Patent Application
20240093917
The present invention relates to a thermoacoustic system according to the preamble of claim 1. Additionally, the invention relates to a method for manufacturing such a thermoacoustic system.
The thermoacoustic system 11, 12 has process volume 130 which is filled with a working fluid, for example a pressurized gas such as helium.
The design of the thermoacoustic system 11, 12 is based on a thermoacoustic network. This network consists of the following components: a thermoacoustic core 140, a compliance, and an inertance.
The thermoacoustic core comprises a cold heat exchanger 102, a regenerator 106, and a hot heat exchanger 104. The regenerator 106 is placed between the cold heat exchanger 102 and the hot heat exchanger 104. Next to the hot heat exchanger 104 on a second side opposite the first side, a thermal buffer zone, TBZ, 108 is arranged. The thermal buffer zone 108 extends between the hot heat exchanger 104 and a TBZ heat exchanger 112. The thermal buffer zone minimizes heat leakage from the hot heat exchanger 104 into the remaining part of the thermoacoustic engine and therefore prevents the heat-up of the engine and insulates the hot heat exchanger 104. The optional thermal buffer zone heat exchanger 112 removes any heat leakage from the hot heat exchanger 104 into the thermal buffer zone 108. Under ideal circumstance, gas in the TBZ oscillates as plug flow along the length of the TBZ 108 and heat conduction through the gas and the wall of the TBZ 108 is very small. However, the performance of such a thermoacoustic system (i.e., thermoacoustic engine 11 or thermoacoustic heat pump 12) can be severely degraded by a high degree of heat leakage that is caused by the presence of streaming in the thermal buffer zone 108.
Streaming within the thermal buffer zone 108 refers to a time-averaged mass circulation 110 (streaming vortex) or mass flow, as schematically shown in
From experiments with a thermoacoustic engine it was shown that heat leakage can be as high as 70% of the heat input to the thermoacoustic engine. Computational Fluid Dynamics (CFD-) simulations of a thermoacoustic engine showed the presence of a large time-averaged vortex 110 in the TBZ 108. The simulations showed that the junction below 112 is causing the streaming due to an asymmetric velocity profile when the flow oscillates up and down. A streaming vortex was found in several geometrical designs of the acoustic loop.
In prior art literature other types of streaming characterized by alternative flow mechanisms are reported and several countermeasures are proposed to reduce the effect of streaming in the TBZ. However, these measures cannot prevent the streaming vortex 110.
Mass streaming or Gedeon streaming is a net time-averaged mass flow which circulates around the acoustic loop of the thermoacoustic system and this streaming is commonly called “DC-flow”. An elastic membrane or jet pump is usually used in prior art to suppress the DC-flow. The membrane or jet pump is placed at a location of low velocity to minimize the acoustic losses. Rayleigh streaming is caused by viscous and thermal boundary layer effects and from asymmetric oscillations of the acoustic wave. A tapered tube has been used to minimize this type of streaming in the TBZ. By optimally varying the cross-sectional radius of the tapered tube over the tube's length, the streaming vortex in the TBZ can be effectively reduced. The optimum taper angle is a function of the acoustic pressure. It appears to be not possible however, or at least very complex, to design a tapered tube which can be used optimally for a TBZ over a range of acoustic operation conditions. Jet streaming is caused by sudden flow contractions which causes a jet type of flow. The jet causes a non-uniform flow profile and net streaming flow pattern. Flow straighteners are typically used to redistribute the jet and to minimize this effect of the non-uniform flow profile. The flow straightener is a porous structure; usually a stack of screens. The effectiveness of the flow straighteners however is dependent on the flow conditions which change when the acoustic conditions change (acoustic pressure amplitude). It appears not possible to design flow straighteners that can operate efficiently for different acoustic conditions. Additionally, the flow straighteners can only minimize the effect of the jets on the vortex but can not completely suppress the vortex in the TBZ.
The skilled in the art will appreciate that a streaming vortex or any other of the streaming types also can exist at the compliance side of the acoustic core. Thus, in case of a thermoacoustic heat pump delivering useful heat, some heat is lost by a flow related to the streaming at this position. Hereafter, streaming will also be referred to as mass flow.
It is an object of the present invention to provide a thermoacoustic system that overcomes or mitigates the above detrimental effects.
The above object is achieved by a thermoacoustic system in accordance with claim 1.
The invention provides that the partitioning element divides the process volume (also referred to as thermal tube volume), in sub-volumes separated from each other. The partitioning element is designed to block streaming of the working fluid between the sub-volumes by closing off the cross-section of the pipe section of the process volume where the thermoacoustic core is located. In other words, the flow of gas (mass flow), is interrupted between the two sub-volumes. At the same time the partitioning element is acoustically transparent and allows acoustic waves to pass between the hot heat exchanger and the cold heat exchanger and vice versa.
In this manner, the mass flow of the streaming vortex is blocked and the convective heat flow between the hot heat exchanger and the thermal buffer zone heat exchanger is strongly reduced. As a result, less heat leaks away from the hot heat exchanger and more heat will be available to contribute to the thermoacoustic conversion process in the regenerator, thus improving the efficiency of the thermoacoustic system.
Also, the present invention relates to a method for manufacturing the thermoacoustic system as defined above.
Advantageous embodiments are further defined by the dependent claims.
The invention will be explained in more detail with reference to drawings in which illustrative embodiments thereof are shown. They are intended exclusively for illustrative purposes and not to restrict the inventive concept, which is defined by the appended claims.
