This research is primarily focused on developing potential applications for microwaveinduced thermoacoustic tomography and correcting for image degradations caused by acoustic heterogeneities. Microwave-induced thermoacoustic tomography was first used to verify the feasibility of noninvasively detecting the coagulated damage based on different dielectric properties between normal tissue and lesion treated with high intensity focused ultrasound. Good image contrasts were obtained for the lesions. A comparison of the size of the lesion measured with microwave-induced thermoacoustic tomography and the size measured by a gross pathologic photograph was presented to verify the effectiveness the proposed method. Clinical data for breast tumors were also collected to verify the feasibility of using microwave-induced thermoacoustic tomography in breast cancer imaging. Next, the effects of acoustic heterogeneities on microwave-induced thermoacoustic tomography in weakly refractive medium were investigated. A correction method based on ultrasonic transmission tomography was proposed to correct for the image distortion. Numerical simulations and phantom experiments verify the effectiveness of this correction method. The compensation is important for obtaining higher resolution images of small tumors in acoustically heterogeneous tissues. Finally, the effects of the highly refractive skull on transcranial brain imaging were studied. A numerical method, which considered wave reflection and refraction at the skull surfaces, was proposed to compensate for the image degradation. The results obtained with the proposed model were compared with the results without considering the skull-induced distortion to evaluate the skull-induced effects on the image reconstruction. It was demonstrated by numerical simulations and phantom experiments that the image quality could be improved by incorporating the skull shape and acoustic properties into image reconstruction. This compensation method is important when the thickness of skull cannot be neglected in transcranial brain imaging.

The use of lean, premixed combustion in gas turbines is now widespread due to their low NOx emissions. Such systems are, however, susceptible to a phenomenon called thermoacoustic instability, which occurs as a result of unstable coupling between the combustion chamber acoustics and the flame. It can lead to large amplitude pressure oscillations within a combustor at frequencies in the hundreds of hertz. These pressure oscillations can result in unacceptably large noise levels, flame blow-out, reduced performance and fatigue failure of the combustor walls. This thesis investigates two problems of particular relevance to thermoacoustic instability. (For complete abstract open document)

A modular thermoacoustic engine (TAE) has been designed and fabricated providing a reliable bench test to study thermoacoustic instabilities. Thermoacoustic engines belong to a class of heat engines that produce acoustic power as a result of a fluid dynamic instability. The engine was built with the capability of being converted from a standing-wave thermoacoustic engine to a traveling-wave thermoacoustic engine with relative ease. Different acoustic mediums in the form of gases comprised of Helium, Argon, or Helium-Argon mixtures were used during experimentation across a range of pressures and temperature gradients. It was found that the thermodynamic properties of the acoustic medium greatly influence the thermoacoustic response of the engine in terms of operational frequency and acoustic power output.

Approved for public release; distribution is unlimited / The computerized measurement of thermoacoustically generated temperature gradients in short, thin plates is reported. The computerized data acquisition system is delineated. The temperature difference developed across a stack of short plates was measured as a function of the longitudinal position of the plates in a resonant tube for acoustic pressure amplitudes of 0.5 to 6.6 kPa, and static (or mean) pressures from 100 to 440 kPa, in argon and helium for the first through the third harmonic frequencies of the tube. Measured data were compared with predictions based on work done by Wheat ley and others [J. Wheatley, et al., Journal of the Acoustical Society of America, v. 74, pp. 153-170, 1983] and results reported by Muzzerall (Master's Thesis in Engineering Acoustics, Naval Postgraduate School, Monterey, CA, September 1987). For low acoustic and static pressures, there is good agreement between measured data and theory. As the acoustic pressure amplitudes increase there is a general degradation of agreement up to the point at which it appears saturation of the thermoacoustic effect occurs. / http://archive.org/details/computerizedmeas00kite / Lieutenant Commander, United States Navy

Approved for public release; distribution is unlimited / The objective of the STAR project is to test and space qualify a new continuous cycle cryogenic refrigeration system for cooling of sensors and electronics which is based upon the newly discovered thermoacoustic heat pumping effect. The new refrigerator has no sliding seals, a cycle frequency of about 300 hz, and uses acoustic resonance to enhance the overall power density and efficiency. This thesis is concerned specifically with the design and testing of the electrodynamic transducer which is responsible for the electro-acoustic power conversion. A computer model of the driver/resonator system is presented along with the techniques for measurement of the electrical and mechanical parameters used as input for the model. A final driver design (including dimensional drawings) utilizing a modified JBL 2450J neodymium-iron-boron compression driver and associated leak tight electrical feed-throughs, microphone, accelerometer, pressure gage, pressure housing, and resonator interface is provided. Keywords: Electrodynamic loudspeaker, Thermoacoustic refrigerator / http://archive.org/details/electrodynamicdr00fitz / Lieutenant, United States Coast Guard

