GENERATION OF THERMO-ACOUSTIC WAVES FROM PULSED SOLAR/IR RADIATION by Aowabin Rahman A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Master of Science Department of Mechanical Engineering The University of Utah May 2014 Copyright © Aowabin Rahman 2014 All Rights Reserved The University of Utah Graduate School STATEMENT OF THESIS APPROVAL The following faculty members served as the supervisory committee chair and members for the thesis of Aowabin Rahman. Dates at right indicate the members’ approval of the thesis. Kuan Chen, Chair 03/17/2014 Geoffrey Silcox, Member 03/17/2014 Kent Udell, Member 03/17/2014 The thesis has also been approved by Tim Ameel, Chair of the Department of Mechanical Engineering and by David B. Kieda, Dean of The Graduate School. ABSTRACT Acoustic waves could potentially be used in a wide range of engineering applications; however, the high energy consumption in generating acoustic waves from electrical energy and the cost associated with the process limit the use of acoustic waves in industrial processes. Acoustic waves converted from solar radiation provide a feasible way of obtaining acoustic energy, without relying on conventional nonrenewable energy sources. One of the goals of this thesis project was to experimentally study the conversion of thermal to acoustic energy using pulsed radiation. The experiments were categorized into “indoor” and “outdoor” experiments, each with a separate experimental setup. The indoor experiments used an IR heater to power the thermo-acoustic lasers and were primarily aimed at studying the effect of various experimental parameters on the amplitude of sound waves in the low frequency range (below 130 Hz). The IR radiation was modulated externally using a chopper wheel and then impinged on a porous solid, which was housed inside a thermo-acoustic (TA) converter. A microphone located at a certain distance from the porous solid inside the TA converter detected the acoustic signals. The “outdoor” experiments, which were targeted at TA conversion at comparatively higher frequencies (in 200 Hz–3 kHz range) used solar energy to power the thermo-acoustic laser. The amplitudes (in RMS) of thermo-acoustic signals obtained in experiments using IR heater as radiation source were in the 80–100 dB range. The frequency of acoustic waves corresponded to the frequency of interceptions of the radiation beam by the chopper. The amplitudes of acoustic waves were influenced by several factors, including the chopping frequency, magnitude of radiation flux, type of porous material, length of porous material, external heating of the TA converter housing, location of microphone within the air column, and design of the TA converter. The time-dependent profile of the thermo-acoustic signals also showed “transient” behavior, meaning that the RMS amplitudes of TA signals varied over a time interval much greater than the time period of acoustic cycles. Acoustic amplitudes in the range of 75–95 dB were obtained using solar energy as the heat source, within the frequency range of 200 Hz–3 kHz. iv TABLE OF CONTENTS ABSTRACT ......................................................................................................................... iii ACKNOWLEDGEMENTS………...……….…………………………………………vii Chapters 1. INTRODUCTION ............................................................................................................ 1 2. LITERATURE REVIEW ................................................................................................. 8 2.1 Photo-acoustic oscillations ................................................................................... 8 2.2 Thermo-acoustic oscillations and thermo-acoustic converters............................. 12 2.3 Research needs and motivations ......................................................................... 16 3. MATHEMATICAL FORMULATION AND NUMERICAL SIMULATIONS ........ 20 3.1 The Rosencwaig and Gersho (RG) model ............................................................... 20 3.2 2D numerical simulations of pressure oscillations inside the air column ............. 25 3.3 Thermal relaxation time and thermal boundary layer ............................................. 37 4. EXPERIMENTAL SETUP AND PROCEDURES ...................................................... 42 4.1 TA laser using IR heater as heat source .............................................................. 42 4.1.1 Construction of TA laser powered by IR radiation ....................................... 42 4.1.2 Experimental procedures ............................................................................. 45 4.2 Solar to acoustic energy converter ...................................................................... 48 4.2.1 Construction of solar-powered TA laser....................................................... 48 4.2.2 Experimental procedures ............................................................................. 50 4.2.3 Alignment of pyrometer and lens-chopper-TA converter module ................. 53 4.3 Conversion of microphone outputs to decibels ................................................... 53 5. RESULTS AND DISCUSSIONS....……………………………………………...…56 5.1 Analysis of acoustic signal obtained ................................................................... 56 5.2 Thermal radiation entering the TA converter ...................................................... 59 5.3 Absence of acoustic waves when the air column was open to ambient air ........... 63 5.4 Factors affecting amplitude of thermo-acoustic waves………………………….63 5.4.1 Chopping frequency…………………………………………………………63 5.4.2 Presence of acoustic signal without any porous solid ................................... 65 5.4.3 IR power level ............................................................................................. 