1. MECHANICAL ENGINEERING
Ergonomics & Safety
Aowabin Rahman
Department of Mechanical Engineering
December 9, 2013
GENERATION OF SOUND WAVES FROM
PULSED SOLAR/IR RADIATION

2. MECHANICAL ENGINEERING
• Introduction & Background
• Objective
• Motivation
• Experimental Setup
• Results and Discussion
• Conclusions

3. MECHANICAL ENGINEERING
Introduction
• Thermo-acoustics oscillations are pressure oscillations
caused by temperature variations.
• Thermo-acoustic (TA) energy converters can be used as
heat engines or as heat pumps.
• TA converters are growing in usage, as they are simple
in construction and do not release greenhouse gases.
• TA converters can be run using solar or waste energy.

4. MECHANICAL ENGINEERING
Introduction
• Photo-acoustic oscillations are a type of TA oscillations
caused by pulsed thermal radiation on a solid (or liquid)
sample.
• Periodic heat transfer from the solid to the surrounding
air results in pressure fluctuations in air, which are
detected as acoustic signals.
• Applications of photo-acoustics have been largely
limited to material detection and characterization.

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Background
• A.G. Bell : Photophone
• M.W. Sigirist: “Thermo-elastic process”
• Rosencwaig and Gersho: The RG Model

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Background – The RG Model
• A small “vibratory piston” near the solid surface
responds to periodic heating.
• Absorption of radiation in gaseous medium and physical
vibration of the solid do not contribute significantly to
acoustic signals generation.
• Rosencwaig and Gersho analytically solved 1-D heat
transfer equations for solid and gaseous medium.

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Background – The RG Model
• The thermal displacement of the gas-piston was then
determined using the ideal gas law, which was then
used to compute the amplitude of pressure fluctuations.
Schematic diagram of TA laser to demonstrate RG model

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Background – Differences with RG Model
• The RG Model was developed for a bulk solid, not a
porous material.
• The model uses the method of complex combinations to
solve heat equations, which does not deal with the
mean or “dc” temperature.
• The RG model deals with heat flow in one dimension
only and does not consider the “cooling” effect.
• The RG model ignores pressure and temperature non-
uniformities (except in the boundary layer).

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2-D Axisymmetric Domain for COMSOL simulations

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Background – Differences with RG Model
• Simulations in COMSOL were performed to illustrate the
limitations of the RG model.
• Periodic heat flux (with frequency, f) was imposed on
surfaces 1 and 2.
• Amplitude of heat flux on surface 2 was assumed to be
5% to that on surface 1.
• Convective and radiative cooling were assumed on
surfaces 2.
• Surface 3 was assumed to be insulated.

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Background – Differences with RG Model
Temperature profiles at probe 1 (r = 0, z = 0.014)
and 2 (r = 0, z = -0.01) for f = 100 Hz
Pressure within the cylindrical domain at any z-
location for f = 100 Hz

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Pressure as a function of z-location for f = 1 kHz

13. MECHANICAL ENGINEERING
Objectives
• Develop thermo-acoustic lasers which would use
IR/solar radiation as heat source.
• Study the parameters which affect the amplitude of
thermo-acoustic waves (using IR radiation as heat
source).
• Use TA converters to obtain acoustic waves from solar
radiation over a wide frequency range (200 Hz – 3 kHz).

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Motivation
• High-amplitude acoustic waves could be obtained
directly from solar energy.
• This acoustic energy can be used for industrial
processes such as acoustic cleaning, materials
processing and sono-assisted CO2 capture.
• Multiple TA lasers (powered by pulsed radiation) could
potentially be coupled “effectively”.

15. MECHANICAL ENGINEERING
Experimental Setup
• Separate experimental setups were used for “indoor”
and “outdoor” experiments.
• Indoor experiments used radiation from IR heater as
heat source.
• They were primarily used to study the influence of
various experimental parameters in the low acoustic
frequency range (50 – 130 Hz).
• Outdoor experiments used solar radiation as heat
source and obtained acoustic waves in the high
frequency range (200 Hz – 3 kHz).

