Orest G. Symko

Design and development of high-frequency thermoacoustic engines for thermal management in microelectronics

Thermoacoustic heat engines provide a practical solution to the problem of heat management in microcircuits where they can be used to pump heat or produce spot cooling of specific circuit elements. There are basically two types of thermoacoustic engines, a prime mover where heat is converted to acoustic energy, and a heat pump or cooler where sound can pump heat up a temperature gradient. Such devices are relatively simple, they can be efficient, and they are readily adaptable to microcircuit interfacing. Since this type of engines is usually operated in a resonant mode, the operating frequency determines its size. The devices presented here are pumped at frequencies ranging from 4 to 24 kHz. They have been developed for interfacing with microcircuits as heat pumps or spot coolers. Results of their performance are presented and suggestions for further improvements are discussed.

Helmholtz-like resonators for thermoacoustic prime movers

In a thermoacoustic prime mover, high acoustic output power can be achieved with a large-diameter stack and with a cavity with a large volume attached at the open end of the resonator containing the stack. The combination of resonator and cavity makes the device Helmholtz-like, with special characteristics of the resonant frequencies and quality factor, Q. Analysis of its acoustic behavior based on a model of a closed bottle presents features that are useful for the development of such prime movers for energy conversion from heat to sound. In particular, the arrangement produces in the cavity a high sound level, which is determined by the Q of the system. Comparison with a half-wave resonator type of prime mover, closed at both ends, shows the advantages of the Helmholtz-like device.

Acoustic approach to thermal management: miniature thermo acoustic engines

An 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 factors, 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

Ultrasonic thermoacoustic energy converter

Thermoacoustic 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 3.4 mm long with a 1mm diameter bore, i.e. the engine inner volume of 2.67 mm(3). The generated sound levels reached intensities in the range of 143-150 dB in the low ultrasonic range. The miniaturization of thermoacoustic engines will lead to the development of device arrays.

Size considerations in interfacing thermoacoustic coolers with electronics

Small thermoacoustic coolers/heat pumps show much promise for heat management in microcircuits, especially since they can be miniaturized for interfacing with circuits. Usually they are operated in the resonant mode with sound pumping heat; the device dimensions are reduced when the acoustic pump frequency is raised. Results on the development of this technology will be presented for devices operating in the frequency range of 4 kHz to 25 kHz. Since the efficiency and cooling power depend on geometric factors, scaling in the miniaturization plays an important role. The cooling power varies with the total effective cross-sectional area of stack relative to sound field and hence on the cross-sectional area at stack position. We have developed and tested devices which vary in length from 4 cm down to 0.8 cm, the cross-sectional area being fixed by available acoustic drivers. An important issue is that of direct interfacing of single units with a microcircuit versus an array of smaller basic thermoacoustic units. Advantages of such arrays range from lifetime of drivers, long-term reliability of the device, to closer interfacing with chip on microcircuits. Results show that indeed the cooling power depends on the cross-sectional area of stack in a single unit. Also, the cooling power depends on level of acoustic drive; in the ultrasonic range it can be raised since the power density of such units is typically high.

Miniature traveling wave thermoacoustic engine.

Five high frequency annular thermoacoustic traveling wave devices have been developed and characterized. A 1.27 cm bore diam 2 kHz engine was optimized, increasing its maximum acoustic output from 140 to 167 dB. A new 1.60 cm bore diam 2 kHz engine has been assembled with the expectation of even higher acoustical power output because power is proportional to cross‐sectional area. In the 3 kHz regime, two geometries were investigated, an oval shape and more square version, which is easier to assemble. These devices were found to have comparable outputs suggesting that the square geometry is a viable alternative, and, thus, all additional devices were constructed this way. A 4 kHz engine was also studied. In order to measure the effects of the heat exchangers and regenerator assembly on the acoustic flow, a resonator tube was constructed. This tube was fitted with the device innards and driven by a speaker. An absorption coefficient of 0.4 was measured. This information along with the 916 T across the regen...

Acoustic approach to thermal management

Miniature thermoacoustic engines can be used quite effectively for thermal management of certain systems. Heat applied to a stack of high surface area material inside an acoustic resonator generates sound when the temperature gradient along the stack exceeds a critical value. Here heat is injected to a hot heat exchanger in thermal contact to one end of the stack. A cold heat exchanger in contact with the other end of the stack is anchored to ambient temperature by means of cooling fins. When heat is coupled to a ¼ wave resonator, 4.3cm long, an intense sound is generated at ∼2.0kHz. An acoustic cavity is attached to the open end of the resonator to provide extra positive feedback for oscillation. This self sustained oscillator starts at a temperature difference along the stack of ΔT ≈ 100°C, about 15 seconds from the time heat is applied to the hot heat exchanger. The generated sound is converted to electricity using a 3cm diameter piezoelectric material, PZT, in the unimorph configuration. Thus the device provides thermal management for the source of heat by converting heat to sound at an efficiency, which is a substantial fraction of the Carnot efficiency and directly converting the sound to electricity.

Coupling of midaudio frequency thermoacoustic prime movers.

Multiple unit midaudio frequency thermoacoustic engine arrays have been studied in terms of synchronized self‐sustaining oscillators. Thermoacoustic engines can be acoustically coupled through shared gas when attached to a common cavity. The coupling between devices was studied by varying the cavity volume, and by varying the separation distance between devices in an array. Frequency and phase measurements for two‐engine arrays have suggested that the engines entrain at a common frequency with approximately zero phase difference implying the onset of synchronization. Amplitude measurements of synchronized arrays suggest that the acoustic amplitude is equal to the sum of the amplitudes of each uncoupled engine, and agree with predictions. Synchronization of four or more units in an array implies that the coupling between engines is global and depends on the volume of the cavity and the density of the gas within the cavity. The detuning between two engines was increased until synchronization no longer occur...

Particle image velocimetry study of acoustic field in miniature traveling wave device.

Particle imaging velocimetry (PIV) techniques were employed to investigate the flow patterns inside a 2 kHz annular thermoacoustic device. This thermoacoustic device uses air at one atmosphere as its working fluid and smoke for seeding particles. For these measurements, the sound level output of the device was kept to 140 dB to help maintain the suspension of seeding particles. Several types of flow patterns were observed, including Gedeon streaming, start‐up noise characterized by instabilities such as vortices, and a traveling wave throughout accompanied by radial standing waves localized near corners. Such measurements are challenging in small high frequency devices. In order to satisfy the Shannon‐Nyquist theorem, a 2 kHz engine would need to be sampled at a minimum of 4 kHz; however, a new technique was developed that allows for the measurements to be made using a camera with a maximum frame rate per sec of 300. This new method makes it possible to perform measurements on a flow without the need for ...

High frequency operation of thermoacoustic coolers and prime movers

By operating thermoacoustic engines at high frequencies, 4 kHz and higher, the devices have characteristics which are important for many applications. Since they are resonant systems, the power density increases with frequency. Reduction of device size provides quick thermal response time in both the cooler and the prime mover. Moreover, small device size makes it practical to incorporate them into arrays, which can handle large powers. Most important is the fact that small devices make it simple for operation at high pressures in working gas without exceeding strength of materials limitations. This leads to high power densities. Results will be presented to illustrate how the above features affect device performance for the frequency range of 4 kHz to 21 kHz. Measurements using Particle Image Velocimetry of streaming, instabilities, and resonator mode interactions will be discussed for this high frequency range. Ultimately as the operating frequency is raised, device efficiency is limited by heat conduct...