Numerical modelling of thermoacoustic engines and coolers

Abstract

Thermoacoustic systems offer a promising alternative for sustainable energy conversion, leveraging the interaction between temperature gradients and acoustic waves to facilitate both power generation and cooling applications. These devices operate without moving parts, enhancing reliability and longevity, making them preferable for applications such as low-grade waste heat recovery. However, challenges remain in optimising their performance, particularly in reducing the required heating temperature and improving overall system efficiency. Traditional one-dimensional linear model such as DeltaEC remains useful for preliminary estimations, but their simplified assumptions neglect non-linear flow effects and overestimate performance, especially in configurations where strong convective or secondary flow behaviours are present. This thesis presents a comprehensive numerical investigation of loop thermoacoustic engine and cooler systems, utilising high-fidelity time-domain computational fluid dynamics (CFD) simulations. The research establishes a robust and detailed modelling framework capable of capturing complex fluid-dynamic and thermo-dynamic phenomena in oscillating flows. Special attention is given to validating numerical methodologies, optimising computational strategies, and investigating secondary flow phenomena that influence system efficiency. By addressing the limitations of traditional modelling approaches, this study provides new insights into the non-linear behaviours that impact overall performance. The study begins with the verification and validation of a two-dimensional CFD model for simulating thermoacoustic systems, including comparative assessments of turbulence models, and initial and boundary conditions. Building on this foundation, a detailed numerical investigation of a three-stage loop travelling-wave thermoacoustic engine system is presented. A novel dynamic initial condition is introduced to enhance computational efficiency and convergence stability. The simulations demonstrate the model's effectiveness in replicating experimental results under various operating conditions, particularly regarding thermal performance. CFD results show strong agreements with experimental measurements in configurations affected by DC flow. Further analysis using a membrane to suppress DC flow quantitatively demonstrates its impact on engine performance. The findings reveal that DC flow is a primary contributor to system losses, while localised convective effects such as jet-driven streaming further degrade performance. The final phase of this research involves a feasibility study of a loop thermoacoustic cooler system employing a two-stage loop travelling-wave configuration. A transient CFD model is developed and validated against both a linear model and limited experimental measurements. The study evaluates mesh and time-step sensitivity, confirming that cooling can be achieved with a well-constructed computational mesh. However, discrepancies in predicted exergy efficiency are observed between CFD, linear model and experiment. Convective losses, primarily due to DC flow and jet-driven streaming, are identified as key limiting factor, with system-level exergy efficiency falling below 2 %. Overall, this research contributes to advancing understanding of loop thermoacoustic systems and offers a validated numerical framework for future investigations. The transient CFD approach developed here enables in-depth analysis of complex, non-linear oscillating flow behaviours and offers a powerful tool for guiding component design and improving the performance of future thermoacoustic engines and coolers. Nonetheless, the high computational cost associated with high-fidelity CFD remains a limitation, highlighting the need for further research.