Thermoacoustic machines (sometimes called "TA machines") are thermosacoustic devices that use high-amplitude sound waves to pump heat from one place to another, or otherwise use heat differentials to induce high amplitude sound waves. In general, thermoacoustic engines can be divided into standing waves and current wave devices. Both types of thermoacoustics devices can be subdivided into two classes of thermodynamics, prime mover (or just a heat engine), and heat pumps. The prime mover creates work using heat, while the heat pump creates or transfers heat using the work. Compared to steam refrigerators, thermoacoustic refrigerators do not have ozone coolers or toxic coolants and little or no moving parts therefore do not require dynamic sealing or lubrication.
Video Thermoacoustic heat engine
Operation
Device overview
Thermocouple devices consist essentially of heat exchangers, resonators, and piles (on stand-up devices) or regenerators (on a running wave device). Depending on the type of machine, the driver or loudspeaker may be used also to generate sound waves.
Consider a closed tube at both ends. Interference can occur between two waves moving in the opposite direction at a certain frequency. Interference causes resonance to create standing waves. Resonance occurs only at a certain frequency called resonance frequency, and this is mainly determined by the length of the resonator.
The stack is a section consisting of small parallel channels. When the stack is placed at a specific location in the resonator, while having a standing wave on the resonator, the temperature difference can be measured throughout the pile. By placing heat exchangers on each side of the pile, heat can be moved. The reverse is also possible, by creating a temperature difference in the stack, sound waves can be induced. The first example is a simple heat pump, while the second is the prime mover.
Heat pumping
In order to make or move heat, work must be done, and acoustic strength provides this work. When the stack is placed inside the resonator, there is a decrease in pressure. The interference between incoming and reflected waves is now imperfect as there is a difference in the amplitude that causes the standing waves to travel small, giving the wave acoustic power.
In an acoustic wave, the adiabatically packed gas packs and expands. Pressure and temperature change simultaneously; when the pressure reaches maximum or minimum, so does the temperature. Heat pumping along the pile in standing waveform devices can now be described using the Brayton cycle.
Below is a counter-clockwise Brayton cycle consisting of four processes for a refrigerator when a gas package is followed between two stack plates.
- Adiabatic compression of gas. When the gas parcel is moved from the far right to the leftmost position, the adiabatic package is compressed and thus the temperature increases. In the leftmost position, the package now has a higher temperature than the warm plate.
- Isobaric heat transfer. The parcel temperature is higher than the plate temperature causing it to transfer heat to the plate at a constant pressure drop temperature.
- Adiabatic gas expansion. The gas is moved back from the leftmost position to the far right position and due to adiabatic expansion, the gas is cooled to a lower temperature than the cold plate./li>
- Isobaric heat transfer. The parcel temperature is now lower than the plate temperature causing heat to be moved from the cold plate to the gas at constant pressure, increasing the parcel temperature back to its original value.
The travel wave device can be explained using the Stirling cycle.
Temperature gradient
A machine and heat pump usually use a stack and heat exchanger. The boundary between the main drive and the heat pump is provided by the temperature gradient operator, which is the average temperature gradient divided by the critical temperature gradient.
Gradien suhu rata-rata adalah perbedaan suhu di tumpukan dibagi dengan panjang tumpukan.
The critical temperature gradient is a value that depends on certain characteristics of the device such as frequency, cross-sectional area and gas properties.
If the temperature gradient operator exceeds one, the average temperature gradient is greater than the critical temperature gradient and the pile operates as the prime mover. If the gradient operator temperature is less than one, the average temperature gradient is smaller than the critical gradient and the pile operates as a heat pump.
Theoretical efficiency
In thermodynamics, the highest efficiency that can be achieved is the efficiency of Carnot. The efficiency of a thermoacoustic machine can be compared with Carnot's efficiency using a temperature gradient operator.
Efisiensi mesin thermoacoustic diberikan oleh
Koefisien kinerja pompa panas thermoacoustic diberikan oleh
Maps Thermoacoustic heat engine
Derivasi
Using the Navier-Stokes equation for liquids, Rott was able to derive special equations for thermoacoustics. Swift continues with this equation, lowering the expression for acoustic strength in thermoacoustic devices.
Efficiency in practice
The most efficient thermocouple devices built to date have efficiencies close to 40% of the Carnot limit, or about 20% to 30% overall (depending on the temperature of the heat engine).
High thermal temperatures can be made possible with thermosacoustic devices because there are no moving parts, thus allowing Carnot's efficiency to be higher. This can offset some of the lower efficiency, compared to conventional heat engines, as a percentage of Carnot.
The ideal Stirling cycle, approximated by a running wave device, is inherently more efficient than the ideal Brayton cycle, approached by a standing wave device. However, narrow pores are required to provide good thermal contact in the traveling wave regenerator, compared to standing standing piles that require imperfect thermal contact, also causing greater friction losses, reducing practical engine efficiency. Toroidal geometry is often used in current wave devices, but not required for stand-standing devices, it can also cause losses due to Gedeon's flow around the loop.
Researching in thermoacoustics
Modern research and development of thermoacoustic systems is largely based on the work of Rott (1980) and later Greg Swift (1988), in which linear thermo- lustic models were developed to form basic quantitative understandings, and numerical models for computation. Commercial interest has resulted in niche applications such as small to medium-sized cryogenic applications.
History
The history of the thermoacoustic hot air machine began around 1887, when Lord Rayleigh discussed the possibility of pumping heat with sound. Further research occurred less until Rott's work in 1969.
A very simple thermoacoustic thermal air machine is a Rijke tube that converts heat into acoustic energy. But this device uses natural convection.
Latest research
Orest Symko at the University of Utah started a research project in 2005 called Thermal Acoustic Piezo Energy Conversion (TAPEC).
Scores Ltd earned Ã, Â £ 2M in March 2007 to research cooking stoves that will also provide electricity and cooling using Thermo-acoustic effects for use in developing countries.
A radioisotope-heated thermoacoustic system has been proposed and made prototype for an inner space exploration mission by Airbus. This system has slightly theoretical advantages over other generator systems such as existing thermocouple-based systems, or proposed Stirling engines used in ASRG prototypes.
See also
- SASER, Sound Amplification by Stimulation Radiation Emission
References
Further reading
External links
- Los Alamos National Laboratory, New Mexico, USA
- Penn State University, USA
- Sound Power, American Scientist Online
- Thermoacoustics at University of Adelaide, Australia, web archive backup: Discussion Forum
- The University of Adelaide
- Hear that? Refrigerators Are Cooling, Wired Magazine Articles
- "Experiments on a Standing-Thermoacoustic Machine"
Source of the article : Wikipedia
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