhesive to ensure good thermal contact and to ensure that on the one hand, the adhesive layer 11 which allows the fixing of the first fluxmeter 1 to the surface S is as thin as possible and secondly, to avoid air bubbles in order to minimize the additional thermal resistance that constitutes this layer of adhesive 11 between the surface S and the first flowmeter 1. Thus, a measurement of the temperature Ts of the surface S is allowed by the thermocouple 12 disposed on the side of the first lower face 3 more representative. For a surface temperature range S, Ts of the order of 100 ° C, an Araldite AV 138M type glue with HV 998 hardener, thermal conductivity of 0.34 W / mK or Loctite Hysol 9450 A & B thermal conductivity 1.7 W / mK is suitable. For a higher temperature range the gluing means is to be adapted by choosing an adhesive whose temperature resistance is suitable.

With regard to the second fluxmeter 2, the remarks of the two previous paragraphs are equally valid, except that the presence of a means for measuring the temperature Ts of the surface S is not necessary, as it is already present in the first flowmeter 1.

The first and the second fluxmètrel 2 shown in FIG. 1 are here rectangular in shape. Another form of flowmeter may however be suitable (circular, square, elliptical, ...). The first and the second fluxmeter 1, 2 also have, in a plane parallel to the surface S, a larger dimension 7.

By larger dimension 7 is meant the largest distance separating two points of a geometrical figure, for example the diagonal of a polygon, the diameter of a circle or even the largest axis of an ellipse.

Preferably, the largest dimension 7 is less than 2 cm. Indeed, the size of the flux meters 1, 2 must be sufficiently small to be able to consider valid the hypotheses allowing the separate measurement of the radiative and convective heat fluxes: same surface temperature S S, same convective flow cl), received by the two flowmeters 1 , 2.

It is essential that, apart from the first and second layers 14, 15 and their corresponding emissivity E1, E2, the first fluxmeter 1 and the second fluxmeter 2 are substantially identical, that is to say, among others, have the same geometric shape and dimension, the same ep thickness of plastic layer 9, the same number of thermocouple 10 for measuring the temperature gradient, a similar bonding ... so that even in the presence of slight aerodynamic disturbances, that always have the same convective flow cl), seen by the two flowmeters 1, 2.

Another important parameter is the spacing between the first and the second fluxmeter 1, 2. Once glued, on the surface S, the first and the second flowmeter 1, 2 are arranged side by side with a spacing 13. By side to coast, we hear close to each other, without necessarily being in contact. Preferably, the spacing 13 is less than 3 cm to be able to consider valid the hypothesis of a convective flow cl), received by the two fluxmeters 1, 2 identical.

In order to be able to determine, through the device of the invention, the contributions of the radiative and convective flows exchanged, the principle of the flow separation method is then as follows:

For the first flowmeter 1, the expression of the first total heat flux (pi passing through it is: cp, = cpc + E, 6 (Ts ûTr4) (2) With: El: emissivity of the first layer 14. G: constant of Stephan-Boltzmann.

Ts: temperature of the surface S. Tr: average radiative temperature. cp ~: convective flow.

In the same way, for the second fluxmeter 2, the expression of the total heat flux cp2le through is: cp2 = cf), + E26 (Ts - Tr4) (3) With: E2: emissivity of the second layer 15.

In expressions (2) and (3), Ts is the temperature of the surface S is therefore known. On the other hand, the average radiative temperature Tr is, at this stage of the demonstration, unknown.35 It is assumed that the two fluxmeters 1, 2, receive the same convective flux cp ~, and that the temperature Ts of the surface S is substantially equivalent. which is a reasonable assumption if the dimensions of said fluxmeters and their spacing 13 are in the orders of magnitude specified in this document.

On the other hand, because of the differences in emissivity E1 and E2, the radiative fluxes are different between the two fluxmeters. Thus, for the first flowmeter 1: cpr1 = E16 (Ts û (4) And for the second flowmeter 2: cpr2 = E26 (Ts û Tr4 ~ (5) 15 In its general formulation, the total heat flux cp as an affine function of the emissivity E, that is to say a function cp of the type cp = b + aE, we can then, from the two measurements (pi and cp2 express this function f and determine the value of f (E = 0) which is equal to the convective flow cl), (Figure 3).

Once the convective flow cl) has been determined, the average radiative temperature Tr is then determined from either the measurement of the first total flux (pi or the second total flux cp2 passing through one of the two fluxmeters 1, 2. thus, for example, from the first total flux (p1 ((P1 - cpc E16 This allows us to easily deduce the average radiative temperature Tr.

