Optimization of the Loop in a Travelling- Wave Thermo-acoustic Engine

Introduction A number of experimental realizations of thermo-acoustic engines that use an acoustic loop to feed energy back to a regenerator have been reported, mostly of the Thermo-Acoustic Stirling Heat Engine (TASHE) type. An alternative configuration is the simple wavelength loop with the linear alternator in a short side-branch, but with close to travelling-wave phasing on both sides of it. The advantage over the TASHE is that a great deal more power can be extracted for the same stroke-length of the alternator. However, a difficulty with this configuration lies in maintaining the optimum conditions in the regenerator, particularly when the temperature at the hot-end is changing. Attempts by the author to achieve consistent power generation with atmospheric air and a radiative heating system that would be cheaply realizable with a wood-burning stove have been frustrated, with very variable performance, and a thermal to electrical conversion efficiency of less than 1% [1]. This contribution focusses upon finding the optimum acoustic path in the loop feeding the regenerator, so as to give the most favourable conditions within it. Computations with a model of the regenerator have proved capable of reproducing the pressure distribution around the loop with reasonable accuracy [2]. However, attempts to use the model to determine the best loop configuration have been negated by the complex interaction of the parameters. An alternative, semi-empirical approach is described here.