final report

Dublin Institute of Technology

Faculty of Science

Development of a Thermoacoustic Refrigerator

_________________________Richard Duffy

May 2014

Project report submitted in partial fulfilment of

examination requirements leading to the award of

Ordinary degree in Industrial and Environmental Physics

Supervisor: Francis Pedreschi

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Abstract

An inexpensive tabletop thermoacoustic refrigerator for demonstration purposes was built

from a boxed loudspeaker, Perspex tubing and sheet, carbon fibre rods, rubber plug and two

thermocouples. The purpose of a thermoacoustic refrigerator is to cause temperature

variations across the thermoacoustic stack using sound waves of a certain frequency. The

stack is placed in a resonance tube in a specified position to manipulate the sound waves

striking it into oscillating gas parcels inside the stack causing them to transfer heat up the

walls of the stack and give a cooling effect below the stack. Temperature differences of more

than 10 °C were achieved after running the apparatus for several minutes. The efficiency of

the device was increased by introducing an amplifier to the system for more speaker power,

by changing the speaker’s impedance and by placing the stack near the pressure maximum in

the tube. While the model could have been more efficient, and acts more like a heat pump

than a refrigerator, with more of an increase in temperature above the stack than a cooling

effect below the stack, this demonstration creates the temperature gradient needed for a

thermoacoustic refrigerator and the key principles for a thermoacoustic refrigeration system.

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ACKNOWLEDGMENTS

Firstly I would like to thank Dr Francis Pedreschi for his support and guidance throughout the

duration of the project. His enthusiasm and approachability over the 6 weeks made it a

pleasure to have him as my supervisor. To Dr Elizabeth Gregan who supported us all year

and give us great advice to achieve our goals. Thanks to the senior lab technician Joseph

Keogh who sourced the materials for the project and his knowledge of the lab equipment

were crucial in the success of the project. Also my family and friends who helped keep me

motivated with their invaluable support.

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TABLE OF CONTENTS

Abstract (ii)

Acknowledgements (iii)

CHAPTER 1 – General Introduction

1.1 BRIEF HISTORY……………………………………………………………………..…2

1.2 THERMOACOUSTIC PHENOMENON………………...……………………………3

1.3 THERMOACOUSTIC STACK………………………………………………………....6

1.3.1 PIN STACK ARRAY………………………………………………………….........7

1.3.2 HONEYCOMB STACK……………………………………………………………..8

1.4 RESONANCE FREQUENCY……………………………………………………………8

1.5 LENGTH OF TUBE…………………………………………………………………….9

1.6 CLOSED END PIPES…………………………………………………………………..10

1.7 THE SPEAKER…………………………………………………………………………11

CHAPTER 2 – Materials and Methods

2.1 MATERIALS………………………………………………………………………….14

2.2 BOXED LOUDSPEAKER……………………………………………………….......16

2.3 CARBON FIBRE STACK……………………………………………………….…….18

2.31 CATALYTIC CONVERTER STACK ………………………………………………..19

2.32 PHOTOGRAPHIC FILM STACK……………………………………………………20

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2.4 CALIBRATION OF THERMOCOUPLES……………………………………………21

2.5 AMPLIFING SOUND WAVE…………………………………………………………21

2.6 INCREASING THE EFFICIENCY……………………………………………………..23

2.7 EXPERIMENTAL SET UP……………………………………………………….…25

CHAPTER 3 - Results

3.1 INTRODUCTION……………………………………………………………………..27

3.2 TESTING OF ISOLATED TUBE ……………………………………………………...28

3.3 CARBON FIBRE STACK TEST ……………………………………………………….30

3.4 CATALYTIC CONVERTER STACK TEST …...............................................................32

3.5 EFFECT OF INCREASING THE AMPLIFIER GAIN……………………………….34

3.6 EFFECT OF STACK POSITION ………………………………………………..……..36

3.7 EFFECT OF SPEAKER IMPEDANCE AND SIZE ……………………………………38

3.8 TESTING THE 3rd AND 5th HARMONICS……………………………………………..39

CHAPTER 4 – Concluding Remarks

4.1 DISCUSSION……………………………………………………………....................43

4.2 FUTURE WORK ………………………………………………………….……….…..44

4.3 CONCLUSION …………………….………………………………………………..…45

BIBLIOGRAPHY………………………………………………………………..….….…47

RISK ASSESSMENT………………………………………………………………………….…48

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List of figures

Figure 1.2 Thermoacoustic Refrigerator

Figure 1.21 P – V diagram showing the four stages in the thermoacoustic refrigerator cycle