The present research analyzes random porous thermoacoustic stack systems analytically, experimentally, and numerically with a primary objective to develop a comprehensive analytical porous media modeling for random porous (such as Reticulated Vitreous Carbon (RVC) foams) environment. Mathematical models are developed for flow, thermal, and energy fields within the random porous medium stack. The Darcy and Brinkman-Forchheimer-extended Darcy models are used for modeling the momentum equation and local thermal equilibrium assumption between the porous matrix and trapped fluid in the void space for energy equation. The expressions of temperature, energy flux density, and acoustic work absorbed or produced by a thermoacoustic device are compared with existing literature and observed good agreements. After obtaining the flow and thermal fields’ information, the present study examines the entropy generation distribution within the stack. One important item revealed in this study is that entropy generation inside the porous medium completely follows the trend of the imaginary part of Rott’s first function profile. Another major contribution of this research is to identify the location of maximum entropy generation which is identical to the location of maximum thermoacoustic heat and work transport. The expression of Nusselt number for steady flow cannot be used in oscillatory random porous medium because of the phase difference between the temperature gradient at the wall and the temperature difference between the wall and the space averaged temperature. The present research experimentally examines novel stack configuration by considering “alternating conducting and insulating materials” as stack in thermoacoustic devices. The objective of considering such stack arrangement is to reduce the conduction heat transfer loss from the hot end of the stack to the cold end, thereby increasing the performance of the stack. Eight different heterogeneous stack arrangements are studied in this research. The performance of the heterogeneous stack arrangement is compared with the typical homogeneous stacks. This research shows that heterogeneous stacks can be used in thermoacoustic devices particularly in small (millimeter) scale thermoacoustic devices. Numerically the present study investigates the influence of working fluid, geometric, and operating conditions on stack performance by solving the full Navier-Stokes, mass, energy equation, and equation of state.

The present research analyzes random porous thermoacoustic stack systems analytically, experimentally, and numerically with a primary objective to develop a comprehensive analytical porous media modeling for random porous (such as Reticulated Vitreous Carbon (RVC) foams) environment. Mathematical models are developed for flow, thermal, and energy fields within the random porous medium stack. The Darcy and Brinkman-Forchheimer-extended Darcy models are used for modeling the momentum equation and local thermal equilibrium assumption between the porous matrix and trapped fluid in the void space for energy equation. The expressions of temperature, energy flux density, and acoustic work absorbed or produced by a thermoacoustic device are compared with existing literature and observed good agreements. After obtaining the flow and thermal fields’ information, the present study examines the entropy generation distribution within the stack. One important item revealed in this study is that entropy generation inside the porous medium completely follows the trend of the imaginary part of Rott’s first function profile. Another major contribution of this research is to identify the location of maximum entropy generation which is identical to the location of maximum thermoacoustic heat and work transport. The expression of Nusselt number for steady flow cannot be used in oscillatory random porous medium because of the phase difference between the temperature gradient at the wall and the temperature difference between the wall and the space averaged temperature. The present research experimentally examines novel stack configuration by considering “alternating conducting and insulating materials” as stack in thermoacoustic devices. The objective of considering such stack arrangement is to reduce the conduction heat transfer loss from the hot end of the stack to the cold end, thereby increasing the performance of the stack. Eight different heterogeneous stack arrangements are studied in this research. The performance of the heterogeneous stack arrangement is compared with the typical homogeneous stacks. This research shows that heterogeneous stacks can be used in thermoacoustic devices particularly in small (millimeter) scale thermoacoustic devices. Numerically the present study investigates the influence of working fluid, geometric, and operating conditions on stack performance by solving the full Navier-Stokes, mass, energy equation, and equation of state.

In thermoacoustic devices, thermal energy is directly converted to an acoustic wave (mechanical energy) or an acoustic input is converted into thermal energy. This is a result of heat interaction between a solid material and adjacent gas, within the so-called ‘‘thermal penetration depth” of the compressible oscillatory flow. Thermoacoustic technology is receiving growing interest in research for its many advantages, such as having no moving parts, being environmentally friendly and the possibility of using renewable energy for its operation (Adeff and Hofler, 2000). However, this technology is still at the development stage and needs more research to produce feasible and practical devices that are ready for domestic and industrial applications. A looped-tube travelling-wave thermoacoustic engine was designed using DELTAEC (Design Environment for Low-amplitude ThermoAcoustic Energy Conversion). The device was equipped with a ceramic regenerator, which is commonly used in catalytic converters for automotive applications, with square channels. The results of preliminary testing of the device were compared with theoretical values estimated from the numerical model. Very close agreement was observed at the qualitative level and reasonable agreement was observed at the quantitative level. After the validation stage, the device was equipped with three selected low-cost porous materials for performance testing and studies. In addition to the ceramic regenerator that was tested before, regenerators made from stainless steel scourers, stainless steel wool and wire mesh screens were tested. This last type is widely available and commonly used in this application. To facilitate meaningful comparison, the regenerators were made in two sets: one having a common hydraulic radius of 200 μm and the other of 120 μm. In total, six regenerators were successfully tested. Before the performance experiments, all of the regenerators were tested in a steady air flow rig that was built for this purpose, to estimate their relative pressure drop due to viscous dissipation. The relative performance of the regenerators was then investigated. The testing focused on the onset temperature difference, the maximum pressure amplitude generated and the acoustic power output as a function of mean pressure as it varied from 0 to 10 bar gauge pressure. This comparative testing revealed a poor relative performance for the regenerators made of scourers and steel wool, while the cellular ceramic regenerator- 10 -seems to offer an alternative for traditional regenerator materials, which may reduce the overall system cost. The literature reports many different observations of nonlinear phenomena by various researchers, a fact which drove the candidate to carefully monitor the behaviour of the device at all stages and led to an interesting finding of a number of nonlinear behaviours during the start-up of the device. These behaviours included an “on-off” effect and “fishbone-like” oscillations in addition to the normal smooth start-up process. The new findings and the detailed observations are reported in chapter 6 of this thesis. The existence of these phenomena focused attention on identifying the key parameters affecting the existence and type of behaviour, which were found to be the mean pressure and the input power, in addition to the material of the regenerator. An attempt was also made to study the phenomena quantitatively. The observations suggest that there are strong interactions between the acoustic and temperature fields within the regenerator, which may be responsible for the reported quasi-periodic unsteady behaviour of the engine.

