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.