Young Sang Kwon

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.

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...

Optimizing miniature thermoacoustic coolers

Because of limited sound intensity output from commercial drivers at midaudio and ultrasonic frequency ranges used in thermoacoustic coolers, it is important to optimize their performance. To achieve this, studies were conducted on heat transfer at the cold heat exchanger and at the hot heat exchanger. Measurements were taken on stray heat influx to the cooler by mechanisms of convection, conduction, radiation, and streaming, and from the driver. PIV studies show the contributions of streaming, both from the driver and other parts of the devices. A special 4‐layer copper mesh heat exchanger was designed and tested to cope with heat pumped acoustically from the cold heat reservoir as well as the streaming from the piezoelectric driver. For typical sound levels of 155–160 dB achieved inside the resonator by commercial bimorph piezoelectric drivers, a cooling power density of 0.5 watts/cm2 was achieved with air at 1 atm. Pressurizing the working gas to 17 atm raises the cooling density. The choice of working...

Ultrasonic thermoacoustic cooler

The development of a thermoacoustic cooler in the ultrasonic range is presented. This cooler was designed to operate at a drive frequency of 24 kHz using air as the working fluid; the resonator is 7.1 mm long and it contains a cotton wool stack with copper heat exchangers at each end. Since the ultrasonic driver is a key element in this device, a major effort was made to optimize its performance and coupling to the resonator. It is a resonant piezoelectric monomorph loaded with a metallic cone for impedance matching to the resonator. By its design it is capable of intense sound levels, of order 140 dB and higher. The attained cooling power scales with the sound power levels and a COP larger than one is achieved. Studies of heat losses consist of PIV imaging of acoustic streaming (such as Eckart) in this device and back heat flow along the stack. The developed device shows much promise for rapid cooling of small samples. [Work supported by the Office of Naval Research.]

Investigation of stack materials for miniature thermoacoustic engines

In a thermoacoustic engine one of the most critical components is the stack that interacts thermally with a sound field. Its effective surface area determines directly the cooling power and efficiency while the thermal properties determine the losses. Moreover, as the engines are miniaturized, stack performance on a small scale becomes a limiting factor, especially when a stack length of 1000 μ or less has to maintain temperature differences of around 30°C. We have investigated as stack material cotton wool, glass wool, ceramics, silica aerogel, carbon fibers, and carbon powder by measuring flow resistance at high acoustic frequencies, 4–20 kHz, and thermal transport properties. The mechanical properties and ease of machining to small dimensions, of the order of 1 μ, have been studied. In selecting the ideal material for very small devices, a compromise needs to be made between thermoacoustic and mechanical properties. [Work supported by ONR.]

Admixture of traveling wave and standing wave components in a miniature thermoacoustic heat engine

Because of the importance of the thermoacoustic prime mover for all sorts of applications, we have investigated its performance at different operating conditions. Of special interest is the onset of oscillation temperature difference and its dependence on the geometry of the device. This was investigated in miniature heat engines, with resonant frequencies of 2 – 5 kHz, with temperature differences where one end of the device was below room temperature and also when the other end was above room temperature. All the devices were operated with air at 1 atm in the wave resonator configuration. The onset temperature ratios depended strongly on geometry, stack, heat exchangers, and on the degree of traveling wave admixture to the standing wave produced by the resonator.

Fluctuation before onset of oscillations in a thermoacoustic device

The onset of oscillations in a resonant device where a temperature gradient produces a sound is a fundamentally interesting problem as it consists of a transition where random gas motion, when biased, produces an almost pure sound. This was studied in small 1/4‐wave resonant tubes containing a fibrous stack across which a temperature gradient was maintained by means of heat exchangers at each end. The temperature gradient biases the random motion of the gas to trigger a standing wave in the resonator. The acoustic spectrum was measured as the temperature gradient approached the onset for oscillations. Factors affecting the transition to oscillation consist of stack gain, quality factor of the resonator, magnitude of the temperature gradient, and acoustic load. The directed diffusion of the gas along the stack leads to a series of sharp pressure pulses whose stochastic behavior triggers resonant oscillations in the tube. Onset temperature differences less than 50 °C have been observed and this could furthe...