Analysis of thermoacoustic cooler

Abstract

Acoustic waves consist of pressure, displacement and temperature oscillations as a response to the pressure variations. The interaction of these effects in gas close to a solid surface generates thermoacoustic oscillations. At the solid surface heat can be extracted or supplied to the gas. The result of this interaction is that acoustic work is absorbed in order to transport heat, generating a temperature gradient along the solid surface. While in the reverse case a sound wave is sustained in case of a large temperature gradient along the solid surface. Utilizing this effect, heat can be transferred from a low temperature source to a high temperature source by a thermoacoustic refrigerator, while the opposite process can be achieved by a thermoacoustic prime mover which converts heat applied on solid surface to acoustical work. Thermoacoustic effects have attracted the scientist’s attention for over two centuries. But a quantitatively accurate understanding was not achieved until Rott’s (1969) studies, which has been confirmed experimentally by Yazaki et al (1979). Many thermoacoustic engines have been constructed and analyzed till now with an increasing interest. One reason for the increasing interest in thermoacoustics is from its potentiality of environment protection. Conventional refrigerators with freon gas as their working fluid are being prohibited because of its destruction of ozone layer. But the thermoacoustic engines do not use any harmful working fluid. They use mainly helium, or helium with argon, neon, xenon or such inert gaseous. The other reason is that conventional systems have moving parts, so they are less reliable and more expensive than thermoacoustic devices. Because thermoacoustic devices use no moving parts, no close tolerances, no exotic materials, the interests on thermoacoustic engines have increased a lot. In this study a thermoacoustic cooler working with atmospheric air was designed and constructed. A systematical approach was followed to design the device which had been proposed by Tijani. The system was constructed with a speaker as the acoustical source, resonator, stacks at different lengths, stack holder, signal generator, amplifier, temperature and pressure measurement system. After constructing the thermoacoustic device, pressure distributions in the resonator at various resonance frequencies were examined. The resonance frequencies of the system were approximately measured at 112 Hz, 180 Hz and 300 Hz starting from the first harmonics. The theoretical and experimental pressure distributions were compared. Changes of pressure antinode position with respect to acoustical source amplitude at resonance frequencies were studied theoretically and experimentally. The analytical and experimental results were compared. Three stacks at different lengths were constructed. After that the temperature distributions on the stack at different stack positions were examined and compared to the theoretical results with and without taking care of axial conduction effects. The maximum temperature difference, measured between the tips of the stacks are 20.4oC (10 cm length stack), 21.4oC (15 cm length stack), 27.6oC (22 cm length stack). It was concluded that the axial conduction effects are important in a system at low power. It was also measured the steady state temperature distribution on stack, and it was seen that it was linear as it should be. Later it was examined how the existence and the position of the stack affects the pressure distribution and the resonance frequency of the system. It was concluded that the existence and the position of the stack in the resonator affects the resonance frequency and pressure antinode position. In the experiments, it was measured that the resonance frequency of the system was changed between 108 Hz and 112 Hz depending on the stack position while the resonance frequency of the empty resonator is 112 Hz. Despite the fact that pressure antinode position from open end of the resonator was measured 72 cm at empty resonator resonance frequency, it was observed that the pressure antinode position decreased down to 66 cm as a result of the stack inside the resonator. At the end it was examined how slight frequency changes affects the system performance. It was measured that a few Hertz frequency changes does not affect the system performance a lot, but there is a sharp decrease at the system performance if the frequency changes are larger than a few Hertz.

Özet

Ses dalgaları içerisindeki basınç salınımları ile birlikte gerçekleşen sıcaklık salınımları sonucu akışkan ve akışkana temas eden katı yüzey arasındaki ısıl etkileşimler termoakustik olarak adlandırılır. Termoakustik sistemlerin maliyetlerinin düşük, yapılarının basit olması, atmosfere zarar veren gazlar kullanmaması, hareketli parçalarının bulunmaması gibi sebeplerle, klasik soğutma sistemlerine göre pratikteki verimlerinin düşük olmalarına rağmen üzerlerindeki ilgi artarak devam etmiştir. Termoakustik soğutucular geleceğin soğutma teknolojilerinden birisi olmaya adaydır. Bu çalışmada atmosferik hava ile çalışan basit bir termoakustik soğutucu dizayn ve inşa edilmiş, daha sonra sistem üzerinde çeşitli inceleme ve analizler yapılmıştır. Sistemin rezonans frekansları birinci harmonikten itibaren yaklaşık olarak 112 Hz, 180 Hz, 300 Hz olarak ölçülmüştür. Akustik kaynak olarak bir hoparlör kullanıldığından ideal rezonans tüpünden farklı olarak kapalı uçta hız nodunun oluşmadığı görülmüştür. Daha sonra sistemdeki basınç dağılımının şeklinin akustik kaynak genliğine bağlı olarak değişimi incelenmiştir. Ardından üç farklı uzunlukta yığın kullanılarak farklı yığın pozisyonlarında yığın üzerindeki sıcaklık dağılımları ve yığın uçları arasındaki sıcaklık farkları ölçülmüş ve teoriyle karşılaştırılmıştır. Yığınların her iki ucu arasında en yüksek 27.6oC sıcaklık farkı ölçülmüştür. Daha sonra rezonans tüpü içerisinde yığının varlığının ve pozisyonunun sistemin basınç dağılımı ve rezonans frekansı üzerine etkileri incelenmiştir. Yığının sistem içindeki varlığının ve pozisyonunun sistem çalışma frekansını ve basınç antinod pozisyonunu etkilediği gözlenmiştir. Son olarak sistemin çalışma frekansındaki kaymaların sistem performansı üzerine etkileri incelenmiştir.