Figure 1.31 Pin stack array inside resonance tube

Figure 1.32 Honeycomb stack design

Figure 1.5 Closed Cylinder

Figure 1.7 30W/ 8Ω Speaker

Figure 2.2 Thermoacoustic Refrigerator

Figure 2.3: Pin stack array made with carbon fibre tube

Figure 2.31: Catalytic converter stack

Figure 2.32: Photographic film stack

Figure 2.5: PA 100 Amplifier

Figure 2.51: Sound wave on Oscilloscope

Figure 2.6: Changing the speaker

Figure 2.61: New speaker set-up

Figure 2.7: Experimental set up

Figure 4.1: Russell and Weibull experimental data

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List of Graphs

Graph 3.2: Testing of isolated tube

Graph 3.21: Resonance tube without stack with 169Hz signal applied

Graph 3.3: Carbon fibre stack at 3cm from closed end



Graph 3.4: Catalytic converter stack in 50cm tube at 169Hz


Graph 3.5: Carbon fibre stack with amplifier gain of 3


Graph 3.6: Catalytic converter stack in optimum position

Graph 3.61: Catalytic converter stack not in optimum position

Graph 3.7: Higher impedance speaker at optimum conditions

Graph 3.8: 3rd harmonic f3 at optimum conditions

Graph 3.81: 5th harmonic f5 at optimum conditions

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CHAPTER 1

General Introduction

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1.1 BRIEF HISTORY

“Thermoacoustic refrigerators are systems which use sound waves to produce cooling power

(1)”. If the system has the ability to convert acoustics into energy it is hence, called a

thermoacoustic refrigerator. During the last two decades thermoacoustic refrigeration is

explored as a new cooling technology. The thermoacoustic device contains no adverse

chemicals or environmentally unsafe elements that are characteristics of the current

refrigeration systems. Thermoacoustics deals with the conversion of sound energy to heat

energy and vice versa. There are two types of thermoacoustic devices: thermoacoustic engine

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

energy and the energy is available for the useful work. In this device, heat flows from a

source of higher temperature to a sink at lower temperature. In a thermoacoustic refrigerator,

the reverse of the above process occurs, i.e., it utilizes work (in the form of acoustic power)

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

For this project we will concentrate on the latter, thermoacoustic refrigeration. The efficiency

of the thermoacoustic devices is currently lower than that of their conventional counterparts,

which needs to be improved to make them competitive. Although thermoacoustic

refrigerators have many advantages which include:

Mechanical simplicity

No lubricants needed

Use of cheap and readily available gases (air)

Power saving by proportional control

Lower life cycle cost

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Another major benefit includes the environmental aspect; the international restriction on the

use of CFC gives thermoacoustic devices a strong advantage over traditional refrigerators.

The gases used in these devices (air etc) are totally harmless to the ozone and have no

greenhouse effect.

1.2 THERMOACOUSTIC PHENOMENON

Acoustic waves are oscillations in a medium that cause it to experience pressure,

displacement and temperature variations. In order to produce thermoacoustic effect, these

oscillations in a gas should occur close to a solid surface. A stack is placed inside the

thermoacoustic device in order to produce such a solid surface. The thermoacoustic

phenomenon occurs by the interaction of the gas particles and the stack plate. The sound

wave (driven from a loudspeaker) is used in order to create temperature gradient across the

stack, which is used to transfer heat from low temperature medium to a high temperature

medium.

A thermoacoustic refrigerator consists of a tube filled with a gas, air for this system. This

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

create an acoustic standing wave inside the tube.

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Figure 1.2: Thermoacoustic Refrigerator

To be able to create or move heat, work must be done, and the acoustic power provides this

work. When a stack is placed inside the resonator a pressure drop occurs. Interference

between the incoming and reflected wave is now imperfect since there is now a difference in

amplitude causing the standing wave to travel a little, giving it acoustic power. In the

acoustic wave, parcels of gas adiabatically expand and compress.

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Pressure and temperature change simultaneously; to understand the thermoacoustic cycle we

must consider the four processes in the Brayton cycle.

Figure 1.21: P – V diagram showing the four stages in the thermoacoustic refrigerator

cycle (2)

Solid circle shows the parcel state at the beginning of process and the dashed circle shows

the parcel at the end of the process.

1. Adiabatic compression of the gas. (temperature of gas increases). The temperature of

the gas parcel is now higher than that of the stack wall and heat flows from the parcel

to the wall.

2. Isobaric heat transfer. (constant pressure with decreasing temperature). The parcels

temperature is higher than that of the stack causing it to transfer heat to the stack.

3. Adiabatic expansion of the gas. (gas is cooled). The temperature of the gas is lower

than that of the stack.

4. Isobaric heat transfer. (constant pressure, temperature of gas increased back to its

original value) Heat is transferred from the stack back to the gas.

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1.3 THERMOACOUSTIC STACK

The stack is the most important and influential component in a thermoacoustic refrigerator.

This will determine the cooling effect at the set frequency of the fridge. The key to improving

the efficiency of the fridge is developing the stack. The primary constraint in designing the

stack is the fact that stack layers need to be a few thermal penetration depths apart, with four

penetration depths been the optimal separation. (2) The thermal penetration depth, dk , is

defined as the distance that heat can diffuse through a gas during the time t = 1/π f , where f is

the frequency of the standing wave.(2)

d k=√ kπf ρ Cp

(1)

k = Thermal conductivity

ρ = Density of the gas

cp = Isobaric specific heat per unit mass

If stack layers are too far apart the gas cannot effectively transfer heat to and from the stack

walls. If the layers are too close together viscous effects hamper the motion of the gas

particles.

