"Device for measuring the velocity of a flow of fluid'

TEXT OF THE DESCRIPTION

The invention relates to a device for measuring the velocity of a flow of fluid, particularly a device of what is known as the heat removal type, in which a heat exchanger element interacts with the flow of fluid, the heat exchange flow between the heat exchanger element and the fluid being measured, and the said heat exchange flow being correlated with the velocity of the flow of fluid.

The object of the invention is to provide a device of the above type which is of inexpensive construction, easily manufactured and miniaturizable, and which does not cause a large amount of mechanical interference with the flow of fluid.

The invention achieves the above objects with a device of the , type described initially, in which the heat exchanger element consists of an exchange surface, preferably substantially flat and oriented or orientable in a direction parallel to the flow of fluid so that it is in contact with the fluid, the exchange surface being associated with means of heating it to a predetermined mean temperature different from that of

SUBSTITUTESHEET(RULE28)

the fluid, and means of detecting the temperature gradient of the said exchange surface in at least one of the directional components of the flow of fluid parallel to the exchange surface. The technique of measuring the temperature gradient, induced by the flow of the fluid, on the exchange surface and not in the fluid itself, allows the placement of the temperature detector means on the side of the said surface opposite the flow of fluid or, in a particular embodiment, on the exposed side, but protected by a thin layer of ceramic or glass material, thus resulting in a good resistance to environmental agents and to the chemical corrosion characteristics of the fluid being measured. To enable the velocity of the flow of fluid in both of its directional components parallel to the exchange surface to be determined, and to derive from this the modulus of the velocity and the direction of the flow of fluid in the plane defined by the exchange surface, it is possible to provide a plurality of means of detecting the temperature gradient along the surface, along two or more axes parallel to the said surface and intersecting each other at predetermined angles. The temperature gradient detecting elements may consist of discrete thermoelectric sensors or transducers fixed, preferably on the side opposite the flow of fluid, in thermal contact with delimited areas of the surface which are spaced apart and disposed

symmetrically about the median area of the surface, in the direction along which the temperature gradient is to be detected.

The means of heating the exchange surface may consist of thermoelectric elements disposed in thermal contact with delimited areas of the side of the exchange surface opposite the flow of fluid, these thermoelectric elements being controlled by a control unit. Preferably, the exchange surface is heated in such a way that there is a predetermined temperature difference between it and the flow of fluid, means of detecting the mean temperature of the said surface and means of detecting the temperature gradient along said exchange surface being provided and connected to the control unit, which is provided with calculation means and means for setting the desired temperature difference.

The calculation means of the control unit are also connected to the means of detecting the temperature gradient for the calculation of the temperature gradient and, from this, the velocity and/or direction of the flow of fluid correlated with it.

Preferably, the disposition of the temperature gradient detecting means, and that of the means of heating the surface, are symmetrical about the centre or the central axis transverse to the direction of flow of the exchange surface. The means of detecting the mean temperature of heating of the surface are disposed

SUBSTITUTE SHEET(RULE2Θ)

in the median area or along the central axis. When means of detecting the temperature gradient in more than one direction are provided, both the shape of the plate and the said disposition of the temperature gradient detecting means are rotationally symmetrical about the central axis of the exchange surface.

In a variant, the exchange surface may be made concavely or convexly curved in at least one direction and may also be closed on itself, or may form a segment of a duct for the flow of fluid.

The fluid subjected to measurement of velocity or direction may be a liquid or a gas. In particular, the invention provides for the use of the device for making an anemometer or similar. The invention also relates to other characteristics which further improve the above device and which form the subject of the dependent claims.

The particular characteristics of the invention, and the advantages derived therefrom, will be more clearly understood from the description of certain preferred embodiments, illustrated by way of example and without restriction in the attached drawings, in which

Fig. 1 shows a measuring head of the device according to the invention;

Fig. 2 shows a rear view of the exchange surface carried by the measuring head shown in Fig. 1; and

Fig. 3 shows a partial perspective view of a segment of an exchange surface in a variant embodiment,

