THERMO-ACOUSTIC REFRIGERATOR

ARPITA ASTHANA (ASSISTANT PROFESSOR)MECHANICAL ENGINEERINGSHRI BALWANT INSTITUTE OF TECHNOLOGY,SONEPAT

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.

KEYWORDS•Thermo-Acoustics, •Sound waves, •Oscillating flow, •Heat Exchangers

INTRODUCTION A thermo-acoustic refrigerator (TAR) is a refrigerator that uses sound waves in order to provide the cooling.

In a TAR, the working fluid is a helium-argon mixture, and the compressor is replaced by a loudspeaker.

‘Thermoacoustic’ or Thermoacoustics explains the relation between heat and sound. It is a science that explains how sound energy can be converted to heat or vice versa, and teaches how to realize the conversion practically by means of special kind of engines and refrigerators.

‘Loudspeaker’ is one of the device used to drive thermoacoustic refrigerator.

‘Refrigeration’ means to remove heat from cold place and to dump it to relatively hot place (thereby further cooling the cold place) by means of some kind of ‘mechanical work’.

Description of Technology•Thermo acoustic refrigeration systems operate by using sound waves and a non-flammable mixture of inert gas (helium, argon, air) or a mixture of gases in a resonator to produce cooling. •Thermo acoustic devices are typically characterized as either ‘standing-wave’ or ‘travelling-wave’. •The main components of standing wave device are a closed cylinder, an acoustic driver, a porous component called a "stack, and two heat-exchanger systems. •Application of acoustic waves through a driver such as a loud speaker, makes the gas resonant. As the gas oscillates back and forth, it creates a temperature difference along the

Constructional Details•Acoustic Driver

•Gas–spring system

•Resonator

•Stack

•Spiral Stack

•Parallel Plate Stack

•Heat Exchangers

Working Principle and Cycle1.Thermo Acoustics

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

2. Thermo Acoustic CycleThe cycle by which heat transfer occurs is similar to the Stirling cycle.

Penetration Depth An essential variable in building a thermo acoustic refrigerator is the spacing between the walls of the stack. The thermal penetration depth is the distance heat can diffuse in a gas over a certain amount of time.

The thermal penetration depth for an oscillating heat source is a function of the frequency of th standing wave, f , the thermal conductivity, k and density, ρ of the gas, as well as the isobaric specific heat per unit mass of the gas, cp , according to the equation: δk =[ k/π.f.ρ.cp]1/2

Critical TemperatureThe critical temperature is the temperature at which no heat will be transferred through the stack. The function for the critical longitudinal temperature gradient is:

Δ Tcrit = p/x.ρ.cp

Sound Waves and Pressure

Thermo acoustics is based on the principle that sound waves are pressure waves. These sound waves propagate through the air via molecular collisions. The equation for the frequency of a wave travelling through a closed tube is given by: f = v/4.L

where f is frequency, v is velocity of the wave, and L is the length of the tube.

Fig.: Shows the relationship between the phase of the wave, the pressure, and the actual arrangement of the molecules. The black line shows the phase of the sound wave, the red shows the pressure and the dots below represent the actual molecules.

Measurement Procedure Measurement of Resonance Frequency

The first thing to determine is the fundamental acoustic resonance

frequency of the refrigerator which is the operation frequency of the system.

The resonance frequency is defined as the resonance for which the phase difference is zero.

Measurement of Electro Acoustic Efficiency

The determination of the electro acoustic efficiency of the driver requires knowledge of the input electrical power and the acoustic output power.

the equations used for the electro acoustic efficiency calculations are the electrical power input to the driver:

Pe = ½.[V.I.cosϴ]

where V is the amplitude of the voltage over the driver, I is the amplitude of the current through the coil, and θ is the phase difference between the voltage and the current.

The acoustic input power is given by:

W = ½ [p.U.cosΨ]

where p is the amplitude of the dynamic pressure, U is the volume velocity amplitude and Ψ is the phase difference between the pressure and the volume velocity.

The electro acoustic efficiency is then given by:

ηea = W/Pe

Advantages 1. The working fluid is typically helium or other inert, benign gases such as air which are environment-friendly unlike common refrigerants.

2. The simplicity of the design makes it robust, small, and lightweight.

3. It has almost no moving parts, which translates into a longer working life with fewer repairs. In turn, this makes the system less expensive.

4. The loudspeaker is TAR’s only moving part which is more durable than a compressor.

5. It has the ability to attain a higher level of the limiting Carnot efficiency than current refrigeration methods.

Disadvantages 1. The downside of the TAR is that these failed to achieve efficiencies as high as those of standard refrigerator units.

2. The coefficient of performance of most advanced TAR is only 1 when compared to 3-4 of modern refrigerators.

3. Another major problem of TAR is that it is either fully on or off.

4. It leaked an incredible amount of sound that causes ear pain but produces only a small temperature gradient.

5. These refrigerators were able to cool the air for a short amount of time before the cooled air started raising its temperature.

Improvements 1. Insulate the sound leaks by isolating the system.

2. Replace the closed cap with a speaker to increase the efficiency by co-generation.

3. If both ends of a stack are connected to a heat exchangers thus coupling the stack to a heat source and sink, the transfer of heat would be more efficient.

4. Use conductive material for hot section of resonator.

5. Widen the resonator and use a cone to reduce the losses due to rapid area change.

6. More practical and efficient reliable temperature sensors such as thermostats should be used.

7. The composition of stack material may also be changed to any conducting materials like gold, silver or copper.

Conclusion In this paper, the manufacturing procedure of a thermo acoustic refrigerator is discussed. The construction of the different parts of the refrigerator is described in detail. The system has been assembled and the first performance measurements have been done. The measurements show that the system behaves very well as expected. A low temperature of -65 0C is achieved. The refrigerator is used to study the effect of some important thermo acoustic parameters, such as the Prandtl number using binary gas mixtures, and the stack plate spacing.

The device worked as a proof of concept device showing that a thermo acoustic device is possible and is able to cool air, but for only a short period of time. If the device is build up with better materials, such has a more insulating tube, better results can be obtained. In order to create a working refrigerator, it is required to attach a heat sink to the top of the device, thus, allowing the excess heat to dissipate to the surroundings. However, this device demonstrate that thermo acoustic device have the ability to create and maintain a large temperature gradient, more than 20 degrees Centigrade, which would be useful as a heat pump or refrigerator.

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