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1.3.1 PIN STACK ARRAY

The pin stack array was constructed using carbon fibre tubes. For optimum performance a

material with low thermal conductivity is required. The internal diameter of the tubes was

1mm and optimum separation four thermal penetration depths. This is the gas corridor the air

travels through.

1 x 10−3 m4

= 2 x 10−4m (dk) (2)

dk = Thermal penetration depth

From this we can calculate the optimum frequency from the diameter of the tubes.

Then determine the length of the tube needed to create resonance at this frequency.

Figure 1.3.1: Pin stack array inside resonance tube(3)

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1.3.2 HONEYCOMB STACK

This stack is new to the market and is being introduced in thermo applications. We

constructed the design by using the catalytic converter from the exhaust of a car.

Figure 1.3.2: Honeycomb stack design (3)

1.4 RESONANCE FREQUENCY

Resonant frequency is the natural frequency of vibration determined by the physical

parameters of the vibrating object. (4) The resonant frequency of air columns depend upon the

speed of sound in air as well as the length and geometry of the air column. The speed of

sound in dry air is approx 334.1 m/s. For the purpose of this project this is accurate and we do

not need to consider room temperature variation effects.

The frequency of the system can be calculated using dk (equation 1)

dk = √ KΠ f p Cp

(2)

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Rearrange for f gives

f = K

Π p Cp dk2 (3)

Where K = thermal conductivity, p = density of gas, Cp = isobaric specific heat per unit mass

The density of air and isobaric specific heat per unit mass were calculated using an online

calculator at room temperature, which was measured with a mercury thermometer.

f = 0.0257 w /mk

(3.14 )(1.205kg

m3 )(1.005KJ

Kg . k )(2 x 10−4m)2

f = 169 HZ

1.5 LENGTH OF TUBE

We can now calculate the length (L) of the tube needed

f = n V4 L (4)

Rearrange for L gives L = n Vf 4

L = (1 )(340)(4 )(169)

L = 0.5m

Where n = Harmonic number ( 1,3,5...) This tube produces only odd harmonics because it is

closed.

V = Speed of sound in air, f = resonance frequency , 4 = ¼ wavelength for closed end

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1.6 CLOSED END PIPES

The air at the closed end of the pipe must be a node (not moving) since the air is not free to

move there and must be able to be reflected back.

There must also be an antinode where the opening is , since that is where there is maximum

movement of the air.

Figure 1.5: Closed Cylinder. (5)

The red line represents sound pressure and the blue line represents the amplitude of the

motion of the air.

The pressure has a node at the open end, and an antinode at the closed end.

The amplitude has a node at the closed end and an antinode at the open end.

Therefore, optimum stack position in the tube should be close to the pressure maximum, but

away from the particle displacement minimum.

Even harmonics are absent as they would be out-of-phase , causing destructive interference

instead of constructive interference.

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1.7 The Speaker

The ohm (Ω) is the unit of measure for impedance, which is the property of a speaker that

restricts the flow of electrical current through it. (6) Study shows that the temperature

differences between the hot and cold sides of the stack increase with speaker power.

The amplifier will deliver maximum power to the speaker when the speaker impedance

matches the internal impedance of the amplifier. Too low impedance will result in weak

output and poor tone. If the speaker impedance is higher than that of the amplifier, its

output power will again be less than its capable of. (6)

For optimum speaker performance in our system the speaker impedance should equal the

amplifier impedance.

To calculate the impedance of an amplifier

Output impedance

The resistance was measured with a digital multimeter, with the speaker being the load on the

system. The load resistance is the resistance of the speaker.

Voltage measurement at the points at OUT:

V1 = Open-circuit voltage (Rload = ∞ Ω, that is without Rload, switch S is open)

Rload = Load resistance (Rtest is resistor to measure Ω value)

V2 = Loaded circuit voltage with resistor Rload = resistance Rtest

Zsource = The output impedance can be calculated

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8Ω x ( 16.9 mv7.7 mv

−1¿ = 9.6 Ω (5)

Figure 1.7: 30W/ 8Ω Speaker

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CHAPTER 2

Materials and Methods

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2.1 MATERIALS

30W Speaker

60W Speaker

Carbon fibre tubes

Catalytic converter

Digital multimeter

Earplugs

Face mask

Lab coat

MDF wood

PA 100 Amplifier

Perspex tubing/ sheets

Power drill

Rubber O rings/cork

Safety goggles/gloves

Screws

Super glue

Silicon

Silver Varnish

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Styrofoam

Tektronix oscilloscope

Thermocouples x 2

Unilab signal generator

Vacuum grease

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2.2 BOXED LOUDPEAKER

The box for the loudspeaker was constructed using MDF wood; the sides were screwed

together using a power drill. The top of the box was drilled for the loudspeaker to fit snugly

into it. The speaker was fitted in and sealed with silicon. A Perspex sheet was fitted on top

of the speaker with a drilled hole big enough for the resonance tube. The Perspex was fitted

using silicon.