SUBSTITUTE SHEET(RULE28)

on which the temperature detecting elements and the heating elements are integrated by the method of construction of what are known as hybrid integrated circuits. With reference to Figs. 1 and 2, a device for measuring the velocity and direction of a flow of fluid comprises a heat exchange surface which consists of a sheet or thin plate 1, with predetermined thermal conductivity, supported so that it is oriented or orientable substantially parallel to the direction of the flow of fluid, so that it is in contact with the flow of fluid along one of its faces. In the embodiment illustrated, capable of determining both the velocity of the flow of fluid and the directional components of the said flow of fluid in the plane defined by the plate 1, the plate has a rotationally symmetrical shape in plan view. The face of the plate 1 opposite the flow of fluid is preferably screened from the flow and carries in the central area of the plate a sensor or transducer 2 for detecting the mean temperature of the plate with which it is in thermal contact. Thermoelectric heating elements 3 extending along an annular surface closed on itself and concentric with the plate 1 are provided in a radially outer area around the sensor or transducer 2 of the mean temperature of the plate 1, at a certain distance from it, and in thermal contact with the same face of the plate 1. Moreover, further sensors or transducers A to detect the local temperature of the plate 1, in

SUESTITUTESHEET(RULE28)

particular four sensors or transducers 4 which are aligned in pairs in axes perpendicular to each other and associated with marginal areas of the plate, and spaced symmetrically about the centre of the plate, are provided on the same face -of the plate 1 in radially outer areas with respect to the centre of the plate. Each pair of sensors or transducers 4 forms an element for detecting the temperature gradient of the exchange surface in the direction of the axis of alignment of the sensors or transducers 4. The sensors or transducers 4 for detecting the temperature gradient are connected by a control and calculation unit 5 to which the said transducers are connected 6, and in which are provided, in the form of stored tables or special algorithms, the functions of calculating the gradient from the two opposite local temperatures detected by each pair of sensors or transducers 4 and the functions of correlating the gradient with the particular velocity of the flow of fluid. The calculation and control unit 5 may also be provided with means of entering or selecting data characteristic of the various types of fluid subjected to measurement, and with means of displaying the velocity and directional components of the flow of fluid as calculated. The control and calculation unit is also connected 7, 8, 9 to the thermoelectric heating elements 3 and the sensor or transducer 2 of the mean temperature of the plate 1, as well as a further sensor or transducer 10 of the mean temperature of the flow of

fluid, which is disposed at a certain distance from the plate 1 in a position such that it intercepts the flow of fluid. The calculation and control unit is provided with a program by means of which the plate 1 is heated to a mean temperature different from and preferably higher than that of the flow of fluid, and which always provides a constant value of the difference between the said two mean temperatures. To determine the direction and the velocity or intensity of the flow, it is sufficient to measure the temperatures of the plate in the four positions disposed symmetrically with respect to the centre in which the sensors or transducers 4 are provided. The temperature difference between two symmetrical positions, caused by the transfer of heat to the flow of fluid which is in contact with the plate

1, is related by a monotonic mathematical function to the component of velocity of the flow of fluid, in the direction defined by the axis of joining of the said two positions. The correlation function is constant with time and strictly correlated for all the transducers characterized by this embodiment. The velocity of the flow of fluid is determined by trigonometric functions from the modulus and sign of the orthogonal components in the two axes of alignment of the two pairs of sensors cr transducers 4.

The mean temperature of the plate 1 is measured by means of the sensor or transducer 2 dedicated expressly to this measurement. It is also possible to provide a plurality of further specifically dedicated sensors or

transducers of the mean temperature distributed over the surface of the plate 1.

Alternatively, it is possible to omit the sensor (s) or transducer (s) 2 dedicated to the detection of the mean temperature, using in combination the sensors or transducers 4 forming the temperature gradient detectors.

However, the use of the sensor (s) or transducer(s) 2 dedicated to the detection of the mean temperature has the advantage of reducing to a minimum the measurement delay due to the time required for the propagation of heat along the surface of the plate. A consistent measurement delay would be critical for the stability of the system of controlling the temperature of the plate 1.

In a further variant, the mean temperature sensor 2 may consist of a continuous heat-sensitive transducer extending along the exchange surface, with a path in the form of a . spiral or a continuous line closed on itself, preferably concentric in the area of the plate 1 between the temperature gradient sensors or transducers 4 and the heating element or elements 3.

