WO2011096801A2

WO2011096801A2 - Heat exchanger

Info

Publication number: WO2011096801A2
Application number: PCT/NL2011/050071
Authority: WO
Inventors: Ronald Batenburg; Moulay El Hassan Tijani; Symen Spoelstra
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 2010-02-03
Filing date: 2011-02-03
Publication date: 2011-08-11
Also published as: WO2011096801A3; NL2004187C2

Abstract

Thermo acoustic device comprising a heat exchanger for transferring thermal energy between a first fluid in a first fluid circuit and a second fluid in a second fluid circuit. The first fluid circuit and the second fluid circuit are connected to each other in a thermally conductive manner. The first and second fluid circuits are formed by a stack of plate elements (11, 12, 13) which form first channels (6a, 6b) for the first fluid circuit and a plurality of channels (3) for the second fluid circuit. The sum total of cross sections of the plurality of channels (3) takes up at least 25% of a front surface of the second fluid circuit.

Description

Heat exchanger

Field of the invention

The present invention relates to a thermo acoustic device comprising at least one heat exchanger for transferring thermal energy between a first fluid in a first fluid circuit and a second fluid in a second fluid circuit, wherein the first fluid circuit and the second fluid circuit are connected to one another in a thermally conductive manner.

Prior art

European patent publication EP-A-0 563 951 discloses an oil cooler for cooling

(engine) oil by means of (cooling) water. Oil and water channels are thermally connected to one another, and are formed by stacking three different kinds of plates which comprise slots and ribs. During operation, water flows in circular channels in a radial direction and oil flows back at right angles thereto through small holes and via a central bore in the oil cooler.

JP 7 243788 describes a heat exchanger which is composed of platelets and in which the platelets form fluid channels in the longitudinal direction (for air). In the intermediate platelets, a pattern of oil channels is formed by the holes. Folded fins can be placed in the air channels in order to improve heat transfer. The function of this heat exchanger is clearly that of an oil cooler.

Summary of the invention

The present invention aims to provide a heat exchanger which can be used in thermo acoustic applications, such as in combination with or as part of a heat pump or an engine.

According to the present invention, a heat exchanger of the kind defined in the preamble is provided, wherein the first and second fluid circuits are formed by a stack of plate elements which form first channels for the first fluid circuit and a plurality of channels for the second fluid circuit, and wherein the sum total of cross sections of the plurality of channels takes up at least 25% of a front surface of the second fluid circuit. First channels are for example provided in the plate elements for the first fluid which flows around the plurality of channels of the second fluid circuit. As a result thereof, an efficient heat transfer can take place in the heat exchanger which is of a design which is as compact as possible. Such a construction of the heat exchanger makes it possible to produce the heat exchanger in a simple manner (stacking of plate elements), and at the same time results in a heat exchanger in which the second fluid circuit is sufficiently open (or transparent), as a result of which the second fluid is subjected to little friction, if any, during operation and the attenuation of an acoustic wave in the second fluid is negligible with thermo acoustic applications. The openness of the construction of the second fluid circuit can also be referred to as the porosity of the heat exchanger to the second fluid.

European patent publication EP-A-0 678 715 discloses a heat exchanger for a thermo acoustic heat pump, composed of curved hollow lines with fins in between. This heat exchanger is difficult to produce and can easily become damaged, both during assembly and in use.

In an embodiment, the plate elements are flat (for example punched or cut-out) plate elements, wherein two successive plate elements comprise aligned holes which, after assembly, form the plurality of channels of the second fluid circuit, and partly overlapping grooves which, after assembly, form the first channels.

In an embodiment, the length of each channel is at least equal to a peak-to-peak distance of the fluid displacement in an acoustic wave which is present in the second fluid during operation. This construction ensures a minimal attenuation of the acoustic wave during operation, as a result of which the heat exchanger can operate efficiently.

In an embodiment, the length of each channel is less than 5 cm, and in a further embodiment, the length of each channel is greater than 0.5 cm. This provides an efficient heat exchanger across a wide range of second fluids and operational circumstances.

The plurality of channels in the second fluid circuit runs parallel to a longitudinal axis of the heat exchanger, from a start opening up to an end opening of each channel. As a result thereof, the acoustic wave is subjected to as little attenuation as possible during operation.