Figure2.2: Thermoacoustic Refrigerator

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Two circular Perspex rings were constructed with holes drilled in the centre to hold the

resonance tube. Using a lathe; notched groves in the Perspex were made to hold the rubber

O rings for an air tight seal.

The resonance tube was cut to length using a hacksaw.

A small hole was drilled in the side of the box for the thermocouple; the thermocouple went

up the tube and sat below the stack.

A rubber cork is placed in top of the tube with a hole drilled in it to fit the thermocouple

which sits above the stack. This hole was sealed with silicon.

The seals were also sealed with a vacuum grease to improve efficiency.

The system was placed on top of Styrofoam to dampen the sound level exposure.

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2.3 CARBON FIBRE STACK

Carbon fibre tubes were ordered from www.easycomposites.co.uk. They were cut using a

power tool with a fine grit edge. Safety goggles were worn. Insulation tape was used to

constrict movement of the tubes. The pin stack constructed was 50mm in length and a rubber

o ring was used for a seal.

Figure 2.3: Pin stack array made with carbon fibre tube

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2.31: CATALYTIC CONVERTER STACK

A catalytic converter was recovered from a car exhaust. It was cut to fit the resonance tube

using a handheld power tool with a sharp cutting edge. Safety goggles and a face mask were

worn as it contained harmful toxins.

cack

Figure 2.31: Catalytic converter stack

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2.32: PHOTOGRAPHIC FILM STACK

This stack was used in the original paper on tabletop thermoacoustic refrigerator by Daniel

A. Russell and Pontus Weibull.(2) The stack was designed using photographic film , fishing

line and a copper rod as the centre piece. Super glue was used to stick the fishing line to the

photographic film.

Figure 2.32: Photographic film stack

Testing of the stack proved problematic as the stack got damaged when changing the stack

position. Preliminary results were poor so this was not tested any further.

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2.4 CALIBRATION OF THERMOCOUPLES

For accurate results the two thermocouples were calibrated before the experiment was

conducted. A mercury thermometer was used as a control and the adjustment screw on the

thermocouples was changed to match the temperature on the thermometer.

2.5 AMPLIFING SOUND WAVE

The maximum temperature gradient achieved using the UNILAB signal generator was 2.9 °C

(see results). An amplifier was introduced to our system to improve the power output of the

speaker and increase the thermoacoustic effect. This increased our temperature gradient to 9

°C ( see results).

Figure 2.5: PA 100 Amplifier

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The signal was viewed on the oscilloscope to see what the maximum gain achievable is

before saturation occurs. Gain = output/input. The max gain of the amplifier before

saturation occurs, A = 7.

Figure 2.51: Sound wave on Oscilloscope

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2.6 INCREASING THE EFFICIENCY

To improve the efficiency of the system the speaker was changed. The speaker was very wide

for the small opening in the tube and some of the acoustic wave energy was being absorbed

by the Perspex walls.

Figure 2.6: Changing the speaker

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A piece of wood was placed between the Perspex top and the speaker to accommodate the

change in size of the speaker.

Figure 2.61: New speaker set-up

The new speaker had also higher impedance. The original speaker was 3Ω, whereas the new

speaker was 8Ω which is much closer to the desired 9.6Ω of the amplifier for maximum

performance. This increased our temperature difference a further 2.1°C giving us a change of

10.7°C (see results).

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2.7 EXPERIMENTAL SET-UP

This is the experimental set up used in the testing of the thermoacoustic refrigerator.

Figure 2.7: Experimental set up

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CHAPTER 3

Results

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3.1 INTRODUCTION

This section reports the results of the study. Following the testing off the system the carbon

fibre stack proved most efficient with the optimum stack position being 8cm from the closed

end. The efficiency of the system was increased by the addition of the amplifier and by

changing the speaker impedance. The study also viewed the difference in temperature

difference between the first, third, fifth harmonics. The system worked for the purpose

designed and demonstrated the thermoacoustic effect successfully with a maximum

temperature gradient of 10.7°C after 10mins being achieved.

NOTE: For the following sets of data Tc and Th will refer to the cold and hot sides of the

stack respectfully.

Data was recorded for time intervals at which significant changes happened, after this time

the temperature gradient between both ends of the stack all but stopped increasing.

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3.2 TESTING OF ISOLATED TUBE

Firstly the system was tested without the stack in place or the speaker connected to check for

any temperature variations. The two thermocouples were placed inside the tube in the

positions they would sit when the stack is in the tube.

0 2 4 6 8 10 12 1416

16.5

17

17.5

18

18.518.1 18.1 18.2 18.2 18.3 18.2 18.2 18.1 18.1 18 18.1 18.1

17 17 17 16.9 17 16.9 17 17 17 16.9 16.9 16.9

Temp vs Time

Temperatire TcTemperature Th

Time (s)

Tem

per

atu

re (

C)

Graph 3.2: Testing of isolated tube

As can be seen from the above graph the temperature does fluctuate inside the tube without

the stack or speaker connected. However, the variation is small with a maximum fluctuation

of 0.2 degrees Celsius for both Tc and Th . This could be due to ambient temperatures which

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is the temperature in the room and around the thermoacoustic refrigerator. Room temperature

was monitored using a mercury thermometer and changes were very small and considered not

important to the experiment. Data was recorded for 12 minutes as the fluctuations in this time

was steady and changes were not expected to happen after this time.

