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

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 a standing 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. In particular the invention relates to heat transmission from relatively small solid bodies, which occur in large amounts and which are caused to fluidize under the influence of the low-frequency sound wave. The apparatus for carrying out the method includes a low-frequency sound generator, which comprises an exigator part (1, 12, 32, 33) and a resonator part (2, 3, 4, 10, 11, 20, 30, 31). The resonator part is acoustically virtually closed and is so designed that the body which is to be exposed to the enforced heat transmission is placed within an area inside the resonator part where the standing sound wave displays a volume velocity anti-node.

Description: METHODANDARRANGEMENTFORANENFORCEDHEATTRANSMISSION BETWEENBODIESANDGASES The present invention relates to a method and an apparatus for enforced heat transmission between a body, solid or liquid, and an ambient gas. In particular, the invention relates to heat transmission from relatively small solid bodies, which occu in large amounts, and where it is desirable for a fluidization of the forward flowing bodies to be materialized in order thus to improve both the heat transmission and t motion of the bodies. The enforced heat transmission is achieved in that the ambie gas is set in oscillatory motion which is generated by a standing sound wave of low frequency and in that the forward flowing bodies are placed in that part of the soun wave where the oscillatory motion is greatest. A fundamental problem in heat transmission, for example from a warm body to an 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 larg thermal effects, high gas flow rates are required, which implies that a large air flow be necessary. At the same time, however, the temperature rise in the air will be slig The large flow entails that cooling will be expensive and, in consequence of the slig temperature rise, the energy in the heated air can seldom be utilized. It is previously known from V. B. Repin, "Heat exchange of a cylinder with low-frequency oscillations", Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi, No. 5, p 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 advantageou 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 transmissi It is also obvious that the heat transmission increases with increasing particle velocities. The reason why the prior-art method of employing low-frequency sound f heating or cooling of bodies has not hitherto enjoyed any practical importance is th 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 be cooled, or alternatively, heated. The object of the present invention is to solve the above-mentioned problem and t realize a method and an apparatus for achieving an enforced heat transmission by transferring high thermal effect per surface unit from a body to ambient gas, especi for applications in which the body consists of a quantity of small solid bodies, for example in the form of granules or as pellets or drops. Instead of increasing the he transmission by aspirating the gas over the surface of the body at high speed, the enforced heat transmission is achieved by imparting to the ambient gas a low frequency oscillation. For the purposes of clarifying the present invention, three different embodiments thereof with regard to cooling will be described. The nature of the present invention and its aspects will be more readily understoo from the following brief description of the accompanying drawings, and discussion relating thereto: Fig. 1 shows a solid body in a standard air flow; Fig. 2 shows a solid body in an air flow which has been exposed to an infrasound field; Fig. 3 shows an embodiment of an apparatus according to the invention; Fig.4 shows another embodiment of an apparatus according to the inventio Fig.5 shows a third embodiment of the invention which can be used in an installation for cooling of plastic granules; Fig. 6 shows a fourth embodiment of an apparatus according to the inventio which is particularly suitable for freezing of vegetables; Fig. 7 shows an apparatus for cooling of foundry sand according to the invention. As was mentioned above, an enforced heat transmission may be achieved betwe the surface of a body and an ambient gas if the gas is influenced so as to reciproc with the aid of a standing sound wave generated in the gas. Fig. 1 shows a solid b at a temperature T 0 which is exposed to an air flow. A particle in the air flow is mar as a dot and the position of the air particle at various points in time is marked by \-\ - The temperature of the air flow is T- j before it has passed the body, and T2 after th body has been passed. Fig. 2 shows the same solid body when it has been expose 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 \ \y here. As will be apparent, 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 time If the body is of a higher temperature than the air flow, the air particle will absorb m 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 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 gre while the velocity of the oscillating motion is low. At a certain point, both the particle 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 velocit and the sound pressure, respectively, is indicated by the amplitude of each respecti oscillatory motion. As a rule, the amplitude of the partice velocity assumes a maxim value, i.e. has