Patent Publication Number: US-2023133558-A1

Title: Thermoacoustic device

Description:
FIELD OF THE INVENTION 
     The present invention relates to a thermoacoustic device. Also, the invention relates to a method for manufacturing such a thermoacoustic device. 
     BACKGROUND 
     A traveling wave thermoacoustic device consists in general of a regenerator unit or thermoacoustic core, comprising a regenerator and two heat exchangers, a feedback loop, and a pressure vessel holding a gas volume and that houses all components. The regenerator is arranged between the two heat exchangers. The heat exchangers are configured to transfer heat to or from the thermoacoustic device. Within the thermoacoustic device a conversion process between acoustic power and thermal power, and vice versa, takes place in the regenerator. A thermoacoustic device can be configured as an engine or as a heat pump. 
     The performance of such a thermoacoustic device can be characterized by two parameters: efficiency of the conversion process and power density. An ideal thermoacoustic device has both a high efficiency and a high power density. However, this combination is not always feasible. 
     Acoustic power is amplified or attenuated by a temperature ratio across the regenerator unit generated by a temperature difference between the two heat exchangers. In addition, an acoustic circuit is provided within the thermoacoustic device, which is characterized by a compliance and an inertance (feedback inertance). 
     A highest efficiency for the conversion in the thermoacoustic device can be obtained with a high resistance at the regenerator unit, which results in low gas velocities in the regenerator unit (low flow losses) and a small phase difference (about 0°) between velocity and pressure of the gas flowing through the regenerator unit. In order to obtain this traveling wave phasing, the magnitude of an impedance of the gas in the feedback inertance should be small compared to the resistance at the regenerator unit. 
     The power of the thermoacoustic device is controlled by a velocity of gas through the regenerator unit, while keeping all other parameters constant (geometry, average pressure, drive ratio, frequency, working medium). Increasing the compliance of the gas volume will lead to higher volume velocities but also to larger power losses. In fact, acoustic losses in the regenerator are proportional to the square of the velocity of the gas. 
     The preferred solution would be a thermoacoustic system with high volume velocities through the regenerator unit, without compromising the efficiency too much. In addition, the solution should not lead to prohibitively large costs associated with large gas volumes or large components. 
     U.S. Pat. No. 6,032,464 describes a traveling-wave device that is provided with the conventional moving pistons eliminated. Acoustic energy circulates in a direction through a fluid within a torus. A side branch may be connected to the torus for transferring acoustic energy into or out of the torus. A regenerator is located in the torus with a first heat exchanger located on a first side of the regenerator downstream of the regenerator relative to the direction of the circulating acoustic energy; and a second heat exchanger located on an upstream side of the regenerator. A mass flux suppressor is located in the torus to minimize time-averaged mass flux of the fluid. 
     It is an object of the present invention to overcome or mitigate one or more e disadvantages from the prior art. 
     SUMMARY OF THE INVENTION 
     The object is achieved by a thermoacoustic device for transfer of energy by an acoustic wave, comprising a process volume, the process volume being filled with a working fluid through which the acoustic wave propagates, comprising: an acoustic network comprising a loop configured with a passage; the loop being a tube configured as acoustic circuit provided with a compliance volume, an inertance volume, and a thermoacoustic core; the passage providing an opening in the loop spaced apart from the thermoacoustic core, wherein the loop connects the thermoacoustic core and the passage via two separate paths; wherein a first side of the thermoacoustic core is at a first path length from the passage in one of the two paths, and a second side of the thermoacoustic core is at a second path length in the other of the two paths; wherein the thermoacoustic device comprises a spring-type partitioning element within the loop; the spring-type partitioning element being configured to close off the cross-section of the tube and to be impermeable for the working fluid while allowing transmission of pressure waves in the working fluid through the spring-type partitioning element. 
     According to the invention, a spring-type partitioning element, i.e., a partition comprising an element with mechanical properties defined by a spring constant thereof, hereafter referred to as a (elastic or mechanical) spring is placed in the acoustic circuit near the thermoacoustic core (or regenerator unit). The spring enforces larger volume flows through the regenerator without adding gas volume to the system. 
     The position of the spring-type partitioning element should be near the thermoacoustic core to be effective. Besides increasing the volume velocities and therewith power density, the spring-type partitioning element also improves the phasing between pressure and velocity of the gas in the regenerator and therefore has a beneficial effect on the efficiency as well. The spring-type partitioning element can also be used to suppress DC flow in the travelling wave engine or heat pump. Therefore, a jet pump or membrane can be omitted which will simplify the system and lower its costs. 
