We demonstrate a prototype acoustic cooler that uses Stirling cycles executed by a traveling wave with high acoustic impedance thermoacoustically induced in a looped tube. The tube has no moving parts, only a pair of stacks sandwiched between two heat exchangers: one amplifies the acoustic power and the amplified wave supplies the driving energy to pump heat directly within the second stack. Because it uses extremely simple hardware consisting of a few parts, the cooling device is potentially a powerful tool for applications such as conventional cooling systems.
An acoustic field spontaneously induced in a thermoacoustic prime mover consisting of a looped tube and resonator is determined through simultaneously measurements of pressure P and velocity U. A thermal efficiency of the thermoacoustic prime mover of this type has been reported to reach 30%. The measurements of the acoustic field in the present system revealed that a phase lead of U relative to P takes a negative value of about Ϫ20°in the regenerator where the output power of the prime mover is generated. It was concluded that the possession of a negative phase lead at this position is taken as a clue in a significant increase in the output power. Moreover, the analysis in the thermoacoustic mechanism shows that a precise position for the location of a second regenerator acting as a heat pump exists in the looped tube. Indeed, by locating the second regenerator at the position, a thermoacoustic cooler was constructed. The thermoacoustic cooler could generate a low temperature of Ϫ25°C without involving any moving parts.
To what extent can we lower the critical temperature ratio (CTR) necessary to start a thermoacoustic engine? We present an experimental method for predicting the CTR before the temperature ratio arrives at it using quality factor measurements. Based on the experimental quality factors, we tried to decrease the CTR of a thermoacoustic Stirling engine consisting of a looped tube and a branch resonator. Installation of the multiple regenerators at suitable positions can markedly enhance acoustic power production while overcoming energy dissipation. Results show that the CTR is decreased from 1.76 to 1.19 using five differentially heated regenerators.
Using thermoacoustic energy conversions, both amplification and damping of acoustic intensity are demonstrated. A differentially heated regenerator is installed near the velocity node of the resonator and thereby a high specific acoustic impedance and a traveling wave phase are obtained. It is shown that the gain of acoustic intensity resulting from the traveling wave energy conversion reaches 1.7 in a positive temperature gradient and 0.3 in a negative gradient. When the regenerator is replaced with a stack, it is found that the gain reaches 2.3, exceeding the temperature ratio (=1.9) of both ends of the stack. This is brought about by the addition of standing wave energy conversion. The present results would contribute to the development of new acoustic devices using thermoacoustic energy conversion.
Two-sensor method proposed by Fusco et al. ["Two-sensor power measurements in lossy ducts," J. Acoust. Soc. Am. 91, 2229-2235 (1992)] is a novel technique that determines acoustic intensity of a gas column in a wide duct from measurements of pressure based on the boundary layer approximation. For further development of this method, its validity is experimentally tested through comparison with the direct method measuring the pressure and the velocity simultaneously, and its formulation is modified to include the narrow duct range where the duct radius is smaller than the viscous boundary layer thickness of the gas. It is shown that the modified two-sensor method enables quick and accurate evaluation of the acoustic intensity seamlessly from narrow to wide duct ranges.
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