High-fidelity simulation of a standing-wave thermoacoustic-piezoelectric engine

TL;DR

This paper presents high-fidelity simulations of a standing-wave thermoacoustic-piezoelectric engine, accurately modeling energy conversion processes and validating results against experiments, to aid in design optimization.

Contribution

It introduces a detailed simulation framework with advanced boundary conditions and verifies growth rates and limit cycles, advancing thermoacoustic engine modeling.

Findings

Grid-convergent growth rates verified against Rott's theory

Thermoacoustic heat leakage and frequency shifts observed at limit cycle

Piezoelectric energy extraction reduces pressure amplitude in simulations

Abstract

We have carried out wall-resolved unstructured fully-compressible Navier--Stokes simulations of a complete standing-wave thermoacoustic piezoelectric (TAP) engine model inspired by the experimental work of Smoker et al. (2012). The model is axisymmetric and comprises a 51 cm long resonator divided into two sections: a small diameter section enclosing a thermoacoustic stack, and a larger diameter section capped by a piezoelectric diaphragm tuned to the thermoacoustically amplified mode (388 Hz). The diaphragm is modelled with multi-oscillator broadband time-domain impedance boundary conditions (TDIBCs), providing higher fidelity over single-oscillator approximations. Simulations are first carried out to the limit cycle without energy extraction. The observed growth rates are shown to be grid-convergent and are verified against a numerical dynamical model based on Rott's theory. The latter is based on a staggered grid approach and allows jump conditions in the derivatives of pressure and velocity in sections of abrupt area change and the inclusion of linearized minor losses. The stack geometry maximizing the growth rate is also found. At the limit cycle, thermoacoustic heat leakage and frequency shifts are observed, consistent with experiments. Upon activation of the piezoelectric diaphragm, steady acoustic energy extraction and a reduced pressure amplitude limit cycle are obtained. A heuristic closure of the limit cycle acoustic energy budget is presented, supported by the linear dynamical model and the nonlinear simulations. The developed high-fidelity simulation framework provides accurate predictions of thermal-to-acoustic and acoustic-to-mechanical energy conversion (via TDIBCs), enabling a new paradigm for the design and optimization of advanced thermoacoustic engines.