CA2001721A1

CA2001721A1 - Method and arrangement for an enforced heat transmission between bodies and gases

Info

Publication number: CA2001721A1
Application number: CA 2001721
Authority: CA
Inventors: Roland Sandstrom; Per Strid
Original assignee: Infrasonik AB
Current assignee: Infrasonik AB
Priority date: 1988-11-01
Filing date: 1989-10-30
Publication date: 1990-05-01
Also published as: WO1990005275A1; SE463786B; SE8803974D0; JPH04501456A; EP0441816A1; CN1022440C; CN1042411A; AU4429389A; SE8803974L

Abstract

ABSTRACT

The invention relates to a method and an apparatus for enforced heat transmission between a body, solid or liquid, and an ambient gas. The enforced heat transmission is achieved in that the gas is set in oscillatory motion which is generated by astanding sound wave of low frequency and in that the body is placed in that part of the sound wave where the oscillatory motion is greatest. The apparatus for working the method includes a low-frequency sound generator, which comprises an exigator part 4, 5, 23, 24 and a resonator part 2, 3, 21, 22. The resonator part 1, 3, 21, 22 is provided with an opening which is located in a region where the low-frequency sound wave displays a volume velocity anti-node. That body which is to be exposed to the enforced heat transmission is advanced through the opening.

Description

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r METHOD AND ARRANGEMENT FOR AN ENFORCED HEAT TRANSMISSION
BETWEEN BODIES AND GASES

The present invention relates to a method and an apparatus for enforced heat transmission between a body, solid or liquid, and an ambient gas. The enforced heat transmission is achieved in that the gas is set in oscillatory motion which is generated by a standing sound wave of low frequency, and in that the body is placed in that part 5 of the sound wave where the oscillatory motion is greatest.

A fundamental problem in heat transmission, for example from a warm body to an air flow enveloping the body, is that the transferred thermal effect per surface unit from the body to the gas flow will be slight at low gas flow rates. In order to transfer large 10 thermal effects, high gas flow rates are required, which implies that a large air flow will be necessary. At the same time, however, the temperature rise in the air will be slight.
The large flow entails that cooling will be expensive and, in consequence of the slight temperature rise, the energy in the heated air can seldom be utilized.

15 It is previously known from V. B. Repin, "Heat exchange of a cylinder with low-frequency oscillations", Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi, No. 5, pp.
67-72, September-October 1981, that heat transmission may be improved by generating a sonic field in the gas. It is also previously known that it is advantageous - if such a sonic field is of low frequency.
It will be obvious from the two parameters sound pressure and particle velocity in a sonic field that it is the particle velocity which provides the enforced heat transmission.
!t is also obvious that the heat transmission increases with increasing particlevelocities. The reason why the prior-art method of employing low-frequency sound for ` 25 heating or cooling of bodies has not hitherto enjoyed any practical importance is that there have not been any usable methods or apparatus for generating sound with a sufficiently high particle velocity throughout the entire surface of the body intended to be cooled, or alternatively, heated.

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The object of the present invention is to solve the above-mentioned problem and to realize a method and an apparatus for transferring high thermal effect per surface unit from a body to ambient gas. Instead of increasing the heat transmission by aspkating the gas over the surface of the body at high speed, the enforced heat transmission is 5 achieved by imparting to the ambient gas a low frequency oscillation.

For the purposes of clarifying the present invention, one embodiment thereof will be described in which hot wire from a wire rolling mill is cooled, as well as one embodiment thereof for cooling of cement clinker. Naturally the present invention may 10 also encompass further embodiments of the method and the apparatus.

When the steel wire leaves the rolling mill, it is at a temperature of approx. 850 C and must by cooled to 300 C in order to be handled. A number of different processes are currently employed for such cooling. According to one method, the wire is laid out in 15 spiral bights of approx. 1 m diameter on a roller conveyor where the wire bights are conveyed forwardly at a speed of approx. 0.5 m/s, at the same time as cooling air is blown onto the wire with the aid of a plurality of large fans placed beneath the roller conveyor. In order to cool to the desired temperature, a cooling distance of approx.
60 m is required.
Among the drawbacks inherent in this prior-art method are that the installation is expensive and extremely bulky, and that an immense air flow must be injected into the premises, a procedure which consumes substantial amounts of power and also entails environmental disadvantages in respect of draught, varying air temperature 25 and whirling of dust into the air. Further disadvantages are that the cooling - from the metallurgical point of view - is not always sufficiently rapid and is uneven, and that the entire thermal effect of the hot wire is lost.

