Abstract – A thermo
acoustic refrigeration system is one of the harmless types of refrigeration
system, which offers a wide range of scope for further research. Some key advantages
include no emission of harmful ozone depleting gases like CFCs and Freon and
the presence of no moving parts. This field is gathering the attention of many
researchers as it combines both the disciplines of thermal and acoustics.
Researchers have found the influence of various parameters of the components,
the working fluid, and the geometry of the resonator on the performance of the
device. Simulations using software also being developed from time to time. The
main objective of the paper is to present a detailed overview on the
arrangement and functioning of the refrigeration system using high intensity
sound waves.


Key Words:
Acoustics, Refrigeration, Resonator, CFC, Freon. 

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creating comfortable home environments to manufacturing fast and efficient
electronic devices, air conditioning and refrigeration remain expensive yet
essential, services for both homes and industries. However, in an age of
impending energy and environmental crises, current cooling technologies
continue to generate greenhouse gases with high energy costs.


1.1  Working


acoustic refrigeration is an innovative alternative for cooling that is both
clean and inexpensive. Through the construction of a functional model, we will
demonstrate the effectiveness of thermo acoustics for modern cooling.
Refrigeration relies on two major thermodynamic principles. First, a fluid’s
temperature rises when compressed and falls when expanded. Second, when two
substances are placed in direct contact, heat will flow from the hotter
substance to the cooler one. While conventional refrigerators rely on pumps to
transfer heat on a macroscopic scale, thermo acoustic refrigerators depends on
sound to generate waves of pressure that alternately compress and relax the gas
particles within the tube. Although the model constructed for this research
project does not achieve the original goal of refrigeration, the experiment
suggests that thermo acoustic refrigerators could one day be viable
replacements for conventional refrigerators.


1.2  Principle


acoustics is based on the principle that sound waves are pressure waves. These
sound waves propagate through the air via molecular collisions which causes a
disturbance in the air thereby creating constructive and destructive
interference. The constructive interference compresses the air molecules while
the destructive interference expands them. This principle is the basis of
thermo acoustic refrigerator.

          One method to control these pressure
disturbances is with standing waves. These waves are natural phenomena
exhibited by any wave in a closed tube. When the incident and reflected waves
overlap they interfere constructively producing a single wave form. This wave
causes vibration in the isolated sections. These waves form nodes and
antinodes. The maximum compression of air occurs at antinodes. Due to this
antinode property standing waves are useful as only a small input power is
required to produce a large amplitude wave which has enough energy to cause a
visible thermo acoustic effect.




Thermo acoustics combines the branches of acoustics and
thermodynamics together to move heat by using sound. While acoustics is
primarily concerned with the macroscopic effects of sound transfer like coupled
pressure and motion oscillation, thermos acoustics focuses on the microscopic
temperature oscillations that accompany these pressure changes. Thermos
acoustics take advantage of these pressure oscillations to move heat on
microscopic level. This results in a large temperature difference between the
hot and cold sides of the device and causes refrigeration.

The most important part of this device is the stack. The stack
consists of a large number of closely spaced surfaces that are aligned parallel
to the resonator tube. The purpose of the stack is to provide a medium for heat
transfer as the sound wave oscillates through the resonator tube. The purpose
of the stack is to provide a medium where the walls are close enough so that
each time as a packet of gas move, the temperature differential is transferred
to the wall of stack.

2.1 Thermo Acoustic Cycle

The cycle by which heat transfer occurs is similar to the Stirling
cycle. The figure traces the basic thermos acoustic cycle for a packet of gas,
a collection of gas molecules that act and move together. Starting from point
1, the packet of gas compressed and moves to the left. As the packet is
compressed, the sound wave does work on the packet of gas, providing the power
for the refrigerator. When the gas packet is at the maximum compression, the
gas ejects the heat back into the stack since the temperature of the gas is now
higher than the temperature of the stack. This phase is the refrigeration part
of the cycle, moving the heat farther from the bottom of the tube.

The second phase of the cycle, the gas is returned to the
initial state. As the gas packet moves backward towards the right, the sound
waves expands the gas. Although some work is expended to return the gas to the
initial state, the heat released on the top of the stack is greater than the
work expended to return the gas to the initial state. This process results in a
net transfer of heat to the left side of the stack. Finally, in the 4th
step the gas packets of gas reabsorb heat from the cold reservoir to repeat the
heat transfer process.

2.2 Penetration Depth

The ideal spacing in a stack is thermal penetration depth. The
thermal penetration depth is the distance heat can diffuse in a gas over a
certain amount of time. For example, if a block of aluminium is at a constant low
temperature and suddenly one side is exposed to a high temperature, the
distance that the heat penetrates the metal in one second is the heat
penetration. As the time passes, the heat penetrates farther into the material,
increasing the temperature of the interior section

The thermal penetration depth for an oscillating heat source
is a function of the frequency of the standing wave ƒ, the thermal conductivity
?, and density ?, of the gas, as well as the isobaric specific heat per unit
mass of the gas cp,
according to the equation.


2.2 Critical Temperature

The critical temperature is the temperature at which heat will
be transferred through the stack. If the temperature difference induced by the
sound wave is greater than this critical temperature, the stack will function
as a refrigerator, transferring heat from the cold end of the tube to the warm
end. if the temperature is less than the critical temperature then the stack
will function as an acoustic engine, moving heat from the warm region to the
colder region and creating sound waves. This temperature is important in
determining the properties of a thermos acoustic device, since efficiency
depends on a temperature differential caused by the sound waves that is larger
than the critical temperature so that a large cooling effect is created.




The main components used


1.       Resonator

2.       Stack

3.       Speaker

4.       Amplifier

5.       Plexi Glass

6.       Aluminium
Stopper Film

7.       Temperature Sensor


The 2D drawing of the
thermo acoustic refrigerator is shown in the figure above and the components of
the control unit is shown in the figure below.




Our device worked as a proof of concept device showing that a
thermo acoustic device is possible and is able to cool air, but only for a
short period of time. If we were able to build the device with better
materials, such as a more insulated tube, we might have been able to get better
results. In order to create a working refrigerator we probably would have to
attach a heat sink to the top of the device, thus, allowing the excess heat to
dissipate to the surroundings. However, our device did demonstrate that thermo
acoustic device have the ability to create and maintain large temperature
gradient, more than 20 degrees Centigrade, which would be useful as a heat




We thank Mr. Sreejith K,
who is the professor in mechanical engineering at Jyothi Engineering College
Cheruthuruthy, for his guidance and support. We are also thankful to the entire
Mechanical Engineering Department of Jyothi Engineering College for their
suggestions and help.




Standing Waves, Rod Nave, Georgia State
University.                                              Available:http://hyperphysics.phyastr.gsu.edu/hbase/waves/standw.html 17 July 2006





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