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. 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: METHODANDARRANGEMENTFORAN ENFORCEDHEATTRANSMISSION BETWEEN BODIESANDGASES 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 he transmission is achieved in that the gas is set in oscillatory motion which is genera by a standing sound wave of low frequency, and in that the body is placed in that p of the sound 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 fro 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 sli The large flow entails that cooling will be expensive and, in consequence of the sli 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 advantageo 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 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 to realize a method and an apparatus for transferring high thermal effect per surface u from a body to ambient gas. Instead of increasing the heat transmission by aspirati the gas over the surface of the body at high speed, the enforced heat transmission i 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 m 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 must by cooled to 300 °C in order to be handled. A number of different processes a currently employed for such cooling. According to one method, the wire is laid out i 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 i 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 and whirling of dust into the air. Further disadvantages are that the cooling - from t metallurgical point of view - is not always sufficiently rapid and is uneven, and that t entire thermal effect of the hot wire is lost. The nature of the present invention and its aspects will be more readily understood from the following brief description of the accompanying drawings, and discussion relating thereto: Fig. 1 shows a solid body in a standard airflow; 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 so Fig. 4 shows a plant for cooling cement clinker using low-frequency sound. 5 As was mentioned above, an enforced heat transmission may be achieved betwee 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 which is exposed to an air flow. A particle in the air flow is mar 0 as a dot and the position of the air particle at various points in time is marked by t- j The temperature of the air flow is T-| 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 expos 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 t- j -t here. As will be apparent, 5 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 tim If the body is at 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. 0 . 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 particl 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 veloci and the sound pressure, respectively, is indicated by the amplitude of each respect 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, 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 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 H lower. The reason why frequencies above 50 Hz are less interesting is that a close 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 slig The sound resonator is preferably of a length corresponding to a half wavelength o 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 ter exigator is here employed to indicate that part of a generator for iow-frequency sou 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 pate applications 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 fro which the above-considered heat transmission is intended to take place is advanc through this opening. Hence, the surface is then located in a particle velocity anti-node of the above-mentioned standing low-frequency sound wave. A stationary air current flows through the resonator tube, one portion of the air curr deriving from the driving air which emanates from the exigator, and its other portio from the air which flows in at the opening through which the heat transmission surf is advanced. It is also possible to use a special cooling air fan. When the surface passes through the resonator tube, it will be swept partly by the stationary air flow partly by the oscillating airflow which is generated by the standing sound wave. If t amplitude of the particle velocity of the sound is considerably greater than the velo 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 surface several times. This implies that the air is heated or alternatively cooled mu more than if only the stationary airflow would have swept the surface. The result w be that a greater heat transmission takes place between the surface and the air un a given period of time than would be the case under normal conditions. One of the advantages inherent in employing the superposed air motion to be foun in the sound pressure node of a standing sound wave is that this is a relatively sim ■ manner of realizing the oscillatory air motion. Another advantage in employing a" standing sound wave is that it is only at that point where the surface is located that high velocity of the oscillating air motion is desirable. In the rest of the system, the high air velocity solely entails friction losses. An apparatus for implementing the method according to the present invention will now be described in greater detail with reference to the embodiment shown in Fig. which illustrates a plant for cooling steel wire. Low-frequency sound is generated by one or more low-frequency sound generator consisting of an exigator part 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 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. 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 sou Acoustically, the plant consists of a virtually closed system. The steel wire is laid o on a roller conveyor or other conveyor belt in accordance with standard usage and 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 table, their o 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 en of each respective resonator, there is a so-called exigator 4, 5. This exigator may b 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 the same driving shaft in such a manner that a phase lag of 180° is obtained betw the exigators when these operate. Since the exigators operate in counterphase, a standing sound wave of the same frequency will be generated in each resonator. two resonators of quarter-wave type thereby together form one resonator of half-w type of the same resonance frequency as the resOnance frequency of the individu resonators and one single common standing sound wave is generated. Cooling air is supplied with the aid of a cooling air fan 12 which conveys the coolin 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 ea respective resonator tube, and this emanation is located in the lower region of eac respective resonator tube. A nose portion 9 is located between the lower regions o the two resonator tubes and mainly beneath the discharge of the cooling air duct i 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 al and