This paper investigates experimentally the onset of thermoacoustic instability and ensuing emergence of self-excited oscillations of air in a looped tube with a pair of identical stacks installed inside on diametrically opposite positions. Each stack is sandwiched by hot and cold (ambient) heat exchangers so that a temperature gradient may be imposed on it in the same sense. The loop is circular except for two short straight sections in which the stack and the heat exchangers are housed. Four sets of stacks of 100, 300, 600, and 900 cells=in 2 are used to examine influences of stack's pore radius on the onset of instability and the steady state of oscillations. It is found that thermoacoustic self-excited oscillations emerge spontaneously for all sets of the stacks in a 1-wave mode where the loop length corresponds to one wavelength of oscillations. It is also found that for the stacks of 900 cells=in 2 , a 2-wave mode also emerges besides the 1-wave mode. Temperatures of the heaters are constantly monitored and excess pressures are measured at four positions. The onset of instability is examined in light of marginal conditions obtained theoretically and step-up behaviors are discussed in terms of the temperature ratio and the growth rate. It is revealed that the pressure profiles of oscillations at the steady state propagate along the loop in the same sense as the temperature gradient and that the peak-to-peak pressure takes a maximum near the stack. As it increases, emergence of acoustic shock waves is observed except for the case with the stacks of 100 cells=in 2. Physical mechanisms of shock formation are discussed in terms of eigenfrequencies of oscillations in higher modes.

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