6 thermoacoustic refrigeration

SEMINAR REPORT 2013 THERMOACOUSTIC REFRIGERATION

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CHAPTER 1

INTRODUCTION

From creating comfortable home environment to manufacturing fast and efficient

electronic devices, air conditioning and refrigeration remain essential services for both

homes and industries.

It is becoming increasingly important in the design and development of

refrigerating systems to consider environmental impacts. To eliminate the use of

environmentally hazardous refrigerants, research efforts are focussing more on the

development of alternative refrigerants and alternative refrigeration technologies. An

approach in the category of alternative technologies is thermoacoustic refrigeration

which produces cooling from sound.

The thermoacoustic effect was first discovered in the 19th

century when heat

driven acoustic oscillations were observed in open-ended glass tubes. These devices were

the first thermoacoustic engines, consisting of a bulb attached to a long narrow tube. It

was in the 1980’s that thermoacoustic refrigerator was first developed, when a research

group at the Los Alamos National Laboratory showed that the effect could be used to

pump heat. The technology has seen rapid growth since then, developing it to a

promising asset as a clean and environmentally friendly refrigeration method.

1.1 LITERATURE SURVEY

Emmanuel c. Nsofor and Azrai Ali (2009) studied on the performance of the

thermoacoustic refrigerating system with respect to some critical operating parameters.

Experiments were performed on the system under various operating conditions. The

experimental setup consists of the thermoacoustic refrigerating system with appropriate

valves for the desired controls, instrumentation and the electronic data acquisition

system. The resonator was constructed from aluminium tubing but it had plastic tube

lining on the inside to reduce heat loss by conduction. Significant factors that influence

the performance of the system were identified. The cooling produced increases with the

temperature difference between the two ends of the stack. High pressure in the system


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does not necessarily result in a higher cooling load. There exists an optimum pressure

and an optimum frequency for which the system should be operated in order to obtain

maximum cooling load. Consequently, for the thermoacoustic refrigeration system, there

should be a related compromise between cooling load, pressure and frequency for best

performance.

Ramesh Nayak.B. et al. (2011) proposed the design of a Thermo Acoustic

Refrigerator (TAR) stack. The design strategy has been described along with the values

of the important working gas parameters as well as the non-dimensional parameters. The

design and optimisation of thermo acoustic refrigerator for a cooling power of 10 watt

was designed. An attempt has been made to design the TAR by using CATIA. Further

modelling and optimization of the design is carried out using MATLAB.

Jonathan Newman et al. (2006) explored the basic principles of thermoacoustic

refrigeration, to produce a small thermoacoustic refrigerator out of readily available

parts. The model constructed for this research project employed inexpensive, household

materials. Although the model did not achieve the original goal of refrigeration, the

experiment suggests that thermoacoustic refrigerators could one day be viable

replacements for conventional refrigerators.


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CHAPTER 2

PRINCIPLE

Thermoacoustics is based on the principle that sound waves are pressure waves.

These sound waves propagate through air via molecular collisions. The molecular

collision cause a disturbance in the air, which in turn creates constructive and destructive

interference. The constructive interference makes the molecules compress, and the

destructive interference makes the molecules expand. This principle is the basis behind

the thermoacoustic refrigerator.

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.

There are two types of thermoacoustic devices namely thermoacoustic engine and

thermoacoustic refrigerator. In a thermoacoustic engine, heat is converted into sound

energy and this energy is available for the useful work. In a thermoacoustic refrigerator

the reverse process occurs, i.e. it utilises work in the form of acoustic pewr to absorb heat

from a low temperature medium and reject it to a high temperature medium.


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CHAPTER 3

THERMOACOUSTIC EFFECT

Acoustic waves experience displacement oscillations, and temperature

oscillations in association with the pressure variations. In order to produce

thermoacoustic effect these oscillations in the gas should occur close to a solid

surface so that heat can be transferred to or from the surface. A stack of closely

spaced parallel plates is placed inside the thermoacoustic device in order to provide

such a solid surface. The thermoacoustic phenomenon occurs by the interaction of the

gas particles and the stack plate. When large temperature gradients are created across

the stack, sound waves are generated i.e. work is produced in the form of acoustic

power(forming a thermoacoustic engine). In the reverse case, the acoustic work is

used in order to create temperature gradients across the stack, which is used to

transfer heat from a low temperature medium to a high temperature medium(as the

case of thermoacoustic refrigerator).