The nature of the present invention and its aspects will be more readily understood 30 from the following brief description of the accompanying drawings, and discussion relating thereto:

Fig. 1 shows a solid body in a standard air flow;

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Fig. 2 shows a solid body in an air flow which has been exposed to an infrasound field;
Fig. 3 shows an installation for cooling of metal wire using low-frequency sound;
Fig. 4 shows a plant for cooling cement clinker using low-frequency sound.

As was mentioned above, an enforced heat transmission may be achieved be~ween the surface of a body and an ambient gas if the gas is influenced so as to reciprocate with the aid of a standing sound wave generated in the gas. Fig. 1 shows a solid body at a temperature To which is exposed to an air flow. A particle in the air flow is marked 10 as a dot and the position of the air particle at various points in time is marked by t1-t7.
The temperature of the air flow is T1 before it has passed the body, and T2 after the body has been passed. Fig. 2 shows the same solid body when it has been exposed to the same air flow, but under the influence of infrasound. The position of the air particle at different points in time is also marked by t1-t7 here. As will be apparent, 15 each air particle which passes the solid body, because of the pulsating air current generated by the low frequency sound, will pass not just once but a plurality of times.
If the body is at a higher temperature than the air flow, the air particle will absorb more and more heat each time it passes the solid body, and the temperature of the body will be correspondingly reduced. Enforced heat transmission will thus be obtained.
In certain parts of the standing sound wave, the velocity of the oscillating motion of the gas, the so-called particle velocity, is great, while the pressure variations, the ~ so-called sound pressure, are slight. In other parts, the pressure variations are great while the velocity of the oscillating motion is low. At a certain point, both the particle 25 velocity and the sound pressure will thus vary with time and, under ideal conditions, will describe a sinusoidal oscillatory motion. The highest value of the particle velocity and the sound pressure, respectively, is indicated by the amplitude of each respective oscillatory motion. As a rule, the amplitude of the partice velocity assumes a maximum value, i.e. has a so-called particle velocity anti-node, at the same time as the30 amplitude of the sound pressure assumes a minimum value, i.e. has a so-called sound pressure node.

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It is desirable, in accordance with the foregoing, that the particle velocity assumes as high a value as possible in order that maximum enforced heat transmission be obtained. In a standing sound wave, there may be several positions where the particle velocity amplitude assumes its maximum level. In a standing sound wave 5 whose length corresponds to a quarter or a half wavelength, the amplitude of the particle velocity has a maximum only at one point. In order to obtain as high anenforced heat transmission as possible, the surface from whence the heat transmission is to take place should therefore be sited at a position as close to the particle velocity anti-node as possible.
In the method according to the present invention, an enforced heat transmission between a body, solid or liquid, and a gas, as shown in Fig. 2, is realized in that a standing, low-frequency sound wave is generated in one or more sound resonators.The term low-frequency sound is here taken to mean sound at a frequency of 50 Hz or 15 lower. The reason why frequencies above 50 Hz are less interesting is that a closed half-wave resonator has such small dimensions at high frequencies that the apparatus will be uninteresting from the point of view of capacity. Since possibly disruptive sound fades at lower frequencies, a frequency of 30 Hz or lower should preferably be used. At this frequency, disturbances may be considered as very slight.
20 The sound resonator is preferably of a length corresponding to a half wavelength of the generated low-frequency sound, but other designs of the sound resonator are also possible. The sound wave is obtained in that air pulses are generated by a so-called exigator located at a sound pressure anti-node in the resonator. The term exigator is here employed to indicate that part of a generator for low-frequency sound 25 which generates a particle velocity in one point in a resonator where a high sound pressure prevails, see for example Swedish patent No. 446 157 and Swedish patentapplications Nos. 8306653-0, 8701461 -9 and 8802452-6. Somewhere in the resonator a particle velocity anti-node will occur simultaneously with a sound pressure node and, at that point, the resonance tube may be open. The surface from 30 which the above-considered heat transmission is intended to take place is advanced through this opening. Hence, the surface is then located in a particle velocity anti-node of the above-mentioned standing low-frequency sound wave.