     Convective heat losses in the thermal buffer tube of an engine or heat pump will be lowered by the additional thermal resistance of the partitioning element. This will lead to a further increase of the system efficiency. 
     It has to be understood that the spring element can be any type of spring; including cylindrical spring, conical spring, wave spring, flexure bearing, or any type of elastic membrane. 
     The present invention also relates to a method for manufacturing a thermoacoustic device in accordance with claim  16 . 
     Advantageous embodiments are further defined by the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not as a restriction of the inventive concept. The scope of the invention is only limited by the definitions presented in the appended claims. 
         FIG.  1    shows a cross-section of a thermoacoustic device in accordance with the prior art; 
         FIG.  2    shows an impedance analogy of the thermoacoustic device in accordance with the prior art; 
         FIG.  3    shows a cross-section of a thermoacoustic device according to an embodiment of the invention; 
         FIG.  4    shows an impedance analogy of the thermoacoustic device of  FIG.  3   ; 
         FIG.  5    shows a cross-section of a thermoacoustic device according to an embodiment of the invention; 
         FIG.  6    shows a cross-section of a spring-type partitioning element in accordance with an embodiment of the invention; 
         FIG.  7    shows a cross-section of a spring-type partitioning element in accordance with an embodiment of the invention; 
         FIG.  8    shows a cross-section of a spring-type partitioning element consisting of a thin plate in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG.  1    shows a cross-section of the thermoacoustic device in accordance with the prior art. 
     The thermoacoustic device  100  according to the prior art comprises a resonance tube  2  that is configured at one connecting end or passage  4  with a loop shaped tube or loop  6 . Within the loop a thermoacoustic core  8  is arranged. At the connecting end the resonance tube  2  branches into a first leg  10  and second leg  12  that extend to a first side  8   a  and a second side  8   b  of the thermoacoustic core  8 , respectively. The thermoacoustic core  8  is positioned “off-center in the loop”, i.e., the loop connects the thermoacoustic core  8  and the passage  4  via two separate paths: the first side  8   a  of the thermoacoustic core  8  is arranged at an end of the first leg  10  at a first path length L 1  (indicated by dashed line) between the thermoacoustic core  8  and the connecting passage  4 . The second side  8   b  of the thermoacoustic core  8  is arranged at an end of the second leg  12  at a second path length L 2  (indicated by dashed line) between the thermoacoustic core  8  and the connecting passage  4 . 
     The loop  6  acts as an acoustic circuit with a process volume V which is filled with a working fluid, for example a pressurized gas such as helium. The working pressure can be about 5 MPa (50 atm), for example. 
     Within the acoustic circuit, in the second leg  12  adjacent to the thermoacoustic core  8  a compliance volume  14  and an inertance volume (or: inertance tube)  16  are defined as explained above. The compliance volume  14  is defined to be positioned between the thermoacoustic core  8  and the inertance volume  16 . 
     Typically, the thermoacoustic core  8  comprises a cold heat exchanger  8   c,  a regenerator  8   d,  and a hot heat exchanger  8   e.  The skilled in the art will appreciate that in the drawing the case of a heat pump is depicted. In case of an acoustic engine the position of the hot and cold heat exchangers is reversed. In this regard, “cold heat exchanger” and “hot heat exchanger” refer to the relative temperature of the respective heat exchangers during use: in use, the cold heat exchanger will have a lower temperature than the temperature of the hot heat exchanger. 
     The thermoacoustic core  8  may be part of either a thermoacoustic engine configuration, a thermoacoustic heat pump configuration, or a thermoacoustic cooler configuration. 
     The regenerator  8   d  is placed between the cold heat exchanger  8   c  and the hot heat exchanger  8   e.  Next to the hot heat exchanger  8   e  on a second side facing away from the regenerator unit  8   d,  a thermal buffer zone (not shown) may be arranged. 
       FIG.  2    shows an impedance analogy of a prior art thermoacoustic device  100  in accordance with  FIG.  1   . 
     In its simplest form, a traveling wave thermoacoustic device  100  as explained with reference to  FIG.  1    is visualized by its impedance analogy  101  which schematically shows an electric network. 
     The electric network comprises a resistance R, an inductance L and a capacitance C. 
     The regenerator unit of the thermoacoustic device is characterized mainly by the resistance R. The compliance and the inertance of the gas volume are characterized by a capacitance C and an inductance L, respectively. 
     The resistance R is arranged in parallel with the inductance L between a first node N 1  and a second node N 2  in the network. The parallel combination of the resistance R and the inductance L is connected in series with the capacitance C at the second node N 2 . 
       FIG.  3    shows a cross-section of a thermoacoustic device  50  according to an embodiment of the invention. 
