Abstract – A thermoacoustic refrigeration system is one of the harmless types of refrigerationsystem, which offers a wide range of scope for further research. Some key advantagesinclude no emission of harmful ozone depleting gases like CFCs and Freon andthe presence of no moving parts. This field is gathering the attention of manyresearchers 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 thedevice. Simulations using software also being developed from time to time. Themain objective of the paper is to present a detailed overview on thearrangement and functioning of the refrigeration system using high intensitysound waves. Key Words:Acoustics, Refrigeration, Resonator, CFC, Freon. 1. INTRODUCTION Fromcreating comfortable home environments to manufacturing fast and efficientelectronic devices, air conditioning and refrigeration remain expensive yetessential, services for both homes and industries. However, in an age ofimpending energy and environmental crises, current cooling technologiescontinue to generate greenhouse gases with high energy costs.
1.1 Working Thermoacoustic refrigeration is an innovative alternative for cooling that is bothclean and inexpensive. Through the construction of a functional model, we willdemonstrate the effectiveness of thermo acoustics for modern cooling.Refrigeration relies on two major thermodynamic principles. First, a fluid’stemperature rises when compressed and falls when expanded.
Second, when twosubstances are placed in direct contact, heat will flow from the hottersubstance to the cooler one. While conventional refrigerators rely on pumps totransfer heat on a macroscopic scale, thermo acoustic refrigerators depends onsound to generate waves of pressure that alternately compress and relax the gasparticles within the tube. Although the model constructed for this researchproject does not achieve the original goal of refrigeration, the experimentsuggests that thermo acoustic refrigerators could one day be viablereplacements for conventional refrigerators.
1.2 Principle Thermoacoustics is based on the principle that sound waves are pressure waves. Thesesound waves propagate through the air via molecular collisions which causes adisturbance in the air thereby creating constructive and destructiveinterference. The constructive interference compresses the air molecules whilethe destructive interference expands them.
This principle is the basis ofthermo acoustic refrigerator. One method to control these pressuredisturbances is with standing waves. These waves are natural phenomenaexhibited by any wave in a closed tube. When the incident and reflected wavesoverlap they interfere constructively producing a single wave form. This wavecauses vibration in the isolated sections.
These waves form nodes andantinodes. The maximum compression of air occurs at antinodes. Due to thisantinode property standing waves are useful as only a small input power isrequired to produce a large amplitude wave which has enough energy to cause avisible thermo acoustic effect. 2. THERMOACOUSTICS Thermo acoustics combines the branches of acoustics andthermodynamics together to move heat by using sound. While acoustics isprimarily concerned with the macroscopic effects of sound transfer like coupledpressure and motion oscillation, thermos acoustics focuses on the microscopictemperature oscillations that accompany these pressure changes. Thermosacoustics take advantage of these pressure oscillations to move heat onmicroscopic level.
This results in a large temperature difference between thehot and cold sides of the device and causes refrigeration.The most important part of this device is the stack. The stackconsists of a large number of closely spaced surfaces that are aligned parallelto the resonator tube. The purpose of the stack is to provide a medium for heattransfer as the sound wave oscillates through the resonator tube. The purposeof the stack is to provide a medium where the walls are close enough so thateach time as a packet of gas move, the temperature differential is transferredto the wall of stack.2.1 Thermo Acoustic CycleThe cycle by which heat transfer occurs is similar to the Stirlingcycle.
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 point1, the packet of gas compressed and moves to the left. As the packet iscompressed, the sound wave does work on the packet of gas, providing the powerfor the refrigerator. When the gas packet is at the maximum compression, thegas ejects the heat back into the stack since the temperature of the gas is nowhigher than the temperature of the stack.
This phase is the refrigeration partof the cycle, moving the heat farther from the bottom of the tube.The second phase of the cycle, the gas is returned to theinitial state. As the gas packet moves backward towards the right, the soundwaves expands the gas.
Although some work is expended to return the gas to theinitial state, the heat released on the top of the stack is greater than thework expended to return the gas to the initial state. This process results in anet transfer of heat to the left side of the stack. Finally, in the 4thstep the gas packets of gas reabsorb heat from the cold reservoir to repeat theheat transfer process.2.2 Penetration DepthThe ideal spacing in a stack is thermal penetration depth.
Thethermal penetration depth is the distance heat can diffuse in a gas over acertain amount of time. For example, if a block of aluminium is at a constant lowtemperature and suddenly one side is exposed to a high temperature, thedistance that the heat penetrates the metal in one second is the heatpenetration. As the time passes, the heat penetrates farther into the material,increasing the temperature of the interior sectionThe thermal penetration depth for an oscillating heat sourceis 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 unitmass of the gas cp,according to the equation. 0.52.2 Critical TemperatureThe critical temperature is the temperature at which heat willbe transferred through the stack. If the temperature difference induced by thesound wave is greater than this critical temperature, the stack will functionas a refrigerator, transferring heat from the cold end of the tube to the warmend. if the temperature is less than the critical temperature then the stackwill function as an acoustic engine, moving heat from the warm region to thecolder region and creating sound waves.
This temperature is important indetermining the properties of a thermos acoustic device, since efficiencydepends on a temperature differential caused by the sound waves that is largerthan the critical temperature so that a large cooling effect is created. 3. COMPONENTS AND2D DRAWING The main components usedare, 1. Resonator2. Stack3. Speaker4.
Amplifier5. Plexi GlassTube6. AluminiumStopper Film7.
Temperature Sensor The 2D drawing of thethermo acoustic refrigerator is shown in the figure above and the components ofthe control unit is shown in the figure below. 4. CONCLUSIONS Our device worked as a proof of concept device showing that athermo acoustic device is possible and is able to cool air, but only for ashort period of time. If we were able to build the device with bettermaterials, such as a more insulated tube, we might have been able to get betterresults.
In order to create a working refrigerator we probably would have toattach a heat sink to the top of the device, thus, allowing the excess heat todissipate to the surroundings. However, our device did demonstrate that thermoacoustic device have the ability to create and maintain large temperaturegradient, more than 20 degrees Centigrade, which would be useful as a heatpump. ACKNOWLEDGEMENT We thank Mr. Sreejith K,who is the professor in mechanical engineering at Jyothi Engineering CollegeCheruthuruthy, for his guidance and support.
We are also thankful to the entireMechanical Engineering Department of Jyothi Engineering College for theirsuggestions and help. REFERENCES 1. Standing Waves, Rod Nave, Georgia StateUniversity. Available:http://hyperphysics.phyastr.gsu.
edu/hbase/waves/standw.html 17 July 2006 2. http://hyperphysics.phy-astr.gsu.