a so-called particle velocity anti-node, at the same time as the amplitude of the sound pressure assumes a minimum value, i.e. has a so-called sound pressure node. It is desirable, in accordance with the foregoing, that the particle velocity assumes 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 whose length corresponds to a quarter or a half wavelength, or alternatively a part o 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 an enforced 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 possib 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 a closed, or in any case acoustically virtually closed, sound resonator. The term low-frequency sound is he taken to mean sound at a frequency of 50 Hz or lower. The reason why frequencie above 50 Hz are less interesting is that such a closed half-wave resonator has su small dimensions at high frequencies that the whole apparatus will be uninterestin from the point of view of capacity. Since possible 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. 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. 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 which generates a particle velocity in one point in a resonator where a high sound press prevails, see for example Swedish patent No.446 157 and Swedish patent applications Nos. 8306653-0, 8701461-9 and 8802452-6. Somehwere in the resonator a particle velocity anti-node will occur and here the body is supplied whi is to be exposed to an enforced heat transmission. If the body in question consist substance which occurs in the form of granules, pellets or similar, the particle velo of the sound can also act fluidizingly on the substance in question. In the case when the body in question, which constitutes an obstacle to the sound becomes all too large, this is revealed in that the sharpness of the resonance of t resonator becomes poorer, which means that the ratio between the amplitude of t particle velocity in the anti-node and node respectively decreases, in a condition large losses there is therefore no reason to generate the " standing sound wave wit the aid of a long resonance tube. By placing the exigator closer to the particle vel anti-node the resonance tube can be shortened. In the practical designing of the sound resonator there are several possibilities. Examples of different embodiments are illustrated in Figs. 3-5, the principles of whic are briefly described here. In all cases an acoustically closed system is aspired to. Fig. 3 shows a generator for low-frequency sound with an exigator 1 and a resonato 2 with a length corresponding to a half of a wavelength of the generated low-frequency sound. A particle velocity anti-node occurs in an area close to the middle of the resonator and consequently the substance which is to be exposed to enforced heat transmission is supplied just above the middle of the resonator and drained just below the middle thereof. Fig. 4 shows a resonator which functions in th same manner as the resonator in Fig. 3 with the difference that the lower half of the resonator has been replaced by a resonator of Helmholtz type. Here there is thus a tube resonator 3 with a length corresponding to a quarter of a wavelength combined with a Helmholtz resonator 4 which is so dimensioned that it is tuned for the same resonance frequency as the tube resonator, implying that the tube resonator and th Helmholtz resonator in this case jointly form one resonator. In Fig. 5 and 6 the Helmholtz resonator in Fig. 4 has been given a funnel shape so that the substance which is to be exposed to enforced heat transmission is collected up by the Helmholtz resonator 10, 20 and, through an opening in its bottom, is passed on. Fig. shows another variant in which two resonators 30, 31 each with a length corresponding to a quarter of a wavelength, have been placed side by side so that their open ends are in communication with each other. Two exigators 32, 33 generat a standing sound wave of the same frequency in each resonator. By permitting thes exigators to operate in counterphase, there is generated one single common standin sound wave. In principle, this joint resonator functions in the same manner as a half-wave resonator. In a case with a sound resonator of irregular shape the appearance of the amplitude of the particle velocity is influenced so that the original sinus wave shape becomes difficult to recognize. The volume velocity of the sound, however, is not influenced in the same way and instead it retains its sinus wave shape, which in periodicity coincides with the amplitude of the particle velocity. In tire case of a sound resonator of irregular shape it may thus be more appropriate and easier to identify the area where the largest heat transmission can be obtained as the area where the volume velocity has an anti-node. The invention will now be described in greater detail with reference to three embodiments which concern cooling. Fig. 5 illustrates an apparatus for cooling of plastic granules. An infrasound genera of the type described in Swedish patent application 8802452-6 can, for example, b used. This comprises a tube resonator 11 , which preferably has a length equivalen a quarter of a wavelength, at one end of which an exigator 12 is mounted. At its ot end it is fitted with a diffusor 13 which is directly mounted on a cooling tower 14 through the upper end of which hot plastic 15 in the form of granules is supplied vi supply pipe 16. Together with the diffusor, the cooling tower and the Helmholtz