Thermoacoustic heat engines (TAHEs) are external combustion engines primarily designed to convert thermal power into acoustic power and, eventually, into mechanical, electric or other forms of high grade power. TAHEs rely on the presence of a porous core, often referred to as “stack”. A temperature gradient is established along the porous core and quasi-adiabatic heat exchanges occur between the solid walls of the pores and the surrounding gaseous medium undergoing pressure fluctuations. The internal geometry of the stack has tremendous impact on the efficiency of thermal-to-acoustic power conversion.
In this study, the selective laser melting (SLM) has been used to produce stacks. The SLM is an additive manufacturing (AM) technique designed for 3D metal printing. It is based on high power- density laser which melts and fuses metallic powders together. Three sets of stacks, provided with different hydraulic radii and internal geometries, have been produced. Each set is constituted by two stacks with similar hydraulic radii, one with internal parallel plates and one with internal oblique pin array. The SLM provides precise control of the features of the printed object, allowing to explore geometries which are difficult to manufacture with conventional technologies but, possibly, more effective in the heat exchange process. This is the case of pin (and oblique pin) array geometries, which provide a reduced amount of viscous losses when the working fluid has Prandtl number Pr < 1, e.g. air.
The printed stacks have been tested in a small scale standing-wave TAHE set up in Tallinn University of Technology (TalTech). Temperatures are monitored in proximity of the hot and cold heat exchangers, as well as the sound pressure within the engine resonator. The measured quantities are shown in time and frequency domain to analyze the onset and the stability of the thermoacoustic phenomenon.