A thermoacoustic refrigerator consists of a tube filled with a gas. This tube is

closed at one end and an oscillating device(a loud speaker) is placed at the other end

to create an acoustic standing wave inside the tube. Standing waves are natural

phenomena exhibited by sound waves. In a closed tube, columns of air demonstrate

these patterns as sound waves reflect back on themselves after colliding with the end

of the tube. When the incident and reflected waves overlap, they interfere

constructively, producing a single waveform. This wave cause the medium to vibrate

in isolated sections as the travelling waves are masked by the interference. Therefore

these standing waves seem to vibrate in constant position and orientation around

stationary nodes. These nodes are located where the two component sound waves

interfere to create areas of zero net displacement. The areas of maximum net

displacement are located halfway between two nodes and are called antinodes. The

maximum compression of the air also occurs at the antinode. Due to these node and

antinode properties, standing waves are useful because only a small input of power is

needed to create a large amplitude wave to cause thermoacoustic effect.


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CHAPTER 4

BASIC CONSIDERATIONS

4.1 THERMODYNAMIC CONSIDERATION

A thermoacoustic device consists of an acoustic driver attached to an acoustic

resonator tube filled with the working fluid. Inside the resonator tube, a stack of thin

parallel plates and two heat exchangers(hot and cold) are installed for the heat

transfer. The schematic of a typical thermoacoustic device is shown in fig.

Fig1 (a)Schematic of a thermoacoustic refrigerator,(b)velocity and pressure variation

across the resonance tube, (c)temperature variation across the resonance tube,

(thesis,Concordia university)

The acoustic driver, connected to one end of the resonator tube, excites the working

fluid and creates a standing wave inside the tube. Hence the gas oscillates inside the

resonator with expansions and compressions. The length of the resonator tube is

typically set equal to one-half of the wavelength of the standing wave, i.e.


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The standing wave creates velocity nodes at the two ends of the tube and a

pressure node at the middle of the tube as in the fig . if a stack of parallel plates is

placed inside the tube, the gas will be at a higher pressure at the end of the stack,

which is closer to the end of the tube(i.e. left side of the stack in fig), than the other

end of the stack. This high pressure results in an increase in the temperature of the gas

and the excess heat is transferred to the stack, causing an increase in the temperature

of the stack at that end and an average longitudinal temperature gradient along the

stack is established.

4.2 ACOUSTIC THEORY

The understanding of acoustic wave dynamics, i.e. the pressure and velocity

fields created by an acoustic wave, is necessary to understand the working of a

thermoacoustic device. The acoustical theory deals with the study of the longitudinal

acoustic waves. The longitudinal acoustic waves are generated as a result of the

compression, and expansion of the gas medium. The compression of a gas

corresponds to the crust of a sine wave, and the expansion corresponds to the trough

of a sine wave. An example of how these two relate to each other is shown in the

figure.

In a longitudinal wave, the particle displacement is parallel to the direction of

wave propagation i.e. they simply oscillate back and forth about their respective

equilibrium position. The compression and expansion of a longitudinal wave result in

the variation of pressure along its longitudinal axis of oscillation. A longitudinal wave

requires a material medium such as air or water to travel. That is, they cannot be

generated and/or transmitted in a vaccum. All sound(acoustic)waves are longitudinal

waves and therefore, hold all the properties of the longitudinal waves discussed

above. Three characteristics of the acoustic waves are necessary for the understanding

of the thermoacoustic process. These properties are amplitude, frequency and

wavelength.


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Fig2. Comparison of a longitudinal acoustic wave with a sine wave (thesis, Concordia

university)

The displacement of a wave from its equilibrium position is called the wave

amplitude. It is also a measure of the wave energy. Larger the amplitude, higher will

be the wave energy. Thus, the energy of an acoustic wave can be estimated by

measuring its amplitude. The energy or intensity of an acoustic wave is measured in

terms of decibel. If the given acoustic wave is comprised of the superposition of

different sine waves, then the amplitude and hence the energy of the given wave can

be estimated by integrating the energy in all the frequency components of the given

wave. The time period of a wave is the time required for the complete passage of a

wave at a given point. The fundamental wave frequency is the inverse of the time

period. In other words, it is the number of waves that pass a given point in a unit time.


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It is measured in hertz(Hz), i.e. the number of waves that pass a given point in one

second.

The wavelength is defined as the horizontal distance from the beginning of the

wave to the end of the wave. It can also be measured as the distance from one wave

crest to the next wave crust, or one wave trough to the next wave trough. In acoustics,

we can define wavelength as the distance between the two successive compressions

or expansions.

The compression and expansion of an acoustic wave result in pressure

variations along the waveform. This pressure variation is the key process that causes

the thermoacoustic phenomenon. These pressure variations can also be used to

estimate the sound intensity.

