Non-linear phenomena occurring during the start-up process of a travelling-wave looped-tube thermoacoustic engine

This article reports the investigation of non-linear start-up processes in a looped-tube thermoacoustic engine. The engine utilises air as working gas, and has the fundamental frequency of around 111 Hz. It is observed that the mean pressure and the input heat power are the two key parameters controlling the start-up behaviour of the engine. When these are varied, a range of non-linear transient phenomena are observed, which include the ‘on–off’ effect, ‘fishbone-like’ oscillations (i.e. the quasi-periodic pressure amplitude bursts followed by a quasi steady state), and the normal smooth start-up process. The experiments show that the ‘fishbone-like’ bursts are a new mode of pressure amplitude growth. A series of experiments have been conducted to investigate in detail the influence of mean pressure, input heat power and regenerator type on the occurrence of such quasi-periodic amplitude bursts. It is observed that the duration of the pressure amplitude bursts depends on the combination of the heating power and the mean pressure. The observations suggest that there are strong interactions between the acoustic and temperature fields within the regenerator. It is thought that these can lead to the reported quasi-periodic unsteady behaviour of the engine.