The plate 1 may be formed from any material, preferably from a material having good qualities of resistance to environmental agents and to the chemical corrosion characteristics of the fluid being measured, and must have a mean value of thermal conductivity. The dimensions of the plate, particularly its thickness, must be chosen according to the specific

SUBSTITUTE SHEET(RULE28)

characteristics of thermal conductivity of the material used, in order to maximize the surface temperature gradient of the plate due to the flow of fluid interacting with it. Ceramic materials, particularly what is known as alumina (AI2O3) or similar materials, and stainless steels and other materials with coefficients of thermal conductivity between 10 and 90 W/m°C, have proved to be particularly advantageous as materials for the plate 1. The thickness of the plate is affected not only by the thermal conductivity characteristics and the type of material, but also by the dimensions of the plate. For example, if an alumina plate of circular shape with a diameter d from 15 to 25 mm is used, the thickness of the plate 1 should preferably be between 0.2 and 1.0 mm, and in particular between 0.2 and 0.3 mm.

The plate 1 may also be made from the substrates commonly used for the construction of hybrid circuits, having thicknesses of the order of 10 mils (thousandths of an inch) .

If materials with higher thermal conductivity are used and the other dimensions are kept constant, the thickness must be reduced; conversely, if the thickness is kept constant, the dimensions must be increased in respect of length, width or diameter, depending on the geometrical shape.

The sensors or transducers 2, 4 used for detecting the local temperature and the mean temperature of the plate 1 may be discrete prefabricated electronic

SUBSTITUTE SHEET(RULE28)

- to - components available on the market, such as platinum, nickel, molybdenum or similar temperature-sensitive resistors, or thermistors or thermocouples, or possibly combinations of these components. These may advantageously be obtained in miniaturized versions on ceramic substrates facilitating their fixing in thermal contact with the plate 1, particularly by means of heat-conducting adhesives of the type available on the market. The heating elements 3 may consist of resistors for surface mounting, particularly of at least one or more resistors of this type connected in series, in parallel or series/parallel, to obtain a power of the order of 2 to 8 watts. These may also be fixed to the plate 1 by means of a heat-conducting adhesive.

According to the variant embodiment shown in Fig. 3, the assembly comprising the plate 1 and the sensors or transducers 2, 4, and the heating elements 3 may be made in a way similar to the method of construction of what are known as hybrid integrated circuits, by the known thick-film or thin-film method.

By means of this integration method, therefore, it is possible to make a hybrid integrated sensor for measuring the velocity and the direction in two dimensions of a flow of fluid.

In this case, the sensors or transducers 2, 4 and the heating elements 3 consist of a film of suitable material 11 which is applied between conducting tracks

SUBSTITUTESHEET(RULE28)

- π -

12 to which the linking cables from the control unit are connected.

In particular, the sensors or transducers 2, 4 for measuring the temperature may consist of a thin film of temperature-sensitive resistive material, such as platinum, nickel, molybdenum or similar metals or semiconductor elements, either discrete or integrated, on the hybrid circuit.

The heating elements consist of thick films of suitable material such as cermet or organometallic material.

The conducting tracks consist of layers of metals, preferably noble metals, while the substrate may consist of various materials such as alumina or others. The manufacturing methods are those normally used for hybrid circuits, such as serigraphic methods, application by cathodic sputtering in a vacuum, the use of inks, and others.

Alternatively, the substrate may consist of a metallic plate 1 covered with a thin ceramic layer on the side where the electrical components are applied.

Construction in the form of a hybrid integrated circuit makes it possible to dispose, in particular, the temperature sensors or transducers 2, 4 on the side of the plate 1 in contact with the flow of fluid. In this 'case, the sensors or transducers 2, 4 are protected by a thin layer, from 10 to 30 μm, of ceramic or glass material. This arrangement increases the

sensitivity of the measurement of the temperature gradient .

In such an embodiment, the conducting tracks may pass to the opposite side of the plate, that is the one screened from the fluid, by means of metallized holes, normally used in hybrid circuits, those holes being preferably filled with suitable material to avoid the passage of fluid.