In a further embodiment, one or more of the plurality of channels comprise secondary heat-conducting elements (for example in the form of fins) made of a material, a main surface of which runs parallel to the longitudinal axis of the heat exchanger. The material may be thin sheet material, but may also comprise equivalent materials, such as metal foam or cylindrical channels. This results in an improved heat transfer by the second fluid. In an embodiment, the secondary heat-conducting elements are, for example, corrugated or folded by curved sheet material. In yet a further embodiment, the curved sheet material has a centre-to-centre distance (or pitch distance) in a direction at right angles to the longitudinal axis of the heat exchanger, and the maximum centre-to-centre distance is dependent on a thermal depth of penetration of the second fluid. In an embodiment, the centre-to-centre distance is situated in the range between one to five times the thermal depth of penetration.

In an embodiment, the first fluid circuit comprises first channels which, at least partly, run coaxially to the longitudinal axis of the heat exchanger, for example in a plane substantially at right angles to the longitudinal axis. As a result thereof, it is possible to use the entire cross section of the heat exchanger effectively. Alternatively, the first channels can also be in the form of straight channels.

In an embodiment, the first fluid is a liquid (for example water) and the second fluid is a gas (for example helium).

In a further aspect, the present invention relates to a thermo acoustic device comprising at least one heat exchanger according to one of the present embodiments. This thermo acoustic device can function as an engine or as a heat pump.

Short description of the drawings

The present invention will now be explained in more detail by means of a number of exemplary embodiments with reference to the attached drawings, in which:

Fig. 1 diagrammatically shows a view of a thermo acoustic device in which the present invention is applied;

Fig. 2 shows a front view of an embodiment of a heat exchanger according to the present invention;

Fig. 3 shows a side view, partly in cross section, of the heat exchanger from Fig. 2; and

Fig. 4 shows an exploded perspective view of an embodiment of the heat exchanger.

Detailed description of exemplary embodiments

The present invention relates to a heat exchanger which is, for example, suitable for applications in the field of thermo acoustics. In thermo acoustic systems, heat is converted into acoustic energy or, conversely, acoustic energy is used to pump heat up. The heat transfer generally takes place between the heat exchanger and a working medium. The working medium oscillates as a result of an acoustic wave. In the following description, embodiments of heat exchangers are described which can be used in thermo acoustic systems.

Fig. 1 diagrarnmatically shows a view of a thermo acoustic system 20, which comprises a heat exchanger 10 according to an embodiment of the present invention. The thermo acoustic system 20 illustrated by way of example in Fig. 1 comprises a regenerator 22 and two heat exchangers 10 which are placed in an acoustic resonator 21. The regenerator 22 (a porous construction) is the heart of the system 20 where the thermodynamic conversion process takes place. The heat exchangers 10 are provided for the heat-exchange with the surroundings (heat sources). A thermo acoustic system 20 can function as an engine or as a heat pump. In a thermo acoustic engine, an acoustic wave 24 is spontaneously generated and amplified by means of a temperature difference applied across the regenerator 22 by means of the two heat exchangers 10. In a heat pump, an acoustic wave 23 (acoustic energy) is used to pump heat through the regenerator 22 from a low-temperature (cold) heat exchanger 10 (on the right-hand side in Fig. 1) to a high-temperature heat exchanger 10 (on the left-hand side in Fig. 1).

Fig. 2 shows a front view of a heat exchanger 10 according to an embodiment of the present invention and Fig. 3 shows a partial cross-sectional view of the heat exchanger 10 from Fig. 2. The heat exchanger 10 is cylindrical in shape and has a longitudinal axis 15. Furthermore, the heat exchanger 10 comprises a supply 1 and a discharge 2 between which a first fluid circuit is situated. A plurality of channels 3 with a start opening 4 and an end opening 5 forms a second fluid circuit parallel to the longitudinal axis 15. The length of each of the plurality of channels 3 is indicated in Fig. 3 by the letter d. Each of the plurality of channels 3 may furthermore have a substantially constant cross section along the length, as a result of which the second fluid encounters as little resistance as possible. The cross section may be rectangular, circular, semi-cylindrical - as is the case in the illustrated embodiments - or have any other desired shape.

In order to achieve as little resistance as possible, the sum total of cross sections of the plurality of channels 3 takes up at least 25% of a front surface of the second fluid circuit. Fig. 2 shows a front view of the heat exchanger 10 which forms the front surface of the second fluid circuit. Thus, the second fluid circuit of the heat exchanger 10 is sufficiently open (or transparent), as a result of which the second fluid is subjected to little friction, if any, during operation. As a result thereof, the attenuation of an acoustic wave in the second fluid is negligible with thermo acoustic applications. The openness of the construction of the second fluid circuit can also be referred to as the porosity of the heat exchanger 10 to the second fluid.

In operation, there is a first fluid (for example a liquid, such as water) in the first fluid circuit and a second fluid (for example a gas, such as helium) in the second fluid circuit, in which case thermal energy is transferred between the first and second fluids by a thermally conductive connection. In operation, the acoustic wave stands in the second fluid circuit which, for reasons of efficiency, has to encounter as little resistance as possible.