     In  FIG.  3    entities with the same reference number as shown in the preceding  FIGS.  1 - 2    refer to corresponding or similar entities. 
     The thermoacoustic device  100  according to the invention comprises a loop shaped tube or loop  6  that is configured with an opening or passage  4 . Within the loop a thermoacoustic core  8  is arranged. At the passage  4  the connecting tube  2  branches into a first leg  10  and second leg  12  that extend to a first side  8   a  and a second side  8   b  of the thermoacoustic core  8 . A structure (not shown) is connected to the loop via the connecting tube  2 . The thermoacoustic core  8  is positioned “off-center in the loop”, i.e., the loop connects the thermoacoustic core  8  and the passage  4  via two separate paths: the first side  8   a  of the thermoacoustic core  8  is arranged at an end of the first leg  10  at a first path length L 1  between the thermoacoustic core  8  and the connecting passage  4 . The second side  8   b  of the thermoacoustic core  8  is arranged at an end of the second leg  12  at a second path length L 2  between the thermoacoustic core and the connecting passage  4 . 
     In this arrangement, the first path length L 1  is relatively shorter than the second path length L 2 . Optionally, the first path length L 1  substantially corresponds to the second path length L 2 . 
     The structure that is connected to the loop may comprise a resonance tube, or a resonator equipped with a driver, for example a mechanical driver such a piston, mass-spring mechanical resonator or a piston compressor. 
     In this embodiment, the thermoacoustic device  50  is similar to the thermoacoustic device  100  as shown in  FIG.  1   , but additionally comprises a spring-type partitioning element  20  that is arranged within the first leg  10  of the loop  6  between the thermoacoustic core  8  and the connecting passage  4 . The spring-type partitioning element  20  is configured to block flow of gas through the element  20 . In other words, the spring-type partitioning element  20  is designed to be impermeable for gas. In addition, the spring-type partitioning element  20  has mechanical properties in accordance with a spring element to allow transmission of pressure waves through the element  20 . 
     As a result, the spring-type partitioning element  20  is configured to enforce relatively larger volume flows through the regenerator  8  without adding gas volume to the device in comparison with the thermoacoustic device  100  according to the prior art. Besides increasing the volume velocities and therewith power density, the spring-type partitioning element  20  also improves the phasing between pressure and velocity and therefore has a beneficial effect on the efficiency as well. 
     Advantageously, the application of a spring-type partitioning element  20  results in a thermoacoustic device  50  that can be kept compact, has a relatively higher power density and improved efficiency. 
     Also, the spring-type partitioning element provide that DC flow in the travelling wave engine or heat pump is suppressed. Convective heat losses in the thermal buffer zone will be lowered by the additional thermal resistance of the partitioning element. 
     The impedance analogy of a possible implementation of the thermoacoustic device  50  according to the embodiment of  FIG.  3    is illustrated in more detail with reference to  FIG.  4   . 
       FIG.  4    shows an impedance analogy  52  of the thermoacoustic device  50  of  FIG.  3   . In the impedance analogy the thermoacoustic device is visualized by a schematic electric network. 
     Similar to the electric network shown in  FIG.  2   , the electric network comprises a resistance R, an inductance L and a capacitance C, in which the resistance R corresponds with the resistance of the regenerator  8 , the inductance L with the inertance volume  16  and the capacitance with the compliance volume  14 , respectively. The resistance R is arranged in parallel with the inductance L between a first node N 1  and a second node N 2 . The parallel combination of the resistance R and the inductance L is connected in series with the capacitance C at the second node N 2 . 
     According to the embodiment of the invention shown in  FIG.  3   , the impedance analogy additionally comprises a second capacitance S. 
     The second capacitance S is arranged in series with the resistance R, between the resistance R and the first node N 1 . 
     The second capacitance S at this position in the electric network corresponds with the compliance added by the spring-type partitioning element  20  in the first leg  10  of the loop  6 . 
       FIG.  5    shows a cross-section of a thermoacoustic device according to an embodiment of the invention. 
     In  FIG.  5    entities with the same reference number as shown in the preceding  FIGS.  1 - 4    refer to corresponding or similar entities. 
     According to the embodiment shown in  FIG.  5   , the spring-type partitioning element is arranged in the second leg of the loop near the second side of the thermoacoustic core, between the thermoacoustic core and the location of the compliance volume. 
     In this position the spring-type partitioning element still enhances the acoustic circuit in comparison with the acoustic circuit from the prior art, but may be less effective than in the position within the first leg as shown in  FIGS.  3  and  4   . 
     An optimal magnitude of the spring constant of the spring-type partitioning element is depending on the acoustic parameters of the system (compliances, inertance, resistance of the regenerator) but also on the temperature ratio across the regenerator. 