resonator 10, the tube resonator forms a resonator corresponding to a half-wave resonator. The diffusor and the cooling tower are situated within an area in which a volume velocity anti-node occurs. The hot plastic granules 15 fall by the force of gravity down through the cooling tower 14. The tower is furnished with a number o inclined obstacles 17 which momentaneously catch up the plastic granules so that transport time of the plastic granules through the area with a high volume velocity i prolonged. The obstacles consist preferably of trays fitted with nets, but the obstac may also have other designs which permit air to pass through them while the plasti granules are unable to pass through them, e.g. pipes, beams or similar. At the low end of the tower there is a Helmholtz resonator 10, which functions like a funnel a catches up the plastic granules for further transport to a container. At the upper pa the Helmholtz resonator, cooling air is supplied from a fan through a duct 18. This rises up through the cooling tower and is heated by the plastic granules. The heat cooling air is discharged through a duct 19.. Enforced heat transmission is obtained between the granules and the gas influenc by low-frequency sound, in this case air. When the granules are caught up by the obstacles the air motion generated by the sound accomplishes a fluidization of the granules. Fig. 6 shows another embodiment which is particularly suitable for the freezing of vegetables. The cooling air has been replaced by a closed system of pipes for a cooling agent such as water, ammonia, freon. The pipes 21 are installed between t inclined obstacles 17 and by allowing the piping system to constitute a part of a heat-exchanger system the heat given off by the bodies, in this case the vegetables can also be utilized. Fig. 7 shows an embodiment in which hot sand from a foundry is cooled. The apparatus consists of two resonators 30, 31 both of which have a length corresponding to a quarter of a wavelength. Located at the upper end of each respective resonator is an exigator 32, 33 which in this case also can appropriately of the type described in Swedish patent application 8802452-6 or equivalent. These two exigators 32, 33 are driven by a common motor 34 such that they operate in counterphase with each other. By this means a single common standing wave is generated in the two resonators, which are situated side by side so that their open ends 35, 36 are in communication with each other through a joint space 37. In the lower part of each respective resonator and in the proximity of the joint space 37, a zone is obtained with a volume velocity anti-node which constitutes the actual cooli zone. Installed in the cooling zone are obstacles in the form of pipes 38, 39 which a conveyed to and fro several times within the cooling zone and thus form two pipe systems. A cooling agent such as water, ammonia, freon or similar, flows through these pipes. The sand to be cooled is supplied to the apparatus from above through pipe 40 which has two branches 41 , 42 emanating immediately above the two pipe systems. The supplied sand passes slowly down through the pipe systems with the aid of the force of gravity and is cooled during this passage. The outside of the pipe systems then constitutes a convection surface so that a heat transmission takes pla first between the grains of sand and the air.inside the resonators and thereafter between the air and the convection surface. The heat absorbed by the coolant is subsequently led off to be used, for example, for heating. When the sand has been cooled and has passed the obstacles 38, 39 it is collected and removed through a duct 43 situated at the lower portion of the joint space 37. In the embodiments of the present invention described in the foregoing, the enforce heat transmission has solely been illustrated in the form of cooling and freezing processes, but the present invention may naturally also be used for other types of processes in which an enforced heat transmission is desirable, for example heatin drying, etc.

Claims: 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, characterized in that said sound consists of a low-frequency standing sound wave. 2. Method as claimed in claim 1 characterized in that said sound wave ha only one volume velocity anti-node. 3. Method as claimed in any one of the preceding claims characterized in that said surface is located in a zone of the standing sound wave which is situated i the proximity of a volume velocity anti-node. 4. Method as claimed in any one of the preceding claims characterized in that all of the dimensions of said body are considerably less than a quarter of the wavelength of said sound wave. 5. Method as claimed in claim 4 characterized in that said body is a granu pellet, drop or similar. 6. Method as claimed in any one of the preceding claims characterized in that a large number of bodies pass through said sound wave in the form of a continuous flow. 7. Method as claimed in claim 6 characterized in that the flow has an inlet the sound wave and an outlet from the sound wave and that the inlet is situated at a level which lies above the outlet. 8. Method as claimed in claim 7 characterized in that said flow of bodies i transported between the inlet and the outlet only with the aid of gravitation forces. 9. Method as claimed in claim 8 characterized in that the transport time the bodies between the inlet and the outlet is prolonged through obstacles placed i the transport path of the bodies. 