Ultrasonic thermoacoustic energy converter

pre-printThermoacoustic prime movers have been developed for operation in the low ultrasonic frequency range by scaling down the device size. The developed engines operate at frequencies up to 23 kHz. They are self-sustained oscillators whose dimensions scale inversely with operating frequency. The smallest one being 3.4mm long with a 1mm diameter bore, i.e. the engine inner volume of 2.67 mm3 . The generated sound levels reached intensities in the range of 143 dB - 150 dB in the low ultrasonic range. The miniaturization of thermoacoustic engines will lead to the development of device arrays

Acoustic approach to thermal management: miniature thermoacoustic engines

Journal ArticleAn acoustic approach to thermal management in electronics can be efficient and it can be directly interfaced with electronic devices. It is based on two types of thermoacoustic heat engines, which are being developed for microcircuit applications. One type of device, the prime mover, converts heat to sound; energy is radiated away acoustically. This is achieved with essentially no moving parts. The other type of device, a heat pump or refrigerator, moves heat from one reservoir to another reservoir using sound waves. Both devices are resonant and hence their size scales inversely with operating frequencies. Devices presented here operate in the frequency range of 4 kHz to 21 kHz, depending on their size. The components are simple and they can be fabricated using microcircuit techniques. They consist of an acoustic resonator, heat exchangers, a stack of high surface area material for heat storage, and a working gas such as air or helium or gas mixture (He - Ar). The cooler has a loudspeaker to generate the sound for pumping heat, while the prime mover has a coupler to the source of heat. Working devices range in size from 2 cm to a few millimeters. Their efficiency, which depends on geometrical actors, is an appreciable fraction of Carnot. An important feature is a high power density. Working models and modeling show that power densities of several watts per cubic centimeter can be achieved by optimizing the parameters and working conditions. Performance characteristics of miniature prime movers and refrigerators will be presented

Master of Science

thesisThermoacoustic lasers convert heat from a high-temperature heat source into acoustic power while rejecting waste heat to a low temperature sink. The working fluids involved can be air or noble gases which are nontoxic and environmentally benign. Simple in construction due to absence of moving parts, thermoacoustic lasers can be employed to achieve generation of electricity at individual homes, water-heating for domestic purposes, and to facilitate space heating and cooling. The possibility of utilizing waste heat or solar energy to run thermoacoustic devices makes them technically promising and economically viable to generate large quantities of acoustic energy. The research presented in this thesis deals with the effects of geometric parameters (stack position, stack length, tube length) associated with a thermoacoustic laser on the output sound wave. The effects of varying input power on acoustic output were also studied. Based on the experiments, optimum operating conditions were identified and qualitative and/or quantitative explanations were provided to justify our observations. It was observed that the maximum sound pressure level was generated for the laser with the stack positioned at a distance of quarter lengths of a resonator from the closed end. Higher sound pressure levels were recorded for the laser with longer stack lengths and longer resonator lengths. Efforts were also made to develop high-frequency thermoacoustic lasers

Prototype Design for Thermoacoustic Flashover Detector

The thermoacoustic flashover detector integrates the phenomenon of thermoacoustics into a fire fighting application. This report presents the prototype design for the thermoacoustic flashover detector to ultimately be implemented in a firefighter's gear. Upon increases in compartment fire heat flux and temperature corresponding to the onset of flashover, the device will produce a loud warning tone to alert the firefighter that flashover is impending. This is critical because post-flashover, the fire transitions to an untenable environment for a firefighter, as well as compromised structural integrity of the building. The current design produces a tone at 115 dB at about 500 Hz upon heating from an external band heater and cooling via an ice/water bath. At 38 mm from the device, this sound level is louder than the 85 dB from fire alarms and distinct from the 3000 Hz tone of smoke detectors. The minimum power input to the device for sound onset is 44 Watts, corresponding to a temperature difference of 150 degrees Celsius at a mean temperature of 225 degrees Celsius across a 2 cm long porous steel wool stack. The temperatures at the hot and cold ends of the stack are 300 and 150 degrees Celsius respectively, which is achieved with a response time of ~100 seconds. The sound is sustained as long as there is a minimum power input of 31 Watts. Although the measurement uncertainties are estimated at 10 degrees Celsius for the temperatures and 5 Watts for the power input, this design provides a foundation for future improvement and quantification of the device. The mechanisms of the thermoacoustics at work and the materials selected for the prototype are presented. Different power level inputs to the device are analyzed and temperatures for operation are determined. Suggestions for future optimization and integration of the device into firefighters' gear are presented