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Z0017Zl A stationary air current flows through the resonator tube, one portion of the air current deriving from the driving air which emanates from the exigator, and its other portion from the air which flows in at the opening through which the heat transmission surface is advanced. It is also possible to use a special cooling air fan. When the surface 5 passes through the resonator tube, it will be swept partly by the stationary air flow and partly by the oscill~ting air flow which is generated by the standing sound wave. If the amplitude of the particle velocity of the sound is considerably greater than the velocity of the stationary air flow, and if the motion amplitude of the sound is considerably greater than the thickness of the surface, the same airborne elements will pass the 10 surface several times. This implies that the air is heated or alternatively cooled much more than if only the stationary air flow would have swept the surface. The result will be that a greater heat transmission takes place between the surface and the air under a given period of time than would be the case under normal conditions.

15 One of the advantages inherent in employing the superposed air motion to be found in the sound pressure node of a standing sound wave is that this is a relatively simple manner of realizing the oscillatory air motion. Another advantage in employing astanding sound wave is that it is only at that point where the surface is located that a high velocity of the oscillating air motion is desirable. In the rest of the system, the 20 high air velocity solely entails friction losses.

An apparatus for implementing the method according to the present invention willnow be described in greater detail with reference to the embodiment shown in Fig. 3 which illustrates a plant for cooling steel wire.
Low-frequency sound is generated by one or more low-trequency sound generators consisting of an exigator par~ and a resonator part. Within the resonator tube, a standing sound wave occurs which shows sound pressure nodes where the sound pressure is at its minimum. The resonator tube has an opening where these nodes 30 are located, the opening being designed such that the wire may pass through the resonator tube and thereby be subjected to infrasound-influenced cooling air.

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Fig. 3 shows in greater detail a plant in which steel wire, which is to be cooled, is allowed to pass across a cooling table 1 where it is subjected to low-frequency sound.
Acoustically, the plant consists of a virtually closed system. The steel wire is laid out on a roller conveyor or other conveyor belt in accordance with standard usage and is 5 advanced across the cooling table at uniform speed in a plane perpendicular to the plane of the paper. Two tube resonators 2, 3 are disposed above the tabie, their open ends discharging above the table. The resonators are preferably of a length corresponding to a quarter of a wavelength of the generated sound. At the other end of each respective resonator, there is a so-called exigator 4, ~. This exigator may be 10 of the type which is described in Swedish Patent application No. 8802452-6.
Together with the resonator 2, 3, the exigator 4, 5 forms a low-frequency sound generator. Both of the exigators 4, 5 are jointly driven by a motor by means of one and the same driving shaft in such a manner that a phase lag of 180 is obtained between the exigators when these operate. Since the exigators operate in counterphase, a15 standing sound wave of the same frequency will be generated in each resonator. The two resonators of quarter-wave type thereby together form one resonator of half-wave type of the same resonance frequency as the resonance frequency of the individual resonators and one single common standing sound wave is generated.