10. Method as claimed in claim 9 characterized in that said obstacles ma consist of nets, pipes, beams or the like. 11. Method as claimed in claim 9 or claim 10 characterized in that the bo are fiuidized on said obstacles on account of the influence of the low-frequency sound. 12. Method as claimed in any one of claims 1-11 characterized in that in sound wave is provided a motionless convection surface and in that heat transmission takes place between the gas and said convection surface. 13. Method as claimed in claim 12 characterized in that the convection surface constitutes the outer side of a pipe through which passes a cooling agent such as cooling water, ammonia, freon or similar. 4. Apparatus for working the method according to claim 1 with a low-frequency sound generator comprising an exigator part and a resonator part characterized in that the resonator part is acoustically virtually closed and designed that the body which is to be exposed to the enforced heat transmission i located within an area inside the resonator part where the standing sound wave displays a volume velocity anti-node. 15. Apparatus as claimed in claim 14 characterized in that all the dimens of said body are significantly smaller than a quarter of the wave length of said sou wave. 16. Apparatus as claimed in claim 15 c h a r a c t e r i z ~ e d in that said body is a granule, pellet , drop or similar. 17. Apparatus as claimed in any one of claims 14-16 characterized in that large number of bodies pass through said sound wave in the form of a continuous flow. 18. Apparatus as claimed in claim 17 characterized in that the flow has an inlet to the sound wave and an outlet from the sound wave and in that the inlet is at level which lies above the outlet. 19. Apparatus as claimed in claim 18 characterized in that the said flow of bodies are transported between the inlet and the outlet only with the aid of gravitatio forces. 20. Apparatus as claimed in claim 19 characterized in that the transport ti of the bodies between the inlet and the outlet is prolonged through obstacles placed in the transport path of the bodies. 21. Apparatus as claimed in claim 20 characterized in that said obstacles may consist of nets, pipes, beams or similar means which allow gas but not the bodies to pass through. 22. Apparatus as claimed in any of claims 20 or 21 characterized in that th bodies are made to fluidize on said obstacles on account of the influence of the low-frequency sound. 23. Apparatus as claimed in any one of claims 20-22 characterized in that cooling gas is supplied to the resonator from a cooling gas inlet situated below the obstacles and in that the cooling gas is discharged from the resonator via a cooling gas outlet situated above the obstacles. 24. Apparatus as claimed in any one of claims 20-22 characterized in that installed between the obstacles is a pipe system througti which a cooling agent is transported. 25. Apparatus as claimed in any one of claims 20-22 characterized in that obstacles consist of a pipe system through which a cooling medium is transported. 26. Apparatus as claimed in any one of claims 24-25 characterized in tha outer side of the pipes of said pipe system constitutes a convection surface and in said pipe system constitues a part of a heat-exchanger system. 27. Apparatus as claimed in any one of claims 14-26 characterized in tha resonator part consists of a tube resonator with a length corresponding to a half wavelength of the sound of the generated low-frequency sound wave. 28. Apparatus as claimed in any one of claims 14-26 characterized in tha resonator part comprises a tube resonator with a length corresponding to a quarter a wavelength of the generated low-frequency sound wave and in that, at its lower facing away from the exigator, it is connected to a Helmholtz resonator, and in that two resonators together form a resonator with the same resonance frequency as t resonance frequency of the individual resonators. 29. Apparatus as claimed in any one of claims 27-28 characterized in tha tube resonator consists of several parts with mutually different diameters. 30. Apparatus as claimed in any one of the claims 28-29 characterized in the Helmholtz resonator is funnel-shaped and in that the outlet of the bodies is situated at the bottojn of the Helmholtz resonator. 31. Apparatus as claimed in any one of claims 14-23 and 25-26 c h a ra c t e r i in that the resonator part comprises two tube resonators each of which has a lengt corresponding to a quarter of the wavelength of the generated low-frequency soun 32. Apparatus as claimed in claim 31 characte rized in that the two tube resonators each have their own exigator and in that said exigators operate in counterphase so that a common standing sound wave of low frequency sound is generated inside the two tube resonators. 33. Apparatus as claimed in claim 32 characterized in that the tube resonators are situated adjacent to each other so that their respective open ends, facing away from the exigators, communicate with each other through a common space. 34. Apparatus as claimed in claim 33 characterized in that the common space at the open ends of the tube resonators consists of a container which collects the bodies for discharge via an outlet.

Inventors: MAXE ANN, SANDSTROEM ROLAND, STRANDBERG TORE, MAXE, ANN, SANDSTROEM, ROLAND, STRANDBERG, TORE
Applicants: INFRASONIK AB
Application year: 1989
Main classification: F28F13
IPC codes: A23L1, A23L3, B01J19, F25B9, F25D13, F28F13