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Thermo Acoustic Refrigeration


Published on Jan 10, 2016

Abstract

Thermo acoustic have been known for over years but the use of this phenomenon to develop engines and pumps is fairly recent. Thermo acoustic refrigeration is one such phenomenon that uses high intensity sound waves in a pressurized gas tube to pump heat from one place to other to produce refrigeration effect. In this type of refrigeration all sorts of conventional refrigerants are eliminated and sound waves take their place.

All we need is a loud speaker and an acoustically insulated tube. Also this system completely eliminates the need for lubricants and results in 40% less energy consumption. Thermo acoustic heat engines have the advantage of operating with inert gases and with little or no moving parts, making them highly efficient ideal candidate for environmentally-safe refrigeration with almost zero maintenance cost. Now we will look into a thermo acoustic refrigerator, its principle and functions .

Basic Functioning

In a nut shell, a thermo acoustic engine converts heat from a high-temperature source into acoustic power while rejecting waste heat to a low temperature sink. A thermo acoustic refrigerator does the opposite, using acoustic power to pump heat from a cool source to a hot sink. These devices perform best when they employ noble gases as their thermodynamic working fluids. Unlike the chemicals used in refrigeration over the years, such gases are both nontoxic and environmentally benign. Another appealing feature of thermo acoustics is that one can easily flange an engine onto a refrigerator, creating a heat powered cooler with no moving parts at all.

The principle can be imagined as a loud speaker creating high amplitude sound waves that can compress refrigerant allowing heat absorption. The researches have exploited the fact that sound waves travel by compressing and expanding the gas they are generated in.

Suppose that the above said wave is traveling through a tube. Now, a temperature gradient can be generated by putting a stack of plates in the right place in the tube, in which sound waves are bouncing around. Some plates in the stack will get hotter while the others get colder. All it takes to make a refrigerator out of this is to attach heat exchangers to the end of these stacks.

It is interesting to note that humans feel pain when they hear sound above 120 decibels, while in this system sound may reach amplitudes of 173 decibels. But even if the fridge is to crack open, the sound will not be escaping to outside environment, since this intense noise can only be generated inside the pressurized gas locked inside the cooling system. It is worth noting that, prototypes of the technology has been built and one has even flown inside a space shuttle.

Thermo acoustic refrigerators now under development use sound waves strong enough to make your hair catch fire, says inventor Steven L Garrett. But this noise is safely contained in a pressurized tube. If the tube gets shattered, the noise would instantly dissipate to harmless levels. Because it conducts heat, such intense acoustic power is a clean, dependable replacement for cooling systems that use ozone destroying chlorofluorocarbons (CFCs). Now a scientist Hofler is also developing super cold cryocoolers capable of temperatures as low as -135°F (180°K). he hopes to achieve -243°F (120°K) because such cryogenic temperatures would keep electronic components cool in space or speed the function of new microprocessors.

THERMO ACOUSTIC EFFECT

Acoustic or sound waves can be utilized to produce cooling. The pressure variations in the acoustic wave are accompanied by temperature variations due to compressions and expansions of the gas. For a single medium, the average temperature at a certain location does not change. When a second medium is present in the form of a solid wall, heat is exchanged with the wall. An expanded gas parcel will take heat from the wall, while a compressed parcel will reject heat to the wall.

As expansion and compression in an acoustic wave are inherently associated with a displacement, a net transport of heat results. To fix the direction of heat flow, a standing wave pattern is generated in an acoustic resonator. The reverse effect also exists: when a large enough temperature gradient is imposed to the wall, net heat is absorbed and an acoustic wave is generated, so that heat is converted to work.

The principle may find applications in practical refrigerators, providing cooling, heat engines providing heat or power generators providing work. A great advantage of the technique is that there are no or only one moving part, in the cold area, which results in high reliability and low vibration levels. Also the use of inert gases make them environmentally safe and hence more in demand.

FUNCTIONING IN DETAIL

Thermo acoustic refrigerators now under development use sound waves strong enough to make your hair catch fire, says inventor Steven L Garrett. But this noise is safely contained in a pressurized tube. If the tube gets shattered, the noise would instantly dissipate to harmless levels. Because it conducts heat, such intense acoustic power is a clean, dependable replacement for cooling systems that use ozone destroying chlorofluorocarbons (CFCs). Now a scientist Hofler is also developing super cold cryocoolers capable of temperatures as low as -135˚F (180˚K). he hopes to achieve -243˚F (120˚K) because such cryogenic temperatures would keep electronic components cool in space or speed the function of new microprocessors.

The interaction between heat and sound has been underestimated even by Sir Isaac Newton. This became clear, when Laplace corrected Newton’s earlier calculation of the speed of sound in air. Newton had assumed the expansions and compressions of a sound wave in a gas happen without affecting the temperature. Laplace accounted for slight variations in temperature that in fact take place, and by doing so he derived the correct speed of sound in air, a value that is 18% faster than Newton’s estimate.

Thermo Acoustic Refrigeration

A thermo acoustic refrigerator functions as follows. First, customized loudspeakers are attached to cylindrical chambers filled with inert, pressurized gases such as xenon and helium. At the opposite end of the tubes are tightly wound "jelly rolls" made of plastic film glued to ordinary fishing line. When the loudspeakers blast sound at 180 decibels, an acoustic wave resonates in the chambers. As gas molecules begin dancing frantically in response to the sound, they are compressed and heated, with temperatures reaching a peak at the thickest point of the acoustic wave. That's where the super hot gas molecules crash into the plastic rolls. After transferring their heat to the stack, the sound wave causes the molecules to expand and cool. "Each one of these oscillating molecules acts as a member of a 'bucket brigade,' carrying heat toward the source of the sound," says Garrett. Cold temperatures can then be tapped for chilling refrigerators, bedrooms, cars, or electronic components on satellites and inside computers, according to Garrett. Someday, he says, turning up the air-conditioner could be accomplished by adjusting a volume-control knob














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