The speaker was then connected with the applied resonance frequency of 169 Hz.

0 2 4 6 8 10 1217.817.9

1818.118.218.318.418.518.618.718.8

18.1 18.118.2 18.2

18.3 18.318.4

18.518.6

18.7

18.1 18.118.2 18.2

18.3 18.318.4

18.518.6

18.7

Temp vs Time


Time (s)

Tem

per

atu

re (

C)

Graph 3.21: Resonance tube without stack with 169Hz signal applied

The graph above if figure 3.21 shows a temperature fluctuation greater than that of figure 3.2.

This is due to the system being subject to the 169Hz signal applied. A rise in temperature is

evident with a maximum difference of 0.6 degrees in the tube after 10 minutes. This test was

done without the stack to see the effect of the applied frequency so the thermocouple Tc was

removed. Data was recorded for 10 minutes as temperatures did not rise after this time.

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3.3 CARBON FIBRE STACK TEST

The next test was the carbon fibre stack placed at different positions in the resonance tube to

search for the optimum stack position for maximum performance. The ideal condition is for

the stack to be close to the pressure maximum but away from the particle displacement

minimum. The UNILAB signal generator was used in this process.

The first test the stack was placed at 3cm from the closed end of the tube to the centre of the

stack.

0 2 4 6 8 10 1221.5

22

22.5

23

23.5

24

24.5

25

23.9

24.3 24.4 24.4 24.4 24.424.1 2423.9

23.223 22.9 22.9 22.9

22.7 22.6

Temp vs Time


Time (s)

Temperatire (C)


After 10miutes of testing the temperature gradient ΔT = 1.4°C.

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The second test the stack was placed at 5cm from the closed end to the centre of the stack.

0 2 4 6 8 10 1221.5

22

22.5

23

23.5

24

24.5

25

25.5

26

23.7

25.2 25.2 25.2 25.2 25.225.5

25.7

23.723.4

23.2 23.1 23 23 22.9 22.9

Temp vs Time


Time (s)

Temperature (C)



The third test the stack was placed at 8cm from the closed end to the centre of the stack.

0 2 4 6 8 10 1219

20

21

22

23

24

25

21.8

22.923.2 23.4 23.5 23.6 23.7 23.9

21.8

20.8 20.8 20.8 20.8 20.9 21 21

Temp vs Time


Time(s)

Temperature (C)

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These tests show that optimum position for the carbon fibre stack was 8cm from the closed

end. Further stack positions were tested but performance degraded significantly any further

distance from the closed end.

3.4 CATALYTIC CONVERTER STACK TEST

This test was to check the effect of changing the tube length and resonance frequency using

the catalytic converter stack. The catalytic converter stack was 25mm in length where the

carbon fibre stack was 50mm. The prime stack position was calculated to be 8cm for the

carbon fibre so the test was done at 4cm for catalytic converter as it’s only half the length.

The first test was using 50cm tube at 169 Hz

0 1 2 3 4 5 6 7 8 9 1023.5

24

24.5

25

25.5

26

26.5

25.2

25.9 25.9 25.9 25.8 25.8 25.8

25.2 25.1 2524.8

24.6 24.5 24.4

Temp vs Time


Time(s)

Temperature (C)

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After 9 minutes of testing the temperature gradient ΔT = 1.4°C.

The second test was using 25cm tube at 343 Hz. The resonance frequency was adjusted to the

tube length using formula f = n v4 L .

0 1 2 3 4 5 6 7 8 9 1022.5

23

23.5

24

24.5

25

25.5

26

26.5

2726.5 26.6 26.6 26.7 26.7 26.7 26.726.5

25.9

25.3

24.824.5

24.123.9

Temp vs Time


Time(s)

Temperature (C)



These tests show that the catalytic converter was more efficient in the 25cm resonance tube

with 343Hz signal applied. This could be due to the stack length being half of that of the

carbon fibre. Further study of stack geometry would make interesting future work.

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3.5 EFFECT OF INCREASING THE AMPLIFIER GAIN

The amplifier was introduced to the system to increase the power of the signal from the input

to the output of the speaker.

The first test was using 50cm tube at 169 Hz and carbon fibre stack.

Gain = 3

0 2 4 6 8 10 120

5

10

15

20

25

30

35

24.326.6 27.3 27.8 28.2 28.6 28.9 29.1 29.3 29.5 29.7

24.3 23.2 23.1 23 22.9 22.9 22.9 22.8 22.8 22.8 22.9

Temp vs Time


Time (s)

Tem

pera

ture

(C)



The amplifier increased performance of the system hugely. The maximum temperature

gradient achieved using UNILAB signal generator was 2.9°C, this increased when using the

PA100 amplifier to 6.8°C.

