Testing of a three-stage looped-tube thermoacoustic sound generator

Abstract

Thermoacoustic sound generators are devices that produce sound waves by using thermal energy. The output acoustic oscillations can be used to drive a linear alternator to generate electricity. The devices are environmentally friendly because they do not produce any exhaust gases and they can utilize waste heat to generate the sound. This paper presents an experimental testing of a three-stage thermoacoustic sound generator of a looped-tube type. Each thermoacoustic core was composed of a regenerator, a hot heat exchanger and an ambient heat exchanger. The regenerators were made of a tight stack of 30-mesh stainless-steel screens, with a length of 35 mm. The looped-tube was made of stainless-steel pipes with inner diameter of 68 mm and total length of 4.3 m. Atmospheric air was used as working gas within the looped-tube. Three electric heaters, each with input power of around 394 W, were used as the heat sources and installed at the three hot heat exchangers. It was found that the device started to generate sound when the average temperature difference between the hot and ambient sides of the regenerators reached 255 °C. After 45 minutes operation, the average temperature difference was 538 °C, producing sound waves with a maximum pressure amplitude of 2.6 kPa and acoustic power of around 4.5 W. Additionally, it was observed a sound frequency jump during operation, that was 83 Hz until the temperature difference reached 526 °C and jumped to 339 Hz after that. During transition, both frequencies existed with relatively lower pressure amplitudes.