between the lower parts of the resonator tubes as a bulge-like projection. The of the conical nose portion is secured in the wall which is common to both of the cooling air ducts and the curved portion constitutes an extension of this wail which thereby, is divided into two walls. With the aid of the nose portion, favourable cooli 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 even more favourable, and in order to reduce the risk of hissing sounds which coul be 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 it inner side located within the cooling air duct, be provided with a curved plate 10, 1 The nose portion 9 has a substantially planar underside which is turned to face the cooling table, this having the effect that the cooling air oscillates reciprocally along underside of the nose portion and thus a greater portion of the cooling table and th 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 th _ steel wire which is advanced in recumbent spiral bights is exposed to more powerf 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. The heated cooling air is removed with the aid of a fan 13 which, for example, may located beneath the cooling table, and its thermal effect content may be extracted employed for various purposes, for example in that tt is allowed to pass through a heat exchanger. In order further to increase the cooling effect, water may be sprayed into the coolin air in the proximity of the pertinent cooling region. Instead of using cooling air to dispose of the thermal effect emitted from the bodies convection surface, such as a pipe system containing a flowing cooling agent such 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 clinkers 20 which are advanced on a conveyor belt. This plant does not constitute acoustically closed system. Otherwise, the plant operates in the same manner as t 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 conveyor belt which advances the clinkers. At the same time as the surface which i 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 clink are a considerably greater obstacle than the steel wire in Fig. 3. If the impedance 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 is reduced. 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. 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 amplitud of the particle velocity declines drastically when the resonator opens outwardly, i.e. its opening. Even though, in the case using a quarter wave resonator, there is still 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 diamet of the resonator but retains its sinus wave form, which in periodicity coincides with particle velocity amplitude. It may therefore be more appropriate and simpler to identify that region where the greatest heat transmission may be obtained as that a where the volume velocity has a anti-node. In the embodiments of the present invention described in the foregoing, the enforc heat transmission has solely been illustrated in the form of cooling processes, but t present invention may naturally also be used for other types of processes in which 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.

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 tha said sound consists of a low-frequency standing sound wave. 2. Method as claimed in claim 1 characterized in that said sound wave only one volume velocity anti-node. 3. Method as claimed in any one of the preceding claims characterized i that said surface is located in an area of the standing sound wave which is situate the proximity of a volume velocity anti-node. 4. Method as claimed in any one of the preceding claims characterized i that at least two of the dimensions of said body are considerably less than one-fou of the wavelength of said sound wave. 5. Method as claimed in any one of the preceding claims characterized i 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 soun wave. 6. Method as claimed in any one of the claims 1-4 characterized in that surface consists of a part surface of the total surface of the body; and that one of th dimensions of the body is not considerably less than one-fourth of the wavelength said sound wave. 7. Method as claimed in claim 6 characterized in that said body is advan 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 characterized in that said body preferab 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 characterized 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. 0. Method as claimed in any one of the claims 1-8 characterized in that i the sound wave is arranged a motionless convection surface being the outer side o pipe through which passes a cooling agent such as cooling water, ammonia, freon the like. 11. Method as claimed in claims 10 characterized in that the pipe forms a 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 characterized in that the resonator part is provided with an opening which i located in a region where the low-frequency sound wave displays a volume velocit anti-node and through which opening that body which is to be exposed to enforced heat transmission is advanced. 13. Apparatus as claimed in claim 12 characterized in that the resonator comprises two tube resonators each of which has a length corresponding to one-fo 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 characterized 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 two tube resonators. 15. Apparatus as claimed in claim 14 characterized 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 communica with one another. 16. Apparatus as claimed in claim 15 characterized in that a nose portion disposed between the tube resonators and at their openings. 17. Apparatus as claimed in claim 16 characterized in that the nose porti substantially conical in configuration with a planar underside along which cooling a which is influenced by the low-frequency sound flows. 18. Apparatus as claimed in any one of claims 13-17 characterized in tha cooling air is supplied through one or more discrete cooling air ducts located betw the tube resonators. 19. Apparatus as claimed in claim 17 characterized in that the nose porti 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 characterized in that the flow 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 characterized in it is provided with a convection surface in the form of the outer side of a pipe throug which passes a cooling agent such as cooling water, ammonia, freon or the like. 22. Apparatus as claimed in claims 21 characterized in that the pipe form 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 characterized in that body which is to be cooled is continuously advanced on a roller conveyor, a convey belt or the like, which passes through the opening of the tube resonators. 24. Apparatus as claimed in any one of claims 12-23 characterized in that an acoustically virtually closed system.

Inventors: SANDSTROEM ROLAND, STRID PER, SANDSTROEM, ROLAND, STRID, PER
Applicants: INFRASONIK AB
Application year: 1989
Main classification: F28F13
IPC codes: F28F13