67 5.4.4 Properties of the porous material ................................................................. 68 5.4.5 Transient behavior of thermo-acoustic signals ............................................. 71 5.4.6 Effect of preheating the TA converter .......................................................... 75 5.4.7 TA converter design .................................................................................... 77 5.4.8 Location of microphone ............................................................................... 80 5.4.9 Length of porous material................................................................................82 5.5 Solar to acoustic energy conversion......................................................................... 84 5.5.1 Filter configurations .................................................................................... 87 5.5.2 Analysis of signal obtained .......................................................................... 87 5.5.3 Magnitude of solar radiation flux in comparison to radiation flux in IR experiments ........................................................................................................ 102 5.5.4 Transient behavior of thermo-acoustic signals (outdoor tests) .................... 103 5.6 Uncertainty analysis ................................................................................................ 104 6. CONCLUSIONS……………………………………………………………………………106 Appendices A. MICROPHONE CALIBRATION PLOTS ................................................................ 109 B CAD DIAGRAMS OF THERMO-ACOUSTIC CONVERTERS............................ 115 REFERENCES .................................................................................................................. 120 vi ACKNOWLEDGEMENTS I would like to acknowledge the contributions of several coworkers who collaborated with this research. I would like to thank my advisor, Dr. Kuan Chen, who provided valuable ideas on theoretical aspects of thermo-acoustics and contributed with practical suggestions on experimental procedures and interpretation of experimental data. I would also like to acknowledge the contributions of Faisal Fathiel, who designed and machined TA converters and the experimental rig for the solar to acoustic energy converter, Dr. NJ Kim, who built the rig for the IR heater, and Mohammad Albonaeem, who helped with the uncertainty analysis of the experimental data. This work was partially supported by a grant (No. 2011-0029820) from the National Research Foundation of Korea, which is greatly appreciated. CHAPTER 1 INTRODUCTION Thermo-acoustic oscillations are pressure oscillations that occur due to temperature variations. Sound waves are generally regarded as a combination of pressure and velocity oscillations of air molecules in space; however, these are coupled with temperature oscillations as well [1]. Conversely, temperature fluctuations in air induced by external sources can cause thermo-acoustic oscillations. The origins of thermo-acoustics can be traced back to 1777, when Bryon Higgins [2] observed that igniting a hydrogen flame inside a tube open at both ends produces sound waves that vary with the position of the flame within the tube. Experiments by Rijke and Sondhass confirmed Higgins’ observations, and Lord Rayleigh suggested a qualitative explanation in 1896 [3]: “If heat be given to the air at the moment of greatest condensation, or be taken from it at the moment of greatest rarefaction, the vibration is encouraged. On the other hand, if heat be given at the moment of greatest rarefaction, or abstracted at the moment of greatest condensation, the vibration is discouraged.” Thermo-acoustic energy converters are of interest to engineers as they can be used as heat engines, where heat flowing through a porous media is converted to sounds [4], or as heat pumps, where externally generated pressure oscillations are used to transfer heat from a region of lower to higher temperature [4]. These thermo-acoustic devices 2 are easy to construct, and they do not release greenhouse gases. Thermo-acoustic devices can also be used for refrigeration purposes, particularly for cryogenic applications [5]. Methods of generating large-scale acoustic energy were not explored in great detail in the past, mainly due to concerns about the high cost of generating acoustic energy from electricity as well as potential safety hazards [6]. However, recent developments in solar-powered thermo-acoustic-lasers (TA lasers) [7] and refrigerators driven by waste energy, such as heat from exhaust gases from automobile engines [8,9], or heat from power plants, provided economical means of producing acoustic energy on a large-scale. Employing thermo-acoustic engines and heat pumps in such applications can reduce energy consumption as well as greenhouse gas emissions [10]. As a result, thermo-acoustic systems have recently been considered for industrial applications, such as liquefaction of natural gas and separation of gases [5]. Photo-acoustic oscillations are a type of thermo-acoustic oscillations that are generated when pulsed radiation is incident on a given sample, resulting in localized absorption of radiation within the sample and subsequent heat transfer to the surrounding medium [11]. The periodic nature of heat transfer results in pressure oscillations that can be detected as acoustic waves. The photo-acoustic effect was observed by Alexender Graham Bell, who used the phenomenon in his invention of the “photophone” [12, 13]. Bell’s photophone reflected a beam of sunlight using a mirror to a selenium cell, which was connected to an ordinary telephone receiver. While working with the photophone, Bell noticed that mechanically modulating the incident light resulted in sound waves near the sample where light was incident. John Tyndall and Wilhelm Roentgen [14] picked up on Bell’s research and proved that this phenomenon