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A gas in a tube spontaneously oscillates when the temperature gradient applied along the wall of the tube is higher than the critical value. This spontaneous gas oscillation is caused by the thermal interaction between the gas and the tube wall. The stability limit of the thermally induced gas oscillation is numerically investigated by using the linear stability theory and a transfer matrix method. It is well known that an acoustic wave excited by the spontaneous gas oscillation occurring in a looped tube is different from that in a straight tube with two ends; a traveling acoustic wave is induced in a looped tube, whereas a standing acoustic wave is caused in a straight tube. The conditions for the stability limits in both tube types were calculated. The calculated and measured conditions were compared and were found to be in good agreement. Calculations performed by varying the value of the Prandtl number of the gas were used to determine the reasons for the existence of the stability limits of the looped and straight tubes.

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The experimental evaluation of the wave number and characteristic impedance of stacked-screen regenerators is described. First, a two-by-two transfer matrix of a stacked-screen regenerator was estimated from pressure measurements performed at four different positions; then, the wave number and characteristic impedance of the regenerator were evaluated using a "capillary-tube-based" theory that models a stacked-screen regenerator as an array of pores having a uniform cross section. The evaluation was applied to seven types of stacked-screen regenerators. The experimental results show that these stacked-screen regenerators can be modeled as arrays of circular-cross-section tubes. Moreover, an empirical equation used to estimate the radius of the circular cross section of the tubes comprising the modeled stacked-screen regenerators was addressed.