Improvement of energy conversion efficiency of thermoacoustic engine by a multistage stack with multiple pore radii

Abstract

In the conventional thermoacoustic engine, the pore radius of stack is almost constant in the axial direction. Therefore, we focus on the improvement of energy conversion efficiency of thermoacoustic engine by proposing a new type of multistage stack with multiple pore radii. The stack is composed of several stages, each of which is a bundle of a number of narrow tubes with specified pore radii. The pore radius is determined so that its ratio to the thickness of boundary layer on the tube wall may be a suitable value for the enhancement of the thermoacoustic oscillation in the tube. Owing to the temperature gradient along the axis of the stack, however, the thickness of the boundary layer changes along the axis and hence the suitable pore radius also changes in the axial direction. We therefore introduce a multistage stack with multiple pore radii, thereby realizing a desired ratio of pore radius and boundary layer thickness everywhere in the stack. The energy conversion efficiency of the multistage stack is experimentally studied on a straight tube type thermoacoustic engine and compared with that of a conventional single-stage stack. In the experiments, in spite that a sufficiently large temperature difference from ambient temperature near low-temperature heat exchanger was not attained, we have been able to confirm a slight improvement of energy conversion efficiency. Furthermore, we used a numerical method with transmittance matrix to include the effect of multistage stack, and obtained a good agreement between the experimental and numerical results. They show the future possibilities of the stack design aiming to the higher efficiency of the thermoacoustic engine.