20 Cooling air is supplied with the aid of a cooling air fan 12 which conveys the cooling air to the cooling table via two ducts 7 and 8 located between the two resonators.
Each one of these ducts has a lower emanation in the wall which is common to each respective resonator tube, and this emanation is located in the lower region of each respective resonator tube. A nose portion 9 is located between the lower regions of 25 the two resonator tubes and mainly beneath the discharge of the cooling air duct in each respective resonator tube, the nose portion preferably being designed as a cross-section of a cone or other similar configuration. This nose portion extends along and between the lower parts of the resonator tubes as a bulge-like projection. The top of the conical nose portion is secured in the wall which is common to both of the 30 cooling air ducts and the curved portion constitutes an extension of this wall which, thereby, is divided into two walls. With the aid of the nose portion, favourable cooling air flow characteristics will be obtained into the lowermost portion of the resonator tubes where the air is exposed to the low-frequency sound. In order to render this flow - ~ , ,, ; , ; ' . ',, - ,~ ~
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even more favourable, and in orderto reduce the risk of hissing sounds which could bc generated by the particle velocity of the sound adjacent a sharp edge, the wall which is common to each respective cooling air duct and resonator tube may, on its inner side located within the cooling air duct, be provided with a curved plate 10, 11.
5 The nose portion 9 has a substantially planar undsrside which is turned to face the cooling table, this having the effect that the cooling air oscillates reciprocally along the underside of the nose portion and thus a greater portion of the cooling table and the steel wire located thereon is exposed to cooling air than would have been the case without the nose portion. Furthermore, the favourable effect will be realized that the 10 steel wire which is advanced in recumbent spiral bights is exposed to more powerful cooling at the outer edges of the table where the wire is more closely wound and, consequently, requires greater cooling effect in order for the wire to be of uniform quality.

15 The heated cooling air is removed with the aid of a fan 13 which, for example, may be located beneath the cooling table, and its thermal effect content may be extracted and employed for various purposes, for example in that it is allowed to pass through a heat exchanger.

20 In order further to increase the cooling effect, water may be sprayed into the cooling air in the proximity of the pertinent cooling region.

Instead of using cooling air to dispose of the thermal effect emitted from the bodies, a convection surface, such as a pipe system containing a flowing cooling agent such as 25 cooling water, ammonia, freon or similar, may be installed in the proximity of the - cooling area. By allowing this pipe system to constitute a part of a heat exchanger system, the heat given off by the bodies can also be utilized.

Fig. 4 shows one embodiment for enforced cooling of, for example, hot cement 30 clinkers 20 which are advanced on a conveyor belt. This plant does not constitute an acoustically closed system. Otherwise, the plant operates in the same manner as the plant for cooling steel wire, with the difference that the two resonators 21, 22, each with their respective exigator 23, 24, and the motor 25 are installed beneath the . ., , . :

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200~7 conveyor belt which advances the clinkers. At the same time as the surface which is to be subjected to heat transmission is placed in the particle velocity anti-node, this constitutes an obstacle for the standing sound wave. In this case, the cement clinkers are a considerably greater obstacle than the steel wire in Fig. 3. If the impedance 5 becomes excessive, this is expressed in that the sharpness of the resonance of the resonators becomes poorer, which implies that the relationship between the amplitude of the particle velocity in the anti-node and the node respectively isreduced. It will be understood that, in a situation implying large losses, there is no reason to generate the standing sound wave with the aid of a long resonance tube.
10 By placing the exigator closer to the particle velocity anti-node, the length of the resonance tube may be shortened.

An open resonator as in the embodiment described above implies that the amplitude of the particle velocity declines drastically when the resonator opens outwardly, i.e. at 15 its opening. Even though, in the case using a quarter wave resonator, there is still a particle velocity anti-node at the open end of the resonator, this may be difficult to identify. On the other hand, the sound volume velocity is not affected by the diameter of the resonator but retains its sinus wave form, which in periodicity coincides with the particle velocity amplitude. It may therefore be more appropriate and simpler to20 identify that region where the greatest heat transmission may be obtained as that area where the volume velocity has a anti-node.

In the embodiments of the present invention described in the foregoing, the enforced heat transmission has solely been illustrated in the form of cooling processes, but the 25 present invention may naturally also be used for other types of processes in which an enforced heat transmission is desirable, for example freezing, heating, drying, etc.
Examples of other fields of application are cooling of extruded aluminium or plastic profiles.

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Claims

1. Method for an enforced heat transmission, by means of sound, between the surface of a body, solid or liquid, and an ambient gas c h a r a c t e r i z e d in that said sound consists of a low-frequency standing sound wave.

2. Method as claimed in claim 1 c h a r a c t e r i z e d in that said sound wave has only one volume velocity anti-node.

3. Method as claimed in any one of the preceding claims c h a r a c t e r i z e d in that said surface is located in an area of the standing sound wave which is situated in the proximity of a volume velocity anti-node.

4. Method as claimed in any one of the preceding claims c h a r a c t e r i z e d in that at least two of the dimensions of said body are considerably less than one-fourth of the wavelength of said sound wave.

