Synopsis: The oscillatory flow in a thermoacoustic sound wave generator is described. Oscillatory flow plays an important role in thermoacoustic phenomena, in which direct energy conversion from heat to work (oscillatory flow or sound wave) and vice versa are performed with no moving parts. A stack is essential in a thermoacoustic sound wave generator, which is the most important part of thermoacoustic equipment. Clearly, there is an optimum stack size for the oscillatory flow of a

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... c sound wave generator from the viewpoints of both heat transfer and pressure loss between the stack and the working fluid. Five kinds of stainless steel cylindrical tubes with different radii were employed to ascertain the optimum stack size. The results of experiments clarified an optimum stack size of 2.0 mm outer diameter, 1.6 mm inner diameter and 80 mm length. This stack size was verified from the viewpoint of oscillatory flow using the numerical formula calculation software, Mathematica. In addition, the use of a regular polygon instead of a circular cross section for the stack was proposed and analyzed using the hydrodynamic equivalent diameter. It was confirmed that a stack with a square or regular hexagonal cross section is practical for use in thermoacoustic equipment. In particular, regular hexagonal conduits with a side length of 0.924 mm are the most suitable shape for the stack of a thermoacoustic sound wave generator.