Thermal losses considerations in thermo-acoustic engine design

Abstract: Thermo-acoustic cooling as an environmentally friendly refrigeration system is one of the research areas being pursued. Although not commercially available and simple to fabricate, the designing of thermo-acoustic coolers involves significant technical challenges. Many fundamental issues related to the thermo-acoustic effects and the associated heat transfer must be addressed. The most inhibiting characteristic of current thermo-acoustic cooling devices is the lack of efficiency. The stack has been identified as the heart of the device where the heat transfer takes place. Improving its performance will make thermo-acoustic technology more attractive. Most of the existing efforts have not taken thermal losses to the surroundings into account in the derivation of the models. Five different parameters describing the stack geometry and the angular frequency of the standing wave are considered. This work explores the use of a multi-objective optimization approach to model and to optimize the performance of a simple thermo-acoustic engine. The present study highlights the importance of thermal losses in the modelling of small-scale thermo-acoustic engines using a multi-objective approach. The unique characteristic of this research is the computing of all efficient optimal solutions describing the best geometrical configuration of thermo-acoustic engines

THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE THERMOACOUSTIC PROCESS

This thesis presents a study of thermoacoustic processes. Thermoacoustic science, which can serve as a renewable and sustainable source of energy, involves thermodynamics, acoustics and their interactions. This research investigated the thermoacoustic phenomenon through theoretical and experimental investigations. The theoretical study is comprised of two parts. The first part focused on the development of a comprehensive algorithm for the design, development and performance evaluation of thermoacoustic devices. The developed algorithm is capable of designing and optimizing individual thermoacoustic heat engines and refrigerators and coupled engine-refrigerator systems. In the second part of the theoretical study, the theoretical model of thermoacoustic couples predicting stack temperature difference was modified by incorporating more realistic physical processes that were consistent with practical applications. Significant improvement in the accuracy of the stack temperature difference predictions was observed with the modified model as compared to the previous models through experimental validation. Detailed experimental investigations were conducted to enhance the fundamental understanding of the thermo-fluid behavior in thermoacoustic couples. The first part of the experimental study was focused on the investigation of the influence of drive ratio and stack position on the stack temperature field. The results provided the first evidence of the two-dimensional temperature distribution on both end faces of the stack. A physical explanation for the change in the stack temperature difference profile from sinusoidal to sawtooth form with an increase in the drive ratio was provided. It is concluded that the acoustic dissipation in the stack which influenced the stack cold-end temperature was responsible for this behavior. In the second part, experiments were conducted to investigate streaming velocity fields in a thermoacoustic device using a synchronized PIV technique. The results showed that not only the presence of a stack but also the type and geometrical characteristics of a stack can significantly change the structure and magnitude of acoustic streaming. For both stacks, the streaming velocity field in the region adjacent to the hot-end of the stack was stronger with higher spatio-temporal variations as compared to that adjacent to the cold-end of stack, at almost all the drive ratios

Master of Science

thesisAcoustic 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. iv 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

Investigation of Thermoacoustic Performance of Standing and Traveling Wave Thermoacoustic Engines