In
A thermoacoustic core 140 may be part of either a thermoacoustic engine configuration or a thermoacoustic heat pump configuration.
The thermoacoustic core 140 is arranged within a pipe portion 101 of the process volume and comprises a cold heat exchanger 102, a hot heat exchanger 104, a regenerator 106, and a thermal buffer zone heat exchanger 112.
The cold heat exchanger 102 is arranged in the pipe portion 101 at a first distance L1 from a first side of the hot heat exchanger 104. At an opposite second side of the hot heat exchanger 104 at a second distance L2 the thermal buffer zone heat exchanger 112 is arranged. The regenerator 106 is positioned between the hot heat exchanger 104 and the cold heat exchanger 102.
The designation of “cold heat exchanger” and “hot heat exchanger” refers to the relative temperatures of the respective heat exchangers during use: in use the cold heat exchanger 102 will have a lower temperature than the temperature of the hot heat exchanger 104.
The pipe portion 101 in which the cold, hot and buffer zone heat exchangers 102, 104, 112 are arranged is filled during use with a working fluid, typically a gas. Such a gas may comprise helium, and may be pressurized, for example at about 50 bar (˜5 MPa).
As explained in the introductory part, in the thermoacoustic core of the prior art thermoacoustic system, a streaming vortex or turbulence 110 in the gas can occur during use, which causes leakage of energy (heat) from the thermoacoustic conversion process between the cold and hot heat exchangers 102, 104.
According to the invention, the thermoacoustic core 140 comprises a partitioning element 114 which is arranged in the pipe portion 101 between the hot heat exchanger 104 and the buffer zone heat exchanger 112 for blocking or suppressing the streaming vortex 110, and thus the mass flow. The partitioning element 114 is arranged at a distance L3 from the hot heat exchanger 104, but the distance L3 of the partitioning element 114 to the hot heat exchanger 104 is shorter than the second distance L2.
The partitioning element 114 is configured to block the circulating mass flow of the streaming vortex 110 in the gas between the hot heat exchanger 104 and the thermal buffer zone heat exchanger 112.
Advantageously, by blocking the mass flow the leakage of heat is significantly reduced. In addition, the partitioning element 114 is configured as an acoustic transparent element. By this property the partitioning element 114 allows that acoustic waves running between the hot heat exchanger 104 and the thermal buffer zone heat exchanger 112 can pass the partitioning element 114 without much disturbance, i.e., with minimal loss of acoustic energy
As a result, the propagation of acoustic waves through the thermoacoustic core 140 and through the thermoacoustic system 11; 12 as a whole is not affected.
Due to the blocking of the streaming vortex 110 and the reduction of heat loss, the efficiency of the thermoacoustic system 11; 12 is increased.
The increased efficiency allows to scale down the thermal buffer zone 108 and as a result, design a more compact thermoacoustic system. Also, the resonator volume which can be coupled to the right side of the loop at the junction (in
The partition element 114 is preferably made of a material with a low thermal conductivity or it can be provided with a thermally insulating material which provides an additional resistance to heat flowing from the hot heat exchanger 104 to the thermal buffer zone heat exchanger 112. Example materials with thermal insulation behaviour and sufficient stiffness are composite materials like Teflon, Peek, Acetal, etc.
In
Preferably, the partitioning element 114 closes off the pipe portion 101 in the thermal buffer zone 108. In this case, the partitioning element 114 has a cross-section equal to a cross-section within the thermal buffer zone 108, i.e., the pipe portion between the hot heat exchanger 104 and the thermal buffer zone heat exchanger 112.
The partitioning element 114 is designed to cover the cross-section within the thermal buffer zone 108 and to attach entirely to the wall of the pipe portion at the level of the covered cross-section of the thermal buffer zone 108. In this manner the thermal buffer zone is divided in two sub-volumes 108a, 108b separated from each other. The division prevents flow of the working fluid between the two sub-volumes 108a, 108b.
As shown in
It is noted that in this embodiment, the partitioning element 114 has a substantially circular shape which is suitable for covering and closing off a circular cross-section within the thermal buffer zone 108. The skilled in the art will appreciate that for other non-circular cross-sections of the thermal buffer zone, the cross-section of the partitioning element 114 can be adapted to fit.
In an embodiment, the diameter of the thermal buffer zone 108 is circular, and the partitioning element 114 has a central circular portion 116 attached to an outer annular suspension 118. This configuration is schematically shown in
The annular suspension 118 may be an elastic ring to provide the flexible connection. Alternatively, an intermediate element (not shown) may be provided to connect the elastic ring with the central circular portion 116. An outer circumference of the intermediate element is connected to an inner circumference of the elastic ring, while the inner circumference of the intermediate element is connected with an outer circumference of the central circular portion 116.
Alternatively, as shown in
As shown in
According to an embodiment, the central portion 116, 124, and 126 has a significant mass and the annular suspension have a significant stiffness which combination can be seen as a mass-spring system which is transparent to the wave by resonating at the same frequency as the acoustic working frequency of the system.
According to an alternative embodiment, the partitioning element is arranged in a second thermal buffer zone (not shown) adjacent to the cold heat exchanger, at a side thereof that faces away from the hot heat exchanger. In this embodiment, the partitioning element is configured to block streaming and mass flow through the second thermal buffer zone.
In the foregoing description of embodiments, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the scope of the invention as summarized in the attached claims.
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.
In addition, modifications may be made to adapt a particular layout or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention includes all embodiments falling within the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19202035.2 | Oct 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/078328 | 10/8/2020 | WO |
