Abstract: Thermoacoustic engines (TAEs) are devices designed to be thermoacoustically unstable, that is to say, capable of sponta- neously converting external heat sources into acoustic power, which in turn can be converted to usable electrical power. TAEs do not require mechanically moving parts to operate; periodic compressions and expansions naturally associated with acoustic waves work against a background temperature difference (∆T = Thot − Tcold) to create a Stirling-like thermody- namic cycle that generates self-amplifying acoustic power. The nature of the wave energy propagation in TAEs guarantees close-to-isentropic stages in the overall energy conversion cycle, thereby promoting very high efficiencies. In the available literature, efficiencies of up to 49% of Carnot's theoretical limit have been reported (Tijani & Spoelstra 2011).
The talk will discuss both thermal-to-acoustic and acoustic-to-electric energy conversion processes by relying on high- fidelity unstructured fully compressible Navier-Stokes simulations, and companion linear and nonlinear low-order models. Two theoretical full-scale computational TAE models will be presented: the large-scale traveling-wave engine by Scalo, et al. (2015);the standing-wave thermoacoustic-piezoelectric engine by Lin, et al. (2015). Both devices have been analyzed in the early stages of acoustic energy growth and at the limit cycle, reaching acoustic amplitudes of up to +178dB. The quality of the prediction of frequencies and growth rates in the start-up phase will be discussed in both cases within the framework of Rott's theory (Rott 1980). The effect of acoustic nonlinearities, such as Gedeon streaming, will be discussed only for the traveling-wave engine. Finally, a broadband multi-pole time-domain impedance boundary condition (TDIBC) is proposed as a modeling strategy for piezoelectric energy extraction in the standing-wave engine model.