A standing wave thermoacoustic engine was designed and constructed to examine the effect of curvature on thermoacoustic performance. Sound pressure level at the pressure node of the engine was recorded in conjuction with the temperature at the hot and ambient sides of the stack. Curvature was varied using flexible tubing from 0 to 45. It was found that the curvature had a negative effect on the thermoacoustic intensity, measured using the sound pressure level and the temperature difference between the hot and ambient sides of the stack. Additionally, a strong relationship between the sound pressure level and the temperature behavior was identified. The findings of the investigation were applied to a study of a traveling wave engine. A looped tube design was employed with a regenerator mounted in a straight section of the tube. Thermocouples were mounted in the regenerator to investigate the temperature behavior. Initial results of the thermoacoustic effect were established by calculating the difference in behavior between operation with oscillation and without. These were followed by an investigation of the relationship between the temperature behavior and the positioning of the regenerator in the looped tube. An optimal spacing was identified for positioning the stack in the straight portion of the tube

Thermoacoustic-Piezoelectric Systems with Dynamic Magnifiers

Thermoacoustic energy conversion is an emergent technology with considerable potential for research, development, and innovation. In thermoacoustic resonators, self-excited acoustic oscillations are induced in a working gas by means of a temperature gradient across a porous body and vice versa with no need of moving parts. In the first part of this dissertation, thermoacoustic resonators are integrated with piezoelectric membranes to create a new class of energy harvesters. The incident acoustic waves impinge on a piezo-diaphragm located at one end of the thermoacoustic-piezoelectric (TAP) resonator to generate an electrical power output. The TAP design is enhanced by appending the resonator with an elastic structure aimed at enhancing the strain experienced by the piezo-element to magnify the electric energy produced for the same input acoustic power. An analytical approach to model the thermal, acoustical, mechanical and electrical domains of the developed harvester is introduced and optimized. The performance of the harvesters is compared with experimental data obtained from an in-house built prototype with similar dimensions. In an attempt to further understand the dynamics and transient behavior of the excited waves in the presence of piezoelectric coupling, a novel approach to compute and accurately predict critical temperature gradients that onset the acoustic waves is discussed. The developed model encompasses tools from electric circuit analogy of the lumped acoustical and mechanical components to unify the modeling domain. In the second part of the dissertation, piezo-driven thermoacoustic refrigerators (PDTARs) are presented. The PDTARs rely on the inverse thermoacoustic effect for their operation. A high amplitude pressure wave in a working medium is used to create a temperature gradient across the ends of a porous body located in an acoustic resonator. Finally, PDTARs with dynamic magnifiers are introduced. The developed design is shown, theoretically and experimentally, as capable of potentially enhancing the cooling effect of PDTARs by increasing the temperature gradient created across the porous body

Onset of Self-Excited Oscillations of Traveling Wave Thermo-Acoustic-Piezoelectric Energy Harvester Using Root-Locus Analysis

The onset of self-excited oscillations is developed theoretically for a traveling wave thermo-acoustic-piezoelectric (TAP) energy harvester. The harvester is intended for converting thermal energy, such as solar or waste heat energy, directly into electrical energy without the need for any moving components. The thermal energy is utilized to generate a steep temperature gradient along a porous regenerator. At a specific threshold of the temperature gradient, self-sustained acoustic waves are generated inside an acoustic resonator. The resulting pressure fluctuations excite a piezoelectric diaphragm, placed at the end of the resonator, which converts the acoustic energy directly into electrical energy. The pressure pulsations are amplified by using an acoustic feedback loop which introduces appropriate phasing that make the pulsations take the form of traveling waves. Such traveling waves render the engine to be inherently reversible and thus highly efficient. The behavior of this class of harvesters is modeled using the lumped-parameter approach. The developed model is a multifield model which combines the descriptions of the acoustic resonator, feedback loop, and the regenerator with the characteristics of the piezoelectric diaphragm. A new method is proposed here to analyze the onset of selfsustained oscillations of the traveling wave engine using the classical control theory. The predictions of the developed models are validated against published results. Such models present invaluable tools for the design of efficient TAP energy harvesters and engines