From the ideal gas equation of state,

= RT

where P is the pressure, is the density, T is the absolute temperature, and R is the

universal gas constant. The above equation indicates that if the density variations are

very small, the change in pressure causes a change in temperature. That is, an

increase in pressure causes an increase in temperature and vice versa.


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CHAPTER 5

BASIC COMPONENTS

A thermoacoustic machine generally consists of:

1. Acoustic driver

2. Stack or regenerator

3. Heat exchanger

4. Resonator

5.1 ACOUSTIC DRIVER

The purpose of the loudspeaker is to supply work to the system in the form of sound

waves.

Fig 3 loudspeaker(wikipaedia)

5.2 STACK

In the thermoacoustic refrigerator the stack is the main component where the

thermoacoustic phenomenon takes place. Below shown are two stacks of different

materials used in a standing wave thermo acoustic refrigerator.

The stack material must have a low thermal conductivity and a heat capacity larger

than the capacity of the working gas, in order that the temperature of the stack plates

is steady.


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(a)

(b)

Fig 4. (a)ceramic stack, (b)glass tubing stack(wikibooks)

5.3 HEAT EXCHANGER

The heat exchangers employed in a thermoacoustic refrigerator influence the acoustic

field created in the resonator. There are many design constraints such as porosity of

the heat exchanger and high heat transfer coefficient for efficiency. Due to these

constraints, special kind of heat exchangers are used. One typical micro channel

aluminum heat exchanger is shown below.

http://en.wikibooks.org/wiki/File:Ceramic2.JPG

http://en.wikibooks.org/wiki/File:Stack2.JPG


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Fig 5 aluminium heat exchanger(wikibooks)

5.4 RESONATOR

This part of refrigerator which is only there for maintaining the acoustic wave.

Because it is a dead volume which causes heat loss and adds bulk, quarter wavelength

resonators are preferred over half wavelength.

http://en.wikibooks.org/wiki/File:Hx1.jpg


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CHAPTER 6

WORKING

Fig 6 P-V diagram (GSET Research Journal 2006)

Starting from point 1, the packet of gas is 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 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.

In the second phase of the cycle, the gas is returned to the initial state. As the

gas packet moves back towards the right, the sound wave expands the gas. This

process results in a net transfer of heat to the left side of the stack. Finally, in step 4,

the packets of gas reabsorb heat from the cold reservoir to repeat the heat transfer

process.


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CHAPTER 7

EXPERIMENTAL STUDY AND RESULTS

7.1 INFLUENCING FACTORS

1.Penetration Depth

The thermal penetration depth corresponds to the thickness of the layer around

the stack plate through which the heat can diffuse during a complete oscillating cycle

of a gas parcel. It is defined as

=

where K is thermal conductivity of the gas, Cp is specific heat per unit mass at

constant pressure, is the density of gas at the mean temperature(Tm), and is the

angular velocity.

At a distance greater than the thermal penetration depth from the plate, the gas

does not feel any thermal effects of the plate. In other words, the heat exchange

between the gas parcel and the stack plate occurs only at a distance less than the

penetration depth from the stack plate.

The optimal value for spacing between the stack layers is to .

2. Viscous Depth

The viscous penetration depth is the thickness of the layer around the stack

plate where the viscous effects are significant. It is defined as

=

where is dynamic viscosity of the gas.

The viscous effects are not desirable for the thermoacoustic process. The

viscous effect decreases as the distance from the solid boundary i.e. the stack plate

increases. However, away from the solid boundary, the thermal contact between the

gas and the stack plate decreases which reduces the thermoacoustic heat transfer.


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3.Mean pressure

Since the power density in a thermoacoustic device is proportional to the

average pressure pm, it is favourable to choose pm as large as possible. This is

determined by the mechanical strength of the resonator. On the other hand, k is

inversely proportional to square root of pm, so a high pressure results in a small k

and a small stack plate spacing. This makes the construction difficult.

4.Drive ratio

It is the ratio of the dynamic pressure amplitude to the mean pressure.

D=

5.Normalised stack position

xn= xs

6.Normalised stack length

Lsn= Ls

7.Blockage ratio

B =

where is half the spacing between the stack and l is half the thickness of the stack.


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7.2 EXPERIMENTAL PARAMETERS

7.3EXPERIMENTAL SETUP

The experimental system in general can be broken down into (a) the

thermoacoustic refrigerating system, (b) the test section and (c) the data acquisition

system. There are a number of valves in the system for effecting operations such as

charging and vacuuming the system, water-cooling and controls for running the

experiments.

