Evaluation of random stack materials for use in thermoacoustic refrigerators

In a thermoacoustic refrigerator, energy conversion between thermal and acoustic power is achieved by means of an oscillatory motion of a compressible fluid along a solid body referred to as "stack". Traditionally, stacks have been most often made by arranging large number of thin plates at equal spacing to fill out the cross section of a thermoacoustic resonator. Other geometries such as circular pores, square or hexagonal pores (honeycombs) or pin-arrays can also be considered. Most common irregular geometry includes layers of woven wire mesh stacked along the resonator length. The advantages of thermoacoustic engines over other conventional energy conversion devices lie in their relatively simple hardware assembly, without the need for any dynamic sealing and lubrication. However, the fabrication of stacks, for example made out of very thin parallel plates, is usually costly and impractical, while using pre-fabricated stacks (e.g. ceramic catalytic converter substrates or honeycomb used in aerospace industry) has high materials costs, which limits the cost advantages of thermoacoustic engines. However, many of these problems could be avoided if irregular stack geometries made out of random (very often waste) materials could be used. There is a wide range of such candidate materials, including glass or steel wool, ceramic chippings, waste material from metal machining (swarf, Scourers), beds of glass or metal balls etc. However the main difficulty is the lack of experimental data characterising the performance of such stacks at the design stage. In this paper, the performance of a standing wave thermoacoustic refrigerator with a stack made of a few chosen random materials, is measured and compared to the one with a parallel plate stack. It is hoped that this work will be beneficial for developing low-cost thermoacoustic prime movers and heat pumps.