In a variant embodiment, when it is not necessary to detect the direction of the flow of fluid, but only the velocity component in a predetermined direction, the plate 1 may have a shape elongated perpendicularly to the direction of the flow, while the temperature sensors or transducers 2, 4 and the heating elements 3 extend substantially over the whole length of the plate 1. Their distribution remains similar to that of the preceding example, with the sensor or transducer 2 of the mean temperature of the plate on the longitudinal median axis, the heating elements 3 disposed symmetrically on both sides of the mean temperature sensor or transducer 2 and parallel to it, and in an intermediate position between the said mean temperature sensor or transducer 2 and the corresponding sensor or transducer 4 for the measurement of the temperature gradient. This also extends in the longitudinal direction of the plate, each in the area of the longitudinal peripheral edge of the plate. In this variant embodiment, because of the reduced dimensions in the direction transverse to the flow of fluid, it is

SUBSTITUTE SHEET(RULE28)

possible to omit the mean temperature sensor or transducer 2, using the sensors or transducers 4 for the temperature gradient as well as for the detection of the mean temperature of the plate 1. Therefore, on plate 1 only one central longitudinal heating element and two sensors or transducers parallel to the heating element and disposed symmetrically about it will be provided in the marginal areas of the plate.

When measurements of velocity and direction in the three components of the three-dimensional space are to be made, it is possible to provide a combination of one plate 1 for measurement of the velocity and direction in two components in the plane defined by the plate, and a further, second plate of a type similar to the preceding one or of the type for the detection of velocity in one direction only, disposed at a predetermined angle, preferably in a plane orthogonal to the plane in which the first plate is oriented.

The device for measuring the velocity and/or direction of a flow of fluid may be used in various applications, for measuring either gaseous or liquid flows.

Depending on the type of application, it is possible to provide various designs of the plate or of its support.

One particular type of support, which is also especially suitable for the construction of an anemometer, consists of a flow dividing element which has a fluid dynamic or aerodynamic shaoe caoable of

SUPSTITUTE SHEET(RULE26)

separating from the flow of fluid a partial flow which is in contact with the surface of the plate 1, thus avoiding as far as possible the generation of turbulence. Preferably and advantageously, the supporting element of the plate 1 consists of material with a low coefficient of thermal conductivity, while the plate is fixed preferably by means of an adhesive also having low thermal conductivity, such as a silicone adhesive or similar.

With reference to Fig. 1, the supporting body 15 is provided with a tubular stem 115, which is preferably of a shape rotationally symmetrical about a central axis, and through which the cables 6, 7, 8 for connection to the calculation and control unit 5 are made to pass in such a way that they are protected from the exterior and from the flow of fluid. The supporting end of the plate 1 has an enlargement in the form of a flange 215 which forms a flared extension of the tubular stem 115 and whose periphery 315 is made rounded or conical, so that it forms a curved surface or angle capable of splitting the flow of fluid, separating from it, with a minimum of turbulence, and therefore with a minimum loss of velocity or deviation, a partial flow which is in contact with the plate 1. The plate I is fixed on the central aperture of the flange 215 communicating with the tubular stem 115 and rests on the radial inner edge of the flange 215 on which there is provided a stepped peripheral axial

SUESTITUTE SHEET (RULE 26)

rebate with a depth corresponding to the thickness of the plate 1, so that the plate extends completely flush with the free end of the flange 215.

The supporting body 15 is mounted in a structure 16 shown in broken lines and may be associated with a guide fin 116 disposed at a certain distance from the plate 1 and parallel to the plate, or may be interposed between two parallel fins. Additionally, the whole structure or only the supporting body 15 of the plate 1 may be supported so that they can be oriented in the direction of the flow, either by the action of the flow, or by means of positioning actuators.

In a further variant embodiment, the measuring device may also be used to measure the velocity of flows of fluid guided in ducts or similar, with the plate used next to an internal wall of the duct, or with the plate itself forming part of the inner wall of the duct. In this case, the plate 1 may also be of a concavely or convexly curved form, in any axis or in an axis parallel to the axis of joining of two associated sensors or transducers 4 for measuring the temperature gradient in the corresponding direction. The plate 1 may also have an extension such that the two opposite edges parallel to the axis of curvature are in contact with each other, giving the plate a transverse section of closed, or substantially closed, annular form.

Naturally, the invention is not limited to the embodiments described and illustrated herein, but may be greatly varied and modified, particularly as regards

SUESTITUTE ShEET(nJLE26)

construction, without departure from the guiding principle disclosed above and claimed below.