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The gradient achieved is due more to Th rising than Tc falling. This is the basis on which a

heat pump would operate and not a refrigerator. However, the principle behind the project is

to obtain a temperature difference across a thermoacoustic stack and this is achieved. All that

is needed is a pump to circulate the hot air which will give the refrigeration effect desired.

The same effect can be seen in the following results.

The second test was using 50cm tube at 169 Hz and carbon fibre stack.

Gain = 7 (max before saturation occurs)

0 2 4 6 8 10 120

5

10

15

20

25

30

35

23.926.6 27.2 27.5 28.2 28.8 29.3 29.5 29.9 30.2 30.4

23.9 22.5 22.4 22 21.9 21.9 21.9 21.8 21.8 21.8 21.8

Temp vs Time


Time (s)

Tem

pera

ture

(C)



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The results show that by increasing the amplifier gain from 3 to 7 (max) our temperature

gradient increased from 6.8°C to 8.6°C while keeping the other parameters constant. This

shows that the gain has a direct effect on the performance of our speaker and therefore the

performance of our thermoacoustic refrigerator.

3.6 EFFECT OF STACK POSITION

This test looks at the effect of having the stack in position to out of position.

Catalytic converter stack in optimum position. (4cm)

Ideal performance conditions, f = 343 Hz , tube length 25cm , amplifier gain = 7.

0 2 4 6 8 10 120

5

10

15

20

25

30

35

2527 27.8 28.3 28.9 29.2 29.6 29.9 30.1 30.3 30.5

25 24.9 24.9 24.7 24.7 24.7 24.7 24.7 24.6 24.6 24.5

Temp vs Time


Time (s)

Tem

pera

ture

(C)

Graph 3.6: Catalytic converter stack in optimum position

After 10 minutes of testing the temperature gradient ΔT = 6°C.

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Catalytic converter stack NOT in optimum position. (8cm)

Ideal performance conditions, f = 343 Hz , tube length 25cm , amplifier gain = 7.

0 2 4 6 8 10 1222

22.5

23

23.5

24

24.5

25

25.5

23.6

24.424.7 24.8 24.8 24.9 24.9 24.9 24.9 24.9 24.9

23.6 23.6 23.5 23.4 23.4 23.4 23.4 23.3 23.3 23.2 23.2

Temp vs Time


Time (s)

Tem

pera

ture

(C)

Graph 3.61: Catalytic converter stack NOT in optimum position


This data shows that the performance of the system decreased rapidly when the stack was

placed out of position. After 10 minutes of testing the performance decreased by 4.3°C.

Therefore, stack position is crucial in the set up of the system.

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3.7 EFFECT OF SPEAKER IMPEDANCE AND SIZE

To increase the size of the temperature differential the speaker was changed. (see 2.6

increasing the efficiency)

The new speaker had higher impedance closer to that of the amplifier and a smaller diameter

to better suit the diameter of the resonance tube.

The test was done with the carbon fibre stack under the same conditions which achieved the

maximum temperature difference of 8.6°C. ( Gain of amp = 7, f = 169Hz, tube = 50cm, stack

position = 8cm)

0 2 4 6 8 10 120

5

10

15

20

25

30

35

21.5

26.9 28.1 28.8 29.3 29.7 29.9 30 30.1 30.2 30.3

21.5 20.8 20.6 20.2 20.2 20.1 20 19.8 19.8 19.6 19.6

Temp vs Time


Time (s)

Tem

pera

ture

(C)

Graph 3.7: Higher impedance speaker at optimum conditions


This increase in temperature shows us that changing the speaker made the system more

efficient. This is due to the new speaker having higher impedance closer to that off the

amplifier. (See 2.6 increasing the efficiency)

45 | P a g e

Another important factor is the diameter of the new speaker is smaller and more power will

therefore get up the resonance tube and not absorbed in the Perspex walls.

3.8 TESTING THE 3rd AND 5th HARMONICS

A harmonic of a wave is a component frequency of the signal that is an integer multiple of

the fundamental frequency. (6)

The wave displacement has only quarter of a cycle of a sine wave, so the longest sine wave

that fits into the closed pipe is four times as long as the pipe.

L = λ4 (6)

We can also fit in a wave if the length of the pipe is three quarters of the wavelength, i.e. if

wavelength is one third that of the fundamental and the frequency is three times that of the

fundamental. But we cannot fit in a wave with half or a quarter the fundamental wavelength

(twice or four times the frequency). Therefore this type of tube produces only odd harmonics.

f = n V4 L

f1 (1st harmonic) = (1 )(343)(4 )(0.5) = 169 Hz

f3 (3rd harmonic) = (3 )(343)( 4 )(0.5) = 515 Hz

f5 (5th harmonic) = (5 )(343)( 4 )(0.5) = 858 Hz

46 | P a g e

http://en.wikipedia.org/wiki/Fundamental_frequency

http://en.wikipedia.org/wiki/Integer

http://en.wikipedia.org/wiki/Frequency

http://en.wikipedia.org/wiki/Wave

3rd HARMONIC TEST

Test done under ideal conditions for performance, carbon fibre stack, stack position = 8cm,

tube 50cm, amp gain = 7, new speaker used.

f3 = 515Hz

0 2 4 6 8 10 1218.5

1919.5

2020.5

2121.5

2222.5

23

20.7

21.5 21.7 21.9 22 22.2 22.3 22.4 22.5 22.6 22.7

20.7 20.5 20.4 20.3 20.3 20.2 20.2 20.2 20.2 20.1 20.1

Temp vs Time


Time (s)

Tem

pera

ture

(C)

Graph 3.8: 3rd harmonic f3 at optimum conditions


Performance of the system degraded from 10.7°C to 2.6°C from the first harmonic f1 to the

third harmonic f3.

