Energy conversion performance in looped and stirling traveling-wave and standing-wave thermoacoustic engines

• Full-scale looped and Stirling thermoacoustic engine (TAE) models are developed. • Time-domain looped and Stirling TAE models are validated against experimental data. • Stirling TAE achieves the highest acoustic power and energy conversion efficiency. • Bistability and mode transitions are obtained under varied operational conditions. • Lowering the temperature gradient reduces mass streaming in the Stirling TAE. This study conducts a comprehensive comparison of looped traveling-wave thermoacoustic engines (TWTAEs) and Stirling TWTAEs using two- and three-dimensional (2D and 3D) time-domain models. These models, validated through comparisons with existing experimental results, confirm the accuracy of the developed full-scale numerical TWTAE models. By evaluating the acoustic characteristics and comparing them with conventional standing-wave thermoacoustic engines, this work highlights the superior performance of the Stirling TWTAE, which achieves the highest heat-driven acoustic power and thermoacoustic energy conversion efficiency. This superior performance is attributed to its operation at lower oscillation frequencies (115 Hz) and optimized phase relationships between velocity and pressure oscillations (40 degrees). Additionally, this study explores rich nonlinear phenomena within the flow fields of the Stirling TWTAE. Notably, varying the temperature gradients between 410 K and 485 K introduces a bistable zone with distinct pressure amplitudes. Moreover, mass streaming under varying temperature gradients, and mode transitions due to external perturbations are observed inside the engine. These findings not only quantitatively confirm the high efficiency potential of the Stirling TWTAE but also demonstrate the effectiveness of CFD in the developed full-scale numerical models for predicting and optimizing the acoustic characteristics and nonlinear phenomena of traveling-wave thermoacoustic systems.