     In a preferred position (as shown in  FIG.  3   ), the magnitude of the spring constant of the spring-type partitioning element is mainly determined by the magnitude of the inertance and the temperature ratio across the regenerator. Detailed analysis with thermoacoustic design software (e.g. Delta EC software) can be applied to determine an optimal spring constant value of the spring-type partitioning element. 
     Furthermore, since a zero-mass spring does not exist under practical circumstances, the magnitude of the spring constant of the spring-type partitioning element needs to be adjusted by taking the mass of the spring-type partitioning element  20  into account. The resonant spring constant (belonging to the mass of the spring-type partitioning element) has to be added to the optimal spring constant value of the zero-mass spring to obtain an optimal spring constant value for a spring-type partitioning element with a specific mass. 
       FIG.  6    shows a cross-section of a spring-type partitioning element  20  in accordance with an embodiment of the invention. 
     The spring-type partitioning element  20  according to this embodiment is designed to cover the cross-section of the first leg  10  or alternatively the cross-section of the second leg  12  of the loop and to attach entirely to the wall  22  of the respective leg portion at the level of the covered cross-section. In this manner, the first or second leg portion  10 ;  12  is divided in two sub-volumes  24 ,  26  separated from each other. The division prevents flow of the working fluid between the two sub-volumes, but at the same time allows transmission of pressure waves through the spring-type partitioning element  20  between the two sub-volumes. 
     As shown in  FIG.  6   , the spring-type partitioning element  20  comprises a central section  30  and an outer (annular) section  32  joined to a circumference of the central section by means of a spring or spring arrangement  34 . The outer annular section  32  is to be attached to the wall  22  of the first leg portion  10  or alternatively the wall of the second leg portion  12 . A flexible seal  33  is placed in the annular section to avoid gas flowing through the partitioning element. 
     Preferably the central section  30  has a planar shape, but could have a different shape, for example convex or concave. 
     According to an embodiment, the plane of the central section  30  is displaced in perpendicular direction relative to the level of outer section  32  of the spring-type partitioning element  20  with the spring or spring arrangement  34  located in between the levels of the central section  30  and the outer section  32 . The spring or spring arrangement  34  is configured to allow movement of the central section  30  in a direction transverse to the plane of the outer section  32 . 
     When the spring-type partitioning element  20  is mounted in the first (or second) leg portion, the central section  30  will be allowed to move in the direction parallel to the (local) length of the leg portion. 
     It is noted that in this embodiment, the spring-type partitioning element  20  has a shape that substantially matches of its location in the cross-section of the leg portion  10 ;  12 , for covering and closing off the cross-section. 
       FIG.  7    shows a cross-section of a spring-type partitioning element  36  in accordance with an embodiment of the invention. 
     In  FIG.  7    entities with the same reference number as shown in the preceding  FIG.  6    refer to corresponding or similar entities. 
     The spring-type partitioning element  36  according to this embodiment is similar to the spring-type partitioning element  20  shown in  FIG.  6   , and further comprises a collar shaped edge portion  38  that is attached around the circumference of the central section  30  of the spring-type partitioning element. A gap seal  40  is arranged between the edge portion and the wall  22  of the leg portion to improve the barrier function for closing off flow of the pressurized gas across the spring-type partitioning element. 
       FIG.  8    shows a cross-section of a spring-type partitioning element consisting of an elastic membrane in accordance with an embodiment of the invention. The elastic membrane is tensioned in radial direction and closes of the cross section. The acoustic wave periodically stretches and deflects the membrane in the transverse direction to the external force exerted by the high acoustic pressure of the working fluid in the loop. 
     The type and thickness of the elastic material of the membrane is chosen such that the membrane obtains the correct spring constant. 
     The membrane has a stiffness that is significantly higher compared to the stiffness of a latex membrane which usually is used to block DC-flow. The required stiffness of the membrane depends on the system design (i.e. size of the system) and the operational conditions (i.e. system pressure). For relatively small systems with typical diameter of 0.07 m a Viton rubber type of membrane with a thickness of 0.5 mm could be used as partitioning element. 
     The invention has been described with reference to some embodiments. Obvious modifications and alterations will occur to the skilled in the art upon reading and understanding the preceding detailed description. 
     In addition, modifications may be made to adapt a particular layout or a material to the teachings of the invention without departing from the scope thereof. In particular, combinations of specific features of various aspects of the invention may be made. An aspect of the invention may be further advantageously enhanced by adding a feature that was described in relation to another aspect of the invention. 
     Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention includes all modifications insofar as they come within the scope of the appended claims.