This gives an efficiency drop of approx 75%.

47 | P a g e

5th HARMONIC TEST

Test done under ideal conditions for performance, carbon fibre stack, stack position = 8cm,

tube 50cm, amp gain = 7, new speaker used.

f3 = 858Hz

0 2 4 6 8 10 1218.6

18.8

19

19.2

19.4

19.6

19.8

20

19.519.6 19.6 19.6

19.7 19.7 19.719.8 19.8 19.8 19.8

19.519.4 19.4 19.4

19.3 19.3 19.319.2 19.2 19.2

19.1

Temp vs Time


Time (s)

Tem

pera

ture

(C)

Graph 3.81: 5th harmonic f5 at optimum conditions


Performance of the system degraded from 2.3°C to 0.7°C from the third harmonic f3 to the

fifth harmonic f5.

This gives an efficiency drop of approx 75%.

In summary, the performance of the system decreases by approx 75% per overtone. This was

due to the standing wave pattern changing as the harmonics increased while the stack position

remained in the optimum position for the first harmonic and was not adjusted accordingly.

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CHAPTER 4

Concluding Remarks

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4.1 DISCUSSION

The project aims where achieved with the thermoacoustic refrigerator being built at low cost

and it provided a sufficient temperature gradient of 10.7°C to show the working principles of

the system. Carbon fibre proved a more efficient material for the stack than the catalytic

converter. The important factors in designing the heat stack include stack position, which is

crucial that the placement is near the pressure maximum in the resonance tube. For efficiency

purposes it is important to consider the power output of the speaker, a amplifier can give

more power to the speaker and thus a greater performance in the system. The speaker’s

impedance must also be close to that of the amplifier for desired performance. The amplifier

increased the temperature from 2.9°C to 8.6°C, and matching the impedance increased it a

further 2.1°C to our maximum gradient achieved of 10.7°C. Our temperature gradient

decreased on average by 75% per overtone. Therefore the fundamental tone n =1 is the most

efficient resonance frequency to work for the system.

In comparison to Russell and Weibull paper in the American Association of Physics

Teachers(2), this system worked more like a heat pump than a refrigerator with a large

increase in temperature above the stack and only a small cooling effect below the stack. We

can see from the following diagram this was not the case for Russell and Weibull who system

had a greater cooling effect below the stack like a typical refrigeration device.

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Figure 4.1: Russell and Weibull experimental data (2)

However, the system designed is suitable for refrigeration, a simple heat pump could be used

to pump the hot air away and get the desired refrigeration effect.

The reason for the difference in performance is unknown and this is an interesting topic for

future work.

4.2 FUTURE WORK

Further development of the stack to increase performance, including stack length

optimization. Different resonator shapes to maximise power going into the tube could also be

investigated. General improvements on the seals could also improve the system. Investigate

the difference in performance between this system and Russell and Weibull system. (2)

51 | P a g e

4.3 CONCLUSION

The project was a success with reasonable and desired outcomes achieved; the temperature

gradient measured across the stack was 10.7°C. This temperature difference could be felt by

touching both ends of the stack which is a strong indication of the temperature gradient on

both ends of the stack. Both the carbon fibre and the catalytic converter were constructed

successfully and worked as a stack with the carbon fibre proving more efficient. The project

was built at low cost and was made more efficient than the original system designed by

introducing an amplifier for more speaker power and by changing the speaker impedance.

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BIBLIOGRAPHY

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1. http://www.nevis.columbia.edu/~ju/Paper/Paper-thermoacoustic/Construction %20therm%20refrigerator.pdf

2. http://www.acs.psu.edu/drussell/publications/thermodemo.pdf

3. http://www.nevis.columbia.edu/~ju/Paper/Paper-thermoacoustic/Construction %20therm%20refrigerator.pdf

4. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html

5. http://www.phys.unsw.edu.au/jw/pipes.html

6. http://www.prestonelectronics.com/audio/Impedance.htm

7. http://en.wikipedia.org/wiki/Harmonic

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http://en.wikipedia.org/wiki/Harmonic

http://www.prestonelectronics.com/audio/Impedance.htm

http://www.phys.unsw.edu.au/jw/pipes.html

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html

http://www.nevis.columbia.edu/~ju/Paper/Paper-thermoacoustic/Construction%20therm%20refrigerator.pdf




RISK ASSESSMENT

Researcher Details

Name (use block capitals): RICHARD DUFFY

Title: MR

Faculty/ School/Department SCHOOL OF PHYSICS

Location of Work

LAB KE 1-039 KEVIN STREET DUBLIN 8

Title and Description of Work

Give brief details of task, materials and equipment, frequency and duration. Continue on separate sheet if necessary or attach method statement, protocol etc.