In an embodiment, the optimum length d of the heat exchanger 10 in the acoustic direction is determined by the peak-to-peak displacement of the second fluid, given by: Peak- peak = 2·-?—.

ραω

In this formula, p is the acoustic pressure amplitude at the location of the heat exchanger 10, ω is the angular velocity (2πί), a is the speed of sound, and p is the density of the second fluid (gas). Good results can be achieved using a heat exchanger 10 where the length of the channels 3 is between 0.5 cm and 5 cm. Depending on the choice of the second fluid and other operational circumstances, this is sufficient to include the peak-to-peak displacement of the second fluid.

In the illustrated embodiment, the channels 3 have a (semi-)circular groove shape, as a result of which a large part of the front surface of the heat exchanger 10 can be flowed through by the second fluid (and is even transparent for the latter), as a result of which only slight attenuation will take place of the second fluid which flows through the channels 3. The pressure drop across the second fluid circuit of the heat exchanger 10 is consequently also very low, which offers advantages for numerous applications, such as thermo acoustic systems and air-treatment systems.

The thermally conductive connection between the first and second fluid circuits is formed by a large number of heat-conducting plate elements 11, 12, 13, as is illustrated in more detail in the exploded perspective view of the heat exchanger 10 in Fig. 4. This figure shows that each of the plate elements 11, 12, 13 is provided with aligned openings 3a which, after assembly of the plate elements 11, 12, 13, form the plurality of channels 3 of the second fluid circuit. For an aligned assembly, the plate elements 11, 12, 13 are, for example, provided with two openings, into which an alignment pin 14 fits.

As can be seen in Fig. 4, the first fluid circuit is formed by a plurality of recesses

6a, 6b in successive plate elements 12, 13, wherein the mutually staggered recesses 6a, 6b form a plurality of first channels through which the first fluid can flow. The first channels 6a, 6b flow around the plurality of channels 3 of the second fluid circuit, substantially coaxially to the longitudinal axis 15 of the heat exchanger 10 and in a plane at right angles to the longitudinal axis 15. The further openings 6c in plate element 12 and further openings 6d in plate element 13 form the connections between the plurality of first channels 6a, 6b and the supply 1 and discharge 2 of the first fluid circuit.

In an alternative embodiment, the recesses 6a, 6b are formed by partly overlapping grooves, for example in the two main surfaces of one plate element 12, 13, in conjunction with an adjacent plate element. A first channel is then formed for each plate element 12, 13.

In an embodiment, the plate elements 11, 12, 13 are made of metal plate parts, into which the various openings and/or grooves have been made by treatments

(punching, milling,...). In an exemplary embodiment, the plate elements 11, 12, 13 are made from stainless steel, so that a long service life can be achieved with a wide range of first and second fluids.

Due to the fact that the plurality of channels 3 of the second fluid circuit flow around the plurality of first channels 6a, 6b of the first fluid circuit, with heat- conducting material of the plate elements 11 , 12, 13 being situated in between, a good transfer of thermal energy between the first and second fluids is achieved during operation. Due to the construction of the first and second fluid circuits ('cross flow'), a compact design of the heat exchanger 10 which makes use of the entire surface of the heat exchanger 10 in a very efficient manner is achieved. If a heat exchanger 10 with a higher capacity is required, this can be achieved in a relatively simple manner by scaling up of the cross section of the heat exchanger 10 while retaining the same length d. The view from Fig. 2 also shows that secondary heat-conducting elements 7 are fitted in one or more of the plurality of channels 3 of the second fluid circuit in order to ensure an improved heat transfer from the second fluid to the plate elements 1 1, 12, 13 of the heat exchanger. In an embodiment, the secondary heat-conducting elements 7 (fins) are formed by thin sheet material, a main surface of which runs parallel to the longitudinal axis 15 of the heat exchanger 10. As a result thereof, as large a total surface of the thin sheet material as possible comes into contact with the second fluid during operation, while the resistance for the second fluid is as small as possible.

In an embodiment, the secondary heat-conducting elements 7 are formed by curved (thin) sheet material, for example curved or corrugated. As is illustrated in the front view of Fig. 2, the curved sheet material then has a centre-to-centre distance s in a direction coaxial to (or at right angles to) the longitudinal axis 15 of the heat exchanger. The centre-to-centre distance s is chosen as a function of a thermal depth of penetration of the second fluid. By way of example, the centre-to-centre distance s may be equal to 0.2 mm.