The following step comprises, knowing the emissivity Es of the surface S, for example from a preliminary measurement by infrared camera or from an emissivity table of the materials, the determination of the radiative flux cpr: cpr = Es Finally, the total heat flux cpg emitted or absorbed by the surface S can be deduced from the equation (1).

There is thus a device for measuring heat flow and an associated method for determining the radiative and convective contributions of the overall flux exchanged between a surface and an external environment. The invention has the additional advantage of being simple, economical, compact and can be easily placed on the surface whose heat flow is known.

A thermal flux measurement device as described in this embodiment can easily be used in environments whose temperature may be around one hundred degrees, such as an under-hood motor vehicle environment or any other industrial application where there is a competition between radiative and convective and where the question arises whether the heating is rather radiation or rather convection.

In addition, the invention is not limited to flowmeters 1, 2 with a normal gradient. In a variant that is not shown, the two fluxmeters 1, 2 are flowmeters with a tangential gradient, that is, whose heat flux they absorb passes through them tangentially, along the plane of their upper face 4, 6, otherwise which determines the total heat flux from a temperature gradient generated along its upper face.

In another variant, the means for measuring the temperature Ts of the surface S may not be integrated with at least one of the two flowmeters 1, 2. Indeed, the means for measuring the temperature Ts of the surface S may be be a temperature measuring means such as a thermocouple separate flowmeters 1, 2 or be a numerical modeling.

According to another variant, the device of the invention comprises a thermo-conductive intermediate support such as a metal foil and adhesive to maintain in thermal contact the first and the second fluxmeter 1, 2 on the surface S. This intermediate support replaces thus the glue layer presented in the embodiment illustrated in FIG. 2, and thus a patch is obtained which is glued on the surface whose radiative and convective heat flux is to be known and which can then be removed once. the measurement carried out. There is thus a removable and reusable measuring device.

Claims (9)

Revendications1. Heat flux measurement device exchanged between a surface (S) and an external medium (E) comprising a first fluxmeter (1) having a first lower face (3) and a first substantially flat and substantially parallel upper face (4), a second fluxmeter (2) having a substantially planar and substantially parallel second bottom face (5) and a second top face (6), the first and second bottom faces (3, 5) being disposed on the surface (S) in thermal contact with it, the first upper face (4) being covered with a first layer (14) having a first emissivity (El), the second upper face (6) being covered with a second layer (15) having a second emissivity (E2) different from the first emissivity (El), characterized in that the first and the second fluxmeter (1, 2) are substantially identical, arranged on the surface (S) side by side so as to be substantially in the same convective flow ((pc) and in that the second emissivity (E2) is different from the first emissivity (El) d a deviation greater than 0.1 2. Measuring device according to claim 1, characterized in that it comprises a thermocouple (12) for determining a temperature (Ts) of the surface (S). 3. Measuring device according to claim 1 or claim 2, characterized in that the difference between the first emissivity (El) and the second emissivity (E2) is greater than 0.4. 4. Measuring device according to any one of claims 1 to 3, characterized in that the first and second flowmeter (1,2) are arranged on the surface (S) with a spacing (13) less than 3 cm. 5. Measuring device according to any one of claims 1 to 4, characterized in that the first and the second flowmeter (1, 2) have a thickness (ep) less than 2 mm. 6. Measuring device according to any one of claims 1 to 5, characterized in that the first and the second flowmeter (1, 2) have in a plane parallel to the surface (S) a larger dimension (7) lower at 2 cm. 7. Measuring device according to any one of claims 1 to 6, characterized in that the first and the second flowmeter (1, 2) are held in thermal contact with the surface (S) by a layer of adhesive (11) thermally conductive. 8. Measuring device according to any one of claims 1 to 6, characterized in that it comprises an adhesive intermediate support and thermo-conductive for maintaining in thermal contact the first and the second flowmeter (1, 2) with the surface (S). 9. Method for determining the radiative and convective heat fluxes exchanged between a surface (S) and an external medium (E), the surface (S) comprising a measuring device according to any one of Claims 1 to 8, characterized in that it comprises the following steps: - the determination of a first total heat flux ((pi) passing through the first flux meter (1), - the determination of a second total heat flux ((P2) passing through the second flux meter (2 ), - determining a convective flux ((pc) exchanged between the surface (S) and the external medium (E) from the first and the second total heat flux ((p1, (p2)); a temperature (Ts) of the surface (S), - the determination of a radiative average temperature (Tr) from the temperature (Ts) of the surface (S), the convective flux ((pc), of the first total heat flux ((pi) and the first emissivity (El), - the determination of an emissivity (Es) of the surface ( S), - the determination of a radiative flux ((pr) exchanged between the surface (S) and the external medium (E) from the average radiative temperature (Tr), the emissivity (Es) of the surface (S) and the surface temperature (Ts) (S).