In this dissertation, applications of thermoacoustic (TA) effect in breast cancer detection and wireless communications are explored. Thermoacoustic imaging (TAI) is a promising candidate for breast cancer detection. TAI creates an image of the internal morphological features of a dielectrically lossy sample by employing generated acoustic waves from absorbed microwave energy in the sample owing to the thermoacoustic effect based on thermoelastic expansion. Malignant tissues, usually embracing higher dielectric loss, absorbing more energy and emanating stronger acoustic waves than the surrounding healthy tissues, may be distinguished in the image. Besides high contrast inherited from microwave imaging and excellent resolution inherited from ultrasound imaging, TAI is also non-ionizing and noninvasive compared with other existing breast cancer imaging modalities. A potential clinically feasible TAI system is more cost-efficient and compact than mammography and especially MRI. Two sets of breast model are investigated by simulations in this work. The first set is made of a slab-shaped breast model. The main purpose of this study is to perform safety evaluation of TAI and calculate the amount of microwave power needed to generate a detectable acoustic pressure. The second set employs four realistic numerical breast phantoms to study the feasibility of applying contrast agents to TAI for breast cancer imaging, which is named as contrast-enhanced TAI (CETAI). The presented results unveil the promising potential of CETAI as a complementary safe, rapid, sensitive, accurate, high-resolution and breast-density-insensitive tomography for 3-D breast cancer detection. Compressive sensing (CS) is applied to significantly reduce the required number of measurements and expedite CETAI applications in breast cancer detection. Results show that the total measurements can be reduced by at least a factor of 13, which is very favorable to potential clinical applications. The second application of TAI explored in this work is wireless communications, which is referred to as thermoacoustic communications (TAC). It is proposed as a potential complementary method to mitigate the challenge in conventional wireless communication from air to water, in which the electromagnetic wave cannot penetrate deep in water. TAC employs a microwave antenna in air to irradiate the water surface with a microwave signal encoded with information to be communicated. Due to the thermoacoustic effect, acoustic waves are subsequently emanated from the water near the surface and propagate in the water with much less attenuation than electromagnetic waves and thus can propagate a longer distance in the water. Finally, an underwater device with an acoustic transducer can detect the generated acoustic signals and the information is acquired by decoding the signals. Its working principle is presented and proof-of-concept experiments are demonstrated. Parametric studies are performed to investigate the dependence of the generated acoustic signals on relevant parameters. Bit rate and link budget of TAC are derived to evaluate the probability of its potential practical applications.

Combustion instability is a process which involves unsteady chemical kinetic, fluid mechanic, and acoustic processes. It can lead to unstable behavior and be detrimental in ways ranging from faster part fatigue to catastrophic system failure. In terms of combustion methodology, combustion instability has been a key issue for lean premixed combustion. The primary objective of this work is to improve understanding of combustion dynamics through an experimental study of lean premixed combustion using a low swirl combustor. This special burner was developed at the Lawrence Berkeley National Laboratory and has recently received significant interest from the gas turbine industry. In these experiments, acoustic perturbations (chamber modes) are imposed on a low swirl stabilized methane-air flame using loudspeakers. The flame response is examined and quantified with OH planar laser induced fluorescence. Rayleigh index maps of the flame are computed for each frequency and operating condition. Examining the structures in the Rayleigh maps, it is evident that, while the flame shows no significant response in some cases, acoustic forcing in the 70-150 Hz frequency range induces vortex shedding in the flame shear layer. These vortices distort the flame front and generate locally compact and sparse flame areas. This information about the flow field shows that, besides illuminating the combustion dynamics, the Rayleigh index is a useful way to reveal interesting aspects of the underlying flow. The experiments also revealed other interesting aspects of this flame system. It was found that the flame becomes unstable when the perturbation amplitude reaches 0.7% of the mean pressure. Decreasing the swirl number makes the flame shape more jet-like, but does little to alter the shear-layer coupling. In a similar fashion, increasing the pressure was found to alter the flame shape and flame extent, but the thermo-acoustic coupling and induced large scale structure persisted to 0.34MPa, the highest pressure tested.