In an embodiment, the sheet material for the secondary heat-conducting elements 7 is copper or a copper alloy, which offers a good thermal conduction.

For the gas side (the second fluid circuit), the heat exchanger 10 also has to be as acoustically transparant as possible, that is to say that the acoustic loss (through viscous and thermal relaxation) has to be small. At the same time, the second fluid must make good thermal contact with the heat exchanger 10. The oscillating nature of the second fluid also determines the optimum dimensions of the channels 3 in the heat exchanger. The cross section of the channels between the fins 7, or the abovementioned centre-to- centre distance s, have to be in the order of magnitude of the thermal depth of penetration δ^. The thermal depth of penetration 8_k is the distance over which the second fluid can exchange heat with the heat exchanger 10 during a thermo acoustic half period and is given by:

where K is the coefficient of heat conduction of the second fluid, ω is the angular frequency (2πί), p is the density of the gas (pressure-dependent), and c_p is the specific heat. Above, a number of embodiments of a heat exchanger 10 according to the present invention have been described. The scope of protection is determined by the elements of the attached claims, and equivalents thereof. Thus, the heat exchanger 10 may, as described above, be cylindrical in shape, but any other shape is of course also possible. Neither is the shape of the plurality of channels 3 in the second fluid circuit limited to the semi-cylindrical channels 3 illustrated in the figures, and may comprise variants thereof, such as elliptical channels 3. Furthermore, the invention has been described with reference to an application in thermo acoustics, but the heat exchanger 10 could also be used in other applications, for example to recover heat from exhaust gases (for example in the exhaust of a vehicle).

Claims

1. Thermo acoustic device comprising at least one heat exchanger (10), wherein the at least one heat exchanger is a heat exchanger for transferring thermal energy between a first fluid in a first fluid circuit and a second fluid in a second fluid circuit, wherein the first fluid circuit and the second fluid circuit are connected to one another in a thermally conductive manner,

wherein the first and second fluid circuits are formed by a stack of plate elements (11, 12, 13) which form first channels (6a, 6b) for the first fluid circuit and a plurality of channels (3) for the second fluid circuit, and wherein the sum total of cross sections of the plurality of channels (3) takes up at least 25% of a front surface of the second fluid circuit.

2. Heat exchanger for transferring thermal energy between a first fluid in a first fluid circuit and a second fluid in a second fluid circuit, wherein the first fluid circuit and the second fluid circuit are connected to one another in a thermally conductive manner,

3. Heat exchanger according to Claim 2, wherein the plate elements (11, 12, 13) are flat plate elements, wherein two successive plate elements (12, 13) comprise:

aligned holes (3a, 3b, 3c) which, after assembly, form the plurality of channels (3) of the second fluid circuit, and

partly overlapping grooves (6a, 6b) which, after assembly, form the first channels.

4. Heat exchanger according to Claim 2 or 3, wherein the length (d) of each channel (3) is at least equal to a peak-to-peak distance of the fluid displacement in an acoustic wave which is present in the second fluid during operation.

5. Heat exchanger according to one of Claims 2-4, wherein the length (d) of each channel (3) is less than 5 cm.

6. Heat exchanger according to one of Claims 2-5, wherein the length (d) of each channel (3) is greater than 0.5 cm.

7. Heat exchanger according to one of Claims 2-6, wherein the plurality of channels (3) in the second fluid circuit runs parallel to a longitudinal axis (15) of the heat exchanger (10), from a start opening (4) up to an end opening (5) of each channel (3).

8. Heat exchanger according to one of Claims 2-7, wherein one or more of the plurality of channels (3) comprise secondary heat-conducting elements (7) made of material, of which a main surface runs parallel to the longitudinal axis (15) of the heat exchanger (10).

9. Heat exchanger according to Claim 8, wherein the secondary heat-conducting elements (7) are formed by curved sheet material.

10. Heat exchanger according to Claim 9, wherein the curved sheet material has a centre-to-centre distance (s) in a direction at right angles to the longitudinal axis (15) of the heat exchanger (10), and the maximum centre-to-centre distance (s) is dependent on a thermal depth of penetration of the second fluid.

11. Heat exchanger according to Claim 10, wherein the centre-to-centre distance (s) is situated in the range between one to five times the thermal depth of penetration (6k).

12. Heat exchanger according to one of Claims 2-11, wherein the first fluid circuit comprises first channels (6a, 6b) which, at least partly, run coaxially to the longitudinal axis (15) of the heat exchanger (10).

13. Heat exchanger according to one of Claims 2-12, wherein the first fluid is a liquid and the second fluid is a gas. Thermo acoustic device comprising at least one heat exchanger (10) according to Claims 3-12.