DEVELOPMENT OF A THERMOACOUSTIC REFRIGERATOR, USING WOOD, PERSPEX, LOUDSPEAKER, SIGNAL GENERATOR, MULTIMETER, CARBON FIBRE. PROJECT DURATION 6 WEEKS MONDAY TO FRIDAY 10-5PM.

55 | P a g e

Hazards

For example: lifting and carrying; repetitive movements; heat or cold; sharp edges; working at heights; noise; electrical. Give a brief description of the injuries that could occur and how.

RISK

(High, medium, Low)

1. LOUD NOISE FROM SPEAKER

2. USING POWER TOOLS FOR CUTTING

3. USING GLUE AND OTHER ADHESIVES

4. ELECTRICAL EQUIPMENT, AMPLIFIER

5. USE OF THE LADE

6. FUMES FROM CATALYTIC CONVERTER

HIGH

HIGH

MEDIUM

MEDIUM

HIGH

MEDIUM

56 | P a g e

Who is at risk?

For example: staff carrying out the task; maintenance and cleaning staff; people nearby; visitors; contractors. Give a brief description of how and when they are at risk.

RISK

(High, medium, low)

1. LAB TECHNICIAN WHEN SPEAKER IS ON

2. STUDENTS WHEN SPEAKER IS ON

3. MYSELF DURING USE OF POWER TOOLS AND LADE

4. CLEANING STAFF WHEN GLUE WAS DRYING

5.

6.

HIGH

HIGH

HIGH

LOW

What physical or mental characteristics may alter the risk?

For example: pregnancy; illness (specify); disability (specify); height; left or right handedness

57 | P a g e

1. WORKING FROM A BENCH INRESASES RISK OF FALLING OBJECTS

2.

3.

4.

5.

6.

What measures are already provided to reduce risks to all those at risk?

(A) Safe working methods, materials, equipment

58 | P a g e

1. SAFETY GOGGLES

2. SAFETY GLOVES

3. LAB COAT

4. EAR PROTETION

5. PROTECTION COVER FOR LADE

6. LAB TECHNICIAN SUPERVISION

B) Location

1. Location: Identify clearly the exact location(s) of the risk

LAB 1-039, KEVIN STREET

2 Mobility: Will the hazard be mobile? How often?

NO, ALL WORK IS DONE IN THE LAB

3 Access: What access arrangements are in place at the location, e.g. locks, electronic safety interlocks etc.

59 | P a g e

LAB IS LOCKED AT LUNCH TIME AND AFTER 5PM ANDBEFORE 10AM

4 Security: Is security required at the location and what level: e.g. alarms, infra-red detection,

security camera(s), panic switches etc

SECURITY CAMERAS IN OPERATION

5 Signs and warnings: What signs and warnings are needed at the location?

NO EATING OR DRINKING IN LAB, DANGER HIGH VOLTAGE

C) Personal protective equipment or clothing

1. LAB COAT

2. SAFETY GOGGLES

3. EAR PROTECTION

4. FACE MASK

5. SAFETY GLOVES

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D) Information, instruction or training

1. FIRE SAFETY DRILLS

2. EXITS CLEARLY MARKED

3. LAB SIGNS

4. FIRST AID AVAILABLE

5.

6.

61 | P a g e

E) Emergency procedures

For example: first aid; fire fighting and evacuation; communications.

1. FIRST AID KIT

2. FIRE SAFETY DRILLS

3. WINDOWS AND DOORS OPEN FOR EXIT

4.

5.

6.

10.8 Are further measures needed to reduce risks?

For example: changes in working methods; materials; equipment; location; protective equipment; training.

ACTION

(DATE)

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1. CLOSE DOORS TO REDUCE NOISE LEVELS

2. GET BETTER EAR PROTECTION

3.

4.

5.

10.9 Sources of information used for this assessment

1. BOOKS

2. INTERNET

3. PUBLISHED PAPERS

4. PREVIOUS THESIS

5.

6.

63 | P a g e

10.10 Person(s) completing this assessment

Signature: _________________________

Print Name: __RICHARD DUFFY______________________

Title: _______MR___________________

Date: _____21-5-2014_____________________

Signature: _________________________

Print Name: ________________________

Title: __________________________

Date: __________________________

10.11 Approved by Safety Officer (or Head of School)

Signature: ___________________________________ Title: __________________________

Print Name: ________________________

Date: ________________________________

10.12 Approved by Head of School

64 | P a g e

Signature: ___________________________________ Title: __________________________

Print Name: ________________________

Date: ________________________________

65 | P a g e

final report

Documents

thermoacoustic stack

honeycomb stack

effect of stack position

carbon fibre stack test

pin stack array

catalytic converter

thermoacoustic phenomenon

g e acknowledgments