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Experimental Setup – Indoor Experiments
• Components
- IR heater and IR lamp
- IR heater cooling system
- Electric motor with speed control
- Chopper (20 holes, hole diameter = 2cm)
- TA Converters
- Microphones (mic 1 and mic 2)
- Data acquisition card (NI-DAQ 2009)

17. MECHANICAL ENGINEERING
Experimental Setup – Indoor Experiments
• TA Converters
- Three designs were used (TA converters 1,2 and 3).
- Design of TA converter 1 was significantly different
from TA converters 2 and 3.
- TA converter 2 and 3 were almost identical in design:
only the diameter of opening in TA converter 3 was larger.
- Multiple TA converters were used to observe the
effect of TA converter design on acoustic amplitude.

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Experimental Setup – TA Converters
Arrangement of components inside a TA converter
Microphone
TA Converter housing
Steel-wool
Glass cover
TA converter cover
O-ring

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Experimental Setup – Indoor Experiments
Cross-sectional view of housing (TA Converter 1) Cross-sectional view of housing (TA converters 2
and 3)

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Experimental Setup – Indoor Experiments
Cover for TA Converters 1 and 2 Steel-wool, grade 0000

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Experimental Setup – Indoor Experiments
• Microphones
- Mic 1 was more “sensitive” at low amplitudes than mic
2, but it reached saturation at around 96 dB.
• Data acquisition card (NI-DAQ 2009) was used to
acquire data which was then analyzed in LabVIEW
Signal Express 2009.
• 5000 samples acquired at a sampling rate of 20 kHz
• LabVIEW “steps” used to measure amplitude,
frequency, total harmonic distortion (THD). Time-domain
data could be exported to Excel for further analysis.

22. MECHANICAL ENGINEERING
Experimental Setup – Indoor Experiments
Schematic diagram of experimental setup
Incident
Radiation
Chopper Hole
Chopper
TA converter housing
Air-column
Porous Material
Glass cover
NI-DAQ 2009
LabVIEW SE
mic

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Experimental Setup – Indoor Experiments
Experimental setup
TA Converter
Chopper
IR heater

24. MECHANICAL ENGINEERING
Experimental Setup – Outdoor Experiments
• Additional Components:
- Generator
- Electric motor (with speed control)
- Variac
- Chopper wheel (60 holes, hole diameter = 1 cm)
- Plastic Frensel Lens
- Solar flux alignment surface
- Cart

25. MECHANICAL ENGINEERING
Lens Cover
Lens
Chopper
Solar flux alignment
surface
Cart
Solar to acoustic energy
converter

26. MECHANICAL ENGINEERING
Experimental Setup – Outdoor Experiments
The solar pyrometer was aligned with the lens-chopper-TA
converter module as follows:
• The axes of the solar pyrometer and the alignment nail
were aligned.
• The lens-chopper-TA converter module was inclined
manually when the brightest focus was observed on the
chopper disc.
• Immediately, the solar flux alignment surface was adjusted
such that minimum shadow of the alignment nail was
observed on the alignment surface.

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Experimental Setup – Indoor Experiments
Solar flux alignment surface

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Experimental Setup – Calibration Tests
• Calibration of microphones were performed to convert
voltage to decibels.
• Sine waves (of known frequency) generated from “Tone
Generator Software” were fed to the speakers.
• Mic at a fixed distance (about 7.6 cm) from each speaker
detected acoustic signals. The amplitudes (in RMS) was
measured in LabVIEW Signal Express.
• Sound level meter (Tenma 72-942) measured amplitudes
in dBA at the same location as the microphone.
• dBA was converted to dB using a “correction factor”.

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Experimental Setup – Calibration Tests
Frequency response with A-weighting
Calibration of microphones

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Experimental Setup – Calibration Tests
“Universal” calibration plot – mic 1

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RESULTS AND DISCUSSIONS

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Results and Discussions
• The acoustic signals detected by the microphone were
periodic and they closely resembled sine/cosine waves.
• The frequency of acoustic signals was almost equal to the
frequency of interceptions of the radiation beam (or the
chopping frequency).
• The chopping frequency corresponds to the frequency of
heat transfer between the solid filaments and the
surrounding air.

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Results and Discussions
Acoustic signal obtained at 127.5 Hz
(chopping frequency = 126 Hz)
FFT analysis of raw signal
Acoustic frequency
Noise

34. MECHANICAL ENGINEERING
Results and Discussions
• No acoustic signals were
obtained when the air-
column was open to
ambient.
• Only a noise signal of 47.0
dB was detected.
Signal obtained when air-column
open to ambient.