5. Method as claimed in any one of the preceding claims c h a r a c t e r i z e d in that all of the dimensions of said body are considerably less than one-fourth of the wavelength of said sound wave; and that the body is transported through the sound wave.

6. Method as claimed in any one of the claims 1-4 c h a r a c t e r i z e d in that said surface consists of a part surface of the total surface of the body; and that one of the dimensions of the body is not considerably less than one-fourth of the wavelength of said sound wave.

7. Method as claimed in claim 6 c h a r a c t e r i z e d in that said body is advanced through said standing sound wave, the various part surfaces of the body being exposed gradually to the sound wave.

8. Method as claimed in claim 7 c h a r a c t e r i z e d in that said body preferably consists of a hot-rolled steel wire, extruded aluminium or plastic profile or any other body which needs cooling.

9. Method as claimed in any one of the claims 1-8 c h a r a c t e r i z e d in that when the heat transmission is a cooling process, the cooling capacity of the gas is increased by evaporation of a liquid supplied to the gas.

10. Method as claimed in any one of the claims 1-8 c h a r a c t e r i z e d in that in the sound wave is arranged a motionless convection surface being the outer side of a pipe through which passes a cooling agent such as cooling water, ammonia, freon or the like.

11. Method as claimed in claims 10 c h a r a c t e r i z e d in that the pipe forms a part of a closed piping system which is connected to a heat exchanger system.

12. Apparatus for working the method according to claim 1 with a low-frequency sound generator comprising an exigator part and a resonator part c h a r a c t e r i z e d in that the resonator part is provided with an opening which is located in a region where the low-frequency sound wave displays a volume velocity anti-node and through which opening that body which is to be exposed to enforcedheat transmission is advanced.

13. Apparatus as claimed in claim 12 c h a r a c t e r i z e d in that the resonator part comprises two tube resonators each of which has a length corresponding to one-forth of the wavelength of the generated low-frequency sound and in that the two resonators have the same resonance frequency and form a common resonator.

14. Apparatus as claimed in claim 13 c h a r a c t e r i z e d in that the two tube resonators each have an exigator; and that said exigators operate in counterphase such that a common standing sound wave of low-frequency sound is generated in the two tube resonators.

15. Apparatus as claimed in claim 14 c h a r a c t e r i z e d in that the tube resonators are located adjacent to one another such that their openings, which are located at their ends facing away from each respective exigator, are in communication with one another.

16. Apparatus as claimed in claim 15 c h a r a c t e r i z e d in that a nose portion is disposed between the tube resonators and at their openings.

17. Apparatus as claimed in claim 16 c h a r a c t e r i z e d in that the nose portion is substantially conical in configuration with a planar underside along which cooling air which is influenced by the low-frequency sound flows.

18. Apparatus as claimed in any one of claims 13-17 c h a r a c t e r i z e d in that cooling air is supplied through one or more discrete cooling air ducts located between the tube resonators.

19. Apparatus as claimed in claim 17 c h a r a c t e r i z e d in that the nose portion deflects the cooling air into the tube resonators in the proximity of their openings where the cooling air is subjected to influence by the low-frequency sound.

20. Apparatus as claimed in claim 18 or 19 c h a r a c t e r i z e d in that the flow of the cooling air is improved in that each cooling air duct is provided, on its inside facing towards the respective resonator tube, with a curved plate.

21. Apparatus as claimed in any one of the claims 12-20 c h a r a c t e r i z e d in that it is provided with a convection surface in the form of the outer side of a pipe through which passes a cooling agent such as cooling water, ammonia, freon or the like.

22. Apparatus as claimed in claims 21 c h a r a c t e r i z e d in that the pipe forms a part of a closed piping system which is connected to a heat exchanger system.

23. Apparatus as claimed in any one of claims 12-22 c h a r a c t e r i z e d in that the body which is to be cooled is continuously advanced on a roller conveyor, a conveyor belt or the like, which passes through the opening of the tube resonators.

24. Apparatus as claimed in any one of claims 12-23 c h a r a c t e r i z e d in that it is an acoustically virtually closed system.