Small high-frequency thermoacoustic oscillators can achieve high power densities in the conversion of heat to sound. In order to achieve high sound power output, it is advantageous to use arrays of such devices. Being a group of self-sustained oscillators, they switch on with a random phase at the threshold for oscillations. Moreover, small variations in the geometry of each device will affect the resonant frequency of each; the array will then not be in unison. This can be corrected by synchronizing them through global coupling; maximum power output will then be achieved. The strength of the coupling is assumed to be the ratio of mass of one thermoacoustic oscillator to the total mass of all devices and of the support to which they are mounted. By varying the strength of the coupling, the acoustic devices show synchronization, which for weak coupling is out-of-phase and at strong coupling is in-phase. Results are presented on the coupling of two prime movers operating near 2.5 kHz as well as for similar devices in an array consisting of five units. The synchronized behavior opens the field for large array systems and their applications.
In order to build more powerful sources of sound for energy conversion, the synchronization of two thermoacoustic heat engines has been studied. Experiments were performed on engines in the acoustic frequency range of 2.6 kHz and also on very small engines in the ultrasonic range of 24 kHz. In both cases, the engines were mounted on a cylindrical cavity, and they were coupled mainly by the acoustic field in the surrounding air at one atmosphere. They were driven by heaters of resistance wire in contact with the hot heat exchanger. At a specific coupling between the 2.6 kHz, engines' synchronization occurred, and also for the 24 kHz devices; frequency pulling in each pair of engines led to a common frequency in each set, i.e., in-phase synchronization. The strength of the synchronization was determined as a function of detuning of engines. Mutual entrainment was observed at the onset of oscillations and this is attributed to drive by fluctuations. Moreover, as a result of synchronization, the critical temperature gradient for onset of oscillations was reduced from that of the individual values. Delays between two oscillators in the start up led to quenching of the generated sound output ( oscillation “death” of Rayleigh).
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