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Results and Discussions
Amplitude of acoustic signals depended on:
- Acoustic frequency
- Presence of porous material in TA converter
- Power level of IR heater
- Properties of porous material
- Transient behavior of TA signals
- External heating of TA converter
- Design of TA converter
- Location of microphone
- Length of porous material

36. MECHANICAL ENGINEERING
Results and Discussions
• Acoustic frequency:
- Increasing acoustic
frequency decreased amplitude.
- Amount of radiation
absorbed per cycle and
exposure time for cooling were
lower at higher frequencies.
- This resulted in a decrease
in difference between the
filament temperature (Ts,mean)
and the air temperature (Tf,mean).
Amplitude vs. Chopping frequency for
steel-wool.

37. MECHANICAL ENGINEERING
Results and Discussions
• Presence of porous material:
- Acoustic signals of much lower amplitudes were detected
when no porous material was present inside the TA converter.
- The amplitude without any porous material was 52.0 dB at
30% IR power level, 127.2 Hz.
- This proved that the aluminum body partly contributed to
acoustic wave generation.
- The decrease in amplitudes were due to a decrease in heat
transfer surface area and comparatively lower temperature
fluctuations on the surface of the aluminum body.

38. MECHANICAL ENGINEERING
Results and Discussions
• Power level of IR heater:
- Increasing IR power level
increased the amount of
radiation absorbed per cycle.
- Linear relation was
observed between acoustic
amplitude and IR power level.
Amplitude v IR power level..

39. MECHANICAL ENGINEERING
Results and Discussions
• Properties of porous material:
- Fine steel-wool (grade 0000, filament width = 0.03 mm)
produced higher amplitudes than coarse steel-wool (grade
3, filament width = 0.09 mm), as it had a higher surface
area for heat transfer.
- Steel-wool produced higher amplitudes than brass-wool
due to its higher absorptivity to incident radiation.

40. MECHANICAL ENGINEERING
Results and Discussions
Acoustic amplitudes for brass-wool
and steel-wool
Acoustic amplitudes at different IR
power levels for coarse (grade 3) and
fine (grade 0000) steel-wool.

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Results and Discussions
• Transient behavior of thermo-acoustic signals:
- “Transient” refers to the variation of amplitudes (RMS)
over a time period much greater than the acoustic cycle.
- Amplitudes increased during the first 1 to 2 s, when the
difference (Ts,mean - Tf,mean) was high.
- As Ts,mean reached a steady value, Tf,mean continued to
increase. The amplitudes decreased as the difference
(Ts,mean - Tf,mean) decreased.
- For acquiring data for transient analysis, 48,000
samples were taken at a sampling rate of 5 kHz.

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Results and Discussions
Time-domain data for acoustic frequency of 116 Hz

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Results and Discussions
Transient effect on acoustic amplitude –
transient and steady state data
Time variations of RMS amplitudes at
different frequencies

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Results and Discussions
Effect of external heating at 125 Hz.
• Heating effect
- Amplitudes dropped when
the TA converter was
externally heated by a heat
gun (250 W) for 5 min.
- External heating increased
the temperature of the
aluminum body as well as the
mean air temperature.

45. MECHANICAL ENGINEERING
Results and Discussions
• Design of TA converters:
- Both TA converters 2 and 3 produced higher acoustic
amplitudes compared to TA converter 1.
- At 127.2 Hz and 65% IR power level, TA converter 2
produced an acoustic signal of amplitude 100.2 dB.
- TA converter 1 produced an acoustic signal of
amplitude 89.4 dB with the same configs .
- Amplitudes with TA Converter 3 (diameter of
opening = 1.6 cm) were higher than those with TA
converter (diameter of opening = 1.3 cm).

46. MECHANICAL ENGINEERING
Results and Discussions
Figure : Effect of TA Converter Opening Diameter

47. MECHANICAL ENGINEERING
Results and Discussions
• For TA converter 3, there was little change in amplitude with
frequency.
• When opening diameter was increased from 1.3 cm to 1.6
cm, the heating area increased and the cooling of hot steel-
wool filaments was reduced.
• As cooling was inhibited, there was no significant
dependence on exposure time.
• For TA converter 3, the increase in absorbed radiation
increased (Tf,mean).