The refrigerating system consists mainly of the resonator tube or resonator,

the stack, the acoustic driver and the heat exchangers. An electrical resistance heater

arrangement was located at the cold side of the resonator to supply the variable load

for the refrigerating system. An audio generator with frequency range from 10 Hz to

1 MHz was used to produce the sound that was transferred to the amplifier. The

amplified sound is transferred to the acoustic driver which powers the thermoacoustic

system. Fluid inside the resonator interacts with the stack plates which are aligned in

the direction of vibration of the standing waves.


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The test section involves specific parts of the system were the measurements

were made. Fig 7 illustrates the locations of thermocouples in the system. One

thermocouple was installed near the electric heater, one was installed at the surface of

the acoustic driver, two were each installed at the inlet and outlet of the heat

exchanger and one was installed at the middle of the resonator tube. All the

thermocouples installed inside the vacuum vessel (labelled in Fig 7 as vessel

addition), were type-T Teflon insulated. These were capable of measuring from low

temperatures up to 300 ᵒC. Outside the vacuum vessel, the thermocouples used were

type-T Nylon insulated.

The data acquisition system includes thermocouples, pressure transducer,

oscilloscope, flow meter, data acquisition board and a personal computer for the data

display. In Fig 7, T, M and P stand for temperature, mass flow and pressure

connections, respectively


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Fig 7 Experimental setup

Experiments were conducted to investigate the thermal response of the system

under various operating conditions. The mean pressure was set initially at the lowest

pressure of 3 bars to begin the experiments. The desired frequency was selected and

then increased slowly from the minimum to the maximum value for the course of

each of the experiments. The cooling load which was controlled using resistance

heating in place of the cold side heat exchanger was initially set at a constant load.

For each experiment, the data readings were taken from the initial time until

conditions became stable. The frequency was then set at the next level and the

experiment was repeated. After running the experiments for the desired frequency

range, the pressure was adjusted to 4bars and the experiment was repeated for the

same set of frequencies and cooling load. Further experiments were performed for

pressure values of 5 bars and 6 bars. The experiments were also repeated in the same

way for various values of the cooling load from the selected lowest cooling load to

the highest. The results show the ranges of the operational parameters.


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7.4 EXPERIMENTAL RESULT

Fig 8 Temperature-time history(at the hot end of the stack) for constant cooling load

and mean pressure for various frequencies

Fig 9 Temperature-time history(at the hot end of the stack) for constant mean

pressure and frequency and variable load


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Fig 10 Temperature-frequency history(at the hot end of the stack) for constant mean

pressure and variable load

Fig 11 Temperature difference at the ends of the stack at constant frequency verses

mean pressure


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Fig 12 Cooling load verses temperature difference at the ends of the stack at constant

frequency and mean pressure

Fig 13 Temperature difference at the ends of the stack verses frquency for constant

cooling load and mean pressure


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7.5 EXPERIMENTAL CONCLUSION

1.The temperature difference between the hot end and cold end of the stack ranged

from 0ᵒC to 15ᵒC.

2. Cooling load increases with the increase in the temperature difference between the

two ends of the stack.

3. For a thermoacoustic refrigerating system, there exist for a given frequency, an

optimum pressure that results in a higher cooling temperature difference and thus a

higher cooling load. This frequency is the resonance frequency.


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CHAPTER 8

CONCLUSION

Thermoacoustic engines and refrigerators were already being considered a

few years ago for specializer application, where their simplicity, lack of lubrication

and their use of environmentally harmless working fluids were adequate

compensation for their lower efficiencies. This latest breakthrough, coupled with

other developments in the design of high power, single frequency loud speakers and

reciprocating electric generators suggests that thermoacoustics may soon emerge as

an environmentally attractive way to power hybrid electric vehicles, capture solar

energy, refrigerate food, air condition buildings, liquefy industrial gases and serve in

other capacities.

In future let us hope that these thermoacoustic devices which promise to

improve standard of living while helping to protect the planet by completely

eliminating the use of refrigerants.


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REFERENCES

1. Tasnim S.H.,S. Mahmud,R.A.Fraser, “Effects of variation in working fluids and

operating conditions on the performance of a thermoacoustic refrigerator.”

International Communications in Heat and Mass Transfer 39, 2012, 762-768

2. Emmanuel C. Nsofor,Azrai Ali, “Experimental study on the performance of the

thermoacoustic refrigerating system.” Applied Thermal Engineering 29, 2009, 2672-

2679.

3. Jonathan Newman, “Thermoacoustic refrigeration”, GSET Research Journal 2006,

1-8.

4. www.wikipedia.org

6 thermoacoustic refrigeration

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