48. MECHANICAL ENGINEERING
large opening diameter
Effect of TA converter size on temperature fluctuations
small opening diameter
Results and Discussions

49. MECHANICAL ENGINEERING
Results and Discussions
• Location of mic:
- Amplitudes decreased
with increasing distance
between steel-wool and
mic.
- Pressure fluctuations
depend on the periodic
volume change and the
total volume.
Effect of mic locations

50. MECHANICAL ENGINEERING
Results and Discussions
• Location of mic:
- Overall volume
increased, when the volume
change remained the same.
(∆V1 = ∆V2 but V1<V2)
- As ∆V2/V2 < ∆V1/V1,
acoustic amplitudes
decreased.

51. MECHANICAL ENGINEERING
Results and Discussions
• Length of porous material:
- Increasing length of steel-wool increased amplitudes up to an
optimal length of about 0.75 cm.
- Beyond the optimal length, amplitudes decreased with
increasing length.
- For length below optimal length:
 Thermal radiation was not fully absorbed.
 The heat transfer area between was steel-wool filaments and
air was too low.
 Contact area between steel-wool filaments and aluminum
body was too low.

52. MECHANICAL ENGINEERING
Results and Discussions
• Length of porous material:
- For length above optimal length:
 Bulk of the incident radiation had been absorbed at optimal
length, so the additional length did not offer an increase in
heat transfer area.
 Air volume increased with increasing length.
 Friction between solid filaments and air increased with
increasing length.

53. MECHANICAL ENGINEERING
Results and Discussions
Effect of length of steel-wool on
acoustic amplitude (TA
converter 1)
Effect of length of steel-wool on
acoustic amplitude (TA
converter 3)

54. MECHANICAL ENGINEERING
Results and Discussions
• Solar to acoustic energy converters (outdoor experiments):
- Acoustic waves were obtained within the range 200 Hz – 3
kHz.
- The data sets were taken on sunny days between May to
September.
- The average solar flux was 1.02 KW/m2, with a standard
deviation of 0.02 KW/m2.
- The solar flux entering the TA converter was approximately
equivalent to 67.3% IR power level.
.

55. MECHANICAL ENGINEERING
Results and Discussions
Acoustic amplitudes vs. frequency (TA
converter 1)
Solar flux corresponding to acoustic
amplitude vs. frequency data

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Results and Discussions
Filtered acoustic signal at 244 Hz
(87.44 dB) – cutoff frequency = 70 Hz
FFT analysis –filtered data at 244 Hz

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Results and Discussions
Filtered acoustic signal at 451 Hz
(85.20 dB) – cutoff frequency = 70 Hz
FFT analysis –filtered data at 451 Hz

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Results and Discussions
Filtered acoustic signal at 3,08 kHz
(79.95 dB) – cutoff frequency = 500 Hz
FFT analysis –filtered data at 3.08
kHz

59. MECHANICAL ENGINEERING
Conclusions
• Pulsed radiation on a porous material generated periodic acoustic
waves. In “indoor” experiments, the acoustic amplitudes were in the
range of 80-100 dB with a frequency of 50 -130 Hz.
• Acoustic signals decreased with increasing frequency when TA
converter opening diameter was 1.3 cm. For opening diameter of
1.6 cm, there was little change in amplitude with frequency.
• Acoustic signals were obtained with no porous material in TA
converter.
• Metal-wools with high absorptivity and high surface area favored
high acoustic generation.
• Acoustic signals showed variation in amplitude over a time interval
much greater than the acoustic time period.

60. MECHANICAL ENGINEERING
Conclusions
• External heating reduced amplitudes of acoustic waves.
• Amplitudes decreased with increasing distance from porous
material.
• Amplitudes increased with length of steel-wool up to an optimal
length, beyond which amplitudes decreased with increasing length.
• Pulsed solar radiation (radiation flux about 1 KW/m2) was used to
generate TA waves of amplitude 75-95 dB and frequency
200 Hz -3 kHz. he amplitudes of generated sound waves (in dB)
decreased with frequency between 200 Hz to 1 kHz. Beyond 1 kHz,
amplitudes showed little variation with frequency..